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OLD DOMINION UNIVERSITY

DEPARTMENT OF MECHANICAL ENGINEERING AND MECHANICS

COLLEGE OF ENGINEERING AND TECHNOLOGY

NORFOLK, VIRGINIA 23529

EFFECTS OF NOSE BI/BTIgFESS AND SHOCK-SHOCK INTERACTIONS

ON BLUNT BODIES IN VISCOUS HYPERSONIC FLOWS

By

D. J. Singh, Graduate Research Assistant

and

S. N. Tiwari, Principal Investigator

Progress Report

For the period ended December 31, 1989

Prepared for

National Aeronautics and Space Administration

Langley Research Center

Hampton, Virginia 23665

Under

Research Grant NAG-I-423

Drs. Ajay Kumar and J. Philip Drummond

Technical Monitors

FLDMD-Computational Methods Branch

January 1990

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OLD DOMINION UNIVERSITY

DEPARTMENT OF MECHANICAL ENGINEERING AND MECHANICS

COLLEGE OF ENGINEERING AND TECHNOLOGY

NORFOLK, VIRGINIA 23529

EFFECTS OF NOSE BLUNTNESS AND SHOCK-SHOCK INTERACTIONS

ON BLUNT BODIES IN VISCOUS HYPERSONIC FLOWS

By

D. J. Singh, Graduate Research Assistant

and

S. N. Tiwari, Principal Investigator

Progress Report

For the period ended December 31, 1989

Prepared for

National Aeronautics and Space Administration

Langley Research Center

Hampton, Virginia 23665

Under

Research Grant NAG-I-423

Drs. Ajay Kumar and J. Philip Drummond

Technical Monitors

FLDMD-Computational Methods Branch

Submitted by the

Old Dominion University Research Foundation

P.O. Box 6369

Norfolk, Virginia 23508-0369

January 1990

I I
0

FOREWORD

This is a progress report on the research project, "Analysis and

Computation of Internal Flow-Field in a ScramJet Engine," for the period

ended December 31, 1989. Special attention during this period was directed

to "Effects of Nose Bluntness and Shock-Shock Interactions on Blunt Bodies in

Viscous Hypersonic Flows." The work was supported by the NASA Langley

Research Center (Computational Methods Branch of the Fluid Mechanics

Division) through the grant NAG-I-423. The grant was monitored by Drs. AJay

Kumar and J. Philip Drummond, FLDMD-Computational Methods Branch, Mall Stop

156.

iii
EFFECTSOF NOSE BLUNTNESS AND SHOCK-SHOCK INTERACTIONS

ON BLUNT BODIES IN VISCOUS HYPERSONIC FLOWS

By

D. J. Singh I and S. N. Tiwarl 2

SUMMARY

A numerical study has been conducted to investigate the effects of blunt

leading edges on the viscous flow field around a hypersonic vehicle such as

the proposed National Aero-Space Plane. Attention is focused on two specific

regions of the flow field. In the first region, effects of nose bluntness on

the forebody flow field are investigated. The second region of the flow

considered is around the leading edges of the scramjet inlet. In this

region, the interaction of the forebody shock with the shock produced by the

blunt leading edges of the inlet compression surfaces is analyzed. Analysis

of these flow regions is required to accurately predict the overall flow

field as well as to get necessary information on localized zones of high

pressure and intense heating. The results for the forebody flow field are

discussed first followed by the results for the shock interaction in the

inlet leading edge region.

The forebody is modeled by slender cones and ogives with a spherically

blunted nose. A combination of Navier-Stokes and parabolized Navier-Stokes

equations is used to compute the flow field. The influence of entropy layer

thickness on the extent of the leading edge effects is also considered. The

extent of downstream effects of leading edge thickness are determined at Mach

iGraduate Research Assistant, Department of Mechanical Engineering and

Mechanics, Old Dominion University, Norfolk, Virginia 23529.

2Eminent Professor, Department of Mechanical Engineering and Mechanics, Old

Dominion University, Norfolk, Virginia 23529.

iv
numbers of i0 and 20 for cone angles of 5 ° , I0", and 20 ° . Three values of

nose bluntness are considered with the smallest nose blunting (0.0025m)

representing the sharp cone/ogive. Depending upon the flow conditions and

the geometry, significant differences have been found between the sharp and

the blunted bodies; the differences persist as far as 236 nose radii

downstream. Also, the bluntness effects decrease with increasing cone angle.

The results show that the wall quantities are not affected much by the

inclusion of high temperature effects through equilibrium chemistry.

For the flow region around the inlet, depending upon the Mach number and

the angle of attack, the forebody shock can interact either with the blunt

cowl leading edge shock or with the shock produced by the blunt leading edges

of the swept sidewall compression surfaces of the inlet. For the interaction

at the cowl leading edge, the forebody shock is assumed planar and the cowl

is modeled by a two-dimensional cylindrically blunted wedge of infinite

width. Use of the full Navier-Stokes equations is made on the cowl forebody

and the thin-layer Navler-Stokes equations are suitably modified for space

marching on the cowl afterbody. The results of the study show that the flow

around the cowl is significantly altered by the impinging shock. The peak

value of pressure is found to be nine times and heating rates eight times the

stagnation point value for the unlmplnged case at Mach 8.03. The peak values

were slightly lower for Mach 5.94 calculations. The use of solution adaptive

grids is made to properly resolve the flow field. The peak heating rates for

the unadapted grid was four times the unimplnged stagnation point value

versus eight times with the adapted grid. A preliminary study was also

conducted to determine the influence of shock-shock interaction on the blunt

leading edges of the swept sidewalls of the inlet. For this configuration,

the flow field is fully three-dlmenslonal. A three-dlmenslonal thln-layer

v
Navier-Stokes code had been used to calculate the flow field. The peak

pressure for this case is found to be 2.25 times and the peak heating three

times the unlmpinged stagnation values. The results of the study are

compared with the available experimental and numerical results. This study

presents the first full three-dlmenslonal analysis of the shock-shock

interaction on the swept inlet sidewalls and the accurate numerical solution

to date using full Navler-Stokes equations for the two-dimensional

interaction on the cowl leading edge.

vi
TABLE OF CONTENTS

FOREWORD ............................................................

SUMMARY .............................................................

TABLE OF CONTENTS ...................................................

LIST OF FIGURES .....................................................

LIST OF TABLES ......................................................

LIST OF SYMBOLS .....................................................

Chapter

I INTRODUCTION ...................................................

2 LITERATURE SURVEY ..............................................

3 DOWNSTREAM EFFECTS OF NOSE BLUNTNESS ...........................

3.1 Entropy Layer .............................................

3.2 Governing Equations .......................................

3.3 Method of Solution ........................................

3.4 Equilibrium Chemistry .....................................

3.5 Validation ................................................

4 EFFECTS OF SHOCK-SHOCK INTERACTIONS ............................

4.1 Classification ............................................

4.2 Governing Equations .......................................

4.2.1 Nondimensionallzatlon ..............................

4.2.2 Coordinate Transformation ..........................

4.2.3 Finite Volume Formulation ..........................

4.3 Method of Solution ........................................

4.3.1 Flux Vector Splitting ..............................

4.3.2 Algorithm ..........................................

4.4 Grid Generation ...........................................

Pa_ag_

iii

iv

vii

ix

xiv

xv

i

5

15

18

21

25

28

29

3O

31

41

42

43

45

46

49

51

55

vii
Chapte_

5

TABLE OF CONTENTS (Continued)

RESULTS AND DISCUSSION .........................................

5.1 Parametric Study of Nose Bluntness Effects ................

5.2 Forebody and the Cowl Shock Interaction ...................

5.3 Forebody Shock and the Inlet Sidewall Shock Interaction...

6 CONCLUSIONS ....................................................

REFERENCES ..........................................................

APPENDICES

A

B

C

EQUILIBRIUM CHEMISTRY .....................................

ADAPTIVE GRID .............................................

RADIATIVE INTERACTIONS IN LAMINAR FLOWS ...................

61

61

I01

138

155

158

165

167

169

viii
LIST OF FIGURES

F__ure

3.1

3.2

3.3

4.1

4.2

4.3

4.4

4.5

4.6

4.7

4.8

4.9

4.10

4.11

4.12

5.1

5.2

5.3

5.4

Physical model for hypersonic flow past a blunted cone ........ 16

Physical model for hypersonic flow past an ogive ............ 17

Typical grid for finite difference calculations .............. 27

Physical model for forebody and cowl shock interaction ........ 32

Physical model for forebody and inlet shock interaction ........ 33

Type I shock-shock interaction ...................... 35

Type II shock-shock interaction ..................... 36

Type III shock-shock interaction ..................... 37

Type IV shock-shock interaction ..................... 38

Type V shock-shock interaction ..................... 39

Type VI shock-shock interaction ..................... 40

Finite volume formulation ......................... 47

Unadapted grid for two-dimensional calculations ............ 58

Adapted grid for two-dimensional calculations ............. 59

Grid for three-dimensional calculations ................. 60

Comparison of wall pressure with the inviscid theory .......... 63

Comparison of shock standoff distance with the inviscid theory .... 64

Comparison of skin friction coefficient with the laminar theory .... 65

Comparison of heat transfer coefficient with the laminar theory .... 66

ix
Figur__£e

5.5

Page

Variation of wall pressure with the axial distance for 0c = 50 at

various Mach numbers .......................... 69

5.6 Variation of shock standoff distance with the axial distance for 0_ = 50

at various Mach numbers ......................... 70

5.7 Variation of wall heat transfer with the axial distance for 8_ = 50 at

various Mach numbers .......................... 71

5.8 Variation of skin friction with the axial distance for 0_ = 50 at various

Mach numbers ............................... 72

5.9 Variation of wall pressure with the axial distance for 0c = 5 °, 100 and

20 o at Moo = 20 .............................. 78

5.10 Variation of shock standoff distance with the axial distance for #_ --

20O at Moo = 20 .............................. 79

5.11 Variation of wall heat transfer with the axial distance for 0_ = 5 °, 10 o

and 20 o at Moo = 20 ............................ 80

5.12 Variation of skin friction with the axial dista,nce for 0c = 5 °, 10 o and

20 o at Moo = 20 .............................. 81

5.13 Variation of wall pressure with the axial distance for 0_ = 5 o at

various Reynolds numbers ........................ 82

5.14 Variation of shock standoff distance with the axial distance for 0_ = 50

at various Reynolds numbers ....................... 83

5.15 Variation of wall heat transfer with the axial distance for 0, = 5 o at

various Reynolds numbers ........................ 84

5.16 Variation of skin friction with the axial distance for 0_ = 5o at various

Reynolds numbers ............................. 85

5.17 Variation of wall pressure with the axial distance for ogive at various

Mach numbers ............................... 87

x
Figure Page

5.18 Variation of shock standoff distance with the axial distance for ogive

at various Mach numbers ......................... 88

5.19 Variation of wall heat transfer with the axial distance for ogive at

various Mach numbers .......................... 89

5.20 Variation of skin friction with the axial distance for ogive at various

Mach numbers ............................... 90

5.21 Variation of wall pressure with the axial distance for O0 = 10 °, 200

and ogive at Moo - 20 ........................... 91

5.22 Comparison of velocity profiles at x = 0.5, 1.0, and 5.0 m for ogive

at Mach 20 ................................. 93

5.23 Comparison of temperature profiles at x = 0.5, 1.0, and 5.0 m for

ogive at Mach 20 ............................. 94

5.24 Comparison of wall pressure for equilibrium and perfect gas ...... 96

5.25 Comparison of shock standoff for equilibrium and perfect gas ..... 97

5.26 Comparison of wall heat transfer equilibrium and perfect gas ..... 98

5.27 Comparison of skin friction for equilibrium and perfect gas ...... 99

5.28 Comparison of temperature profiles for equilibrium and perfect gas

at x - 0.5, 1.0, and 5.0 m for ogive at Mach 20 ............ 100

5.29 Computational domain .......................... 102

5.30 Temperature contours for the cowl forebody, Moo --- 8.03 ....... 105

5.31 Mach number contours for the cowl forebody, Moo - 8.03 ....... 106

5.32 Pressure number contours for the cowl forebody, Moo = 8.03 ..... 107

5.33 Density contours for the cowl forebody, Moo -- 8.03 .......... 108

5.34 Variation of surface pressure for the cowl forebody, Moo -- 8.03 .... 109

• 5.35 Variation of surface heat transfer for the cowl forebody, Moo - 8.03. 110

5.36 Temperature contours for the cowl forebody for Moo - 8.03 with

adapted grid ................................ 112

xf
Figure Page

5.37 Mach number contours for the cowl forebody, Moo = 8.03 with

adapted grid ............................... 113

5.38 Pressure contours for the cowl forebody for Moo = 8.03 with adapted

grid..................................... 114

5.39 Density contours for the cowl forebody, Moo = 8.03 with adapted grid 115

5.40 Velocity vectors for the cowl forebody, Moo = 8.03 with adapted grid. 116

5.41 Variation of surface pressure for the cowl forebody, Moo = 8.03 with

adapted grid................................ 118

5.42 Variation of surface heat traxmfer for the cowl forebody, Moo = 8.03

with adapted grid............................. 119

5.43 Temperature contours on the windward side of the cowl afterbody,

Moo = 8.03................................. 122

5.44 Temperature contours on the leeward side of the cowl afterbody,

Moo = 8.03................................. 123

5.45 Mach number contours on the windward side of the cowl afterbody,

Moo = 8.03................................. 124

5.46 Mach number contours on the leeward side of the cowl afterbody,

Moo = 8.03................................. 125

5.47 Variation of shock standoff distance with axial distance for the cowl

afterbody, Moo = 8.03 .......................... 126

5.48 Variation of surface pressure with axial distance for the cowl afterbody, Moo = 8.03 ............................. 127

5.49 Temperature for the cowl forebody, Moo = 5.94 ............. 128

5.50 Mach contours for the cowl forebody, Moo = 5.94 ............ 129

5.51 Pressure contours for the cowl forebody, Moo = 5.94 .......... 130

5.52 Density contours for the cowl forebody, Moo = 5.94 .......... 131

5.53 Variation of surface pressure for the cowl forebody, Moo = 5.94 . . . 132

xii
Figure . Page

5.54 Variation of surface heat transfer for the cowl forebody, Moo = 5.94. 133

5.55 Mach number contours on the windward side of the cowl afterbody,

Moo -- 5.94 ................................. 134

5.56 Mach number contours on the leeward side of the cowl afterbody,

Moo = 5.94 ................................. 135

5.57 Variation of shock standoff distance with axial distance for the cowl

afterbody, Moo = 5.94 .......................... 136

5.58 Variation of surface pressure with axial distance for the cowl afterbody, Moo = 5.94 ............................. 137

5.59 Pressure contours for the unimpinged case in stagnation (0 = 0) and

z = constant plane ............................. 140

5.60 Temperature, Mach number and the Pressure contours in the stagnation plane(0 = 0) ............................ 142

5.61 Temperature contours in the z -- 2.3, 2.6, and 2.8 cm plane ...... 143

5.62 Local regions of high heating rates and corresponding schlieren photograph ................................... 144

5.63 Surface pressure contours for Type V interaction ............ 145

5.64 Pressure contours in the stagnation plane(8 = O)............ 146

5.65 Comparison of surface pressure with the experimental data ...... 149

5.66 Comparison of surface heat transfer with the experimental data. . . 150

5.67 Variation of the surface pressure in various y = constant planes . . . 151

5.68 Variation of the surface heat transfer in various y = constant planes 152

5.69 Variation of the surface pressure in various z = constant planes . . 153

5.70 Variation of the surface heat transfer in various z = constant planes 154

xiii
LIST OF TABLES

Table Page

5.1 Flow conditions for the forebody calculations ............. 67

5.2 Entropy swallowing distance (x/P_) for 50 cone at Moo = 10 .... 74

5.3 Distance (x/R_) up to which bluntness effect persist for Moo = 10.. 75

5.4 Distance (x/R.) up to which bluntness effect persist for Moo = 20.. 76

5.5 Freestream and stagnation point conditions for the cowl ....... . 104

5.6 Freestreazn conditions for inlet ...................... 139

xiv
a

CDT

Cl

Cp

Ch

e

J

,h

M

P

Pr

Q

q

e.

R,S

Re

R.

S

T

U

LIST OF SYMBOLS

= speed of sound

= drag coefficient of spherical nose

= coefficient of skin friction

=specific heat at constant pressure

= coefficient of heat transfer

= total energy per unit volume

= transformation jacobian

= mass flow rate

= Mach number

= pressure

= Prandtl number

= conservation variable

=heat transfer

= wall heat transfer, nondimensionalized by p_ U_

= viscous terms of Navier-Stokes equations

= Reynolds number

= nose radius

= distance along body surface measured from the stagnation point

=5/R.

= swallowing distance

= temperature

= axial velocity

XV
X

Y

-y

6.h

0

e_

/z

--streamwise coordinate in physical domain measured from

virtualtip of sharp nose

=x/R,,

= normal coordinate in physical domain

= ratio of specific heats

= shock generator angle

= shock standoff distance

= conical shock angle

= angular position on cowl forebody

= cone half angle

= dynamic viscosity

_, r/, _": curvillinear coordinates

P

"7

1"

= density

= stream function

= ratio of specific heats

= viscous shear stress

Subscripts

c

c

/

o

tU

O0

= inviscid sharp cone conditions

-local conditions at the boundary layer edge

- ith specie

-stagnation point conditions

= wall conditions

- freest ream condit ions

xvi
Chapter 1

INTRODUCTION

There is a renewed interest in the hypersonic flow regime after a gap of

over a decade primarily because of the proposed National Aero-Space Plane. This

hypersonic aircraft is to be powered by an airbreathing engine which should be

very closely integrated with the airframe to avoid severe drag penalties. The whole

forebody of the vehicle is used to precompress the airflow before it enters the engine

inlet. The air is further compressed inside the inlets and the supersonic combustion

takes place inside the scramjet combustor. The burned gases are expanded through

the nozzles as well a_ on the aft portion of the undersurface. Integrating the airframe with the propulsion system for the hypersonic aircraft makes the traditional

wind tunnel guided design not only difficult but very expensive. However, due to

recent advances in computer architecture and efficient algorithm, it is now possible to solve more and more complex problems numerically. The Computational

Fluid Dynamics (CFD) has emerged as an extremely valuable engineering tool in

aerodynamic analysis and design. A wide variety of very complicated flows are being simulated by computers. At the NASA Langley Research Center, a great deal

of numerical and experimental efforts are directed toward a better understanding

of the complex flow field in different regions of the Aero-Space Plane. Numerical

modeling of the flow field has proven to be a valuable tool for getting better insight
2

0

into the complex nature of these flows. This approach in conjunction with ongoing

experimental program provides an effective analysis capability.

The forebody geometry is relatively simple and fixed and can be modeled

by long slender bodies of revolution with blunted nose. A reusable vehicle will

probably not have an ablative surface. Thus, the forebody flow field analysis appears

fairly straight forward. But, in addition to the prediction of wall properties, such

as skin friction, heat transfer and pressure, an accurate estimate must be made of

the entire flow field in order to predict the mass and the momentum flux entering

the inlet. The thickness of the boundary layer at the inlet face is important for

predicting the inlet performance. It is, therefore, necessary to incorporate realistic

geometrical modeling and flow conditions in the analysis of the forebody. One of

the geometrical features which can have significant effect on the entire flow field is

the nose bluntness. In hypersonic flows over slender bodies, the influence of nose

bluntness can be significant for hundreds of nose dimensions downstream. Thus, it

is important to develop efficient and reliable methods for predicting the effects of

blunt nose on the entire flow field.

In most studies involving flow past slender bodies, it is assumed that the

leading edge is infinitely sharp. In practice, it is impossible to manufacture a body

with zero leading edge thickness as well as allbodies must have some finitebluntness

in order to sustain the heat generated at hypersonic speeds. The leading edge heat

transfer can be decreased by increasing the leading edge thickness. However, this

resultsin a drastic increase in drag. With the advancement in material technology,

itispossible to reduce the leading edge thickness without encountering intolerable

heating rates in the vicinity of the nose. But the extent to which this bluntness

influences the development of flow downstream is not known precisely for hypersonic flows. In general, such a calculation is very difficultbecause of the various

competing effectssuch as pressure interaction,vorticityinteraction,and curvature
parameters. To calculate the flow over blunted slender bodies, two sets of equations

are utilized in the present investigation. For the nose section, the use of full NavierStokes equations is necessary due to the presence of the subsonic region; but slightly

downstream of the nose region on the body, the flow becomes predominantly supersonic and the space marching parabolized Navier-Stokes equations can be used. A

shock fitting algorithm for both the Navier-Stokes and parabolized Navier-Stokes

equation is used. The real gas effects which might be present at hypersonic speeds

are also considered.

Another impact that a blunt leading edge can have is on the inlet flow

field. The optimum propulsion efficiency occurs when the forebody shock impinges

on the engine cowl lip so that all the precompressed air is captured by the inlet.

Depending upon the Mach number and the angle of attack the forebody shock can

interact with the shock from the compression surface leading edges of the inlet.

This interaction results in a very complex fIow field and can result in large increase

in pressure and heat transfer over certain localized regions. The large temperature

gradients cause thermal stresses which could result in structural failure. In order to

relieve the influence of thermal stresses, some form of active cooling is needed. To

determine the cooling requirements, pressure and heating rates on the body need

to be predicted accurately. Such phenomena also occur on space shuttle and other

high speed vehicles at the point where the bow shock from the nose interacts with

the wing leading-edge shock.

For accurate predictions of shock interference patterns and aerothermal

loads, it is necessary to employ modern algorithms. The classical central difference

numerical algorithms have less than desired resolution in the shock regions. In

recent years, the upwind biased algorithms have become a popular alternative to

the central difference schemes. These methods model the characteristic nature of the

equation. The information at each grid point is obtained from the direction dictated
by the characteristic theory. The particular upwind method used here is the flux

vector splitting due to van Leer which is used to solve the compressible NavierStokes equations with upwind biased differencing for the inviscid terms and central

difference approximation for the viscous terms. The cowl forebody is modeled by a

cylindrically blunted wedge of infinite width which allows a two-dimensional analysis

of the flow field. An adaptive grid technique is used to properly resolve the flow

field features and to accurately predict the surface heat transfer. The use of full

Navier-Stokes equations is made on the cowl forebody and the thin layer NavierStokes equations are suitably modified for the space marching on the cowl afterbody.

For the forebody shock interaction with the inlet sidewall, the compression surface

leading edge is modeled by a swept wedge. In this case, the line of intersection of the

forebody shock and the compression surface leading edge shock is curved making

the flow field fully three-dimensional.

The objective of the present investigation is to develop an accurate solution procedure for determining the zone of influence of the leading edge thickness

and to calculate the shock interference phenomenon with an accurate prediction of

aerothermal loads on the blunt leading edges of slender bodies in viscous hypersonic

flows. The high temperature effects which might be present at hypersonic speeds

are accounted for by using an equilibrium gas model. The influence of entropy

layer on the leading edge effects is also addressed. To accurately predict the shock

interactions, the importance of a suitable grid is discussed. Two types of shock

interference patterns are considered. The analyses ranges from a few simplified approaches to the solution of the combination of full Navier-Stokes and parabolized

Navier-Stokes equations. The solution procedures are validated by comparing the

present results with the available experimental and numerical data.
Chapter 2

LITERATURE SURVEY

In this chapter, the theoretical and analytical works carried out in

the area of the effects of nose bluntness and shock-shock interactions are reviewed.

The effect of blunt leading edge on flat plates and slender bodies is considered first

and then the available work on shock-shock interactions is reviewed.

Early studies on the effects of nose blunting are summarized in classical

books on hypersonic flows [1] - [5]*. They deal with approximate approaches such as

the Newtonian theory, small disturbance theory, and constant density assumptions.

Some empirical correlations have been developed on the basis of experimental data.

But these approaches have limited validity and are very restrictive in nature.

Some specific studies of nose bluntness effects have been carried out for

flat plates [6] - [10]. Hammitt and Bogdonoff [6] carried out experiments on blunted

flat plates. A wide range of Mach number and Reynolds number were considered

for various leading edge thicknesses. It was found that the flow over a flat plate

with thick leading edge is essentially inviscid but viscosity plays an important role

for thin leading edges. Also the effect of leading edge is felt for several thousand

leading edge thicknesses downstream. Vas et al. [7] did an experimental study of

tl_e shock shape and surface pressure distribution about thick two-dimensional and

axisymmetric bodies. The shock shape was found to be parabolic about flat plates

*The numbers in brackets indicate references.

5
with flat and cylindrical leading edge for _ < 10. The pressures on the cylindrical

sections can be predicted reasonably well by the Newtonian theory and the pressure

coefficient ratio on the surface was found to be higher for two-dimensional bodies

than for axially symmetric bodies. Allegere and Bisch [8] studied the effects of the

angle of attack and leading edge on the flow about flat plates at Mach 18. The

Reynolds number considered were 70-15000. It was found that the wall pressure

increases with increasing leading edge thickness and angle of attack.

Cheng et al. [9] did a theoretical and experimental study of the leading

edge bluntness and boundary layer displacement effects for the situation where both

effects are equally important. The interaction of the two effects was treated theoretically by extending the basic shock layer concept. In the outer inviscid flow, a model

consisting of a detached shock wave and an entropy layer was introduced to account

for the bluntness. In the boundary layer the approximate solution was found to be

governed by the local flat plate similarity theory. Bradfield et al. [10] conducted

an experimental investigation to ascertain the effects of leading edge bluntness on

the laminar boundary layer of a flat plate. It was demonstrated that the effect of

the leading edge bluntness is an important consideration in the formulation and

development of the boundary layer on a flat plate at supersonic speeds. The ratio

of leading edge thickness to the value of boundary layer thickness at the measuring

stations may be taken as criterion of the magnitude of this effect.

The flow field over slender bodies has been studied by various investigators. However, most of these studies are limited to rarefied flows [11] - I14] and/or

to a very small nose bluntness [15]-[28]. Vas and Sierchio [11] performed an experimental study to determine the downstream effects of nose bluntness in hypersonic

rarefied flows. The study was carried out at Much 25 and freestream Reynolds

number of 11000 per inch on 5 o half angle sharp and blunted cones. Significant

differences between the sharp and blunted bodies were noted on the surface and
7

flow quantities. McCroskey et al. [12] studied the leading edge flows over slender

bodies in rarefiedhypersonic flows. The primary attention was directed upstream

of the merged layer. Based on the experimental data itwas established that there

is a competition between the displacement and merging effectsin determining the

surface pressure and heat flux. Feik et al.[13] conducted experimental studies on

cones of 5°,100 and 150 half angles at Mach 25 and Reynolds number 7000-15000 per

inch. The effectof bluntness within the merged region and the flow downstream of

the merged region was studied. Significanteffectsof the nose bluntness were found

on the location of merging, the location of shock wave afterthe merging, and both

the surface pressure and heat transfer within and considerably downstream of the

merged region.

Vogenitz et al.[14]conducted numerical experiments on rarefiedflows of

a monoatomic gas about slender cones and flatplates at Mach 10 and 25; cone

half angle ranged from 3°-15°. The gas model was composed of hard sphere model

which accommodates completely to solid surface and reflectdiffusively.Leading

edge flows on sharp slender cone was found to differsubstantially from that on

the flatplates. Burke and Curtis [15]measured staticpressure distributionon the

surface of a blunted 7.5 degree half angle cone for Mach 8-18 and Reynolds number

of 1 x 108 per foot. It was found that the position of minimum pressure and the

rate of pressure recovery depends upon cone angle. Bertram [16] examined the

aerodynamic effectof nose blunting on a 100 cone, the blunting consisted of a plane

cut normal to cone axis. The shock shape was found to have an inflectionat about

40-60 nose radii from the nose and the surface pressure reaches a minimum. For

lower values of Reynolds number, the surface pressure data were not affected to

any considerable extent by viscous effects.Dewey [17]studied the slender vehicles

operating in high-altitude,high-temperature environment. The study showed that

a regime of significantpractical interestexists in which the effectsof leading edge
bluntness and viscous interaction are of comparable magnitude; the liftand drag

depend to a large extent on the relativeei_'ectof the leading edge bluntness and

boundary layer displacement.

Cheng and Pallone [18}investigatedthe inviscidleading edge ei_ectsin hypersonic flow on the basis of small perturbation theory. The resultswere obtained

under the assumption of one dimensional adiabatic flow of an ide_lgas and infinite

shock strength. Various correlationsfor surface pressure and shock shape were developed for several values of "7.Lees and Kubota [19]analyzed inviscidhypersonic

flow over blunt nosed slender bodies. Based on similaritytheory, correlationsfor

shock shape and surface pressure for slender bodies were developed. It was found

that the shape of the bow shock wave, flow fieldand wall quantities are dominated

by the blunt nose over a downstream distance many times greater than the characteristicnose dimension. Cheng [201,on the basis of small perturbation theory,

developed the laws of similitude for hypersonic, inviscid,real gas flow fieldover

blunted flatplates,cones and wedges. The resultsfor real gas flow over a cone at

zero yaw agreed with the laws of similitude developed. Also the thickness of the

singular region near the surfaceof a blunt region was estimated and itseffectson the

pressure prediction was determined. Lees [21]calculated laminar heat transferover

blunt nose bodies at hypersonic flightspeeds for limiting cases of thermodynamic

equilibrium and diffusion as controlling factors for the rate of heat transfer. De

Jaxnette and Davis {221 developed a simplifiedmethod for calculating the laminar

heat transfer over blunt bodies with only freestrea_nMach number and the ratioof

specificheat as input paxameters. The viscous problem was simplifiedby using the

a_xisymmetric analog for three-dimensional boundary layersin conjunction with the

Lees' laminar heating rate relation.The resultsfor 15o and 20o cones at freestream

Mach number of 10.6, for various angles of attack, showed good agreement with

experimental data.
In recent years, certain numerical and experimental efforts have been directed to investigate the effects of nose bluntness on slender bodies [23] - [28 t.

Stetson [23] conducted an experimental study on blunted cones to determine the

leading edge effects on the boundary layer transition. Based upon the results it was

concluded that transition depends upon the 'entropy swallowing distance' which

in turn is a strong function of the leading edge thickness. Nowak [24] did an experimental investigation at Mach 6.7 to determine the effects of Reynolds number,

angle of attack and the nose shape on aerothermal environment of a 12.40 half angle

cone. Three nose tip configurations were tested on the cone; a three inch radius,

a one inch radius and a sharp tip. Heating rate distribution, surface pressure distribution, shock shape and shock layer profiles were measured and compared with

predictions. Surface pressure and heating rates when normalized by their stagnation values were found to be independent of the freestream Reynolds number. With

the increasing bluntness, the surface pressure took longer distance to recover from

the nose overexpansion.

Zoby and Simmonds [25] did engineering calculations over hyperboloids,

paraboloids, ellipsoids and sphere-cones. The heating and constant or variable

entropy conditions were coupled with an approximate inviscid code. Results were

obtained for various angles of attack and good comparison with experimental data

was obtained. Thompson et al. [26] did a numerical study to assess the applicability

of viscous shock layer code and various engineering codes for aerothermal predictions

over slender spherically blunted cones. The numerical results were compared with

experimental data from flight and ground based tests. The effects of nose blunting

and angle of attack on drag and heat transfer were illustrated. Gupta et al. [27]

obtained numerical solution for hypersonic laminar and turbulent flows over slender

bodies. The results were obtained from the viscous shock layer equations at Mach

8-15 for sphere-cones and hyperboloids. Detailed comparisons were made with
10

other numerical results and experimental data to assess the accuracy of viscous

shock layer solutions. Zoby et al. [28] conducted a parametric study of laminar

nonequilibrium heat transfer on slender vehicles. The study included the variation

of altitude-velocity conditions, cone half angle and nose bluntness. The effect of

wall catalycity to reduce the heating rates was also investigated.

A great deal of work is currently underway on shock-shock interactions at

various places due to the practical significance of the phenomenon in complex high

speed flows. In relation to the National Aero-Space Plane, special attention is directed to study the shock-shock interaction at the cowl plate and the inlet sidewalls

of the scramjet engine inlet. In order to avoid the structural failure, it is very crucial

to calculate the thermal stresses arising in the scramjet inlet. To have an optimum

design in terms of structural strength and cooling requirements, accurate calculations of surface pressure and heating rates are of utmost importance. Due to the

complexity of the problem, most of the earlier works are either experimental or of

highly simplified model where empirical inputs have been used. The effects of Mach

number, Reynolds number, impinging shock strength and sweep angle have been

studied on surface pressure and heat transfer over fiat plates, cylinders, hemispheres

and various leading edge geometries. The experimental data has unusually large

scatter due to difficulties in making accurate measurements over the narrow zone

where the impingment occurs. Until recently, the numerical calculations were virtually nonexistent. With the rapid advancement in computer hardware and improved

algorithms, it is now possible to compute such complex flow field accurately.

Teterin [29] conducted experimental study on the effect of incident shock

on the flow field and force effects on the cylinder. The cylinder was mounted on a 250

wedge. In order to isolate the effects of shock-shock interaction from separation,

the separation zone was eliminated by means of a fillet. Schlieren pictures were

obtained and the pressure distribution was measured. Sukovstin and Shestova [301
11

studied effects of bow shock impingment on a body located in the influence zone

of primary body {creating the bow shock) . An approximate integral method was

presented and results were obtained for Mach 3-20.

Edney [31] in 1968 conducted a detailed study of the shock-shock interaction phenomenon at Aeronautical Research Institute of Sweden. Three basic

models, a 30 mm diameter hemisphere/cylinder, a 30 mm diameter fiat faced cylinder, and a 30 o half angle cone/cylinder with 5 mm nose radius and 15 mm base

radius were tested at Mach numbers of 4.6 and 7.0. Edney was the first to classify these interactions into six basic patterns, which can occur when an extraneous

shock interacts with the bow shock of a blunt body. The interactions could result in

shock-boundary layer interaction {Type I, II, V), shear layers {Type III), supersonic

jet {Type IV) or expansion fan-boundary layer interactions {Type VI). All of these

interactions, except for Type VI, increase the surface pressure and heat transfer

over a localized zone. The Type IV interaction is the most severe one, producing

largest increase in pressure and heat transfer. The type VI interaction is the only

one that causes the heat transfer and pressure to decrease due to production of an

expansion fan.

Keyes and Hains [32] continued the work of Edney for higher Mach numbers, Reynolds numbers and various values of specific heat. All six types of interaction were studied. The planar shock waves were allowed to impinge upon the bow

shock surrounding swept fins, hemisphere and wedges. The impinging shock wave

was generated by a wedge. Heat transfer measurements were obtained using a phase

change paint technique. Peak heating rates of 17 times and peak pressure of eight

times the unimpinged stagnation point values were measured. The results showed

that pressure and heating rates are strongly affected by Mach number, specific heat,

impinging shock strength and geometry.

Craig and Ortwerth [33] investigated the leading edge shock impingment
12

problem at Mach 15 in the Airforce Applied Physics Laboratory. This study was

carried out to determine the pressure and temperature amplifications on the leading

edge of hypersonic inlets. The peak heat transfer amplification on leading edge was

found to be less than five. In this study the spacing between the pressure taps

and the thin film thermometer was too large to give adequate resolution. The data

obtained cannot be used due to lack of resolution and unavailability of information

on the position and strength of the impinging shock. Wieting [34] conducted

an experimental study on the shock wave interference over a cylindrical leading

edge at Mach 6.3, 6.5 and 8.0. The model consisted of a three inch diameter

cylinder. The results were presented for heat transfer and pressure distribution for

a two-dimensional shock wave interference on a cylinder. This was considered to be

representative of the cowl leading edge of the scramjet inlet. The local heat transfer

rate and pressure were amplified up to ten times the unimpinged stagnation point

values.

Several semi-empirical approaches have been proposed to theoretically predict the peak heating and pressure [35] - [40} but they rely on several empirical

inputs which must be known apriori. Morris and Keyes [35] developed a computer

program based on semi-empirical correlations to calculate the peak pressure and

heating rates for all six types of interactions. The correlations require various inputs such as width of impinging jet and length of transmitted shock. In general this

information is not known apriori. Crawford [36] improved upon the Edney's graphical approach for predicating peak pressure and heat transfer by plotting a family

of pressure deflection curves with the pressure ratio on a logarithmic scale and flow

deflection on linear scale thus eliminating the need for iterations as Mach number is

uniquely defined on this set of coordinates. Keyes and Morris [37] developed correlations for peak pressure and heating for shock/boundary layer interaction and shear

layer attachments for two-and three-dimensional interactions. The Mach number
13

considered were 6 and 20 over a freestream Reynolds number range from 3.3-25.6

million per meter and specific heat ratio of 1.4 and 1.67. The shock generator angle

was varied from 5 o - 25 °. Bramlette [38] developed an approximate method for

computing pressure for Type III and IV interference. The peak pressure obtained

for various shock generator angles showed good comparison with the experimental

data. Markarian [39] studied the problem of aerodynamic heating caused by shock

wave and boundary layer interactions. It was found that heating rates for this

type of interaction are significantly higher than normally predicted values; also the

heating rates were found to be higher for laminar interaction than turbulent interaction. Some empirical correlations were developed to predict the heating based

on the inviscid pressure rise across the interaction. Bertin et al. [40] studied shock

interference pattern for delta wing orbiters. The surface pressure and heat transfer

distribution in the interaction perturbed region were obtained for velocities from

1167 m/s to 7610 m/s. At high velocities, the Type IV interaction was found to

exist for wing leading edge sweep as low as 27 °. For this case, no locally severe

heating rates were obtained.

There are very few numerica{ studies available on the shock-shock interaction due to the complexity of the problem. Tannehill et al. [41,42] presented the

first successful numerical simulation of the interaction problem based on the explicit

MacCormack method. White [43] solved this problem using the Reynolds-averaged

Navier-Stokes equations (based on an implicit finite-volume method) and obtained

results for Mach 6.0. Klopfer [44] conducted an extensive study for various Mach

numbers using the second-order implicit TVD algorithm but the results for the

peak heat transfer for the Type IV interaction were off by a factor of about 2.25,

although the pressure was in close agreement with the experimental results. Perry

et aJ. [45] used the Roe scheme to solve the Type IV interactions but did not give

any heat transfer results. Moon et al. [46], in a recent paper, provided the results
14

for the Type III interference pattern. The study indicated that the flow for this

type of interaction in the impingement region is turbulent rather than laminar.

Most of the studies mentioned earlier to study the shock-shock interaction

are two-dimensional in nature except [32], thus cannot be used for interaction on

swept inlet sidewalls. The only related numerical study on three-dimensional interaction was carried out by Holst et al. [47]. They solved the three-dimensional shockshock interaction on an infinite cylinder using the explicit MacCorrnack scheme.

The viscosity was assumed an order of magnitude higher than the actual viscosity

to physically thicken the boundary layer and a very coarse grid was employed. The

resulting solution was at best qualitative in nature.

The literature survey clearly indicates the need for an additional extensive

study in this important field of current interest.
Chapter 3

DOWNSTREAM E:F:FECTS OF

NOSE BLUNTNESS

In this chapter the effects of nose bluntness on the flow field and wall

quantities over forebody of a generic hypersonic plane are considered. The calculation of hypersonic viscous flow field past long slender axisymmetric blunt bodies

is of prime interest to the designer of aerospace vehicles. A wide variety of flow

conditions are encountered during the trans-atmospheric flight of these vehicles. As

mentioned previously, the forebody of these vehicles must have finite thickness in

order to sustain the pressure and heating rates encountered at hypersonic speeds.

The forebody is modeled by spherically blunted cones and ogives. The physical

model for hypersonic flow past a blunt-nosed slender body is shown in Fig. 3.1.

The relative positions of boundary, entropy, and shock layers are indicated in the

figure. The first physical problem considered is the flow past a blunt nosed slender cone. Numerical results have been obtained by employing a combination of a

Navier-Stokes code and a parabolized Navier-Stokes code. These are discussed here

briefly and some specific results are presented in Chap. 5.

In the present study, 5 °, 10 °, and 20 ° half-angle cones are selected first with

spherical nose tip as shown in Fig. 3.1. Three nose tips with radius of 0.0025m,

0.025m and 0.05m are considered, with the smallest nose radius tip approximating

the sharp tip. Blunting was accomplished by keeping the cone angle fixed and

15
16

am,- x

Fig. 3.1 Physical model for hypersonic flow past a blunted cone.
17

1

Rc

____J

I_ L _1

y.. x 12- XlRc

L L

tang@nl poinl

Fig. 3.2 Physical model for hypersonic flow past an ogive.
18

increasing the nose radius. The bluntness effectsare determined by comparing the

solutions obtained from the blunted tips with that obtained from the sharp tip. As

shown in Fig. 3.1, the origin of the coordinate system is at the virtual tip of the

sharp cone with x axis along the symmetry line and y axis normal to it. Due to

symmetry of the flow fieldat zero angle of attack,only half of the flow iscalculated.

The second geometry considered is a generic forebody (Fig. 3.2) with three nose

tips of same radii as the cone. Here again the smallest nose radius approximates

the sharp tip body.

3.1 Entropy Layer

Nose bluntness at hypersonic speed causes shock wave to be detached and

curved in the nose region. The entropy of the flow increases across a shock wave,

the stronger the shock wave, the larger the entropy increase. So the streamline

passing through the nearly normal portion of the shock will have larger entropy

increase than a streamline passing through the weaker portion of the shock. This

change in entropy production in the nose region generates a layer of flow with

entropy gradient, known as the entropy layer,which "influences"the development

of the boundary layer in two differentways. First, itcauses a continuous change

of the flow properties at the outer edge of the boundary layer in the strearnwise

direction and second, itproduces a velocityand pressure gradient at the outer edge.

These external gradients invalidatethe classicalboundary layerapproach. However

Ferri [48]pointed out that these gradients are important when the vorticityof the

inviscidflow isof the same order as that of the boundary layer. These conditions

may exist when the Reynolds number is low and the M_h number ishigh. The

thickness of the entropy layer is a function of the leading edge bluntness; even a

small leading edge thickness generates an entropy layer which influencesthe local

flow conditions for a large distance downstream. For some distance downstream of
19

the blunt nose the boundary layer grows inside the entropy layer which eventually

is swallowed by the boundary layer in the case of flow past blunted cones. This

distance is called the entropy swallowing distance. For conical flows, the entropy

swallowing distance is defined [23] as "the location at the cone frustrum where fluid,

which has gone through the strong portion of the bow shock, has been swallowed

by the boundary layer." This is by no means a precise definition as it depends

upon the shock shape, boundary layer assumptions, and definition of the entropy

layer thickness. The presence of the entropy layer has an important effect on the

aerodynamic heating predication.

For hypersonic flows over a blunted slender cone, the curvature of the shock

produces the entropy layer as shown in Fig. 3.1. Once the shock attains conical

shape, it is no longer curved and hence no more entropy gradient is produced.

The thickness of the entropy layer decreases as it moves downstream on the body

and eventually it is swallowed by the boundary layer. The entropy effects in the

boundary layer vanish asymptotically further downstream.

To calculate the entropy swallowing distance without calculating the flow

field, certain assumptions regarding the shock shape and the boundary layer have

to be made. Several investigators have attempted to approximate the shock shape

for slightly blunted cones. Klaimson [49] has given one of the most accurate, yet

simple, expression for the shock shape as

= 1.424cos 0¢ [CDT°5 (co-_0_) ] °'4e (3.1)

For hypersonic speeds, the drag on the hemispherical nose can be calculated as

CDT = 2--Cos 20¢ (3.2)

A comparison of this shock shape with numerically calculated shape has been shown

to give excellent agreement [49] within the region of interest. The value of _, where
2O

the shock approaches the conical angle, is given by

y_ = cos 0_ 0.984CDr sin _ 6_

The angle 6o can be obtained from the relation

1)] °'42e (3.3)

Moo sin _ = 4 + 1.01 (Moo sin 0_ - 3.43) (3.4)

With the known value of !/_, the entropy swallowing distance, So, can be approximated as [50]

]

S_.L = 1.5_'_ Moo }

Re_ .P,/Poof2(r},) 4M,sin20c

In Eq. (3.5), f(r/) is the transformed stream function defined as

(3.5)

¢(s,_) = (2_1⁄2f(_), r= &

u,

(3.6)

where

p,u,e_ for --Pdy

- (2s)1⁄2 p,

-S = Z"p,U,#,R_dS

In the present study, entropy swallowing distance iscalculated by considering the mass flow rate. The entropy layer thickness is calculated in the downstream region, and the point where the entropy layer thickness becomes equal to the

boundary layer thickness is termed as the entropy swallowing point. To estimate

the thickness of the entropy layer, its edge is defined as the streamline that passes

through the point A (Fig. 3.1) at which the shock attains the conical angle as given

by Eq. (3.4). Most of the entropy changes occur before this point. The rate of mass

flow in the entropy layer is given by

rh= 2_ puydy

b

(3.7)

At point A, the mass flow rate is expressed as

,_A= _-y_poouoo (3.8)
21

Since the mass flow rate through the entropy layer isconstant, y_(x) can be calculated using Eqs. (3.7)and (3.8).At point A, y, issame as Yc given by Eq. (3.3)

The entropy swallowing distances calculated using present approach were

compared with the resultsof Rotta [50] for various nose bluntness and Reynolds

number. As will be shown laterthe present approach for calculating the entropy

swallowing distance agrees qualitativelywith the merging distance for skin friction,

surface pressure, wall heat transferand shock shape. The theoreticalcalculationsof

Rotta assume that the pressure on the conical portion of the body isconstant and,

therefore,the overexpansion and recompression on the shoulder are neglected. This

causes the overestimation of the entropy swallowing distance. For small bluntness,

the overexpansion can be neglected. Thus, Rotta's resultsare valid only for very

small values of nose bluntness.

3.2 Governing Equations

To calculate the flow over blunted slender bodies, two sets of equations

are utilized. For the nose section, the use of full Navier-Stokes equations is necessary due to the presence of the subsonic region; but slightly downstream of the nose

region on the body, the flow becomes predominantly supersonic and the NavierStokes equations are simplified for space marching. The Navier-Stokes equations

without body force and external heat addition can be written in nondimensional,

strong conservation form as [51]

where

OU a(E-E.) a(F- g.) a(G-G_)

cg--t-+ cgx + cgy + vqz -- 0 (3.9)

U = {p, pu, pv, pw, pet} T

E -- {pu, pu 2 + p, puv,puw, (pe, + p)u} r
Eu -- {0, Ozz,rzv,_z,,OzzU 4- rzVV + Tz, W 4- qz} T

F = {p._.p.,...p.,'+ p.pv... (p_.+ p).,}T

Fu = {0, 7"zv,O'vV,%., rzvU 4- O'vVV4- %zW 4- qv} T

GU

2. (2c9u Z)v 0w

a_= 3Re Ox OU -_z)

2t_ , Ov cgu Ow

ally )

2. ^ Ow Ou Ov

"" = 5-_ (__ a_ a_)

,.,, = _.[a 4 +_a,,

. [#u Ow"

. JOy Ow'

. OT

q" = M&RelPr(_l- 1) Ox

. OT

qv = M£RelPr('7 - 1) i:gy

. OT

q* = M£ReiPr(_- 1) cgz

-- {0, rz., r_.,¢7.., rz.u 4- %.v 4- auW -F q.} T

and u2 + v2 + w 2

22

et = e + 2 (3.10)

p = (_- 1)pc (3.11)

These equations are nondimensionalized with respect to the freestream

values. The coefficient of viscosity, is obtained using the Sutherland's law. For

perfect gas calculations, Prandtl number is taken as 0.72.

For the spherical nose region of the body, the flow is analyzed by the code

SOFIA [52] . It solves complete Navier-Stokes equations using an explicit, finite

volume, alternating two step scheme and uses local time stepping to accelerate the
23

convergence to steady state. It also incorporates time dependent grid adaptation to

properly resolve the flow gradients without increasing the grid points. Once the flow

field is obtained over the nose section, it is used as initial plane solution to initialize

the parabolized Navier-Stokes equations. In the present investigation, a parabolized

Navier-Stokes code developed by Vigneron et al. [53] and subsequently modified by

Gnoffo [54] is used for analyzing the downstream flow over the bodies. Parabolized

Navier-Stokes (PNS) equations are subset of the complete Navier-Stokes equations

and can be used to predict complex three-dimensional, steady, supersonic, viscous

flow fields. These equations can be solved using a space marching technique as

opposed to the time marching, which is usually employed for the Navier-Stokes

equations. They are valid in both viscous and inviscid regions and thus unlike

boundary layer equations no special effort is needed for viscous/inviscid interactions.

They can be obtained by

(i) Neglecting unsteady terms

(ii) Neglecting streamwise viscous diffusion terms

(iii) Modifying the streamwise flux to permit the stable time like marching of the

equations from the initial data plane..

For these equations to be valid, the flow in the inviscid region should be supersonic

and streamwise velocity component should be positive. Thus, these equations fail

in the presence of strea.mwise separation, although they are valid in the region of

crossflow separation. The streamwise pressure gradient needs special treatment as

will be explained later.

The following two independent variable transformations are applied to allow for conical effects and stretching between the body and shock.

_g

a = z b = -y c-- - (3.12)

X
24

Upon combining Eqs.

cous derivatives in _ direction, the PNS equations are expressed as [53]

aE, OF, aG,

o-T + 0-T =0

where

(3.9),(3.12) and (3.13),neglecting unsteady terms and vis-

(3.14)

a2E

J

a On On _

F, = _-[(a_a a - b-_-_ C_cc)(E - E,,)

an tF _ r,,) + an (G - O,,)] +'_ _

a Oq O_

GI = _'[(-b N - c-_c)(E - E,,)

0F

+_ F,) + _c_(G-G,)] lfJ = O(r/, q)

a(b,c)

The presence of streamwise pressure gradient term allows for upstream

propagation of information in the subsonic regions of the flow; thus, the equations

are not well posed for space marching method. Various remedies have been proposed

to overcome this problem, the most successful one was proposed by Vigneron et

al. [53]. In this approach, a fraction of the pressure gradient term is retained in the

subsonic region mad the remainder is either omitted or is evaluated explicitly using

the backward differencing technique. Thus the term E, is split as

E1 = E_ + P

E: = {p,,,p,,' + _p,p_,p_, (p_,+ p),,}="

P = {0,(1- w)p,O,O,O} r

(3.15)

This treatment of the pressure gradient allows for stable marching scheme without

encountering departure solutions. An eigen value analysis given in [53] indicates

that for stability

a'TM_

< (3.16)

- 1 + (,_- 1)M_

where w is the factor of safety; in the present calculations its value is taken as 0.9.
25

3.3 Method of Solution

The PNS equations are a mixed set of hyperbolic-parabolic equations in the streamwise direction. Equation(3.14) issolved with a finitedifference

method using the Beam and Warming algorithm [55].Itisan implicit,non-iterative

algorithm with approximate factorizationin Delta form. The Vigneron technique

isused to suppress the numerical instabilitiesdue to streaznwise pressure gradients.

Details of numerical procedure are available in [53]. For the sake of completeness

only a briefdescription isgiven here.

form as

Using the Euler implicit scheme, Eq.(3.14) can be written in the Delta

[aE_ a aGl aE_ -1.aE_ au1+ _(b-_,)I(b-U7) [_-h7

.aF1 CgGl.

= -A_(-_- + --_--)- A,P

a aF1 A_

+ _(_U-:)] Ul

(3.17)

This procedure avoids the computation of inverse matrix and introduces two onedimensional operators which are solved using the Thomas algorithm. In order

derivative terms axe neglected in the calculation of Jacobian.

solved in the following four steps.

aE{ a aG1 _U-(

[_-+ _(b-b-T)]_1= RHS(3.17)

A_ OE'_ = [_-T)_

.aE_ ___v (OFl ,

(3.18)

where

U1 = a_U/J, A'UI = U_+1 - U_ and A,P = A'-IP

Since the vectors El, F1, and G1 are homogeneous functions of degree one in U,

the conservative form of the governing equation is maintained. The viscosity _ is

assumed independent of vector U and is a function of the position only. The cross

Equation(3.17) is
26

to damp the spurious oscillations, second order implicit and fourth order explicit

damping terms are added. After adding the dissipation terms the truncation error

of the algorithm is consistent with the first order Euler scheme.

Due to symmetry of the flow field, only one half of the flow field is computed. The generalized coordinates r/and ¢ are defined such that the computational

plane has a square shape of side unity with uniform spacing in both directions. The

outer boundary corresponds to the shock and the inner to the body. The grid

points are clustered near the body to properly resolve the gradients. A typical grid

is shown in Fig. 3.3.

At the body surface, no slip, zero pressure gradient and constant wall

temperature conditions are imposed. At the outer boundary, shock fitting approach

is used and the flow variables behind the shock are calculated by the RankineHugoniot relations. At the plane of symmetry, reflecting conditions are used. As

mentioned earlier, the initial plane solution to initiate the marching procedure is

provided by the Navier-Stokes equations.
27

Fig. 3.3 Typical grid for finite difference calculations.
28

3.4 Equilibrium Chemistry

To include the real gas effectswhich might be present at hypersonic

speeds, calculationsare also made with equilibrium air chemistry. The equilibrium

chemistry package developed in [56]isused in the analysis. In this model, complete

thermodynamic equilibrium isassumed, in which the gas properties and concentration of atoms and molecules are identicalwith their equilibrium values appropriate

to the local conditions. The rates of reaction are assumed very fast in comparison

with the rate of diffusion.Here, the free energy minimization technique with the

method of steepest descent isutilized.The mole fractionsof the constituent species

are calculated as a function of temperature and pressure. The assumption of equilibrium is made on the basis of physics of the problem. In the blunt body flows

near the stagnation point, where the shock isnearly normal, the velocitiesare very

low and temperature isvery high. Consequently the flow attainsequilibrium values

prior to leaving the nose region. The particlescrossing the bow shock farther from

the axis experience higher velocitiesand lower temperature due to weaker shock,

thus reducing the degree of non-equilibrium. On the after body past the sonic line,

the flow experiences expansion thus increasing the velocitywhile reducing pressure

and density, both of these effectstry to quench the chemistry and produce frozen

flows. Thus the afterbody flows can be calculated by using an effectivevalue of "7

determined from the mass concentration. Similar approaches have been taken previously by several authors [57,58].The equilibrium chemistry option can easilybe

incorporated into a perfect gas code with equivalent "7approach where "7isnow defined as the ratioof enthalpy to internalenergy. Further detailson implementation

of the equilibrium chemistry model are given in Appendix A.
29

3.5 Validation

The PNS code was validated by comparing the present results with

those calculated using the laminar theory with reference temperature concept [59].

For the same Reynolds number, the skin friction on a cone and a fiat plate is related

by

cs.,oo..=

Consequently, the heat transfer coefficient is expressed as

C / ,,_o..

Ch,,co,=- 2Pr_

The value of Cf_,ptau can be calculated from

0.664v/_ _

C1e.nlau =

(3.19)

(3.20)

(3.21)

where Re,_,is the Reynolds number based upon the distance along the cone surface

and the subscript e refers to the edge conditions. The surface flow along the cone is

assumed to be constant, then Pe, Tie, and T, can be calculated by inviscid supersonic

cone theory [60]. The quantity C* in Eq.(3.21) is the reference viscosity and is given

by the Sutherland's law as

where

and

C*- (T*_°s 7", + K

\'_] T*+K

K = 200°R

T____*= 0.5 + 0.039M_ + 0.5 T_

T. i",

By using Eqs.(3.19)-(3.21), the theoretical values of C/,.,o,, and Ch,.,o,_, can be

calculated.

These results are compared with the numerically calculated values for

sharp cone at Moo = 10 and are discussed in Chap. 5.
Chapter 4

EFFECTS OF SHOCK-SHOCK

INTERACTIONS

In this chapter, the physical models used to calculate the two-and threedimensional shock-shock interactions in the scramjet inlet are discussed. The various types of interaction patterns as described by Edney [31] are reviewed and how

they occur in the scramjet configuration is discussed. The governing equations and

method of solution used are described. Depending upon the Mach number and the

angle of attack, the forebody shock can interact with the inlet flow; thus producing

a very complex flow field. The primary factors affecting the nature of the interaction

are freestream conditions, strength of the impinging shock, geometry and impingement location. Shock-shock interaction can also occur on space shuttle, missile

launcher and other high-speed aircrafts. As mentioned earlier, this study concentrates on interactions in scramjet inlet but the results obtained are very general in

nature and can be applied to other applications as well. All interactions considered

here are of the free interaction type i.e., the region upstream of the interaction

zone is independent of the source of impinging shock. Two types of interaction can

occur in the scramjet inlet; interaction of the forebody and the cowl shock, and the

interaction of the forebody and the inlet sidewall shock. The interaction of cowl lip

shock with the forebody shock is modeled by a cylindrically blunt wedge with an

oblique shock impinging on it in such a way that the line of interaction is parallel

3O
31

to the cylinder axis. Ifthe end effectsare neglected, the flow in each plane normal

to the interaction lineshould be identical.Therefore, a two-dimensional model as

isshown in Fig. 4.1 isused. The interaction of inletsidewall shock and the forebody shock ismore complicated because the inletsidewalls are swept back and the

forebody shock is normal to the axis. Thus the flow is fullythree-dimensional in

nature. The interactionpoint of the forebody and inletsidewalls shocks isfar away

from the inlet/body junction. Therefore, the effectof the body on the shock-shock

interaction isneglected and the inletsidewalls ismodeled by an infiniteswept blunt

wedge with impinging shock normal to itsaxis a.sshown in Fig. 4.2.

4.1 Classification

Edney [31] in 1968 categorized the shock-shock interactions into six basic

patterns (Types I to VI, Figs. 4.3 - 4.8). The primary factors in determining the

type of interaction are strength of the impinging shock, location of the impingement

and the freestream Mach number. All six types of interactions can occur when an

oblique shock interacts with the bow shock of the leading edge. This classification is

a general one and sometimes it is not clear to which category an interaction belongs.

Types I, II, a_d V interactions result in shock-boundary layer interactions, Type

III results in attaching shear layer interaction, Type IV produces a supersonic jet,

surrounded by subsonic flow and the Type VI interaction results in an expansion

fan-boundary layer interaction. The shock-shock interaction causes the stagnation

point to move and produce a localized zone of high pressure and heat transfer.

Since for a blunt body the peak heating and pressure occur at the stagnation point,

an amplification factor is defined as ratio of the peak value of pressure (or heat

tr_.nsfer) divided by its value at the stagnation point for the unimpinged case.

The Type I interaction occurs when two weak shocks of opposite family

interact. When an oblique shock intersects the leading edge shock well above the
32

BOW SHOCK

BODY

LINE OF

INTERACTION

Fig. 4.1 Physical model for forebody and cowl shock interaction.
33

I

I

I

/

I

I

I

I

IMPINGING

SHOCK

BODY

!

/

I

I

I

I

t

LINE OF"

INTERACTION

BOW SHOCK

Fig. 4.2 Physical model for forebody and inlet shock interaction.
34

upper sonic line, the interaction occurs as is shown in Fig. 4.3. The interaction

produces a shear layer and a transmitted shock; the shock strikes the body, thus

causing shock-boundary layer interaction. If the transmitted shock is strong, it can

cause boundary layer to become turbulent or even separated. Holden [61] correlated

the peak pressure and heat transfer with the relation q/qo = (p/po) °ss over the Mach

number range 2.4 - 13.

A Type II interaction occurs when two shocks of opposite family intersect.

If the impinging shock intersects the leading edge shock just above the upper sonic

line, this type of interaction is produced. This interaction produces two shear layers

and a transmitted shock as shown in Fig. 4.4. The transmitted shock strikes the

body, causing pressure and heating amplification. Depending upon the body shape,

the shear layer can also strike the body, thus resulting in additional amplification

of heating and pressure. In general, the upper shear layer(from pt. A) is stronger

than the lower shear layer( from pt. B).

A Type III interaction occurs when a weak shock wave interacts with the

strong shock wave. When the oblique shock strikes the bow shock inside the sonic

region, this type of interaction occurs. The flow is supersonic above the shear layer

and subsonic below it. A reflected shock exists between the body and the leading

edge shock which turns the flow parallel to the body. The pressure and heating

rates are amplified due to striking shear layer and reflected shock, times.

A Type IV interaction occurs when the impinging shock strikes the leading edge shock near the stagnation zone, where the shock is nearly normal. This

produces a supersonic jet bounded by two shear layers and submerged in subsonic

flow. Near the body, it produces jet bow shock and stagnation zone when it strikes

the body. This produces a very complex flow field with presence of shocks, shear

layers and the jet. Type IV interaction is the most severe case and produces the

largest amplification of heating and pressure.
35

IMPINGING

SHOCK

DISTORTED

BOW SHOCK

SHEAR LAYER

TRANSMITTED

SHOCK

\

SONIC

LINES

BODY

BOW SHOCK M>l

Fig. 4.3 Type l shock-shock interaction.
36

IMPINGING

SHOCK

SHEAR

LAYERS

TRANSMITTED

SHOCK

SONIC

LINES

BOW SHOCK

BODY

M,,> 1

DISTORTED

BOW SHOCK

A

B

Fig. 4.4 Type 11 shock-shock interaction.
37

i

DISTORTED

BOW SHOCK

SHEAR LAYERS

TRANSMITTED

SHOCK

IMPINGING

SHOCK

M,,> 1

SONIC LINE

BOW SHOCK

BODY

Fig. 4.5 Type IIIshock-shock interaction.
38

DISTORTED

BOW SHOCK

JET SHOCK

SHEAR LAYER

EMBEDDED__

SHOCK

SUPERSONIC

JET

IMPINGING

SHOCK

M.>I

SONIC

LINE

BOW SHOCK--

BODY

M<l

M>I

Fig. 4.6 Type IV shock-shock interaction.
39

BODY

SONIC

LINE

M.>I

TRANSMITTED

SHOCK

IMPINGING

SHOCK

SUPERSONIC

JET

BOW SHOCK

SHEAR LAYER

Fig. 4.7 Type V shock-shock interaction.
4O

BODY

BOW SHOCK

SONIC

LINE

M.> 1

IMPINGING

SHOCK

EXPANSION

FAN

SHEAR LAYER

Fig. 4.8 Type VI shock-shock interaction.
41

When the oblique shock intersects the bow shock just below the lower

sonic line (Fig.4.7), Type V interaction is produced. In this case, the shocks are

of the the same family. The interaction produces a thin supersonic jet and a shear

layer. It is very similar to the Type II interaction.

A Type VI interaction occurs, when the two weak shocks of the same

family interact; these shocks are weaker than Type V interaction. Such a situation

exists when a weak impinging shock intersects the bow shock well below the lower

sonic line. An expansion fan is produced from the intersection point, which causes

decrease in pressure and heating rates.

4.2 Governing Equations

The Navier-Stokes equations in cartesian coordinates describing three

dimensional, unsteady compressible flow in the absence of external body force can

be written in differential form as [51]

aQ a(E - Eo) O(F- O(G- _ 0

+ o-x + ay + az

where

Q pv

pw

e

E

F .__

pu

pu 2 + P

puu

putu

pv

pvu

pv 2 + P

pvw

+ p),,

, Z_ =

_Fv=

0

r_z

rz¢

rz,

rzzU + r=vv + r=,w - _=

0

r=v

%v

%,

Tzvu+ %vv + %,w - qv

(4.1)
G

pit)

pwu

pwv

pw _ + P

, Gv _

0

_=

_x

_=",- M°°[ R._["(_-g_,. Oui + Oui" _1 + ____kk6,,]

laM_ ] aT

q;' = - [ReLPr(_- 1)J

42

The molecular viscosity is calculated from the Sutherland's law and the Stoke's

hypothesis for bulk viscosity, ,_ + _ = 0 is invoked. The perfect gas equation of

state is used to define the pressure p via the internal energy as

p= (_/- 1)[e- p(u'+ v'+ w')/2] (4.2)

k J

4.2.1 Nondimensionalization

Equation(4.1) is nondimensionalized as follows

p_m

U _

a¢¢

t_

a¢¢

alp'=

b

T"-_

To_

In the above equations tilde denotes the dimensional quantity and the subscript

denotes the freestream quantity.
4.2.2 Coordinate Transformation

The governing equations are transformed to the body-fitted coordinate

_=/ J = y,Tz_ - z.y_

_v/J = z,Tx ¢ - x,z r

_./J = z,y_ - y.z r

rl=/J = zey¢ - yez_

rl./J = y_x¢ - x_y¢

f./J = z_y, - y_x,

After applying the generalized coordinate transformation to Eq.(4.1), it can be

expressed in the strong conservation-law form as

a0 a(t- J_.) a(F - Fu) a(G - G,) = 0 (4.4)

a-T + O_ + an + a_-

43

fined as follows [51]

system (_, r/, f) using a steady transformation of the type

_ = _(_,y,z)

= _(z,y,z)

The Jacobian of the transformation is denoted by J and is given by

I_ a(_, ,7, _-)

a(z, y,z)

= {xay,tz , + x, yaz , + x._y¢z,- x,y,z,- x,y,z,- x_y, zq] (4.3)

Using the chain rule of partial differentiation, the transformation metrices are de-
44

where the new variables are defined as

Q=Q/J

0 - G = -5- lafl[ ?.(E - Eo)+ _,(F - Fo)+ ?.(a - c.)

for k = (_,rl, f) respectively.

(k,,k,,k,)

The inviscid and viscous flux vectors in generalized coordinate system can now be

defined as

1

pU

pUu + Gp

pUv + _vP

pUw + GP

(e+p)v

pV

pVu + rl,p

pVv + rl,jp

pV w + rt,p

Ce+p)V

1 pWu + f,,p ] 1

= pwv+f_p ] , G, =

(_+ p)w .j

where U, V, and W are contravarient

U = Gu + _vv+ Gw

V = rl.u + rl, v + _7,w

W = _..u + fyv + f.w
45

and

bz, = ujrz,z i - _z, (4.5)

For two-dimensional calculations, E and E_ were set equal to zero, thus reducing

the governing equations to two-dimensions with ; normal to the surface and _7 along

it. For three-dimensional case, viscous terms were dropped in _ and r/ directions

due to coarse grids in axial and circumferential directions.

4.2.3 Finite Volume Formulation

Equation (4.4) is solved using a finite volume method. In this method, the

integral formulation of the conservation laws is discretized directly in the physical

space. The direct discretization ensures that mass, momentum and energy are

conserved at discrete levels. It remains valid in the presence of discontinuities in

the flow field such as shocks and contact surfaces. Eq.(4.4) is expressed in the

integral form as

 fff, odv+ffs ads=o (4.6)

where F = (E - E_)_ + (F - F_)) + (G - G_)_;, and h = n,_ + nv) + n,[¢ is the

unit normal vector pointing outward from the surface S bounding the volume V. A

semi-discrete finite volume representation of Eq.(4.4) leads to

"_-_;,,j,k + (E- E,_)_+1⁄2,S,k - (E, - E,,),-1⁄2j,J,

+(P- _o),,;+1⁄2,,- (P- _,),,_-1⁄2,k

+(c - ¢o),,i,k+1⁄2- (c - G),,i,k-1⁄2= 0 (4.7)

It can be shown that Eq.(4.6) can be written in a form identical to Eq.(4.7) by

writing the surface integrals as the sum of the contribution from six faces of the

hexagonal cell. In the computational domain(G r/, _'),
46

= - = i

Similarly, At/ = Af --_ 1

The fluxes are evaluated at the cell boundaries defined by the grid points

as is shown in Fig. 4.9. So far the method of solution has been identical to the

finite difference method. In most of the finite difference methods the transformation

matrices and Jacobian is calculated by using the two point central difference formulas. This leads to inconsistency in the volume and surface normal calculations such

that the geometric conservation laws axe not satisfied and hence the scheme fails to

capture the freestream when evaluated on arbitrary meshes. In the present method,

geometrical interpretation of the Jacobian and transformation terms is made. The

Jacobian is calculated as inverse of the cell volume, the vector Ak/J is the directed

area of the cell surface to a k = constant coordinate direction and [Ak[/J is the

area, of the cell interface. Here k represents _,tl, and f, respectively.

The volume of a cell is defined by its eight vertex points and is calculated

as the sum of six pentahedrons. Each pentahedron ks formed by one of the six cell

faces and the average point in the cell volume. The directed area of the cell interface

is calculated as one half of the vector cross product of two diagonal vectors joining

the four vertex points. In general, all four vertex points do not lie in the same

plane. So the directed area so formed corresponds to the planar surface obtained

by passing a least square plane through the four points and then projecting them

into the plane. This approach satisfies the geometrical conservation laws and makes

the numerical scheme compatible with the finite volume formulation.

4.3 Method of Solution

Over the past few years, upwind space discretization methods have become a popular alternative to the central space discretization methods. The basic
47

i-1/2, j, k

i

X

0

X

Q evaluated at

centers, ijk

cell

FIuxes evaluated

on the cell faces,

A

e.g. Fi+l/2, J, k

2, j,k

Fig. 4.9 Finite volume formulation.
48

idea behind the development of upwind scheme is that they try to mimic the directional dependence by applying asymmetric space discretization with a bias toward

the direction from which the information is propagating. The upwind differencing

reflects the predominant nature of the Navier-Stokes equations in the limit as Re

--, oo (hyperbolic) and Re --, 0 (parabolic). This improvement in physical treatment comes with increased computational work per iteration but the total number

of iterations is reduced thus offsetting the increased computation time per iteration.

These methods are naturally dissipative and therefore no artificial viscosities with

problem dependent coefficients are needed.

The Navier-Stokes equations consist of inviscid (pressure and convective)

terms and viscous (diffusive) terms. In the upwind methods, the inviscid fluxes

are split into positive and negative components based on eigen values, and then

spatially differenced by backward or forward differencing. The differencing could

be first order, second order fully upwind or third order upwind biased. First order

upwind differencing have a large amount of numerical dissipation which makes it

unsuitable for meaningful calculations. The second order upwind schemes have

second order truncation error; the leading term of this truncation error is dispersive

and the leading numerical dissipative term is actually a third order, fourth derivative

term. Even for third order differencing the scheme is actually second order because

of the second order treatment of diffusion terms. Hence, the numerical dissipation

of the second order upwind scheme is of the same order as the central difference

scheme. The central difference schemes have been shown to be superior than upwind

schemes in the viscous zone [62].

Various upwind algorithms are available in the literature. These are

scheme [63), split coefficient method (SCM) [64], flux difference methods [65]

- [68] and flux vector differencing [69] and [70]. For the physical problem of shockshock interaction, various schemes were tried. The first scheme tried was explicit
49

MacCormack [711with central differencing. The solution had excessive oscillation

at high Mach numbers and required proper tuning of damping coefficient, also due

to explicit nature of the scheme, the time step was very restrictive for the grid

employed. The flux differencing algorithm developed by Roe [68] gave physically

incorrect solution even for unimpinged blunt body flows. The bow shock had a

dimple near the stagnation point. For some cases, the shock did not even stablize

but continued to move upstream until stopped by the inflow boundary. The peak

pressure value was not at the stagnation point but rather away from it. The same

type of phenomenon has been observed for the Roe's scheme by Perry et al. [45].

The critical Mach number around which this phenomenon starts is about 6.0 [72].

This phenomenon is not fully understood yet, and therefore no rigorous correction

to Roe's scheme to eliminate this problem is available. However, some progress has

been made for simple flows by adding eigen value smoothing [45]. van Leer's [70]

flux vector splitting method gave the best results for this problem. Consequently,

this method has been used for two-and three-dimensional shock-shock interactions.

4.3.1 Flux Vector Splitting

Steger and Warming in 1981 [69], realizing the homogenity property of the

Euler equation, proposed a novel way of splitting the inviscid fluxes into positive and

negative subvectors based on the eigen values. The positive subvector is associated

with the non-negative eigen values and negative subvector is associated with the

non-positive eigen values. The split fluxes are then spatially discretized using one

sided upwind differencing. The splitting is not unique and various combinations

are possible as long as the sum of two subvectors is the same as the original vector.

The details of this splitting and its implementation into Euler equation is available

in [69]. The main drawback of this type of splitting is that forward and backward

fluxes although continuous, are not differentiable when an eigen value changes sign.
5O

Such phenomena occurs at stagnation and sonic points. This causes small glitchesin

the solution near these points, van Leer in 1982 [70]proposed a similaxsplittingbut

with continually differentiablefluxes by introducing additional requirements that

the Jacobian must be a continuous function of the Mach number. As mentioned

earlier,upwind differencingisapplied to inviscidfluxesonly, the viscous fluxes are

centrally differenced.

Splittingthe fluxes in _ direction,

= E+ q-E- (4.8)

where

E,+-'F., , E,---O for Me>>-+-1

F,--F, , E+--O for M e<_-1

Here M e isthe contravaxiant Mach number along _ and isgiven as

For subsonic flows. IMel < 1, the split fluxes axe obtained as follows

f_

.f_{_,(-_ + _)/_ + ,}.

f_(k,(-tl" + 2c)/'7 -t- t,,}

/_(L(-_+ 2c)h+ w)

f_

where

f_ = +PcCXt_e -4- 1)=/4

fs_ = .f_ I{--("/-- 1)_ "2-t-2('7- 1)R'c -I- 2c')/("/' - 1)+ (u' -F-v' -f- w')/2]

The conserved state variables on the cellinterfaceaxe obtained as

(4.9)

(4.10)
51

where

_7_Q_ = Q_ - Q_-I

The switch _ is zero for the first order differencing and unity for higher order differencing. The parameter i¢ determines the order of differencing; _:_ = -1 corresponds

to the fully upwind second order, _:_ = 0 corresponds to second order upwind biased, a_ = +1 to second order central differencing, and _ = +1/3 to the third

order upwind biased. Having obtained the value of Q at cell interfaces, the split

flux differences are implemented as follows

6_7i - 6_E_- + _ifk[ (4.11)

A÷ _ ^'l- E- A +

= E (Q),+1⁄2- E (Q-),_1⁄2 + (Q+),+1⁄2-E-(Q ),_1⁄2

The term E+(Q-)_+1⁄2 denotes the forward flux evaluated at the cell interface i +

1 It should be noted that MUSCLE ( monotonic upstream-centered scheme for

conservation laws) approach is used. The conservation variable Q is obtained first

at the cell interface and then the split fluxes E ± are obtained. This is in contrast

to obtaining E + first at the cell center and then interpolating it to get the cell

interface value. The former approach has been shown superior than the latter as

the split fluxes are less differentiable than conserved variables when transitioning

through sonic and stagnation points. Similarly, the fluxes can be split in rl and

directions also.

4.3.2 Algorithm

For the present calculations, the algorithm developed by Thomas et

o.

al. [73] - [75] has been used. The algorithm is outlined for three-dimensional flows

and can be specialized for two-dimensional flow as necessary. As noted previously,
52

viscous terms have been dropped in _ and r/direction; thus Eq.(4.4) can be written

as

_- + -_- + -ST+ a-_ =o (4.12)

band width depends upon the grid size and choice of spatial discretization.

second order accurate spatial discretization, it can be expanded as

I

(3-£7+ ce + c, + c,)zxO" +

A '* A '* " H " + eAQi_2 + Be Q_-: +DeAQ_+t + eAQi+2

+A,_AQ']_, + B,_AQT_ , + D,_AQ_+, + H, TAQi+=

+ A _,aQk_= n + B_A Q___ " + D_,_Q';÷,+H_,_Q,,+_= R(Q")

and

I OR"

()-At OQ )AQ" = R(Q n) (4.13)

OR Off" A ,, 0(7?," , ""

b-_AQ = -[6(( _XQ")+ 6,(--_- Q ) + 6,(.-_-AQ ) - _,(--_AQ )]

Equation(4.12) represents a large banded block 5 x 5 matrix equation, the

Using

(4.14)

where A,B,C,D and H are 5x5 block matrices associated with implicit spatial differencing in the _, r/, and f directions. The solution of Eq. (4.14) involves inversion of

a block pentadiagonal matrix equation. The computational effort is reduced if the

implicit-spatial dincretization is taken as first order accurate. This does not affect

the steady state accuracy which is determined by the spatial differencing of R(Q).

Then, A_, An, As, He, Hn, and He are equal to zero and

ok+

Be = -( _-),-,/:

where

Applying the Euler implicit time integration scheme and linearizing it, one obtains
53

ak + ako_= (-_-¢),+,/,- (--_-),-,z,

ag'-

.D_= -(-_--1,+_/,

Similar expressions can be developed in _ and C directions also.

Using Eq.(4.16), Eq(4.14) can be written as

I A "

(-ff-_ + C e + C,_ + C¢)AQ" + B(AQ__ 1 + D r Q_+I

A _" n

+BnAQ']_ 1 + D, Qi+l + B_AQ_,-1 + D_AQ_+I = R(Q")

(4.15)

(4.16)

Relaxation is implemented in the streamwise direction and the approximate factorization is implemented in the cross flow planes.

For first order implicit space discretization, the algorithm is stable regardless of the sweep direction. With higher order spatial differencing, alternate forward

and backward sweeps are required. For a forward relaxation sweep, AQ__ 1 is known

and AQ_+ 1 = 0, then Eq.(4.16) becomes

/ A "

(ff-_ + C e + C, + C_)AQ" + B,_ Qi-_ + D'_AQi+I

p_(3,_+1 Q__+_,Q,_,Q_+I,Q_+2) (4.17)

Equation(4.17) is written in a compact form as

_ Ok OG ad.)]AQ . Rh,.).+l f).+_ . .

[M + di,_-_ + 6¢(_ aQ = t'_'-''_'-"Q"QT+l'Q'+2)(4"18)

where

I

M = Jh---7+ Ce

Equation (4.18) is approximately factored in r/ and C directions as

at. __ .a_ ado

[M + 6,_-ff_]M [M + g¢(-ff-_ aq )]AQ"

-- a.._i_ 2 ,

(4.19)
54

The solution of Eq.(4.19) then involves the solution of two block tridiagonal equations

ok

[M + 6,_-_]¢ = R( Q_._+_, Q'__+_,Q_, Q,_+I, Q,_+2)

at4

[M + 6¢(_ _-_ )]AQ'_ = ¢

Q.+I = Q. + AQ" (4.20)

For the cowl afterbody calculations the algorithm has been modified so that

the downstream influence of the subsonic part of the boundary layer in an otherwise

supersonic flow is suppressed by restricting the streamwise pressure gradient. A

parabolized solution is then obtained by marching downstream and iterating locally

in each plane until convergence. The Vigneron [53] technique as described previously

in Chap. 3 is used to modify the algorithm. The details of modified algorithm are

available in [74]. The forward flux vector k is now modified as

pU

1 pUu + w_ffip

k = -_ pUv + w_vp (4.21)

pUw + w_p

(e+ p)U

where w is the Vigneron factor and was defined in Eq. (3.16). The additional terms

due to the explicit lagging of the streamwise terms are neglected, then the procedure

is not subjected to the departure solution for arbitrary streamwise mesh spacing.

The modified flux and flux Jacobian are split as

k+CQ-),±,/2= kCQ),±I/2

ok+CQ-) _ Ok(Q)

OQ ,±112 OQ ,±112

(4.22)

and

k-(Q+),±l/2 = 0

OE-(Q+) =0

OQ ,±_/2

(4.23)
55

The solution from the (i-1) station is used as initial guess at ith station and is

iterated locally to obtain the converged solution and the process is repeated at the

next (i÷1) station. The resulting algorithm is fully second order accurate. The

value of _ is tal_en as -1 in order to have fully upwind algorithm.

In order to eliminate the overshoots and undershoots near the shocks,

Van Albada type continuous differentiable flux limiter has been used in the present

calculations. It reduces the scheme to fully one sided. The limiters modify the

upwind biased interpolation and are implemented by modifying Eq.(4.10) as follows

Q[÷_= Q, + -_ (1 - s,¢_)_ +(1 + _,¢_)A_Q,

Q,_1⁄2= O,÷_- i (1 + s,_) _ +(1 - _,_)zx_ Q,÷_ (4.24)

where

2/_ _ +

+

and e -10 -e is a small number to prevent the division by zero in the region of zero

gradients.

4.4 Grid Generation

The first step in any numerical solution is the discretization of the governing equations from a continuous domain to a set of discrete points (called grid

points). The choice of grid points is not an arbitrary one but is governed by physics

of the problem. Grid generation is a procedure for orderly distribution of the sampling stations over a physical field in such a way that all physical phenomena in the

entire region of interest may be represented with sufficient accuracy by this finite

number of sampling stations. Since the computer memory and speed limit the number of grid points, it is very crucial to make best use of the available resources. The

grid points should conform to the boundaries and be concentrated in the regions

of high gradients such as shocks and boundary layers in order to properly resolve
56

the flow field. They must also be oriented in such a way that grid is as close to

orthogonal as possible.

The grids can be generated by either numerically solving the partial differential equations or algebraic equations. In the present study, an algebraic method

is used. In this method, the grid is generated by algebraic interpolation from the

boundaries. Here an explicit functional relationship between the computational and

physical domain is known. It is much faster than the differential equation approach

and offers an easy control of grid spacing and distribution. For the present calculations, various types of grids were employed. For the first set of calculations, an

outer boundary consisting of a circular arc was defined. Since the shock capturing

algorithm has been employed, the outer boundary was chosen in such a way that

there were sufficient number of points between the shock and outer boundary. The

grid points were clustered normal to the body using the exponential stretching

e k" - 1

f(rl)-- e_-I

to resolve the boundary layer. Since the heat transfer and skin friction are strong

functions of grid spacing, it is very important to properly resolve the gradients. For

the impingement case the shock moves closer to the body on one side and away from

the body on the other. Thus a lot of grid points are wasted. In order to alleviate

this problem, a semi adaptive grid was used. Figure 4.10 shows this grid, in this

case the outer boundary is made to conform with the shape of distorted bow shock,

which was obtained by coarse mesh calculations where no attempt was made to

resolve the gradients. This type of grid, reduced the wastage of grid points but it

requires a new grid every time either freestream conditions are changed or the type

of interaction changes. The solution obtained by using this type of grid was much

better resolved near the shock region but in the case of impingement where we have

a supersonic jet enclosed within subsonic flow and extremely high gradients, the

resolution was still less than desired, as will be shown in Chap. 5.
57

0

In order to improve upon the solution, adaptive grid system has been

used, which adapts the grids during the course of the solution in order to follow

the developing gradients in the physical solution. The grid points move as the

solution develops, concentrating the points where they are needed the most. The

total number of points were kept constant. The finaladapted grid for Mach 8.03

case isshown in Fig. 4.11. For this,the method developed by Abolhassani et al.[76]

has been used and is explained in Appendix B. It is a very general method with

capabilityto adapt the grids with various variablessuch as pressure, Mach number,

shear stress etc. and isbased upon the variational approach. Since the solution

varies predominantly in the normal direction,the grids were adapted in only one

direction.

The grid for three-dimensional calculationsisshown in Figure 4.12. The

grid points were clustered near the body in normal direction as well as near the

shock impingement location to resolvethe gradients. As in the previous case, the

outer boundary was moved fax enough so as to avoid any interferencebetween the

shock and the outflow boundary.
58

\

Fig. 4.10 Unadapted grid for two-dimensional calculations.
59

Fig. 4.11 Adapted grid for two-dimensional calculations.
60

Fig. 4.12 Grid for three-dimensional calculations.
Chapter 5

RESULTS AND DISCUSSION

The numerical schemes described in Chaps. 3 and 4 are applied to obtain

steady state results for the forebody and the shock-shock interaction on the cowl

plate and inlet sidewalls. All calculations were performed under the assumption of

a laminar flow. For the forebody, the flow is calculated by the parabolized NavierStokes equations with the starting solution for the nose section provided by the

full Navier-Stokes equations. For two-dimensional shock-shock interaction, the full

Navier-Stokes equations are used and for three-dimensional interaction, the thinlayer Navier-Stokes equations are used. Results for the nose bluntness effects on the

forebody are discussed first followed by the results for the shock-shock interaction

on the cowl plate and inlet sidewalls.

5.1 Parametric Study of Nose Bluntness Effects

The parametric studies were conducted to determine the extent of downstream effects of nose bluntness for various freestream conditions given in Table 5.1

for three different values of nose bluntness. The blunted cone with the smallest

nose radius is considered as a sharp cone and the effects of leading edge bluntness

are compared with respect to this cone. When the surface quantities fall within

five percent of the sharp cone values, the effect of bluntness is assumed to have

vanished. The same criterion is used for the case of ogive also.

61
62

First, the results for a 5o sharp cone at Mach 10 are compared in Figs.

5.1 - 5.4 with the results of inviscid and laminar theory to validate the numerical

procedure as well as to justify the assumption of smallest nose radius cone as a

sharp cone. Next the results for the wall quantities are presented for various Mach

numbers and cone angles in Figs. 5.5 - 5.12. The effects of Reynolds number on

the wall quantities are presented in Figs. 5.13 - 5.16. Similar results for ogives are

presented in Figs. 5.17 - 5.23 for wall and flow field quantities.

The numerical results are validated by comparing the surface pressure and

shock standoff distance with the inviscid theory in Figs. 5.1 and 5.2. The pressure

has been nondimensionalized with respect to the freestream pressure and the shock

standoff distance with respect to the nose radius. The x distance is measured

from the virtual tip of the sharp cone. The results are plotted from the tangency

point onward. The skin friction and the heat transfer coefficient are compared

with the laminar theory in Figs. 5.3 and 5.4. These figures show an excellent

comparison. The theoretical results are valid for sharp nose cone only, while the

numerical calculations were performed for smallest nose radius cone. Except very

close to the nose, the results agree with the sharp cone results. So the assumption

of smallest nose radius as sharp cone is valid slightlydownstream of the nose.

Figures 5.5 - 5.8 show the variation of surface quantities with the

axial distance for a 5o cone with various degree of bluntness and for freestream

Mach numbers of 10 and 20. The surface pressure distributions are presented in

Fig. 5.5. In the stagnation region,the pressure isvery high but as the flow moves

downstream, the shock strength decreases thereby decreasing the pressure. The

flow overexpands near the shoulder; the extent of overexpansion is a function of

the bluntness and freestream conditions. The overexpanded flow then recompresses

back to the sharp cone value. The distance ittakes to reach the sharp cone value

is also a function of the freestrearnconditions and nose bluntness. It should be
63

5

IL I

13.

4

3

2

Numerical

Theory

8C=5"

Rn=O.OO25m

_'=I.L,

M.= 10

• • • • A • • •

u u _ n _ _ n

I I I 1 I I I I

5 10 15 2O

Xo m

Fig. 5.1 Comparison of wall pressure with the inviscid theory.
64

E

L/I

,O

1.25

1.00

0.75

0.50

0.25

eC=5"

Rn=0.0025m

_' =1.4

M.=10

Numerical

Theory

0.00 I i I ,

5 10 15 20

X° m

Fig. 5.2 Comparison of shock standoff distance with the inviscid theory.
65

Cfe

10

10

-I

-2

-3

10

-4

10

Numerical e - 5 °

A Theory c

. R n =0.0025 m

_,_ _, -1.4

_ =10

-5

10 , , ,i , ,, i , , ,,i .... l

4 5 6 7

10 10 10 10 10

8

Re,xe

Fig. 5.3 Comparison of skin friction coefficient with the laminar theory.
66

10

10

Che 10

10

10

-1

-2

-3

-4

-5

10

Numerical

Theory e c =5 °

R =0.0025 m

n

7 =1.4

Moo =10

* , L l l I J I ' I ,,I , , ,,I

4 5 6 7 8

10 10 10 10

Re,x e

Fig. 5.4 Comparison of heat transfer coefficient with the laminar theory.
67

Table 5.1 Flow conditions for the forebody calculations

Quantity Moo = 10 Moo -- 20

Poo, N/m _ 404.7 171.0

Too, k 243.4 261.3

Twazt, k 1000 1000

Pr 0.72 0.72
68

noted that the pressure on the cone is influenced by the nose over a large portion of

the afterbody and is lower than the conical pressure. For all values of leading edge

bluntness considered, the surface pressures have reached the same value at about

160 nose radii downstream.

Figure 5.6 shows the variation of the shock standoff distance as a function

of the axial distance. The shock standoff distances are measured from the body

surface. Here the behavior of the shock shape is seen to be qualitatively similar

to those observed by previous investigators [20]. The shock standoff distance is

affected considerably by the leading edge bluntness. For the sharp cone, it is a

straight line, while for blunted cones it is curved with inflection point about 50

nose radii downstream. For sufficiently large values of x, the shock shape becomes

independent of the nose bluntness. At Mach 20, it takes less distance to attain

conical value than at Mach 10.

The variation of the wall heat flux with axial distance is illustrated in Fig.

5.7 for various freestream conditions and bluntnesses. The wall heat flux has been

nondimensionalized by pooUS_. Figure 5.7 shows that the heating rates decrease

over the body with increasing nose bluntness at a given Mach number. This is

because heat transfer in the nose region is inversely proportional to the square root

of the nose radius and decreases over the afterbody due to lower local pressure. The

results again show that the sharp cone value is attained in a shorter distance for

Mach 20 than for Mach 10. Comparing the distance it takes to attain the conical

value for pressure and heat flux, it is noted that it takes almost the same distance for

both. Since (to a reasonable accuracy) the pressure can be predicated analytically

by using various correlations, it appears that the merging distance for heat transfer

can be estimated without calculating the detailed flow field.

Variation of the skin friction coefficient with the axial distance is shown in

Fig. 5.8. The skin friction is zero at the stagnation point, it increases around the
69

PwlPoo

10 2

10 1

Rn--O.O025 m 8c=5"

0.025 _ --_.c

0.05

I

I

I

f t j"

I

f

1,4.,=20

I0

1 I | I I I I I

100

0 4 8 12 16

Xom

2O

Fig. 5.5 Variation of wall pressure with the axial distance for 8, = 50 at various

Mach numbers.
7O

bsh, m

10 1

IO0

-1 10

10-2

10--3

Rn=0.0025 m

0.025 ec:5"

o.os

I I I I I I I I I

0 4 8 12 16 20

X,m

Fig. 5.6 Variation of shock standoff distance with the axial distance for O_ = 5 0 at

various Mach numbers.
71

10--2

Rn--O.O025 m

0.025

0.05

10--3

Ow

10--4

M_= 2 0

I0

In--5 I I l I I I I I I

Iu

0 4 8 12 16

Xom

2O

Fig. 5.7 Variation of wall heat transfer with the axial distance for 0c = 5 0 at various

Mach numbers.
72

10-2 f

Rn=0.0025 m

II 0.025

m _ __ 0.05

C I 10 -3

_

10__ 4 I I I 1 I I

0 4 8 12

x°m

M== 20

10

I I

16 20

Fig. 5.8 Variation of skin friction with the axial distance for O: = 50 at various

Mach numbers.
73

nose, reaches a maximum and decreases further downstream. It follows the same

general trend as the wall heat flux over the afterbody. Figure 5.8 shows that the

skin friction decreases over the body with increasing nose bluntness at a given Mach

number. This is because the entropy layer increases the thickness of the boundary

layer, thereby reducing the gradients of velocity profile near the body. The skin

friction is found to be most sensitive to the nose bluntness as it takes maximum

distance to reach the sharp cone value among all the flow properties.

A comparison of the entropy swallowing distance with the results of Rotta

[50] for Mach 10 freestream conditions (Table 5.1} is given in Table 5.2 for various

nose bluntness and Reynolds numbers. The table shows a large difference between

the present results and the empirical results of Rotta. This difference increases

with the increasing bluntness. The calculated entropy swallowing distance agrees

with the merging distance for skin friction, surface pressure, wall heat transfer

and shock shape. As mentioned earlier, the discrepancy between the two is due

to the assumption of constant pressure on the conical portion of the body. This

causes the overestimation of the entropy swallowing distance. For small bluntness,

the overexpansion can be neglected. Thus, Rotta's results are valid only for very

small values of nose bluntness. The distances to attain conical values of surface

pressure, shock standoff distance, wall heat transfer and skin friction are given in

Tables 5.3 and 5.4. The effects of cone angle has been investigated for a freestream

Mach number of 20 and cone angles of 5 °, 10 ° and 20 °. Some results for surface

quantities are presented in Figs. 5.9 - 5.12. An interesting general trend is noticed

from these figures. With a small increase in cone angle from 5 ° to 10 °, the merging

distances decrease dramatically. The merging distances decrease even more when

the cone angle is increased from 10 ° to 20 °. This decrease in merging distance

can be attributed to the increase in shock angle with increasing cone angle thereby

reducing the entropy layer thickness.
74

Table 5.2 Entropy swallowing distance (x/Rn) for 50 cone at Moo = 10

R_ Reynolds No. Present Rotta

0.0025 2891 139 224

0.025

14454 206 375

22908 243 470

57816 286 590

0.05 57816 288 590
75

Table 5.3 Distance (x/P_) up to which bluntness effectpersistfor Moo = I0

Quantity P_,rn 0_ = 50 Ogive

(_/= 1.4)

0.025 120 47

0.05 160 48

_ah

0.025 209 63

0.05 218 71

(7),

0.025 233 127

0.05 267 154

Q_

0.025 120 44

0.05 160 48
76

Table 5.4 Distance (x//_) up to which bluntness effect persist for Moo = 20

Quantity R_,rn 0c-50 0c= 10 ° 0c=200 Ogive Ogive

('y = 1.4) (EQUIL)

0.025 123 38 9 48 38

0.05 146 40 15 52 40

_h

0.025 172 43 10 57 44

0.05 182 45 11 67 50

c,

0.025 165 80 25 87 65

0.05 198 108 35 121 119

Q_V

0.025 126 37 10 46 36

0.05 135 40 18 50 37
77

The variation in the wall pressure with axial distance for various cone

angles is shown in Fig. 5.9. The wall pressure increases with the increasing cone

angle due to increased shock strength. Also the overexpansion/recompression increases with the cone angle while the physical distance over which this takes place

decreases. Thus a blunt cone with a larger cone angle will experience a constant

wall pressure over most of its surface as compared to a cone with a smaller angle.

Figure 5.10 shows the variation of shock standoff distance with the axial distance

for 200 cone. The shock standoff distance decreases with the increasing cone angle

over forward portion. Also the distance it takes to attain conical value decreases

with the increasing cone angle. Figure 5.11 shows the variation of wall heat transfer

for various cone angles. It is seen that the heat transfer on the wall increases with

increasing cone angle while merging distances decrease. Thus larger angle cones

will experience almost uniform heat transfer over most of the afterbody. Similar

behavior can be seen for the skin friction in Fig. 5.12. These results indicate that

as the cone angle increases the extent of effects of nose bluntness decreases.

The variation of the wall quantities with the Reynolds number is shown

in Figs. 5.13 - 5.16. In order to change the Reynolds number, the nose bluntness is

kept constant as 0.025m, while the freestream pressure is changed. The Reynolds

number is based on the nose radius. The variation of wail pressure and shock

standoff" distance is shown in Figs. 5.13 and 5.14. As expected, these quantities

were found to be independent of the Reynolds number. The variation of the wail

heat transfer and the skin friction is shown in Figs. 5.15 and 5.16. A large variation

can be seen with increasing Reynolds number. With increasing Reynolds number,

the skin friction and the heat transfer decrease. This is in agreement with the

laminar theory which states that C! and Ch are inversely proportional to v/R-'_.

Figures 5.17 - 5.23 show the effect of nose bluntness on ogive. Calculations have

been made for three nose bluntnesses and two freestream conditions. Qualitatively,
78

P wl_=

10 2

10 1

ec-2O °

i D

0c:10"

I

\

t OC:5"

b<_--_T._=- - -

J _ ._..__. ....

- _.

10 0 I I I I I I I I

0 2 ¢ 6 8

Rn:0.0025 m

0.025

0.05

X,m

10

Fig. 5.9 Variation of wall pressure with the axial distance for 0c = 5 °, 10 ° and 200

at Moo = 20.
79

10 1

10 0

10--1

b$h,m

10 -2

10--3

_- Rn=0.O025 m ec=20*

0.025 Moo =20

--.m 0.05 _ =1.4

4

3

0

0 0.2 0.4 0.6

X,ITI

10--/, I I I i l i I I I I

0 2 & 6 8 10

_ 5X10 -2

bsh, m

x,m

Fig. 5.10 Variation of shock standoff distance with the axial distance for 0c = 200

at Moo -- 20.
80

Ow

10 -1 _

Rn--O.O025

- 0.025

- 0.05

10--2

10--3

10--/,

10_ 5 I

0 2 A 6

X,m

eC=20"

10"

O

I I

8 10

Fig. 5.11 Variation of wall heat transfer with the axial distance for 0_ = 5 °, 100 and

200 at Moo = 20.
81

10--1

10 --2

C t

10--3

- Rn=0.OO25m Moo--20

_ 0.025 _"=1.l.

m. __ 0.05

-'- lo2_._s.

10__ L I I I I I I I I I

0 2 4 6 8 10

x°m

Fig. 5.12 Variation of skin friction with the axial distance for O, = 5°, 10 ° and 200

at Moo = 20.
]:

[L

Re =14454 ec=5"

28908 Rn=O.O25m

57816 ]1'=1.4

5 10 15 20

X, m

Fig. 5.13 Variation of wall pressure with the axial distance for 0= = 50 at various

Reynolds numbers.
83

E

,O

1.25

1.00

0.75

0.50

0.25

Re.=14454 ec=5"

25908 Rn=O.O25m

57816 _'=1.4

0.00 = = l _ = , ,

5 10 15 20

X° m

Fig. 5.14 Variation of shock standoff distance with the axial distance for O, = 50 at

various Reynolds numbers.
84

o 2

II/ Re.=14&5/, ec:5"

Ii_ ..... 28908 Rn=O.O25m

Li_ ----- 57816 lr: 1./,

0 I I i I i i i

5 10 15 20

X. m

Fig. 5.15 Variation of wall heat transfer with the axial distance for 0c = 50 at

various Reynolds numbers.
85

(J

5

4

3

2

x10--4

: \

' \ e-=5"

_I _ _ Rn=0.025m

- _ _'" _

Re. =1/.454 " _ -

" 28908

57816

0 I I I I I I I

5 10 15 20

X. m

Fig. 5.16 Variation of skin friction with the axial distance for 8, = 50 at various

Reynolds numbers.
86

these resultsare very similarto those for the cone, although the merging distances

have decreased. These resultsare also given in Tables 5.3 and 5.4. The variation

of wall pressure with axial distance isshown in Fig. 5.17. Here, the sharp ogive

values are reached earlierthan sharp cone values. Unlike cone, the pressure in this

case does not attain a constant value but decreases gradually with a constant slope.

The bluntness effectsvanish earlierfor Mach 20 than for Mach 10 and the region

influenced by the nose has a lower pressure than that on a sharp body. Figure 5.18

shows the variation of the shock standoff distance. As expected, the shock iscloser

to the body for Mach 20 than for Mach 10. The skin frictionand wall heat transfer

are shown in Figs. 5.19 and 5.20 respectively.In this case also the skin frictionis

found to be most sensitiveto the nose bluntness. It is the last quantity to attain

the sharp ogive values.

Figure 5.21 compares the variation of the wall pressure at Moo --20 for 50

and 10o cone with the ogive for two values of nose bluntnesses. The wall pressure

for the cone attain a constant value irrespectiveof the cone angle and bluntness at

certain distance downstream. But for the case of ogive, the pressure continuously

decreases, i.e.,it does not attain a constant value as in the case of cone for the

range of axial distances considered in the present study.

Figures 5.22 and 5.23 show the variation of the strea_nwise velocity and

temperature as a function of y distance for Mach 20 perfect gas flow over an ogive

with various nose bluntnesses. Itshould be noted that since the shock fittingprocedure isused, the normal profilesare plotted within the shock layeronly. The velocity

and temperature have been nondimensionalized with respect to theirfreestream values. The variation of strearnwisevelocityisshown in Fig. 5.22 for x --0.5, 1.0,and

5.0 m. There isa significantdifferencebetween the profilesnear the nose_ differences tend to diminish with increasing axial distance. The figure shows that the

thickness of the boundary layerincreaseswith increasing bluntness and the shape of
. 87

10 2

I

Rn=O.OO25m OGIVE

0.025 7 =1.c

0.05

100 I I I I ! I I i

0 2 4 6 8

Xom

10

Fig. 5.17 Variation of wall pressure with the axial distance for ogive at various

Mach numbers.
88

101

100

6sh'm 10-- 1

10--2

- Rn=O.OO25m

_ 0.025

_ N .m 0.05

Moo :IQ

2O

10 -3 I I I I I I I I I

0 2 4 6 8 10

X,ffl

Fig. 5.18 Variation of shock standoff distance with the axial distance for ogive at

various Mach numbers.
89

10--2

10 --3

Qw

10--4

- Rn=0.0025m

0.025

__ 0.05

m

B

Moo "20

lO

10-- 5 I ! I I I I I I I

0 2 4 6 8

X,m

10

Fig. 5.19 Variation of wall heat transfer with the axial distance for ogive at various

Mach numbers.
9O

10-2

C I 10 -3

10--4

t" Rn=O'0025m OGIVE

0.025 _/" :1.4

0.05

I I I I I I I I I

0 2 4 6 8 10

x,m

F_g. 5.20 Variation of skin friction with the axial distance for ogive at various Mach

numbers.
91

Pw/_

102 !

Rn=0.0025 m Moo =20

0.05 _ --1.4

, Oc=lO"

-| i_-

101

8C-- 5"

I m _ m -

10 0 I I I | 1 I I I I

0 2 4 6 8 10

x.m

Fig. 5.21 Variation of wall pressure with the axial distance for 0, = 10 °, 200 and

ogive at M_ = 20.
92

the profile is changed near the outer edge of the boundary layer. It is this difference

which causes the wall values to be so different from the sharp cone values. The

same type of effects can be seen in Fig. 5.23 where temperature profiles between

the shock and the body are shown. Here again the differences decrease with the

axial distance.

The results for perfect and equilibrium gas flows are shown in Figs. 5.24

- 5.28. The calculations were performed for Mach 10 and 20, but the results for

Mach 10 were found to be identical for perfect and equilibrium gas, so only Mach

20 results are presented.

The variation of wall pressure, shock standoff distance, heat transfer, and

skin friction at Mach 20 for ogive of O.05m nose radius are shown in Figs. 5.24 -

5.27 for perfect and equilibrium gas flows. The equilibrium gas results do not show

any dramatic differences from the perfect gas results although, as expected, there

are some minor differences. The difference in two solutions tends to increase with

increasing Mach number and bluntness. The most severe case is shown here. These

figures show expected effects of equilibrium gas chemistry on the flow properties.

Figure 5.24 shows that the surface pressure variation is almost identical

for the two cases, except in the recompression zone, where the equilibrium pressure

is slightly higher than the perfect gas value due to higher density. The variation of

shock standoff distance is shown in Fig. 5.25; the shock moves closer to the body

for equilibrium flow calculations. The shock curvature is slightly reduced, thus the

entropy generation is reduced too. This in turn, decreases the merging distances

as can be seen in Table 5.4. The decrease in the shock standoff distance can be

attributed to the increase in density of the gas behind the shock. The variation of

the skin friction and the heat transfer is shown in Figs. 5.26 and 5.27 respectively.

The skin friction is reduced slightly in the overexpansion zone but reverses its trend

in the recompression zone before settling clown to the perfect gas value where the
93

1.0

0.8

0.6

0.4

0.2

x=O.5m

I

# S

#

!

r

1.0 5.0

H m

,_._" ."-I" I/"

r/./

.15

!

Ogive

Moo= 20

7=1.4

Rn

0.0025 m

...... 0.025

0.05

O0 I | I I 5 .10 .20 .25

t I ! J H

.10 .15 .20 .25 .30

L I I | I m

•65 .70 .75 .80 .85

y, m

Fig. 5.22 Comparison of velocity profiles at x = 0.5, 1.0, and 5.0 m for ogive at

Mach 20.
94

x=O.5m

30 - I 1.0

25 ,'_'\ _ 5.0 Ogive

i! '\ !i\ 11I M.=20

20 " \',', "_ 7=1"4

"'_.

T ! _! \ _' Rn

_, \ I_

T_ 15 ! _ \ / _i 0.0025m

10 "l[_ \, \ /1!

',,IIi "\ \..I I! ...... 0.025

",, " 0.05 5 _ ,,,_=_,_., -.%._

O0 5 I I I I I

.10 .15 .20 .25

! 1 | I I ]I

.10 .15 .20 .25 .30

t I I I J HI

•65 .70 .75 .80 .85

y, m

Fig. 5.23 Comparison of temperature profiles at x = 0.5, 1.0, and 5.0 m for ogive

at Mach 20.
95

real gas effects are practically zero. In general, the real gas effects do not affect the

wall quantities to any large extent for the present conditions. The only noticeable

difference is for the case of shock standoff distance.

Figure 5.28 shows the effect of equilibrium gas chemistry on the temperature profile. It should be noted that even though the maximum temperature in

the shock layer decreases for the equilibrium gas as compared to the perfect gas,

the gradients near the wall do not change significantly and therefore the wall heat

transfer is not affected much. The merging distances for these cases are given in

Table 5.4. Due to the shock moving closer to the body, a small decrease in merging

distances is noted for equilibrium flows.
96

P

45

40

35

30

25

20

15

10

5

0

OGIVE

Rn=O.05

EQUIL

=1.4

I I I L

2 4 6 8

Xom

10

Fig. 5.24 Comparison of wall pressure for equilibrium and perfect gas.
xlO -1

4

bsh,m 2

0

OGIVE

_ M=20 f//

_ Rn=O'O5m /_///

_t __" --'1.4

I

_

I I I I

0 2 4 6 8 10

x°m

Fig. 5.25 Comparison of shock standoff for equilibrium and perfect gas.
98

0 W

3.0

1.5

0

xi0--3

OGIVE

M=--20

Rn=O.O5m

m

/- EQUIL

B

I

0 2 _ 6 8 10

I I I

x°m

Fig. 5.26 Comparison of wall heat transfer equilibrium and perfect gas.
99

x10--3

6

Cf 3

0

OGIVE

20

Rn=0.05m

___ EOUIL _ --1./.

!

2

I I I

4 6 8 10

Xom

Fig. 5.27 Comparison of skin friction for equilibrium and perfect gas.
100

T

Too

30-

25-

20-

15-

10-

5-

Ogive

M=_= 20

Rn= 0.05 m

x=0.5m

! 1.0

y =1.4

...... Equil

I I 1 I [

.05 .10 .15 .20 .25

I I I i I H

•10 .15 .20 .25 .30

t I I I I

•65 .70 .75 .80 .85

nl

y, m

F{g. 5.28 Comparison of temperature profiles for equilibrium and perfect gas at x

= 0.5, 1.0, and 5.U m tor ogive at Mach 20.
101

5.2 Forebody and the Cowl Shock Interaction

In this section the results of the forebody and the cowl shock interaction are discussed. A two-dimensional model is used to study the interaction.

The computational domain is shown in Fig. 5.29; the solution over the whole domain is obtained in two steps. First, the cowl forebody solution is obtained using

the Navier-Stokes equations. This solution is then used as the starting solution

for the parabolized Navier-Stokes equations in the second step to calculate the extent of downstream effects of the shock-shock interaction into the inlet. Two sets

of freestream conditions at Mach 8.03 and 5.94 (Table 5.5) are considered due to

availability of the experimental data on the cowl forebody. However, no experimental data is available for the cowl afterbody. The results for Mach 8.03 are discussed

first, followed by Mach 5.94 results. For all cases, the undisturbed blunt body flow

was calculated first and this solution was then used as initial condition for shock

impingement calculations. First the results are presented for unadapted grid and

then with the adapted grid to show the importance of a suitable grid in obtaining

improved solution.

Figures 5.30 - 5.33 show the temperature, Mach number, pressure, and

density contours respectively for the cowl forebody at freestream Mach number

8.03 with unadapted grid shown previously in Fig. 4.10. The surface pressure

and the heat transfer results are shown in Figs. 5.34 and 5.35 respectively. The

improved results with adapted grid are shown in Figs. 5.36 - 5.43. The temperature

and Mach number contours for the cowl afterbody are shown in Figs. 5.43 - 5.46.

The variation of shock standoff and surface pressure on the cowl afterbody are

shown in Figs. 5.47 and 5.48. The leeward and windward side computations were

made independent of each other due to the two-dimensional nature of the flow.

Similar calculations for Mach 5.94 are shown in Figs. 5.49 - 5.57. For this case the

calculations were made for adapted grid only.
102

MI_

BOW SHOCK

Y

Fig. 5.29 Computational domain.
103

The temperature, Mach number, pressure, and density contours are shown

in Figs. 5.30 - 5.33, respectively. The distorted bow shock is clearly evident in the

figures. The bow shock has moved toward the body on the windward side and away

from it on the leeward side. Here, the windward side is defined as the upper side

where the flow passes through the impinging shock before encountering the body.

The interaction of the bow shock and the impinging shock produces a supersonic

jet surrounded by subsonic flows. The jet terminates with jet shocks and impinges

on the body producing a local zone of very high pressure and heating rates. The

stagnation point moves toward the windward side. The location of the stagnation

point depends upon the strength and orientation of the impinging shock. Due to the

coarseness of the grid away from the wall, the shocks and shear layers are smeared

over several grid points.

The variation of wall pressure along the cowl forebody surface is shown

in Fig. 5.34. The pressure is nondimensionalized by the stagnation point pressure

for unimpinged blunt body flow. In order to properly visualize the effect of shockshock interaction, the surface pressure for unimpinged blunt body flow (for the same

freestream conditions) is also shown in the figure. The pressure on the windward

side increases considerably with a localizedzone of high pressure and fallsbelow the

surface pressure for unimpinged case on the leeward side.The peak value of pressure

isabout nine times the stagnation point value. The resultscompare very well with

the experimental data of Wieting [34] and numerical calculations of Klopfer [44].

Figure 5.35 shows the heat transfer along the wall on the cowl forebody.

The heating rates are nondimensionalized with respect to the stagnation point heating for unimpinged case. It shows similar behavior as the surface pressure, i.e., an

increased heating on the windward side and decreased heating on the leeward side.

A localized zone of intense heating is observed on the windward side. The results

compare fairly well with the results of [44] but show poor comparison with the
104

Table 5.5 Freestream and stagnation point conditions for the cowl.

Moo ReD T,,,/Too D 6 po/po_ qo(w/m _)

8.03 387500 2.382 0.0762 12.5 83.5 6.80 E + 5

5.g4 186000 6.857 0.0254 15.0 45.8 2.16 E + 4
105

Fig. 5.30 Temperature contours for the cowl forebody, Moo = 8.03.
106

Fig. 5.31 Mach number contours for the cowl forebody, Moo = 8.03.
i07

Fig. 5.32 Pressure number contours for the cowl forebody, Moo = 8.03.
108

\

"\

\

\

\

\

Fig. 5.33 Density contours for the cowl forebody, Me,, = 8.03.
109

o

i

10

9

8

7

6

4

0

--100

_ Present

Klopllr

0 Experiment

i__ - Unimpinged

--80 -60 --40 --20 0 20 40 60 80

I

100

Fig. 5.34 Variation of surface pressure for the cowl forebody, Moo = 8.03.
II0

o

o"

I0

g

8

5

3

_) _ Present

0

0 ..... Klopter

0 Experiment

O0

----- Unimpinged

0

Fig. 5.35 Variation of surface heat transfer for the cowl forebody, Moo - 8.03.
Iii

experimental data. The trend is predicted but the magnitude is relatively lower,

specially near the jet impingement location. It should be noted that for this case, the

solution was found to be marginally unsteady as the residual did not go down more

than three orders of magnitude. Also the jet oscillated slightly, thereby changing

the location of the peak pressure and heating. Similar behavior was noted for the

Type IV interaction in [44] and I45]. Due to poor comparison of heating rates and

smearing of important flow features, it was decided to use adaptive grids thereby

placing the grid points where they are needed the most. Body curvature, pressure

and density were chosen as weight functions to adapt the grid to the solution. The

body curvature clusters the grid points very close to the body while pressure and

density attract the grid points near shocks and shear layers.Thirty percent of the

points were allocated for adaptation by body curvature, forty percent by pressure

and density and the rest of the points were used for creating uniformity of the grid

so that the grid isnot too coarse in any section. The adapted grid isshown in Fig.

4.10.

The temperature, Mach number, pressure and density contours are shown

in Figs. 5.36 - 5.39,respectivelyfor the adapted grid. Since the grid isfinenear the

shocks and shear layer,the flow features are now captured very well. The location

of the stagnation point, jet and shear layer originatingfrom the shock intersection

can be seen clearly.The shear layeroriginating from the stagnation zone is much

thickeron the leeward side as compared to the windward side.

Figure 5.40 shows the velocity vectors for the cowl forebody flow. Even

though the flow is at zero angle of attack, its direction is changed as it passes

through the impinging shock. The shear layer on the leeward side is seen clearly

originating from the stagnation point.

The variationsof pressure and heating rate along the cowl forebody surface

are shown in Figs. 5.41 and 5.42, respectively.The general features of the pressure
• 112

Fig. 5.36 Temperature contours for the cowl forebody for M_o = 8.03 with adapted

grid.
113

Fig. 5.37 Mach number contours for the cowl forebody, Moo : 8.03 with adapted

grid
114

Fig. 5.38 Pressurecontours for the cowl forebody for Moo = 8.03 with adapted grid.
115

Fig. 5.39 Density contours for the cowl forebody, M_ = 8.03 with adapted grid
116

Fig. 5.40 Velocity vectors for the cowl forebody, Moo = 8.03 with adapted grid.
117

distribution are unchanged, except for the peak pressure which is now the same as

obtained in [44]. It should be noted that for this case the location of the impinging

shock was made to coincide with the experimental location by matching the peak

pressure location. The heating rates show a remarkable improvement over the

previous calculations. The results compare favorably with the experiment. The

discrepancy in the peak values is probably due to the unsteadiness in the flow and/or

the turbulent nature of the jet as indicated recently by Moon et al. [46]. The rate

of convergence for this case was found to be slower than that for the unadapted

grid case. The cowl of the scramjet engine is modeled by a 5° wedge. The cowl

afterbody flow is computed by solving the parabolized Navier-Stokes equations using

the van Leer scheme described previously. The initial plane solution, to initiate the

marching procedure, is provided by the cowl forebody solution. The objective of

carrying out the cowl afterbody calculation is to determine how the flow entering

the engine inlet is affected by the shock-shock interactions.

Figures 5.43 and 5.44 show the temperature contours on the cowl afterbody

for the windward and the leeward sides, respectively. The flow field on the windward

side is quite different as compared to the leeward side. The flow on the windward

side shows typical two-dimensional wedge flow features, such as rays of the constant

Mach number and the shock attaining conical angle slightly downstream of the nose.

The leeward side shows thick shear layer interacting with the boundary layer; this

interaction again produces a weak shear layer whose strength diminishes rapidly

with the axial distance. Similar behavior can also be seen in the Mach contours for

the cowl afterbody in Figs. 5.45 and 5.46.

Figure 5.47 shows the variation of shock standoff distance along the cowl

afterbody on leeward and windward sides for the impinged case. These are compared with the results for the unimpinged case. The shock moves closer to the

body on the windward side and away from it on the leeward side as compared to
118

10

0

n

Q.

5

3

Present

..... Klopler

o Experiment

----- Unimpinged

|

-100 --80 -60 -40 -20 100

!

0 20 40 60 80

o

Fig. 5.41 Variation of surface pressure for the cowl forebody, Moo = 8.03 with

adapted grid.
119

o

o"

I0

8

7

6

5

o

o

Present

..... Klopt er

o Experiment

----- Unimpinged

--100 --80 -60 --40 --20 0 20 40 60 80 100

e

Fig. 5.42 Variation of surface heat transfer for the cowl forebody, Moo = 8.03 with

adapted grid.
120

the unimpinged case. The calculations were carried out up to 40 nose radii downstream. The angle at which the shock enters the inlet is significantly changed due

to the shock-shock interaction which can affect the inlet performance.

The variation of pressure on the cowl afterbody is shown in Fig. 5.48.

The pressure on the windward side increases on the forward portion of the cowl

afterbody; it reaches a peak value and then settles down to a constant value as the

flow recompresses back after the expansion on the cowl forebody. On the leeward

side also, the pressure increases and then gradually attains approximately the same

value as for the unimpinged case. The rate of pressure increase on the forward

portion of the leeward side is found to be slower than on the windward side.

Similar calculationswere carried out for Mach 5.94 freestream conditions

and the resultsare shown in Figs. 5.49 - 5.58. Qualitatively these resultsare very

similar to the Mach 8.03 conditions. The grid for this case isgenerated in the same

way as for the previous case. The temperature, Mach number, pressure and density

contours are shown in Figs. 5.49 - 5.52 for the cowl forebody. As noted previously

the shock moves toward the body on windward side and away from iton leeward

side. The effectsof jet are clearly visiblein the figure. The variations of surface

pressure and heat transfer are shown in Figs. 5.53 and 5.54, respectively. The

numerical resultsare compared with the experimental data reported by Tannehill

et al. [42]from an unpublished experiment by J. W. Keyes of NASA Langley. The

computed values of surface pressure agree very well with the experimental data

although the peak pressure isslightlylower than the experimental peak. Although

not shown here, the peak value of the pressure matches with the numerical calculationsof Tannehill et al.[42].The variation of heat transfer on the cowl forebody

isshown in Fig. 5.54. For heating rates,the experimental data was reported only

at one point. The peak value of the heating rate is6.4 times the stagnation point

value, which iswithin the uncertainty range of the experiment.
121

The results for the cowl afterbody are shown in Figs. 5.55 - 5.58. The

Mach number contours for the windward and the leeward sides on the afterbody are

shown in 5.55 and 5.56. The shear layer on the leeward side is thinner as compared

to the Mach 8.03 due to weaker strength of the interacting shocks. The variation

of shock standoff distance for the cowl afterbody is shown in Fig. 5.57 and the

variation of the wall pressure for the cowl afterbody is shown in Fig. 5.58. The

results show the same trends as for the previous case.
122

ORIGINAL PAGE IS

OF POOR QUALITY

Fig. 5.43 Temperature contours on tile windward side of the cowl afterbody,

Moo = 8.03.
123

ORIGrNAL PAGE rs'

OF POOR QUALITY'

F_g. 5.44 Temperature contours on the leeward side of the cowl afterbody,

Moo = 8.03.
124

ORIGINAL PAGE F_

OF POOR QUALITY

Fig. 5.45 Mach number contours on the windward side of the cowl afterbody,

Moo = 8.03.
125

_RIGINAL PAGE fS

OF POOR QUALITy

Fig. 5.46 Mach number contours on the leeward side of the cowl afterbody,

Moo = 8.03.
126

¢

IB

4_

20

Mm:8.03

_-- Leeward

,. -:: °::°7.::: °

| I ! I I I

10 15 20 25 30 35 _,0

xlR n

Fig. 5.47 Variation of shock standoff distance with axial distance for the cowl

afterbody, Moo = 8.03
127

1.0

0

Or.

Q.

.g

.8

.7

.8

.S

./.

.3

.2

.1

O

, \

/

!

/

m

I%o "8.03

Leeword

..... Unimpinged

----- Windword

I0

I i I

15 20 25 30 3S

xlR n

|

4O

Fig. 5.48 Variation of surface pressure with axial distance for the cowl afterbody,

Mo_ = 8.03
128

ORrGINAL PAGE |S

OF POOR QUALITY

Fig. 5.49 Temperature for the cowl forebody, Moo = 5.94.
129

OE POOR QUALITY

Fig. 5.50 Mach contours for the cowl forebody, M¢¢ = 5.94.
130

ORIGINAL PAGE f£

OF POOR QUALITY

Fig. 5.51 Pressure contours for the cowl forebody, Moo = 5.94.
131

ORIGINAE PAGE I_;

OF POOR QUA'JTY

Fig. 5.52 Density contours for the cowl forebody, Moo = 5.94.
132

0

n

Q.

10

5

0

00

! I I

--80 -60 -/,0

Presenl

0 Experiment

..... Unimpinged

0

-20 0 20 40 60 80

i

100

Fig. 5.53 Variation of surface pressure for the cowl forebody, Moo = 5.94
133

0

O"

O"

10

7

2

1

0

-100

Present

Experiment

Unimpinged

ql

l-

"e'" I

--80 --60 --/,0 --20 0 20 /.0 60 80 100

Fig. 5.54 Variation of surface heat transfer for the cowl forebody, Moo -- 5.94.
134

ORIGINAL PAGE IS

OF POOR QUALITY

M_ Fig. = 5.55 5.94. Mach number contours on the windward side of the cowl afterbody,
135

ORIGINAL PAG_ !_

OF POOR QUALITY

Fig. 5.56 Mach number contours on the leeward side of the cowl afterbody,

Moo = 5.94.
136

C

r'

,0

2O

16

12

Ma_:5,94

! ! | | t I

10 15 20 25 30 35 /.0

x/R n

Fig. 5.57 Variation of shock standoff distance with axial distance for the cowl

afterbody, Moo = 5.94
137

1.0

0

.9

.8

.7

.6

.5

.3

.2

.I

f,,

Moo= 5.9t,

Leeward

..... Unimpinged

----- Windward

I I I I ! I

I0 15 20 25 30 35 _0

xlR n

Fig. 5.58 Variation of surface pressure with axial distance for the cowl afterbody,

Moo = 5.94
138

5.3 Forebody Shock and the Inlet Sidewall Shock

Interaction

In thissection the interactionofthe forebody shock with the inletsidewall

shock isdiscussed. As noted previously,the flow isfullythree-dimensional in nature

for this case due to the orientation of the impinging forebody shock as well as the

sweep of the inlet.Due to sweep of the inletsidewall,the Type V interactionoccurs

and isconsidered in thissection. Results have been obtained fora 25o swept sidewall

and 10o impinging shock generator angle. The freestrearnconditions used for the

present calculationsare given in Table 5.6. For three-dimensional calculationsalso,

the flow fieldis initializedby the undisturbed flow fieldsolution. First the results

are presented for the undisturbed flow and then with the impinging shock to show

the effectof impingement.

Figure 5.5g shows the pressure contours for the unimpinged case. The

stagnation plane (0 = 0) contours are shown in Fig. 5.59(a). The bow shock can

be seen clearlyand isswept back as itfollows the body contours. The flow in each

cross-sectionalplane is identical,so only one such plane isshown in Fig. 5.Sg(b).

The figureshows a typicalblunt body solution. At the stagnation point, the pressure

has itspeak value and the shock standoff distance isminimum. Figure 5.60 shows

the temperature, Mach number and the pressure contours in the stagnation plane

with shock impingement. The temperature contours are shown in Fig. 5.60(a).

Due to stretching,the grid away from the surface iscoarse;consequently, the shock

and the other flow features are smeared over several grid points. The impinging

shock isclearlyvisibleinthe figure.The bow shock moves away from itsunimpinged

position and a shear layer isproduced. The shear layer interactswith the boundary

layer. Also, a transmitted shock isproduced which strikesthe body causing a jump

in pressure and heating rates. These are typicalfeaturesof the Type V interaction.

There are temperature and density gradients across the shear layer. The Niach
139

Table 5.6 Freestream conditions for inlet.

Moo ReD T_, Too R,_

5.94 180000 394 k 59.6 k 0.0125 m I0.0
140

ORIQINAL PA_ IS

OF POOR QUALITY

/

(a) (b)

Fig. 5.59 Pressure contours for the unimpinged case in stagnation (0 = O) and z =

constant plane.
141

number contours are shown in Fig. 5.60(b); the flow is fully subsonic behind the

bow shock and the shear layer can be seen. Figure 5,60(c) shows the pressure

contours in the stagnation plane. Since the pressure is constant across the shear

layer, it is not visible in this figure. Some expansion waves can be seen emanating

from the intersection point. These expansion waves reduce the surface pressure as

the flow expands.

More insight into the numerical solution can be gained by examining the

plots for z = constant planes. Figure 5.61 shows a series of contour plots for increasing values of z. The movement of the impingement point (as it moves away

from stagnation point) can be seen clearly; also, the shear layer as it develops is

very well captured.

Figure 5.62 shows a cylindrical fin coated with temperature sensitive paint

and the corresponding schlieren photograph locating regions of high pressure and

heat transfer for Type V interference at Mach 4.6 and is taken from [31]. It should

be noted that the freestream conditions for this case are slightly different but it

shows a typical Type V interaction. The numerical solutions are shown in Figs.

5.63 and 5.64. The surface pressure contours are shown in Fig. 5.63 and the

stagnation plane pressure contours in Fig. 5.64. The numerical solution captures

the first two (A and B) peaks in pressure and heat transfer. The third peak (C) is

caused due to separation near the nose which is absent in the numerical solution due

to assumption of infinite length of the body. The behavior of bow shock near the

impingement point is very well captured by the numerical scheme. The comparison

of Figs. 5.63(b) and 5.64 shows how well the physical phenomenon is capture by

the numerical scheme.

Figures 5.65 - 5.70 show the variation of pressure and heat transfer along

the surface. They are nondimensionalized with respect to the unimpinged stagnation line values. The unimpinged stagnation line pressure is obtained by the
142

ORIGINAL PAGE' IS

POOR QUALITY

Fig. 5.60 Temperature, Mach number and the Pressure contours in the stagnation

plane(0 = 0).
143

ORIGINALPA_E , OF I:'O0_ IS"

QUALiTy

\

/

s:2.3

s = 2.6 | : 2.8

• Fig. 5.61 Temperature contours in the z = 2.3, 2.6, and 2.8 cm plane.
144

(a)

A

B

C

(b)

A

B

C

Fig. 5.62 Local regions of high heating rates and corresponding schlieren photograph.
145

Fig. 5.63 Surfacepressurecontours for Type V interaction.
146

Fig. 5.64 Pressurecontours in the stagnation plane(O = 0).
147

Rayleigh pitot formula as

eM? 6

p®P"2"°= ( 5 • }(7M_- 1 (5.1)

Where M1 is the normal Mach number. The stagnation line heat transfer

is obtained by the following expression developed by Beckwith and Gallagher [77]

qo = k_ T_,,_ - T_ 2ReD,oo/Zo 2Toopo Po 1⁄4

2D ( M¢o_oo )1⁄2[_(p"_- 1)] (5.2)

where

= + ,=K:M:)

here r is the recovery factor and is taken as 0.85 and subscript e denotes the boundary layer edge conditions.

Figure 5.65 shows the comparison of stagnation line pressure with the

experimental data of Keyes and Hains [32] and numerical calculations of Holst et

al. [47}. In the experimental set up, the interaction point was only 3 cm downstream

from the end of the cylinder (z = 0) and, therefore, there was some relieving effect

(Fig. 5.62). But in the present calculations, the body is assumed as infinite in

length and hence no relieving effect is allowed. Due to this discrepancy in the

model used, the results do not match near the end. However, the results compare

fairly well away from the end point. The peak value of the pressure is caused by

the interaction of the transmitted shock with the boundary layer and it is very well

captured. The flow overexpands and then recompresses back to the unimpinged

value. The peak value of the pressure is about 2.2 times the unimpinged stagnation

line pressure. The variation of the stagnation line heat transfer is shown in Fig.

5.66. In this case also, the comparison is poor near the end for the same reasons as

explained earlier. But away from the end, results compare fairly well. As expected,

the heat transfer follows the same general trend as the pressure. The peak value

of heat transfer is about three times the unimpinged stagnation line heat transfer.

The experimental data is available only for the stagnation plane.
148

The resultsfrom the present calculationsshow a remarkable improvement

over Holst's calculations due to various factors. The flow fieldis much better resolved due to a finer grid, no artificialthickening of the boundary layer has been

attempted and the boundary condition at the end( z-0 ) axe physicallycorrect. The

present calculationsemploy zero gradient conditions at the end which are physically

realistic.In the previous calculations,the flow was maintained at constant conditions corresponding to the unimpinged flow solution, which is incorrect for the

impinging shock case.

The variation of pressure and heat transfer at the surface is shown in

Figs. 5.67 and 5.68 for various values of y. The stagnation line (y--0) has the

maximum amplification of pressure and heating rates as the bow shock has its

maximum strength along this line.Away from the stagnation line,strength of bow

shock decreases. Consequently, the interactionbecomes weaker and weaker, thereby

causing lessamplification of the wall quantities.

The circumferential variations of the wall pressure and the heat transfer

are shown in Figs. 5.69 and 5.70 for various values of z. The impinging shock

intersectsat the normal portion of the'bow shock at z -- 2.4 cm, and therefore,the

peak values of pressure and heating occur in this circumferential plane. At higher

values of z, the impinging shock intersectsaway from the normal portion of the bow

shock, thus reducing the amplification.
149

0

Q.

2.5

2.0

I.S

1.0

0.5

o

C_

o

oo ,o.Oo_ .....

o 0 0",--_

-- Present

Experiment

Holst

0 0

! I I | I

0 0.2 0.4 0.6 0.8 1.0

zlL

Fig. 5.65 Comparison of surface pressure with the experimental data.
150

0

O"

o

Present

0 Experimenl

..... Hoist

; _ ,.\ o

O ! I i ! !

0 0.2 0.4 0.6 0.8 t.O

zlL

Fig. 5.66 Comparison of surface heat transfer with the experimental data.
151

0

Q.

0-

2.5

2.0

1.5

1.0

0.5

i'_

]

y" 0.0 cm

..... 0./.

----- 0.8

1.3

" |_l I1-----

.f

0 i i i i I

0.2 0.4 0.6 0.8 1.0

z/L

Fig. 5.67 Variation of the surface pressure in various y = constant planes
152

o

cr

_r

! !

0.2 1.0

y"O.O cm

..... 0./.

0.8

_.-- 1.3

¢

% I

_. _/_'_ _ _-----_

_ . _ .__

! ! !

O.L 0.6 0.8

zlL

Fig. 5.68 Variation of the surface heat transfer in various y = constant planes
153

2.5

2.0

z-"2.LO cm

2.75

3.00

3.25

0

CL

3r

OL

1.5

1.0

0.5

0 t v v !

20 LO 60 80 100

0

. Fig. 5.69 Variation of the surface pressure in various z -- constant planes
154

o

oz= 2./.0 ¢m

..... 2.75

-- - -- :3.00

3 3.25

1

! ! ! |

20 _0 60 80

!

100

Fig. 5.70 Variation of the surface heat transfer in various z = constant planes
Chapter 6

CONCLUSIONS

This study presents a numerical investigation of the effects of blunt leading edges on the viscous flow field around a hypersonic vehicle such as the proposed

National Aero-Space Plane. Attention is focused on two specific regions of the flow

field. In the first region, effects of nose bluntness on the forebody flow field are

investigated. The second region of the flow considered is around the leading edges

of the scramjet inlet where the forebody shock interacts with the shock produced

by the blunt leading edges of the inlet compression surfaces.

An accurate solution procedure is developed to determine the downstream

influence of forebody bluntness and the conditions, under which it can be modeled

by a relatively simple pointed nose body of revolution, are determined. A wide

range of physical and geometrical conditions are considered. The geometrical models used axe spherically blunted cones of 5 °, 10 °, and 200 angles and ogives with

various degree of bluntness. Several approximate methods as well as combination

of Navier-Stokes and paxabolized Navier-Stokes equations are used to accurately

compute the flow field. The high temperature effects are incorporated by using

equilibrium gas model. The influence of entropy layer generated by the shock curvature on the flow field are also addressed. The results show that the effects of leading

edge bluntness persist several hundred nose radii downstream depending upon the

bluntness, freestream conditions and geometry. It is found that the entropy layer

155
156

generated by the shock curvature affects the development of the boundary layer far

downstream of the nose. These effects decrease with increasing Mach number. Also

the bluntness effects decrease with increasing cone angle. The results show that the

wall quantities are not affected much by the inclusion of high temperature effects

through equilibrium chemistry for the conditions considered in the study.

The interaction of the forebody shock with the blunt cowl shock and with

the shock produced by the blunt leading edge shock of the inlet is also investigated.

The calculation of flow field and aerothermal loads under such conditions is quite

difficult due to complexity of the flow field and the presence of high gradients. A

modern computational algorithm developed by van Leer which has the advantage

of better modeling of physics of the problem has been used. The impinging forebody shock is treated as a sharp discontinuity across which the exact shock jump

conditions (Rankine-Hugoniot) are applied. The compressible Navier-Stokes equations are solved using a shock capturing algorithm. It does not require any prior

information about the shock impingement flow field to be computed as is the case

with some of the previous semi-empirical approaches. The use of solution adaptive

grid technique to properly resolve the flow gradient is made and the importance

of suitable grid in the numerical calculations is shown. The numerical computations are primarily done for Type IV supersonic jet interaction since it represents

the most severe case in terms of aerothermal loads. A preliminary investigation

of the Type V interaction which occurs on the leading edge of the compression

side walls of the inlet is also made. The flow field for this interaction is found to

be fully three-dimensional unlike the cowl plate interaction. Results of the study

show the presence of local regions of very high surface pressure and intense heating.

For Type IV interaction, the peak amplification of pressure is found to be nine

times and heating amplification to be eight times the stagnation point value of the

unimpinged case. The degree of amplification is found to increase with the increas-
157

ing Mach number. For Type V interaction, the amplification factors are found to

be much lower, the pressure is amplified by a factor of 2.25 and heat transfer three

times the stagnation values for unimpinged case. The results compare well with

the experimental data. The study demonstrates the capability of CFD to simulate

extremely complex viscous hypersonic flows.

Based on the present study, several recommendations concerning the extensions of this work are suggested.

(1) The realistic forebody geometry should be used for calculating bluntness effect.

(2) The flow in the present investigation is assumed to be laminar, but in actuality

the flow would be turbulent, so an appropriate turbulence model should be incorporated into the governing equations.

(3) Due to high Mach number flows considered, it would be desirable to examine

the non-equilibrium high temperature effects.

(4) The oscillatory behavior of the supersonic jet in Type IV interaction needs to

be further investigated.

(5) Since the inlet will have more than one shock impinging on it, the effects of

multiple shock wave interaction needs to be determined.

(6) The effects of oscillations in the impinging shock should be investigated.

(7) The use of adaptive grids for three-dimensional calculations to properly resolve

the flow should be made.

(8) For Type V interaction considered, the end effects should be accounted for to

have a better comparison with the experiment by using zonal techniques.

(9) The effects of shock wave interference in the presence of active cooling should

be determined because to relive the thermal stresses, some form of active cooling

would be used.

(10) Various shock interference pattern for three-dimensional calculations should be

computed by varying the sweep angle.
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APPENDICES
APPENDIX A

EQUILIBRIUM CHEMISTRY

A fully vectorized code EQUIL as described in [56] has been used in

the present analysis to include the real gas effects. It is based on free energy

minimization technique [78] in which the method of steepest descent is used. In this

method, with the known values of local temperature, pressure and elemental mass

fractions of the gas, the mole numbers of species are calculated which minimizes

the free energy. The free energy of a mixture of n species containing x_ moles of the

ith species is expressed as

where

F(X) = _ f, (A.1)

i=1

x = (_,_,_ ......_.) (A.2)

f, = x, [c, +ln_] (A. 3)

c,= (R-_), + tnP (A.41

i=1

Equation (A.1) is minimized, subject to mass balance constrain. An initial

assumption is made on the mole numbers of various species, and then an iterative

procedure is followed to find the set of mole numbers which satisfy both the constraints. Once the mole numbers are known, the molecular weight and mass fraction

165
of each species are calculated. The specific heat, free energy, viscosity and thermal

conductivity are calculated using polynomial expressions given below.

Cp __

-- - al + a2T + aaT 2 + a4T 3 + as T4

R

H T T 2 T 3 T' 1

R---T = al + a2_ + a3-'_- + a4--_- + as"_ + a6"_

F T T 2 T a T 4 1

RT - a,(1 - lnT) - a,_ - aa--_ - a,--_ - as_-_ + a6-_ + a7

I.t = bl + b2T + b3T 2

k = C1 "_- c2T

(A.6)

(A. 7)

(A. S)

(A.9)

(A. 10)

The values of constants al, a2, - - -, aT, bl, --, bs, ci and c2 are given in [56]
APPENDIX B

ADAPTIVE GRID

An adaptive grid system was employed to improve the quality of the solution without increasing the number of grid points. The accuracy of the numerical

solution depends on the fineness of the grid. The presence of large gradients causes

the error to be large in numerical approximation of the derivatives. Therefore, it

is desirable to have fine grid spacing near the shocks, shear layers, and boundary

layer. The method developed by Abolhassani et al. I76] has been used. It is a very

general method with capability to adapt the grids with various variables such as

pressure, Mach number, shear stress etc and is based on variational approach. It

is an algebraic method and is formulated in such a way that there is no need for

matrix inversion, which makes it computationally very efficient. The grids were

adapted by using Euler Lagrange equations

Z_f.u -_ constant (B. :)

This equation can be written in normalized form as

where s is the arc length.

spacing where w is large.

f_ wCt)dt (B. 2)

_(s) = f_: w(t)dt

The weight function is used to reduce the grid point

For grid adaptation with several variables, the weight

167
168

function is expressed as

i=l

where n is the number of variables, b_ are the constants and f_ are the variables( or

their derivatives ). The constant 1 is for uniformity. The values of b_ are calculated

based on percentage of grid points allocated to each variable.

grid points for each f_ can be written as

where

b;F,(S_..)

Rj = S,_, + ELI b_F_(S,,,°=) ;3" - 1,2,3, 4 ......

F_(s) = fi(t)dt

The percentage of

(B.4)

The integral is approximated by a trapezoidal rule. After some algebraic manipulations,Eqs. (B.2) and (B.4) can be written as

'(")= _8maz t- P_ + _i=I P"F,(Sc_a=) (S.s)

In order to avoid extremely small grid spacing near the shocks, the weight functions

f_ are multiplied by the factor

--_$

1 - e z'J_7_- (B. 6)

where AS,a,, is the allowable minimum spacing. The function varies from 0 to 1

and is proportional to the spacing.
APPENDIX C

RADIATIVE INTERACTIONS

IN LAMINAR FLOWS

A preliminary study was conducted to include the radiative formulation

in the general governing equations and provide the step by step analysis and solution procedure for realistic problems. The objective of this study was to prepare

the ground work for future studies of inlet flows involving radiative interactions.

The original plan was to compute the inlet flow in combination with shock-shock

interaction as well as radiative heat transfer. But due to time limitation this part of

study could not be completed. However, some very important results were obtained

during the course of this study. A brief introduction to the problem is given here,

the details are available in [79] and [80].

The problem has been formulated in such a way that any nongray gas

model for participating species can be used. The governing equations involving radiative interactions, in general, are integro-differential equations. In order to fully

understand the radiative part, it was decided to use a very simple fluid dynamic

model along with sophisticated nongray gas models. The laminar flow of participating species between two parallel plates was considered. The Tien and Lowder [81]

wide-band correlation model was used to account for the spectral properties. The

walls are assumed as black.

169
170

Specific results were obtained for energy transfer by pure radiation, radiation and conduction, and finally by radiation, conduction and convection. The

participating species considered were CO, OH, CO2, H20 and their mixtures. It

was noted that the extent of radiative interaction is dependent on the nature of

participating species and parameter such as T1, T2/T1, P and PL. Here T1 and

T2 are lower and upper wall temperatures, P is the pressure and L is the plate

spacing. Radiative ability of a gas was found to increase with increasing pressure,

temperature and plate spacing. For most conditions, H20 was found to be a highly

participating species (as compared to CO, OH and CO2)

The analysis, computational procedure, and results presented in [79] and [80 l

provide essential information on how to account for radiative interaction in internal

flows. The models developed in the references can be used to investigate radiative

interaction in complex flows as long as the system temperature remains within the

range 300 - 5,000 K.