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NASA Technical Memorandum 107315

AIAA-97-0546

A Proof of Concept Experiment for

Reducing Skin Friction By Using a

Micro-Blowing Technique

Danny E Hwang

Lewis Research Center

Cleveland, Ohio

Prepared for the

35th Aerospace Sciences Meeting and Exhibit

sponsored by the American Institute of Aeronautics and Astronautics

Reno, Nevada, January 6-9, 1997

National Aeronautics and

Space Administration

A PROOF OF CONCEPT EXPERIMENT FOR REDUCING SKIN

FRICTION BY USING A MICRO-BLOWING TECHNIQUE*

Danny P. Hwangt

National Aeronautics and Space Administration

Lewis Research Center

Cleveland, Ohio 44135

Abstract

A proof of concept experiment for reducing skin

friction has been conducted in the Advanced Nozzle

and Engine Components Test Facility at the NASA

Lewis Research Center. In this unique concept,

called the micro-blowing technique (MBT), an

extremely small amount of air was blown vertically

through very small holes to reduce the surface

roughness and to control the gradient of the flow

velocity profile on the surface thereby reducing skin

friction. Research revealed that the skin was the

most important factor to make this concept

achievable. The proposed skin consisted of two

layers. The inner layer was a low permeable porous

skin for distributing the blowing air evenly while the

outer layer with small holes controlled the vertical or

nearly vertical blowing air. Preliminary experimental

results showed that the MBT has the potential of a

very large reduction in skin friction below the skin

friction of a nonporous plain flat plate. Of the skins

tested, three have been identified as the MBT skins.

They provided very low unblown skin friction such

that a large skin friction reduction, below a flat plate

value, was achieved with very small amounts of

blowing air. The reduction in skin friction of

55 percent was achieved at the Mach number of 0.3

for the exhaust pressure of 0.85 atm, and 60 percent

reduction was obtained for the exhaust pressure of

0.24 arm (corresponding to 10 700-m altitude) at the

same Mach number. A significant reduction in skin

friction of over 25 percent was achieved for the

exhaust pressure of 0.24 atm at the Mach number of

0.7. This implied that the MBT could be applied to a

wide range of flight conditions. It is also believed

that an additional 10 percent reduction could be

obtained by eliminating the gap between the inner

layer and the outer layer. The aspect ratio of the

vertical small holes for the outer layer of the MBT

skin should be larger than 4 based on the preliminary

conclusion from this test. Many experiments are

needed to find out the optimal MBT skin. The

penalty associated with the MBT needs to be

assessed. However, preliminary results indicated that

the MBT could provide a 25 to 35 percent reduction

for real-world application. The concept can be

applied to not only an airplane, but also a missile, a

submarine (micro-blow water instead of air), and an

ocean liner.

m_Lv.m_b__ s

A area of test plate

AR aspect ratio, T/D

Cf total skin friction coefficient, (skin friction

1 2

force)/(_ poouooA)

Cto total skin friction coefficient of nonporous plain

flat plate

D diameter of blowing holes, mm

H shape factor (ratio of displacement thickness to

momentum thickness)

T thickness of plate, mm

u velocity component parallel to surface, m/sec

u** free stream velocity

y vertical distance from surface, cm

po. free stream density

*Patent pending.

tMember AIAA.

This paper is declared a work of the U.S. Government and is not

subject to copyright protection in the United States.

1

American Institute of Aeronautics and Astronautics
Introduction

One of the most challenging areas of research in

aerodynamics is the reduction of skin friction,

especially for turbulent flow. Many techniques and

methods have been tried as summarized in Refs. 1

and 2. However, none of the techniques can provide

significant reduction in skin friction for real-world

application. For the past 20 years, attention has been

focused on surface suction to delay transition so that

a large area of the laminar flow region can provide

skin friction reduction as summarized in Ref. 1.

There are many problems associated with this

technique. Laminar flow is very unstable and tries to

transit into turbulent flow even with a very small

foreign object present. Also, laminar flow is

susceptible to flow separation. Therefore this

technique still remains in the research stage. Another

method is placing riblets on the surface; 1 however,

drag reduction is limited to under 8 percent.

One of the methods that has been ignored is the

surface mass injection (or blowing) because many

researchers believed that the penalty associated with

blowing was very large because of the susceptibility

of flow separation. Despite this shortcoming, many

experiments were conducted in 1970's for a flat plate

with no pressure gradient.3-12 It was well established

that blowing did significantly reduce skin friction

with respect to the skin friction of the unblown

porous plate. However the skin friction of unblown

porous plates tested so far by many investigators was

very high when compared with a nonporous flat plate

value. 13 It is impractical to reduce skin friction by

blowing for these plates with high unblown skin

friction because it requires a very large amount of

blowing air (which blows away the boundary layer)

to reduce the skin friction below a flat plate value.

The innovative skin friction reduction technique

called the micro-blowing technique (MBT) (patent

pending) has been tested in the Advanced Nozzle

and Engine Components Test Facility 14 at the NASA

Lewis Research Center, and the concept has been

proven to achieve over 25 percent reduction in skin

friction for a wide range of simulated flight

conditions. Results of the tests are presented in this

paper.

More assessment about the penalties associated

with this new technique is required, but the

preliminary results show that it is a very promising

new technology for reducing skin friction of both

laminar and turbulent flow.

The Micro-Blowing Technique (MBT)

In this unique concept, an extremely small

amount of air is blown vertically at the surface

through very small holes with high aspect ratio (AR).

This reduces the surface roughness and the gradient

of the flow velocity profile on the surface thereby

reducing skin friction.

The MBT Skin

The most important factor to make the MBT

achievable is the skin. One of the proposed MBT

skins consists of two layers as depicted in Fig. I.

The inner layer is a low permeable porous skin

for distributing the blowing air evenly while the outer

layer with high AR holes controls the vertical or

nearly vertical blowing air. In order to reduce the

skin friction below a flat plate value, the skin friction

of the unbiown porous plate needs to be only slightly

higher than the skin friction of a flat plate. The skin

with less than 10 percent above the skin friction of a

flat plate can be considered as a MBT skin.

Wilkinson13 has tested several permeable

surfaces, closest to our MBT skins in size and

porosity of holes, and found that the unblown skin

friction of these plates (which have holes with AR

<1) was very high especially for high free stream

flow. These plates cannot be considered as the MBT

skins.

Skins Tested

A 30-micron high density polyethylene plate

with a thickness of 9.14 mm was used for the inner

layer throughout the test.

The gap between the inner layer and the outer

layer was about 0.8 mm. It was found later from

Ref. 13 that elimination of the gap between two

plates could provide an additional 10 percent

reduction in skin friction.

Seven outer layers (each 12.36 by 25.06 cm)

have been tested, and the specifications of the plates

are listed in Table I.

NASA PN2 and PN3 skins were laser drilled; the

shape of the hole was irregular as shown in Fig. 2.

The flow direction over the plate is also indicated in

Fig. 2. It is believed that a streamline on the surface

should pass over the holes as often as possible in

order to get the benefit of vertical blowing air.

2

American Institute of Aeronautics and Astronautics
GAC series plates were provided by Northrop

Grumman Corporation; they were designed for

acoustic testing. The smaller openings of the conical

cross section were placed in touch with free-stream

flow in the same way as that used for an acoustic

liner.

Test Facility

The Advanced Nozzle and Engine Components

Test Facility (CE22) 14 was modified for this

experiment. A constant rectangular cross section duct

(20.32 cm wide, 14.2 cm high, and 63.5 cm long)

replaced the usual test article (designed for testing a

nozzle). A 12.7-cm-long transition duct was used to

connect the test section to the facility.

The facility provided stable Mach numbers from

0.3 to 0.7. The exhaust pressure at the exit of the test

section could be adjusted from 1.0 atm to near

vacuum, and the supply total pressure could be as

high as 2.7 arm. A Mach number of 0.7 is very close

to the cruise Mach number of most of the

commercial airplanes, and any skin friction reduction

at that speed which uses the MBT has application to

a real aircraft.

Apparatus and Instrumentation

Figure 3 shows the balance used to measure

skin friction. It was loaned by the Naval Surface

Weapons Center, and the detailed description of the

balance is in Ref. 4. The Linear Variable Differential

Transformer (LVDT) was replaced by a load cell

with a maximum loading of 500 grams. The accuracy

of the load cell was +0.25 percent. The balance was

placed under the test section inside a sealed

compartment, as shown in Fig. 4, to minimize air

leakage through the gap (0.2 mm) between the plate

and the tunnel floor. The top tunnel section was

removed in Fig. 4. The test skins were placed

25.4 cm downstream from the transition duct in the

constant pressure region. At the Mach number of 0.7,

the flow was slightly accelerated as a result of the

boundary layer blockage. Because this was a proof of

concept experiment, no attempt was made to correct

the blockage by enlarging the cross section. There

were two total pressure rakes. The total pressure

rakes were built with tubing which had an outside

diameter of 0.508 ram. A total pressure rake (not

shown) was located 0.635 cm from the side of the

test plate, and the tips of the rake were placed at the

location of the leading edge of the test plate. This

total pressure rake was planned to be used as a total

pressure rake at the leading edge of the test plate.

The thickness of the boundary layer from the

sidewall unexpectedly increased, and the rake was

embedded in the sidewall boundary layer. Therefore,

the measurements of this rake were not usable.

Another total pressure rake, as shown in Fig. 4, was

placed at the centerline on the tunnel floor; the

openings of the tubes were 1.27 cm upstream of the

trailing edge of the plate, and there was a very small

vertical gap (less than 0.2 mm) between the test

plate and the nearest tube. This total pressure rake

was used to calculate the momentum thickness and

the velocities inside the boundary layer. A total

pressure probe, a static pressure probe, and a total

temperature thermocouple were placed at the

entrance of the plate on the top surface of the tunnel

(not shown) for the free stream Mach number

measurement.

A 300-standard-liter/min (SLM) electronic mass

flowmeter, as shown in Fig. 4, was used to measure

the flow rate of blowing air.

Calibration

Another identical load cell was used to calibrate

the load cell inside the balance. The calibration

showed that the friction of the balance was very

small because of the frictionless flexural pivots. The

data were adjusted for this small difference during

the data acquisition process.

The error introduced on the flexible bellows, as

shown in Fig. 3, during 100 percent blowing was very

small; it was not corrected during the test.

Test Matrix

The test plates were tested for 5 Mach numbers

(i.e., 0.3, 0.4, 0.5, 0.6 and 0.7). The exhaust pressures

used were 0.85 atm (Reynolds number/m = 5.36(10)5

to 9.24(10)5) and 0.24 atm (Reynolds number/m =

1.49(10)5 to 3.81(10)5). The tests for 0.85 atm were

terminated at the Mach number of 0.5 because the

skin friction was much higher than the flat plate

value. The micro-blowing flow rates for the test

plates are indicated in Table II.

Results and Discussion

The test results from the exhaust pressure of

0.24 atm are presented first.

A plain stainless steel flat plate without vertical

holes was first tested. The good repeatability of the

facility is shown in Fig. 5 for the results taken on

August 4, 1995 and August 10, 1995. The total skin

friction coefficients obtained from this test for a plain

stainless steel flat plate were compared with the

empirical formula of Ludwieg and Tillman based on

3

American Institute of Aeronautics and Astronautics
theexperimental results of Rotta 15 and are shown in

Fig. 6. The skin friction coefficients from the tests

being reported were the average value based on a

12.36- by 25.06-cm plate, and the momentum

thickness was based on the downstream total

pressure rake which gave higher momentum

thickness than that at the center of the plate.

However, the results were reasonably close.

The total skin friction coefficients of NASA PN2

at the Mach number of 0.7 for different blowing rates

were calculated based on the momentum integral

equation by using the downstream total pressure rake.

Since the measurements of the upstream total

pressure rakes were unusable, only the effect of

blowing on the total skin friction coefficient was

calculated. The results were compared with the

direct measurement by using the balance shown in

Table III.

As mentioned earlier, the calculated Cfwas

based on the downstream boundary layer rake (not at

the center of the plate), and Cf was assumed to be a

constant on the flat plate. However, the comparison

between the measured Cf and the calculated Cf was

excellent.

The momentum thickness of this plate without

blowing was about 1 to 2 mm.

The total skin friction coefficient of a nonporous

flat plate (Cf0) was measured and was considered as

a reference skin friction coefficient. The skin friction

ratios (Cf/C_) were measured at different Mach

numbers for different porous test plates. The skin

friction ratios for unblown cases were shown in Fig. 7

for high altitude exhaust pressure. Only three porous

plates (i.e., NASA PN2, NASA PN3, and GAC 1897)

had unblown skin friction ratios lower than 1.1 (i.e.,

only 10 percent more than a flat plate) at the Mach

number of 0.7. These plates were considered as the

MBT skins. The unblown skin friction ratios were so

high for other skins that the reduction in skin friction

below a flat plate value was not possible for

practical application. Notice that the AR's of the

MBT skins were 4 or higher. These small high AR

holes were able not only to control the vertical

blowing air during the micro-blowing but also to

provide the low skin friction without blowing. The

study of an open cavity flow with an AR of 5 using

the Navier-Stokes code indicates that there are three

vortices existing inside the cavity. Further study is

needed to verify experimentally whether there exist

three recirculating vortices inside the small high AR

holes without blowing. Since both ends of the hole

are open, a slight difference in pressure could push

the vortices out of the hole. Understanding these

micro-physical phenomena is a challenge for

aeronautical scientists. It is believed that the slip

flow occurring on the surface of the MBT skin plays

an important role in reducing the unblown skin

friction. The porosity (percent open area) of the

NASA plates was 23 percent. GAC 1897 had

50 percent porosity based on the large open circle.

The porosity is 4 percent based on the small neck

area of the hourglass-shaped cross section. Whether

these large open areas contribute to the reduction in

unblown skin friction needs to be investigated.

The skin friction ratios of three MBT skins are

shown in Figs. 8 to l0 for the exhaust pressure of

0.24 atm.

Figure 8 shows the skin friction ratio of NASA

PN2. The unblown skin friction ratio at the Mach

number of 0.3 is 5 percent lower than a flat plate

value and increases to 10 percent more than a flat

plate value at the Mach number of 0.7. The low skin

friction at the Mach number of 0.3 could be due to

the lower effective roughness at lower Reynolds

number and the 23 percent open area without skin.

The general trend, as expected, is the reduction in

skin friction when the blowing rate increases. The

reduction is less at higher Mach numbers with the

same blowing flow rate. About 60 percent reduction

below a flat plate was achieved at the Mach number of 0.3 with 100 percent blowing rate

(0.205 kg/m2/sec), while 28 percent reduction was

obtained at the Mach number of 0.7 with the same

blowing rate. The first 50 percent (i.e., below

0.1025 kg/m2/sec) of blowing did reduce 75 percent

of the total reduction at the Mach number of 0.3 and

60 percent of the total reduction at the Mach number

of 0.7. This indicates that, at the lower blowing rate,

the MBT is more efficient for reducing turbulent skin

friction. The micro-blowing air is believed to reduce

the pressure difference inside the small holes with

high AR resulting in reduction in skin friction. The

NASA PN2 plate has circular cylindrical holes which

are perpendicular to the surface. It is believed that

this type of hole can more efficiently remove the

pressure difference inside the hole than the

hourglass-shaped hole (GAC 1897 plate). The microblowing air is also believed to lift the external

streamlines up so that the external streamlines can

flow more smoothly over the surface which is

covered with a thin layer of air. Thus the MBT can

reduce the roughness of the surface. This thin layer of

air is also believed to cause slip flow on the surface.

The blowing air also reduces the gradient of the

velocity profile on the surface resulting in the

reduction of the viscous shear friction. Again, the

straight hole can lift the streamline better than the

hourglass-shaped hole. Consequently, the NASA PN2

plate is more efficient than the GAC 1897 plate.

The results of NASA PN3 (Fig. 9) were identical

4

American Institute of Aeronautics and Astronautics
tothoseof NASAPN2up to the Mach number of 0.6.

Somehow the skin friction ratio of this plate

increased slightly at the highest blowing rate. It

could be the adverse effect of a manufacturing defect

groove across the plate near the trailing edge of the

plate.

The results of GAC 1897 (Fig. 10) were different

from those of the NASA plates. The unblown skin

friction ratio of GAC 1897 was 23 percent higher

than a flat plate value at the Mach number of 0.3,

therefore the reduction of only 50 percent below a

flat plate value was achieved at this Mach number

with 100 percent blowing rate. However this plate

had a very low unblown skin friction ratio at the

Mach number of 0.7 (i.e., only 3 percent above a flat

plate value), and 24 percent reduction in skin friction

was obtained. As mentioned earlier, this plate was

not as efficient as the NASA PN2.

Preliminary results showed that small holes with

the AR of more than 4 can provide such a low

unblown skin friction ratio that significant skin

friction reduction below that of a flat plate can be

achieved.

At very low blowing rates, the boundary layer

growth was reduced because of the reduction in skin

friction, while the boundary layer thickness increased

slightly as a result of the addition of blowing air. The

combined effect is shown in Fig. 11. It shows that the

growth of the boundary layer thickness near the end

of the plate with micro-blowing is very small. This

could indicate that there was more room for

reduction if a larger flow meter were available during

the test.

Figure 12 is the skin friction ratio of NASA PN2

at the exhaust pressure of 0.85 atm. The unblown skin

friction ratios were much higher than the cases with

the exhaust pressure of 0.24 atm. The reduction of 55

percent below a flat plate value was achieved at the

Math number of 0.3. At the Mach number of 0.5, the

unblown skin friction ratio was so high that the skin

friction was unable to be reduced below a flat plate

value.

Figure 13 shows the effect of Reynolds number

(based on the plate length of 25.06 cm) on skin

friction reduction for two pressure levels and two

blowing rates. It is very clear that the reduction in

skin friction diminishes with the same blowing rate

when the Reynolds number increases.

The velocity profiles from the downstream rake

of the test of two skins, GAC 1897 and GAC 2005,

are shown in Fig. 14. GAC 2005 had an unblown skin

friction twice as high as that of GAC 1897 based on

the measurement by the balance as shown in Fig. 7.

However, if the velocity profiles were extrapolated to

zero on the surface, the gradient of the velocity

profile on the surface of GAC 2005 is lower than that

of GAC 1897 which implies lower viscous shear

friction for GAC 2005. This contradiction leads to the

belief that the boundary condition on the surface and

the laminar sublayer just above the surface are more

complicated than many scientists have believed. In

order to improve the accuracy of computational fluid

dynamics (CFD) analysis for predicting skin friction,

a sophisticated turbulence model is needed to extend

the accuracy to the surface and a more realistic

boundary condition is required.

There was a gap of 0.8 mm between the inner

layer and the outer layer for this test. An additional

10 percent reduction in skin friction might be

possible by eliminating the gap based on the paper

by Wilkinson. 13 However, it might be possible to

eliminate the inner layer completely for a real

application to minimize the pressure loss across the

inner layer.

Potential Application

The high external flow on the surface of an

airfoil or a nacelle provides a low pressure region on

the surface. The suction boundary layer control

usually requires large suction power against this low

pressure, while the MBT can utilize this surface

suction force to provide part of blowing air. For the

same skin friction reduction, the higher blowing flow

rate is needed for the higher external flow velocity.

Therefore, the surface can adjust itself giving more

suction force on the surface where the higher blowing

rate is needed. Consequently, the penalty for

supplying very low blowing air is believed to be very

small.

Some of the blowing air could be bled off

downstream to prevent flow separation and to supply

blowing air upstream. There are lots of other

applications which use the MBT besides reducing

skin friction. It is possible to control an airplane by

adjusting the degree of skin friction on the wings

using variable blowing rate. For the objects with a

pressure difference between surfaces, such as a wing

or a nacelle, a passive MBT can be applied to bleed

off air from high pressure surface and to blow out

slowly through the low pressure surface. Most foreign

objects bounce off the surface because of blowing

air. Even if a large foreign object sticks to the

surface, it deteriorates only the small area

surrounding it and does not spread to a large area

such as the case of the Hybrid Laminar Flow Control

(HLFC).

5

American Institute of Aeronautics and Astronautics
Concluding Remarks

A proof of concept experiment for the microblowing technique (MBT) has been successfully

conducted, and the preliminary results show that 25-

to 35-percent skin friction reduction below a flat

plate value could be achieved for a wide range of

flow conditions. Research indicates that the skin is

the most important factor to make the MBT

achievable. Three skins have been identified as the

MBT skins during this test. The hole aspect ratios of

these skins are larger than 4. The MBT did reduce

skin friction by effectively reducing the roughness of

the skin with very low blowing flow rate. More

experiments are required to determine the optimal

MBT skin and to assess the penalty associated with

this technique.

Acknowledgment

The author gratefully acknowledges the

encouragement and the effort to obtain financial

support provided by Mr. Thomas Biesiadny and Mr.

Bobby Sanders. This experiment could not have

proceeded in a timely manner without the funding

given by Mr. Dennis Huff, especially during the early

stage of the development. The author is very grateful

to him. The author would like to appreciate the

valuable discussions given by Dr. Hsiao C. Kao,

Dr. Kamlesh Kapoor, and Dr. Gary Harloff during the

development of this concept. The author would like

to express sincere thanks to Dr. Greg Tillman of

UTRC for reviewing the final draft and providing

valuable comments. The author would like to thank

Mr. Robert Voisinet of the Naval Surface Weapons

Center for providing the balance and Mr. Charles

Parente of Northrop Grumman for supplying the test

skins. The financial support from the NASA Lewis

Research Center Director's Discretionary Fund is

greatly appreciated. The following individuals are

also acknowledged for their contributions during the

execution of the test program: Mr. Tony Herrmann,

Miss Gwynn Severt, Mr. Kurt Loos, Mr. Carlos

Gomez, Mr. Wayne Stopack, Mr. David Lam, and

Miss Wendy Barankiewicz.

References

1. Bushnell, D.M.; and Hefner, J.N.: Viscous Drag

Reduction in Boundary Layers. Progress in

Astronautics and Aeronautics, vol. 123,

American Institute of Aeronautics and

Astronautics, Inc., Washington, D.C., 1990.

2. Special Course on Skin Friction Drag Reduction,

AGARD Report No.786, 1992.

3. Jeromin, L.O.F.: The Status of Research in

Turbulent Boundary Layers With Fluid

Injection. Prog. in Aeronaut. Sci., vol. 10,

1970, pp. 65-189.

4. Voisinet, R.L.P.: Influence of Roughness and

Blowing on Compressible Turbulent

Boundary Layer Flow--Skin Friction Drag.

Final Report. Naval Surface Weapons Report

NSWC TR-79-153, 1979.

5. Bushnell, D.M.; Watson, R.D.; and Holly, B.B.:

Mach and Reynolds Number Effects on

Turbulent Skin Friction Reduction by

Injection. J. Space. Rock., vol. 12, Aug. 1975,

pp. 506-508.

6. Healzer, J.M.; Moffat R.J.; and Kays, W.M.: The

Turbulent Boundary Layer on a Rough, Porous

Plate: Experimental Heat Transfer With

Uniform Blowing. Stanford University Report

No. SW-HMT-18, 1974.

7. Dahm, T.J., et al.: Mass, Momentum, and Heat

Transfer Within a Turbulent Boundary Layer

With Foreign Gas Mass Transfer at the

Surface. Vade Report No. 111, Feb. 1964.

8. Goodwin, B.M.: The Transpired Turbulent

Boundary Layer With Zero Pressure Gradient.

DSc Thesis, Massachusetts Institute of

Technology, May 1961.

9. Kays, W.M.; Moffat, R.J.; and Simpson, R.L.: The

Turbulent Boundary Layer on a Porous Plate:

Experimental Skin Friction With Variable

Injection and Suction. Int. J. Heat Mass

Transfer, vol. 12, 1969, pp. 771-789.

10. Dershin, H.; Gallaher, W.H.; and Leonard, C.A.:

Direct Measurement of Skin Friction on a

Porous Flat Plate With Mass Injection.

AIAA Paper 67-194, vol. 5, no. 11, 1967,

pp. 1934-1939.

11. Simpson, R.L.: Characteristics of Turbulent

Boundary Layers at Low Reynolds Numbers

With and Without Transpiration. J. Fluid

Mech., vol. 42, 1970, pp. 769-802.

12. Jeromin, L.O.F.: An Experimental Investigation of

the Compressible Turbulent Boundary Layer

With Air Injection. ARC Reports and

Memoranda No. 3526, Aeronautical Research

Council, London, England, 1968.

13. Wilkinson, S.P.: Influence of Wall Permeability

on Turbulent Boundary-Layer Properties. AIAA

Paper 83-0294, 1983.

14. Beltran, L.R.; Del Roso, Ri.; and Del Rosario,

R.: Advanced Nozzle and Engine Components

Test Facility. NASA TM-103684, 1992.

15. Schlichting, H.: Boundary Layer Theory.

McGraw-Hill Book Co., Inc., New York, 1960.

6

American Institute of Aeronautics and Astronautics
Plate name

NASA PN2

Table I. - Specifications of Test Plates

Hole cross section shape

(side view)

Straight

Hole size,

D,

0.165

Thickness of plate,

T,

n'fin

1.02

Porosity,

percent

23

Aspect ratio,

AR,

T/D

6.2

NASA PN3 Straight 0.254 1.02 23 4

GAC 2004 Conical 0.381 0.787 21 2.1

GAC 2003 Conical 0.152 0.305 21 2

Conical 0.076

0.229

0.06

mall neck area.

GAC 2005

GAC 2002

0.152

0.394

GAC 1897 0.305

Conical

23 2

31 1.7

Hourglass 50_ 5.1

*The porosity is 4 percent based on the

Table II. - Test Matrix

Mach numbers Blowing flow rate, kg/m2/sec

(percent of maximum blowing rate)_

0

0.3, 0.4, 0.5, 0.6, and 0.7

0.019 (9.3 percent)

0.054 (26.2 percent)

0.066 (32.1 percent)

0.1025 (50 percent)

0.146 (71.4 percent)

0.176 (85.7 percent)

0.205 (100 percent)

_Maximum blowing rate was limited by the size of the flowmeter.

Table IlL - Comparison Between Direct

Measurement and Calculated Cf

Blowing rate,

kg/m2/see

Total skin friction coefficient, C f

0.205

Measured Calculated

Percent

difference

0 0.00441 0.00441 0

0.054 0.0039 0.00397 2

0.066 0.00379 0.00378 0

0.1025 0.00351 0.00348 -1

0.146 0.00323 0.00321 - 1

0.176 0.00307 0.00303 -1

0.00292 0.003

7

American Institute of Aeronautics and Astronautics
Free stream flow

0000(

0000

O000C

0000

000

O000

,oooo

Top view

Outer layer

(Controls vertical flow,

a plate with high porosity

laser drilled vertical holes)

I

Inner layer __' (Controls low flow rate. and

%%%%%%,%,%%%%%%,%%%% %%%%%,%%%%_.,%,,%_.%%%

JSJJ4PJJSJ#'JSJJ_'J4PJJ JS,JJJJS,,4'/'.g.I,,.tJ4P= evenly distributed flow. low P_,,,_,,_,,._,._'_,__,

permeable porous plate) PtJIJAPlLPSSL?IL_A?L_AFSJLF_Ji_SJ#JtSJI_J_II_IS_ _,,_,,_,,_,_,

I/lllttll'lltl'ltll ,_,_,_,_,,_ IllllilI'lllll'll'/_

Blowing air supply

Fig. 1 ._Micro-Blowing Technique (MBT) skin.

Side view

Fig. 2.---Shape of laser drilled holes. Fig. 3._Balance used to measure skin friction.

8

American Institute of Aeronautics and Astronautics
Fig. 4.mExperimental setup.

.. 10 -1

_=

O

O

O

"_ g 10 -2

.E "5

0

F-

:ZFlat plate

results on

-_- 8/10/95

- _- 8/4/95

10 -3

0.3 0.4 0.5 0.6

Mach number

Fig. 5.--Repeatability of test facility.

I I I I

0.7

_ 710 -2

o_

¢" e--

.g "_

Shape factor, H

-- -- 1.2 -', Empirical results of

- _.

.... 1.4 Ludwig and TUlman

i A 1.32 to 1.41 Current test results

"6 o 10-3 I i I i I Llll ] , I , I_1,1

p103 104 105

Momentum thickness Reynolds number

Fig. 6._kin friction coefficient of flat plate.

O

O

b.

¢-

.o

.B

t-

,m

03

1.5

2.5 Aspect Test

ratio, skins

L T/D

........ _"- - 2

..........o., .... D-........ _': .............. GAC 2005

2.0 _... _-_ :__.. aGAC 2002

........... ' "__ 2 GAC 2003

_ b- ............_--_.;i---_ GAC 2004

1.0_

6.2 PN2

• _ .... . and4 ^ and PN3

-- _ .... ' 5.1 _AGAC 1897

0.5 I I I I I

0.3 0.4 0.5 0.6 0.7 0.8

Mach number

Fig. 7._Unblown skin friction ratios.

d

to

.¢-

._c

03

Percent of maximum

blowing rate

--_--0

.....o ..... 9.3

.....-_ ..... 26.2

....._ ..... 32.1

.--_-- 50

---_-- 71.4

1.2- '_ ..... 85.7

......c,...... 100 (0.205 kg/m2/sec)

1.0

.............._ ..... ,_ ......... _:_::_::_i'. _

0.8

0.6 -- j,__i;i:i;i;2J_>

0.4 I

0.3 0.4 0.5 0.6 0.7 0.8

Mach number

Fig. 8._kin friction ratio of NASA PN2 at exhaust

pressure of 0.24 atm.

9

American Institute of Aeronautics and Astronautics
1.2

1.0

5

_ 0.8

C

0

0

C

ffl

Percent of maximum

blowing rate

---v-- 0

.....o .... 9.3

....._,..... 26.2

......<>-- 32.1

...o.. 50.0

---_.-- 71.4

....._ .... 85.7

-- ---_ ...... 100 (0.205 kg/m2/sec)

C"'"

0.6

o.4 I

0.3 0.4 0.5 0.6 0.7 0.8

Mach number

Fig. 9._kin friction ratio of NASA PN3 at exhaust

pressure of 0.24 atm.

Percent of maximum

blowing rate

_0

.....<>-- 26.2

....... _-32.1

....."_-- 50.0 3-

E .--o-. 71.4

---o-- 85.7 T

'?=_E2__, ..... _-.,I oo(o_

_1

'_ o ...... I

125 150 175 200 225 250 275

Velocity component parallel to surface, u, m/sec

Fig. 11 .--Velocity profile at downstream rake; NASA

PN2; Mach number = 0.7.

0 1.2

.o_" 1.0

C

._o

"6

C

¢n

Percent of maximum

blowing rate

---v-- 0

.....o .... 9.3

....... _ .... 26.2

....<>-- 32.1

•..o-. 50.0

---_-- 71.4

....._ .... 85.7

0.8 ..................................... _ .......... -m

o.6 I I t t I

0.3 0.4 0.5 0.6 0.7 0.8

Mach number

Fig.10.--Skin friction ratio of GAC 1897 at exhaust

pressure of 0.24 atm.

o

r_ 1.2

0

1.0

=-

0

0_

4.e

tO

C

Percent of maximum

blowing rate

---_--0

....._-- 50.0

1.4 -- --_ .......... 71.4

......_-- 100 (0.205 kg/m2/sec)

.h..........::i............................ .......o

,,°"°'"i. .o-'[_"°" ....

0.8

0.6 I I I t

0.30 0.35 0.40 0.45 0.50

Mach number

Fig. 12.--Skin friction ratio of NASA PN2 at exhaust

pressure of 0.85 atm.

10

American Institute of Aeronautics and Astronautics
S2

d,m

to

(J

cu)

101

Percent of Exhaust

maximum pressure,

blowing atm

ratio

--v- 0 \

0.24 -©- 100 J

-_ 100 0.85

0..0.0.0 "0 0"

L

100

10-1 I L I J I illl I = I, 1,1,1

105 106 107

Reynolds number based on plate length

Fig. 13.--Effect of Reynolds number on total skin

friction reduction for PN2.

3 m

E

o Test skins Skin friction ratio,

Cf/Cf0 oE2- t

_ GAC 1897 1.0306 ....o .... GAC 2005 2.0013 J/ _5

_1 ""

® ._"

:_ 0 Io..o--4" ....-o-" "

125 150 175 200 225 250 275

Velocity component parallel to surface, u, m/sec

Fig. 14._Velocity profiles of two skins; Mach

number = 0.7; no blowing.

ll

American Institute of Aeronautics and Astronautics
Form Approved

REPORT DOCUMENTATION PAGE OMB NO. 0704-0188

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1. AGENCY USE ONLY (Leave blank) 2. REPORT DATE 3. REPORT TYPE AND DATES COVERED

October 1996 Technical Memorandum

4. TITLE AND SUBTITLE 5. FUNDING NUMBERS

A Proof of Concept Experiment for Reducing Skin Friction By Using a

Micro-Blowing Technique

6. AUTHOR(S)

Danny E Hwang

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

National Aeronautics and Space Administration

Lewis Research Center

Cleveland, Ohio 44135-3191

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

National Aeronautics and Space Administration

Washington, D.C. 20546-0001

WU-27_

8. PERFORMING ORGANIZATION

REPORT NUMBER

E-10412

10. SPONSORING/MONITORING

AGENCY REPORT NUMBER

NASA TM-107315

AIAA-97-0546

11. SUPPLEMENTARY NOTES

Prepared for the 35th Aerospace ScienceS Meeting and Exhibit sponsored by the American Institute of Aeronautics and

Astronautics, Reno, Nevada, January 6-9, 1997. Responsible person, Danny E Hwang, organization code 5850, (216)

433-2187.

12a. DISTRIBUTION/AVAILABILITY STATEMENT

Unclassified -Unlimited

Subject Category 34

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

12b. DISTRIBUTION CODE

13. ABSTRACT (Maximum 200 words)

A proof of concept experiment for reducing skin friction has been conducted in the Advanced Nozzle and Engine Components Test Facility at

the NASA Lewis Research Center. In this unique concept, called the micro-blowing technique (MBT), an extremely small amount of air was

blown vertically through very small holes to reduce the surface roughness and to control the gradient of the flow velocity profile on the surface

thereby reducing skin friction. Research revealed that the skin was the most important factor to make this concept achievable. The proposed

skin consisted of two layers. The inner layer was a low permeable porous skin for distributing the blowing air evenly while the outer layer with

small holes controlled the vertical or nearly vertical blowing air. Preliminary experimental results showed that the MBT has the potential of a

very large reduction in skin friction below the skin friction of a nonporous plain flat plate. Of the skins tested, three have been identified as the

MBT skins. They provided very low unblown skin friction such that a large skin friction reduction, below a flat plate value, was achieved with

very small amounts of blowing air. The reduction in skin friction of 55 percent was achieved at the Mach number of 0.3 for the exhaust pressure

of 0.85 atm, and 60 percent reduction was obtained for the exhaust pressure of 0.24 atm (corresponding to 10 700-m altitude) at the same Mach

number. A significant reduction in skin friction of over 25 percent was achieved for the exhaust pressure of 0.24 atm at the Mach number of 0.7.

This implied that the MBT could be applied to a wide range of flight conditions. It is also believed that additional 10 percent reduction could

be obtained by eliminating the gap between the inner layer and the outer layer. The aspect ratio of the vertical small holes for the outer layer of

the MBT skin should be larger than 4 based on the preliminary conclusion from this test. Many experiments are needed to find out the optimal

MBT skin. The penalty associated with the MBT needs to be assessed. However, preliminary results indicated that the MBT could provide a 25

to 35 percent reduction for real-world application. The concept can be applied to not only an airplane, but also a missile, a submarine (microblow water instead of air), and an ocean liner.

14. SUBJECT TERMS

Micro-blowing; Skin friction reduction; Porous plate

17. SECURITY CLASSIFICATION

OF REPORT

Unclassified

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OF THIS PAGE

Unclassified

19. SECURITY CLASSIFICATION

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Unclassified

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13

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