--._

--- _.

NASA/TM-2001-210690 

RESEARCH LABORATORY 

ARL-TR-2382 

AIAA-2001-1012 

".)-/ L I I I 

Effectiveness of Micro-Blowing Technique 

in Adverse Pressure Gradients 

Gerard E Welch and Louis M. Larosiliere 

U.s. Army Research Laboratory, Glenn Research Center, Cleveland, Ohio 

Danny P. Hwang and Jerry R. Wood 

Glenn Research Center, Cleveland, Ohio 

January 2001 

https://ntrs.nasa.gov/search.jsp?R=20010018967 2017-12-28T14:41:58+00:00Z
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NASA/TM-2001-210690 

RESEARCH LABORATORY 

ARL-TR-2382 

AIAA-2001-1012 

Effectiveness of Micro-Blowing Technique 

in Adverse Pressure Gradients 

Gerard E. Welch and Louis M. Larosiliere 

U.s. Army Research Laboratory, Glenn Research Center, Cleveland, Ohio 

Danny P. Hwang and Jerry R. Wood 

Glenn Research Center, Cleveland, Ohio 

Prepared for the 

39th Aerospace Sciences Meeting and Exhibit 

sponsored by the American Institute of Aeronautics and Astronautics 

Reno, Nevada, January 8-11, 2001 

National Aeronautics and 

Space Administration 

I Glenn Research Center 

I . 

January 2001 
Acknowledgments 

The authors thank Dr. Eric R. McFarland of NASA Glenn Research Center for supporting this work with 

panel code calculations, Mr. Charles A. Herrmann for engineering design work, and Ms. Gwynn A. Severt, 

Mr. Carlos R. Gomez, and Mr. Scott R. Panko for facility set-up, operation, and testing. 

NASA Center for Aerospace Information 

7121 Standard Drive 

Hanover, MD 21076 

Price Code: A03 

Available from 

National Technical Information Service 

5285 Port Royal Road 

Springfield, VA 22100 

Price Code: A03 

Available electronically at http: //gltrs.grc.nasa.gov I GLTRS 
A 

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Cf 

CD 

cp 

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cp 

Cp 

<50 

<51 

<52 

f 

F 

F + 

G 

H 

L 

Effectiveness of Micro-Blowing Technique 

in Adverse Pressure Gradients 

Gerard E. Welch* and Louis M. Larosiliere 

u.s. Army Research Laboratory at Lewis Field 

Danny P. Hwang* and Jerry R. Wood 

NASA John H Glenn Research Center at Lewis Field 

Abstract 

The impact of the micro-blowing technique (MBT) on the skin friction and total drag of a strut in a 

turbulent, strong adverse-pressure-gradient flow is assessed experimentally over a range of subsonic 

Mach numbers (0.3 < M < 0.7) and reduced blOwing fractions (0 5, 2F ICI .

o 5, 1.75). The MBT-treated 

strut is situated along the centerline of a symmetric 2-D diffuser with a static pressure rise coefficient 

of 0.6. In agreement with presented theory and earlier experiments in zero-pressure-gradient flows, 

the effusion of blOwing air reduces skin friction significantly (e.g., by 60% at reduced blowing fractions 

near 1.75). The total drag of the treated strut with blowing is significantly lower than that of the 

treated strut in the limit of zero-blowing; further, the total drag is reduced below that of the baseline 

(solid-plate) strut, provided that the reduced blowing fractions are sufficiently high. The microblowing air is, however, deficient in streamwise momentum and the blowing leads to increased 

boundary-layer and wake thicknesses and shape factors. Diffuser performance metrics and wake 

surveys are used to discuss the impact of various levels of micro-blowing on the aerodynamic blockage 

and loss. 

Nomenclature m = mass flow rate 

= area M = Mach number 

= local skin friction coefficient P = area-averaged pressure 

= integral skin friction coefficient P = mass-averaged pressure 

= total drag coefficient Re L .

o = Reynolds number based on strut chord and 

= (P2 - PI) /( PO.I - PI)' diffuser static pressure 

inlet total conditions 

Re. = Reynolds numbers based on axial position and rise coefficient 

= ideal static pressure rise coefficient 

freestream conditions 

T = temperature 

= integral pressure force coefficient u = axial velocity component 

= boundary layer height v = transverse velocity component 

= displacement thickness W = width of strut 

x = axial direction = momentum thickness y = transverse direction 

= Blasius function r = ratio of specific heats 

= (PV)8 /(pu)", , blowing fraction 1] = (y / x)) Re. , Blasius similarity parameter 

= 2F / C f .O' reduced blowing fraction 1]D = diffuser effectiveness (Eqn. 2). 

= blowing to free stream mass flow rate ratio p = mass density 

= <51 / <52 , shape factor OJ = loss coefficient 

= chord of strut 

Subscripts 

o = stagnation condition 

1 = diffuser inlet (throat) 

2 = diffuser exit 

* Senior member of AlAA. B = blowing air 

Copyright © 200 I by the American Institute of Aeronautics and 

Astronautics, Inc. No copyright is asserted in the United States under 

Title 17, U.S. Code. The U.S. Government has a royalty-free license to 

exercise all rights under the copyright claimed herein for Governmental 

Purposes. All other rights are reserved by the copyright owner. 

S = solid plate (baseline) value 

<Xl = freestream value 

* = variable normalized by zero-blowing value 
Introduction 

The micro-blowing technique (MBT) is a passive 

method for reducing the skin-friction component of 

total drag in which a small investment of air in effused 

through holes of specific shape and distributed porosity. 

The "micro" refers both to the minimal investment of 

blowing air required-typical blowing fractions, F, are 

on the order of 0.1 %-and to the small size of the 

blowing holes relative to the local momentum 

thickness. Hwang demonstrated that MBT can be used 

to reduce the skin friction of a flat-plate in a subsonic, 

zero-pressure-gradient flow by as much as 60%.1 

Significant reductions were obtained throughout a wide 

range of Reynolds and Mach numbers. Similar 

reductions in the skin friction levels of an MBT-treated 

nacelle were reported by Tillman in regions of mild 

adverse pressure gradient. 2 

The micro-blowing technique is described in detail 

elsewhere. 

1 The blowing air is typically effused normal 

to the treated surface. Evidently the addition of this air, 

which is deficient in streamwise momentum, increases 

aerodynamic blockage. In an external flow (e.g., a 

wing) this leads to increased pressure drag while in an 

internal flow (e.g., a diffuser) the pressure recovery is 

decreased. Even so, Hwang and Biesiadny 

demonstrated that MBT could be applied to reduce the 

total drag of an uncambered strut in a zero-pres suregradient test section by 2% at blowing fractions near 

0.2%.3 Their experiment demonstrated that the decrease 

in the skin-friction component associated with microblowing could in some circumstances win out over the 

inevitable increase in pressure drag penalty. 

The work described herein is motivated by the potential 

application of the micro-blowing technique to reduce 

the total drag of surfaces in external flows and the loss 

production in the boundary layers of internal flows. The 

flow fields considered are subsonic and turbulent. The 

primary objective is to assess the impact of microblowing on skin friction and total drag in strong adverse 

pressure gradients typical of highly loaded airfoils and 

diffusers. A second related objective is to investigate 

the viability of using micro-blowing to increase the 

effectiveness of diffusing sections. 

An analytical description of flow over transpirated 

surfaces is first used to discuss the impact of 

transpiration on skin friction, entropy production, 

boundary layer stability, and aerodynamic blockage. An 

experiment to assess the benefits of micro-blowing in 

adverse-pres sure-gradient flows is then described. The 

results from this experiment are presented, followed by 

related discussion. The final section summarizes the 

paper. 

2 

Theory 

The primary purpose of the present investigation is to 

ascertain the efficacy of micro-blowing to reduce skin 

friction and total drag in subsonic, turbulent flows in 

adverse pressure gradients. It is deemed valuable, 

however, to review the expected impact of transpiration 

on boundary layers in general (cf. Schlichting4

). 

Consider a laminar flow over a "micro" transpirated 

surface in a zero pressure gradient. The Blasius 

equation f"' = - t f . f " is solved with the boundary 

conditions that enforce zero axial velocity at the 

surface, 1'(0) = 0 , transpiration (blowing or suction) at 

the surface, f(O) = -2· F · ~Re, , and matching at the 

freestream, f'(oo) = 1. The Blasius function,f(7]) ,was 

found using a Runge-Kutta integration scheme (after 

WhiteS) along with an iterative search for f " (0). The 

velocity profiles for various transpiration factors are 

shown in figures I and 2 and relevant normalized 

boundary layer parameters are shown in Table 1. It is 

evident from the axial velocity profiles in Fig. 1 that 

blowing increases the boundary layer thickness, 

decreases the wall shear stress, and destabilizes the 

flow. Suction has the opposite impact. Transpiration 

impacts both the magnitude and shape of the transverse 

velocity component as shown in Fig. 2. 

"Micro " or "Macro ". As seen in the axial velocity 

profiles (Fig. 1) and by the normalized boundary layer 

thicknesses, 0; and 0; (Table 1), the seemingly small 

amounts of transpiration change the boundary-layer 

thicknesses substantially. Indeed, consistent with 

predictions from the integral boundary-layer theory for 

zero-pressure-gradient flows with and without 

transpiration, 0; "" c / / C / .0 + 2F / C /.0 . Based on this view 

of the impact of transpiration on momentum thickness, 

the magnitude of the transpiration fractions should be 

interpreted relative to the m~anitude of the skin friction 

coefficient in the zero-blowing limit; hence, the reduced 

blowing fraction is defined as r == 2F / C /.0 (cf. 

Hefner and Bushne1l6

). The "micro" transpiration levels 

considered in these examples are in reality "macro" in 

terms of impact on the boundary layer. 

Skin friction reduction and entropy production. By 

considering Table 1, it is evident that blowing reduces 

skin friction (e.g., C; = 0.47 @ r = 0.8) substantially 

while suction increases skin friction ( C; = 1.60 @ r = 

-0.8). The impact of the transpiration on skin friction is 

in agreement with the Falkner-Skan solutions for 

transpirated flows by Nickel (as reported in Ref. 4). The 

micro-blowing reduces loss levels significantly (e.g., 

0/ = 0.61 @ r = 0.8) as well, while suction strongly 

increases the loss levels (e.g., 0/ = 1.55 @ r = -0.8). 
'-"- I 

--- --

Note however that the effused air is necessarily of low 

total pressure (i.e., equal to the free stream static) and 

therefore, although the boundary-layer entropy 

production is reduced, the mass-averaged total pressure 

of the aggregate boundary-layer flow might in some 

cases be lower than that of an untreated boundary layer. 

Stability. The blowing destabilizes the flow field as 

reflected in the increased shape factor relative to the 

zero-blowing case while suction stabilizes the flow. 

Even at "micro"-blowing levels, the boundary layers 

are substantially more susceptible to separate than those 

with zero or negative transpiration (suction). 

AerodynamiC blockage. In the case of an external flow, 

for example a wing, the increased displacement and 

momentum thicknesses and hence the wake thickness 

associated with the injection of blowing air implies 

increased pressure drag. The impact of the blowing on 

total drag will therefore depend on the trade between 

the decreased skin friction drag and the increased 

pressure drag, the impact of the latter which is scaled 

by the thickness-to-chord ratio. Analogously, in a 

diffuser, the increased displacement thickness and 

momentum thickness associated with micro-blowing 

will effectively decrease the aerodynamic area available 

for diffusion. The diffuser pressure rise will be 

dependent on balance between this increased 

aerodynamic blockage and decreased loss production in 

the boundary layers. 

As seen above, normalized transpiration levels of order 

unity lead to significant changes in the near-wall flow. 

Blowing should reduce skin friction and boundary layer 

loss production substantially; however, the boundary 

layer is destabilized and aerodynamic blockage is 

increased. It is anticipated that the same trends hold for 

the turbulent flows in the adverse pressure gradients of 

the experiment described below. 

Experiment Description 

Wind Tunnel 

The wind tunnel described in Ref. 3 was modified to 

investigate the benefits offered by MBT in strong 

adverse-pressure-gradients. New sidewalls were 

designed so that an adverse pressure gradient would be 

established along an uncambered strut situated along 

the tunnel centerline. The profiling of the sidewalls of 

the 2-D diffuser is shown in Fig. 3. The diffuser static 

pressure rise coefficient, cp> measured from throat to 

exit, is approximately 0.6. Static taps are located at 

mid-height along the sidewalls and along the center of 

the top wall. Five static taps are aligned along the top 

wall at the x = 33" location, approximately 6" (1/3-

chord, 7-thicknesses) downstream of the strut trailing 

edge. A traversing total pressure probe is used to obtain 

3 

wake surveys between y = - 2" and y = 2" at this same 

x = 33" position. The measured static pressure 

distribution at mid-height along one of the sidewalls is 

shown in Fig. 4 (baseline case). 

ModeJ Description 

The uncambered strut is located at in the center of the 

diffuser as shown in Fig. 3. The elliptical nose is 

situated near the diffuser throat and the wedge shaped 

tail is situated near the diffuser exit. The strut is 

mounted on an axial force balance located beneath the 

tunnel floor. The strut has two internal chambers, one 

forward and one in the tail, and is 18" long, 5.4" high, 

and 0.8" thick. Airlines route air to (or from) the fore 

and aft chambers from air supplies (or vacuum 

exhaust). In the tests reported herein, the aft chamber is 

sealed by solid plates and isolated from the forward 

chamber. Both sides of the forward portion of the strut 

are treated with a set of flush-mounted MBT plates 

(4.87" high and 9.87" long). Blowing is applied over 

2118~x L~12118. The PN23 plates of Hwang's 

earlier experiment) were tested in the present work. As 

in earlier experiments, low permeability polyethylene 

backing was used to promote pressure equalization in 

the chambers and uniform blowing fraction delivery. 

The leading edges of the plates were 2.0" downstream 

from the leading edge of the strut. A trip strip is used to 

initiate a turbulent boundary layer 1" upstream of the 

MBT plate. To obtain the baseline (solid-plate) 

measurements, two interchangeable solid plates replace 

the two MBT plates over the fore chamber. 

Force Balance Measurement 

and Pressure Calibration 

The force balance provides a direct measurement of the 

reaction to the aerodynamic forces acting on the strut, 

the sub-platform mounting stand, and the balance 

surfaces. Because the strut sits in a diffuser, the net 

force is positive-that is, thrust rather than drag. This is 

counter-intuitive at first and indeed means that if the 

strut broke free it would initially move upstream, 

against the flow! The thrust is due to the pressure 

loading impressed on the strut by the diffusing flowthe nose is in the low-pressure region at the throat and 

the tail is in the higher-pressure region near the end of 

the diffusing section. The force balance measurement 

strategy therefore accommodates both axial thrust and 

drag measurements. The balance was preloaded using 

springs so that thrust forces would register more 

positive while drag forces would register less positive. 

It was also necessary to tare out the pressure loading on 

the mounting stand to account for the sub-platform 

pressure loading. A calibration curve related pressure 

measurements from two sidewall static taps located 

near the leading and trailing edge of the strut to the 

forces on the mounting stand. During this calibration, 

the strut was mounted to the tunnel ceiling so as to 
preserve the pressure gradient impressed by the strut on 

the mounting plate. The force on the mounting plate 

and the two static pressures were then measured at 

various Mach numbers. The resulting linear calibration 

curve is shown in Fig. 5. At any run condition, the 

measured static pressures are used to infer the tare force 

associated with the pressure loading on the mounting 

plate. 

Test Results 

The inlet stagnation pressure was fIxed at 7.5 psia for 

the results presented herein and the total temperature of 

the tunnel and blowing air was near 530 R The chord 

Reynolds number, ReL,o, based on the inlet total 

conditions was approximately fIve million (5.2 E6). 

The turbulence intensity of the tunnel was not measured 

but is known from previous testing to be relatively high 

and to depend upon the load demand in adjacent test 

facilities. Again, the trip strip was used to initiate a 

turbulent boundary layer upstream of the leading edge 

of the MBT plates. Tunnel back pressure was varied to 

set throat Mach numbers in increments of 0.1 between 

0.3 < M < 0.7. The blowing fraction was varied 

between 0 < F < 0.002 in increments of 0.0005. The 

corresponding ratios of bleed mass flow rate to through 

flow mass flow rate were as high as G = 0.5%. The 

maximum blowing fraction was set by the flow. capacity 

of the blowing air supply system. 

In a typical series of tests, solid plates were fIrst 

installed and baseline values were established for the 

various throat Mach numbers. The MBT plates were 

then installed and tests were conducted over the same 

range of Mach numbers at the various blowing 

fractions. A force balance calibration was carried out 

each time the plates were changed, for example, from 

the MBT to the solid plates. There was signifIcant 

scatter in the force balance measurements before and 

after plate change-out whereas for a given plate 

confIguration (solid or MBT) the force measurements 

repeated through repeated sweeps of Mach number and 

blowing fractions with negligible scatter or hysteresis. 

This points to a limit in the accuracy of the force 

balance measurement system at the small load levels 

established on the strut associated with plate changeout. Each case was therefore run several times and the 

mean value and some sense of the deviation was 

obtained. Disparity in force balance measurements 

were as large as 20% at the low Mach numbers at 

which the net forces are smallest. 

Skin friction reduction 

The difference between the integral skin friction 

coefficients of the MBT and solid-plate tests at fixed 

Mach numbers are related by 

4 

W LiC, == LiCD + -LiC 2L p 

(1) 

The integral pressure coefficients are obtained by using 

static pressure data from sidewall taps in the vicinity of 

the tail of the strut. The difference between the integral 

pressure coefficients of two tests at the fixed Mach 

number is essentially equivalent to the difference 

between the static pressure rise coeffIcients (i.e., 

LiC p "'" Lie p)' The drag coefficients are obtained directly 

from the force balance measurements with the pressure 

tare force correction (see Fig. 5). The !:1C I values 

calculated using Eqn. 1 were then doubled to normalize 

to the MBT plates that cover only half the strut 

planform area; that is, the changes in Cl are credited to 

reductions in the shear force at the MBT plates, 

seemingly a very reasonable assumption. To 

contextualize the level of the calculated changes in skin 

friction coefficient, data from the solid-plate 

experiments of Ref. 1 were used to set the skin friction 

coefficient of the baseline (solid-plate) to 0.0039. 

The reduced skin friction data (symbols) are shown in 

Fig. 6; evidently, these data compare well with the 

bounds of Rubesin's data (dashed curves) as reported 

by Hefner and Bushnell.

6 C,.o denotes the skin friction 

coefficient of the MBT-treated strut in the limit of zero 

blowing fraction. Consistent with fmdings in Ref. 1, the 

experimentally determined C 1.0 is essentially equivalent 

to e ,.s (i.e., 0.0039). This highlights the efficiency of 

the MBT plates and means that the skin friction 

reduction with blowing relative to the baseline (solidplate), C I / C l.s' is equivalent to the skin friction 

reduction relative to the zero-blowing limit, CI / CI .

o ' 

Note that the scaling suggested by integral boundary 

layer theory described earlier collapses the data. 

Interestingly, by mUltiplying the reduced blowing 

fraction by a factor of two (solid curve), the analytical 

theory can be used as an accurate basis for the data. As 

found for zero- or mildly adverse-pressure-gradient 

flows, \,2,6 the micro-blowing substantially reduces skin 

friction in this strong adverse-pres sure-gradient flow. 

Diffuser pressure coeffIcient and effectiveness 

The increased aerodynamic blockage associated with 

the effusion of micro-blowing air evidently leads to 

reduced diffuser cp as shown in Fig. 7. Indeed, to fIrst 

order, the cp of the diffuser is reduced with blowing 

fraction ( c p "'" c~ -0) - G). The benefit of reduced 

viscous dissipation (or loss) in the boundary layers has 

evidently fallen below the detrimental impact of 

increased blockage. A comparison of the axial pressure 

distributions in Fig. 4 shows evidence of the loss of 

effective aerodynamic area available for diffusion with 

---.--- ---- --- -- ----~ 
increased blowing fraction. The diffuser effectiveness 

(cf. Shapiro 7) provides an indication of the 

thennodynamic efficiency of the diffusion process and 

is calculated using the fonnula 

I I.::! M 2 1 G [ ]rl(r-I

) _ + 2 2 + P2 1 

77D - L=!.(M2 -M2)(1 +G.T I T -P - ) 2 I 2 8 01 I 

(2) 

where 

2 (1 + 2(y -1)(1 + G)(1 + G· T8 I TO I)]1/2 

(y-l)M = -1 2 . (AIPI 1(~ p2 »2 (M12 + r;1 M;) 

(3) 

As seen in Fig. 8, the effectiveness of the diffuser 

decreases slightly with increasing blowing fraction. 

Total drag reduction 

The total drag reduction-<>r better, thrust 

enhancement--due to micro-blowing is obtained 

directly from the force balance measurement and the 

pressure tare calibration. The total drag reduction 

provides a measure of the benefit derived by reducing 

skin friction while at the same time increasing 

aerodynamic blockage and hence reducing pressure 

recovery in the aft portion of the strut. In Fig. 9, the 

nonnalized drag coefficients are plotted as a function of 

reduced blowing fraction. Values greater than unity 

mean that the net thrust of the treated airfoil is greater 

than the net thrust of the airfoil at zero blowing. In all 

cases, net positive blowing is better than zero blowing. 

Another important comparison is shown in Fig. 10. 

The drag coefficients from treated struts at various 

blowing fractions are compared with the baseline, solidplate values, at three different Mach numbers. The 

range bands give a good indication of scatter in the 

force balance measurements. Considering the mean 

values however, thrust is increased (or drag is 

effectively reduced) relative to the solid-plate thrust, 

provided that the reduced blowing fraction level is 

sufficiently high (e.g., r ~ 0.5 for M= 0.7). 

Wake profiles 

Comparison of wake survey data obtained from the 

traversing total pressure probe at various levels of 

blowing and for the baseline solid-plate case are shown 

in Fig. 11. Wake parameters are shown in Table 2. Note 

that these results are qualitatively consistent with the 

analytical results for the Blasius flow in zero-pressuregradient shown in Table 1. The micro-blowing 

increases the wake thicknesses and shape factors. The 

shape factor appears to approach 1.53 asymptotically. 

Note that the shape factor at the MBT-treated strut in 

the zero-blowing limit is lower than that of the solidplate. At low reduced blowing fractions, the MBT 

treated struts have higher mass-averaged total pressure 

5 

than does the solid-plate strut. Although theory suggests 

that the loss production in the boundary layer is reduced 

by micro-blowing, the mass-averaged total pressure is 

reduced as blowing air is increased because of the 

deposition of the low total pressure blowing air. 

Discussion 

The functional dependence of C; with F+ is shown in 

Fig. 6 and is in excellent agreement with the data 

compiled by Hefuer and Bushne1l6 from a number of 

sources, which spans a wide-range of Mach numbers 

and conditions. The agreement suggests that the 

functional dependence holds independent of Reynolds 

and Mach numbers, pressure gradient, and blowing 

strategy (e.g., hole size or porosity distribution). 

However, as noted in earlier work by HwangI, the 

selection of efficient MBT skin is by no means 

arbitrary. Indeed, the micro-blowing skin was chosen to 

accommodate the blowing air effusion that effects the 

C; vs r behavior (i.e. , skin friction reduction) while 

also assuring that C 1.0 = C I.S • 

The functional dependence of the ratio of the total drag 

of the MBT-treated strut with finite blowing to the total 

drag of the MBT-treated strut with zero-blowing 

( CD / C D.O) on the reduced blowing fraction is shown in 

Fig. 9. The nonnalized total drag is effectively reduced 

by micro-blowing at all levels; further, as shown in Fig. 

10, the blowing effectively reduces the total drag of the 

MBT-treated strut below that of the baseline (solidplate) strut, provided that the reduced blowing fraction, 

F+ , is sufficiently high. The break-even r depends on 

the Mach number. It is noted, however, that the 

thickness-to-chord ratio (WIL) scales the impact of the 

increased pressure drag (or in the present case, reduced 

pressure thrust) associated with the lack of pressure 

recovery due to aerodynamic blockage (cf. Eqn 1 and 

Fig. 4). The attained total drag reduction is in general 

dependent on the thickness-to-chord ratio or the trade 

between skin friction benefit and pressure drag penalty. 

As expected, the addition of the micro-blowing air at 

low total pressure increases aerodynamic blockage and, 

in all cases, results in lower diffuser cp (see Fig. 7). 

The diffuser effectiveness was reduced very slightly by 

the MBT treatment (Fig 8). This last finding runs 

contrary to the slight increase in diffuser effectiveness 

anticipated by the authors and to the notion that MBT 

can be used to increase the effectiveness of diffusing 

sections. It is also noted that the deeper, thicker wakes 

introduced by the MBT-treatment could lead to higher 

mixing losses; hence, in some applications, although 

the entropy production in the boundary layer is reduced, 

the overall entropy production associated with the 

boundary-layer dissipation and wake mixing could be 
increased. Perhaps an accelerating, high-speed section, 

especially one in which the effusion cooling is required, 

might be a fruitful internal flow application of the 

micro-blowing technique. 

Summary 

The experimental data reported herein indicate that the 

MET technique effectively decreases skin friction in 

strong adverse pressure gradients. Consistent with a 

Blasius solution for transpirated flows, wake surveys 

downstream of the micro-blown strut showed that the 

effused blowing air increases the boundary layer 

displacement and momentum thicknesses and shape 

factor. In terms of diffuser performance, the increased 

aerodynamic blockage associated with the blowing air 

results in lower diffuser static pressure rise coefficient 

and effectiveness. For external flows, the reduced 

pressure recovery results in increased pressure drag. 

The trade between increased pressure drag due to 

aerodynamic blockage and reduced skin friction hinges 

on the thickness-to-chord ratio of the airfoil. For the 

long, narrow strut tested herein, drag of the METtreated strut with [mite blowing was effectively lower 

than the drag of the treated strut in the zero blowing 

limit. Further, the total drag of the MET ~treated strut 

was effectively reduced below the total drag of the 

baseline (solid-plate) strut, provided that the reduced 

blowing fractions were sufficiently high. 

Acknowledgements 

The authors thank Dr. Eric R. McFarland of NASA 

Glenn Research Center for supporting this work with 

panel code calculations, Mr. Charles A. Herrmann for 

engineering design work, and Ms. Gwynn A. Severt, 

Mr. Carlos R. Gomez, and Mr. Scott R. Panko for 

facility set-up, operation, and testing. 

References 

IHwang, D. P., "A Proof of Concept Experiment for 

Reducing Skin Friction by using a Micro-Blowing 

Technique," NASA TM-I07315, Jan., 1997. 

2Tillman, T. G., "Drag Reduction on a Large-Scale 

Nacelle using Micro-Porous Blowing," R97-

4.910.0001 , NASA LeRC LET, Task 50, United 

Technologies, Dec., 1997. 

3Hwang, D. P. and Biesiadny, T. 1., "Experimental 

Evaluation of the Penalty Associated with MicroBlowing for Reducing Skin Friction," NASA TM113174, Dec., 1997. 

4Schlichting, H., Boundary-Layer Theory, McGrawHill, Inc., USA, 1979, pp. 389-39l. 

5White, F. M., Viscous Fluid Flow, McGraw-Hill, Inc., 

USA, 1974, pp. 675-678. 

6Hefner, 1. N. and Bushnell, D. M., "Viscous Drag 

Reduction via Surface Mass Injection," ed., Bushnell, 

D. M. and Hefner, 1. N., Viscous Drag Reduction in 

Boundary Layers, AIAA Inc., USA, 1990, pp. 457-476. 

7Shapiro, A. H., Compressible Fluid Flow, Ronald 

Press, USA, 1953, pp. 151-152. 

Table 1. Parameters from Blasius solution of zero-pressure-gradient flow with transpiration ( 2~ Re x = 350 ). 

F 8 t 81 82 H flO o· 8' , 

0 1 2 co c' f F + c;+ r 

-0.001 4.2 1.30 0.543 2.38 0.533 0.75 0.81 1.55 1.604 -0.79 0.81 

0 5.0 l.72 0.666 2.59 0.332 1.0 1.0 1.0 1.0 -- 1.0 

0.001 6.2 2.51 0.840 2.99 0.157 1.46 1.26 0.61 OA73 0.79 1.26 

t o,A, and 0, are reported here in tenus of the Blasius similarity variable. 

Table 2. Normalized wake survey parameters 1I3-cbord downstream of strut (Mach 0,7), 

F + 2 *00 / L 0' 0' H m / ft Po / Po .... Po / Po .... 1 2 

solid 0.104 0.792 0.768 1.44 0.9918 0.9698 0.9697 

0.0 0.138 1.00 1.0 lAO 0.9943 0.9726 0.9725 

0.25 0.158 1.10 1.04 1.49 0.9894 0.9711 0.9709 

0.50 0.15 1 1.17 1.08 1.52 0.9887 0.9688 0.9686 

0.75 0.139 1.27 1.16 1.53 0.9890 0.9630 0.9627 

6 
:::l 

:J 

>: 

~ 

u 

0 

(jj 

> 

iii 

>< lIS 

"C 

Q) 

~ lIS 

E 

... 

0 

z 

...... 

.. 

~ 

i: 

0-

.. '" 

~ e 

~ 

~ 

:E: 

1.2 

1.0 

0.8 

0.6 

...... / ~ ,''---- ........ - ,,"'- .,-

V ~ " ~ 

: .... / ,,' 

,/J " __ Blowing. F=0.OOlS. F+ =12 

0.4 / - / _Sold pial •. F =0 ;,:/ ,- _ . • _ St.dion.F=-O.OO1S. F+::=-1.2 

0.2 

0.0 

" 

'i i----.... / " 

o 2 4 6 8 

'll = y/x*sqrt(Re.) 

Figure 1. Normalized axial velocity profiles from 

Blasius solution of transpirated flows. 

1.8 

1.6 

1.4 

1.2 

1.0 

0.8 

0.6 

0.4 

0.2 

0.0 

0 

__ Blowing. F = 0.0015. F+ = 1.2 

_ Sofidplate, F=O 

___ • Suction. F =.0.0015. F+ =-12 

.)? / '/ I 

2 4 

" " 

,,' 

6 

'll = y/x*sqrt(Re.) 

8 

Figure 2. Normalized transverse relative velocity 

profiles from Blasius solution of transpirated flows. 

X =O X=9" X=27" 

Front chamber 

for blowing air 

X=33" 

IR 

~ iii 

~ 

Figure 3. Schematic diagram of 2-D diffuser (top 

view) showing strut test model and traversing total 

pressure rake for wake surveys. 

10 

10 

7 

:! 

l:: " .!: 

.. 

~ 

0.9 

1- 0.8 0 

Q- 0.7 ;:: 

Q. 0.6 -'-

~ 

.e 0.5 

Buelint. solid pia .. 

1\ ..... .. r.fBT,F+-O 

_ .. .. MBT. F+ - 0.25 

\ __ _ MBT.F+- 050 

_ ... .. MBT, F+ - 0.75 .1 

\ / ...... -.j." . 

i \ ~ (". Increasin2 

.11 0.4 blowing - IE 0.3 ~ 

'" I! 0.2 ::0 

~ 0.1 

\ r fraction 

, F 

/ Q- \ j . 1rC~!.~nt " '" Q 

...l 

0 Stnn 

-0.1 

o 5 10 15 20 25 30 35 

Axial position, incbes 

Figure 4. Local pressure coefficient at mid-height 

along sidewall as a function of axial position at 

Mach 0.7 throat conditions, showing the impact 

of MBT treatment at various blowing levels. 

0 --

-0.02 ~ 

-0.04 

-0.06 ~ 

-0.08 ~ 

-0.1 

-0.12 

~ 

y=-O.l 22x· 0.0008 

~ "'-

-0.14 

o 0.2 0.4 0.6 0.8 

Measured subplatform pressure difference, psia 

Figure 5. Calibration curve for tare force to 

account for pressure loading on mounting stand. 

1 

I 

i 

! 

I 

I 

I 
2.5 ,---,----.----,-----,------r-----, 

2.0 4--__ +-__ -+- Suction f- Blowing +----+----1 

~ ' .I ~ " 

1.5 t----+--"""-::-f---";:---t---f----4-----1 

J "--"-'\ '" o 1.0 +--__ +--__ +1 __ ~..::o.j.c_:_ _+--___j--___j 

_ ._BWiusJolution ~iL. _ BwillS solution with scaled 2A F/CfO r--'.o.' ,,-+~_C.l ' ~'~ _ _ ---' 

.••• Rubcsin data - upper limit ,,-- ~ _.' .' _ 

. _ . _ Rubesin data - lower limit - ~ 

0.5 

0.0 +---I-----jl---___j----+----+---i 

-3.0 -2.0 -1.0 0.0 1.0 2.0 3.0 

1.02 

0.98 

2*F/Cr.o 

Figure 6. Normalized integral skin friction 

coefficient as a function of reduced blowing 

fraction, showing data from present experiment 

at all Mach numbers (symbols), dashed curves 

that bound the Rubesin data as reported in Ref. 

6, and curves of Blasius solution. 

. ~ ., i~ ~ : ... . , '. ". : "l. 

0- . . . ~ 0.96 ~ • ' I . 

:- '. 

r-:.. . .. .:. .' --;----. 

--- . ... 0.94 

0.92 

0.9 

I 

I 

I 

I 

I 

-0.5 0.0 0.5 1.0 1.5 2.0 

2F/C(O 

Figure 7. Diffuser static pressure rise coefficient 

normalized by baseline (solid-plate) value as a 

function of reduced blowing fraction. 

1.02 

0.98 

0.94 

0.92 

0.9 

..(l.S 

. 

; . I~ ~ ,.: .: ! t-- .. . .... . ~ 

. . . 

. 

--..;;:; . ~ r--- .. .' 

0.0 0.5 1.0 1.5 

2F/CI.O 

Figure 8. Diffuser effectiveness normalized by 

baseline (solid-plate) value as a function of reduced 

blowing fraction. 

! 

2.0 

8 

0 

u 

c; 

u 

~ 

1.40 I 

1.35 ! 

1.30 ! 

L25 .. /- i 

1.20 

1.15 

1.10 

LOS 

LOO 

0.95 

y-- . I 

7.. I 

f:7 I 

/ : . f. I 

V I 

I I 

0.90 

-0.5 0.0 O.S LO 1.5 2.0 

1.8 

L6 

L4 

u 

Figure 9. Normalized drag coefficient as function 

of reduced blowing fraction. 

I 

I I 

Ii T .- ... - -' " 

. .. 

~ 1.0 

c; Uo.s 

.-_ ...... 

I-

-:1 ..... - --- -...,. ---

··· ·: ~t ---- 11 I - 1 

II) 

'" .c 

<> 

.= 

~ 

0.6 

0.4 

0.0 

Brosk~ 

atMadJ0.7 ._ ~4~ I 

__ ~5 

Il -~7 

I o 0.4 0.6 o.s u 1.4 l6 

2F/G.o 

Figure 10. Drag coefficient normalized by baseline 

(solid-plate) value as a function of reduced blowing 

fraction. 

2 __ MBT, F+-O 

1.5 _0 _MBT,F+-O.2S I 

0.5 

.. ;. MBT, F+= O.s 

_:II _ MBT,F+ - O.75 IJJ .. ", . B .... Iine, solid plate 

--- I ~ -"Jr' 

0 ~ IF"" 

-0.5 

-1 

-1.5 

- X_)I; _ 

-. -)1(_)(. 

------~ 

b 

-2 

200 250 300 350 400 450 500 

Freestream axial velocity, ftls 

Figure 11. Velocity from total pressure traverse 

at 1I3-chord (7.5 strut-widths) downstream of 

strut, showing impact of various blowing levels. 
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4. TITLE AND SUBTITLE 5. FUNDING NUMBERS 

Effectiveness of Micro-Blowing Technique in Adverse Pressure Gradients 

WU-708-28-13-OO 

6. AUTHOR(S) lLl62211A47A 

Gerard E. Welch, Louis M. Larosiliere, Danny P. Hwang, and Jerry R. Wood 

7. PERFORMING ORGANIZATION NAME(S) AND ADDRESS(ES) 8. PERFORMING ORGANIZATION 

National Aeronautics and Space Administration REPORT NUMBER 

John H. Glenn Research Center 

Cleveland, Ohio 44135-3191 E-12617 and 

U.S. Army Research Laboratory 

Cleveland, Ohio 44135-3191 

9. SPONSORINGIMONITORING AGENCY NAME(S) AND ADDRESS(ES) 10. SPONSORING/MONITORING 

National Aeronautics and Space Administration AGENCY REPORT NUMBER 

Washington, DC 20546-0001 NASA TM-200 1-2 10690 

and ARL-TR-2382 

U.S. Army Research Laboratory AIAA-2001-1012 Adelphi, Maryland 20783-1145 

11. SUPPLEMENTARY NOTES 

Prepared for the 39th Aerospace Sciences Meeting and Exhibit sponsored by the American Institute of Aeronautics and 

Astronautics, Reno, Nevada, January 8-11, 2001. Gerard E. Welch and Louis M. Larosiliere, U.S. Army Research Laboratory, Glenn Research Center, Cleveland, Ohio; Danny P. Hwang and Jerry R. Wood, NASA Glenn Research Center. 

Responsible person, Gerard E. Welch, organization code 5810,216-433-8003. 

12a. DISTRIBUTION/AVAILABILITY STATEMENT 12b. DISTRIBUTION CODE 

Unclassified - Unlimited 

Subject Category: 07 Distribution: Nonstandard 

Available electronically at hltp:llgltrs.grc.nasa.govIGLTRS 

This publication is available from the NASA Center for AeroSpace Information, 301-621-0390. 

13. ABSTRACT (Maximum 200 words) 

The impact of the micro-blowing technique (MBT) on the skin friction and total drag of a strut in a turbulent, strong 

adverse-pressure-gradient flow is assessed experimentally over a range of subsonic Mach numbers (0.3 < M < 0.7) and 

reduced blowing fractions (0 ::; 2FICf,0::; 1.75). The MBT-treated strut is situated along the centerline of a symmetric 2-D 

diffuser with a static pressure rise coefficient of 0.6. In agreement with presented theory and earlier experiments in zeropressure-gradient flows, the effusion of blowing air reduces skin friction significantly (e.g., by 60% at reduced blowing 

fractions near 1.75). The total drag of the treated strut with blowing is significantly lower than that of the treated strut in 

the limit of zero-blowing; further, the total drag is reduced below that of the baseline (solid-plate) strut, provided that the 

reduced blowing fractions are sufficiently high. The micro-blowing air is, however, deficient in streamwise momentum and 

the blowing leads to increased boundary-layer and wake thicknesses and shape factors. Diffuser performance metrics and 

wake surveys are used to discuss the impact of various levels of micro-blowing on the aerodynamic blockage and loss. 

14. SUBJECT TERMS 15. NUMBER OF PAGES 

14 

Skin friction; Drag reduction; Micro-blowing technique 16. PRICE CODE 

A03 

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