AIAA-93-3258 Artificial Transition - A Tool For High Reynolds Number Simulation? Youngguang Teng Nanjing Aeronautica I Institute People's Republic of China

and Hans-Ulrich Meier DNW German-Dutch Wind Tunnel The Netherlands

Al AA Shear Flow Conference

July 6-9, 1993 1 Orlando, FL For permission to copy or republish, contact the American lnstttute of Aeronautics and Astronautics 370 L'Enfant Promenade, S.W., Washington, D.C. 20024

ARTIFICIAL TRANSITION - A TOOL FOR HIGH REYNOLDS NUMBER SIMULATION?

by

Yongguang Teng* and Hans-Ulr lch Meter t*

Abst ract

The problem o f a r t l f l c l a l boundary- layer t r a n - s l t l o n was s tud led on a l a r g e 1:3.5 sca le h a l f model o f t h e CASA 3000 turboprop a i r c r a f t l n the DNW. The experlmental r e s u l t s concern lng the l n f l u e n c e o f a l o c a l roughness lmnersed I n a t u r b - u l e n t boundary l a y e r were supplemented by a theo- r e t l c a l e s t l m a t l o n and a methodica l experlment on a two-dlmenslonal t e s t model l n t h e 1:10 scale model u l n d tunne l o f DNW. Based on these r e s u l t s . some fundamental l l m l t a t l o n s f o r t h e a r t l f l c l a l boundary- layer t r l p p l n g were der lved. I t was found t h a t t h e a p p l l c a t l o n o f e lec t ro -acous t l c generators as a t r a n s l t l o n dev lce can avold most o f t h e shor t - comings r e s u l t l n g from the use o f a r t l f l c l a l roughnesses.

Nomenclature

AG Cf c o r r d E erms f k L RE Re/m t Ue u, U

:rms

s Y 61 V

a c o u s t l c generator s k l n - f r l c t l o n c o e f f l c l e n t a u t o c o r r e l a t l o n f u n c t l o n d lameter o f Preston tube power spectrum dens l t y f u n c t l o n rms v o l t a g e from ho t -w i re probe frequency h e l g h t o f roughness element i n t e g r a l l e n g t h scale o f tu rbu lence roughness element Reynolds number o f u n l t l e n g t h t tme v e l o c l t y ou ts lde boundary l a y e r f rees t ream f l o w v e l o c l t y v e l o c l t y i n boundary l a y e r rms v e l o c l t y i n boundary l a y e r e l e c t r t c a l power f o r AG d l s t a n c e from lead lng edge d l s t a n c e normal t o model su r face dlsplacement thickness o f boundary l a y e r k lnemat l c v l s c o s l t y

1. l n t r o d u c t l o n

I t 1s a well-known phenomenon t h a t the boundary l a y e r deve lop lng on a model used I n subscale slmu- l a t l o n a t low Reynolds numbers compared t o f ree - f l l g h t c o n d l t l o n s has l l t t l e resemblance t o the boundary l a y e r on the f u l l - s c a l e veh lc le . I f an l n v e s t l g a t l o n a t f u l l - s c a l e Reynolds number - e.g. I n a p ressur l zed . cryogenlc wlnd tunne l - I s n o t poss lb le . I t 1s comnon p r a c t l c e t o manlpulate the boundary- layer t r a n s l t l o n l o c a t l o n through a r t l - f l c l a l d l s tu rbances on the model su r face . For t h l s purpose, a r t l f l c l a l t r a n s l t l o n s t r l p s a r e c o n o n l y used. They can cons ls t . f o r Instance, o f sparsely d l s t r l b u t e d carborundum gra lns . b a l l o t l n l g lass beads o r d e f l n e d roughnesses l l k e small c y l l n d e r s . Because these devlces have been developed by t r l a l and e r r o r procedures, t h e l r a p p l l c a t l o n requ l res some s k l l l and experlence. '

*Nanj lng Aeronau t l ca l I n s t l t u t e . People's Republ lc o f Chlna. Cur ren t l y Research Assoclate. Aerospace Englneer lng Department. C a l l f o r n l a S ta te U n l v e r s l t y , Long Beach.

**DNW German-Dutch Wlnd Tunnel, The Netherlands

I n order t o p u t these methods on a more sophls- t l c a t e d basts , t h e AGARD/FDP Worklng Group 09 has t r l e d t o sumnarlze the s t a t e - o f - t h e - a r t o f t h l s s p e c l a l problem o f boundary- layer s l m u l a t l o n I n wlnd tunnels and t o propose methods f o r a slmula- t l o n methodology.( l) Obvlously. t h e maln shor t - coming o f such a procedure 1s t h a t t h e f u l l - s c a l e f l o w cond l t l ons ( t r a n s l t l o n l o c a t l o n and boundary- l a y e r development) have t o be known I n advance f rom t h e f l l g h t t e s t s o r from adequate wlnd tunne l l n v e s t l g a t l o n s c a r r l e d ou t a t f r e e f l l g h t Reynolds numbers. The t r l p p l n g l o c a t l o n and the roughness h e l g h t then have t o be chosen I n such a way t h a t t h e boundary- layer development I s s l m l l a r t o f r e e - f l l g h t cond l t l ons and t h e momentum loss ln t roduced l n t o the boundary l a y e r by t h e t r l p p l n g devlce I s taken l n t o account. I f these c o n d l t l o n s can be f u l f i l l e d . boundary- layer t r a n s l t l o n Imposed by means o f a f l x e d sur face roughness serves t o slmu- l a t e the boundary- layer development on the model sur face. However, t h l s approx lmat lon 1s on ly a p p l l c a b l e f o r one angle o f l nc ldence and one f l l g h t Reynolds number, because t h e boundary- layer t r a n s l t l o n l o c a t l o n may change w l t h these two parameters.

Thus. the search f o r an e f f e c t l v e t r l p p l n g devlce t h a t r e s u l t s I n a mln lmal momentum loss I n t h e boundary l a y e r and 1s c o n t r o l l a b l e d u r l n g a t e s t run 1s an Impor tant s u b j e c t f o r the lmprove- ment o f e x l s t l n g wlnd tunnel t e s t l n g technlques.

I n the past , d i f f e r e n t methods f o r f o r c l n g t r a n s l t l o n have been proposed and app l led . I n genera l . we can d l s t l n g u t s h among t h e f o l l o u l n g methods:

S lng le roughness elements ( n o t c o n t r o l l a b l e ) . Constant h e a t l n g o r b l o u l n g . Pulsed c o n t r o l l a b l e d ls turbances.

I n 1987. an experlmental study on a r t l f l c l a l t r a n s l t l o n by means o f I n t e r n a l acous t l c e x c l t a t l o n was s t a r t e d I n the framework o f the DLR-CAE cooper- a t i o n and r e s u l t e d I n the des lgn o f an e l e c t r o - acous t l c generator . Resul ts concern lng the deslgn and a p p l l c a t l o n o f t h l s e l e c t r o - a c o u s t l c generator* a r e s u m r l z e d I n Reference 2.

The o b j e c t t v e o f t h t s paper 1s f l r s t t o demon- s t r a t e how a t r a n s l t l o n s t r l p a t t h e leadtng edge o f a u l n g can l n f l u e n c e the whole h l g h - l l f t per- formance. The l a t t e r p a r t o f t h e paper w l l l deal w l t h the a p p l l c a t l o n o f t h e acous t l c generator as a c o n t r o l l a b l e t r l p p l n g devlce. The effectiveness o f t h l s dev lce w i l l be q u a n t l f l e d . e s t a b l l s h l n g a comparlson w l t h a d e f l n e d s l n g l e roughness.

2. The I n f l u e n c e o f a T r a n s l t l o n S t r l ~ on The H l g h - L l f t Performance o f a Half-Model Wlnq

I n the framework o f the development o f the CASA 3000 seventy-seater turboprop a l r c r a f t a 1:3.5 scaled h a l f model was l n v e s t l g a t e d I n t h e DNW ( F i g . 1 ) . One o f t h e objectives o f the t e s t was the o p t l m l z a t l o n o f the h l g h - l l f t performance o f the

*US Patent Number 4,909,810, February 05, 1991, European Patent No. 0315806. January 01. 1993.

F l g . 1. The CASA 3000 h a l f model I n the Em x 6m t e s t s e c t l o n o f the DNW.

w l n g - f l a p c o n f l g u r a t l o n . For t h a t purpose, a c a r e f u l I n v e s t l g a t l o n about t h e boundary- layer t r a n s l t l o n f l x l n g was performed. I n order t o o b t a l n a r e p r o d u c l b l e and de f lned roughness s t r l p , a method was used whlch was developed by M r . Quast (DLR). The p r e f a b r l c a t e d so-ca l led zlg-zag s t r l p s e s s e n t l a l l y c o n s l s t o f p l a s t l c tapes w i t h d l f f e r - en t th lcknesses. They a re a p p l i e d on a r o u t l n e bas ls f o r g l l d e r s . Wlth the CASA h a l f model, the c o r r e c t work lng o f the s t r l p s a t f l x e d p o s l t l o n s was checked o u t by f l ow v l s u a l l z a t l o n by means o f a dye/kerosene, pet ro leum m l x t u r e pa ln ted on the sur face behind the s t r l p s . Along the s t r l p s a few gaps o f about 10 cm span were l e f t t o I d e n t i f y the undls turbed n a t u r a l t r a n s l t l o n o f the leadlng-edge boundary l a y e r . Thls method a l lows c o n t r o l o f the e f f e c t l v e n e s s o f the t r l p p l n g dev lce b u t g lves no l n f o r m a t l o n about the d ls turbance ln t roduced l n t o the boundary l a y e r . How l a r g e and I n f l u e n t l a l these disturbances are can c l e a r l y be seen from f o r c e measurements app ly lng d l f f e r e n t s lzes and l o c a t l o n s o f t h e z lg-zag s t r l p s . I n Ftg. 2 th ree curves f o r l l f t c o e f f l c l e n t c f ( ~ ) f o r the d l f - f e r e n t boundary- layer t r a n s l k l i n condl t l o n s a re shown. From these r e s u l t s I t becomes ev ldent t h a t C L = f ( a ) f o r n a t u r a l t r a n s l t l o n and a r t l f t c l a l t r a n s l t l o n caused by a strip w l t h a th lckness o f 0.25 mn d l f f e r s on ly s l l g h t l y a t nega t l ve angles o f lnc ldence, and t n the reg lon o f maxlmum l l f t . I n comparlson w l t h these r e s u l t s a t r a n s l t l o n s t r i p o f 0.4 mn causes a s l g n l f l c a n t change o f t h e t o t a l l l f t I n t h e r e g l o n o f h lgh angles o f lnc ldence. However, t h e r e s u l t s u l t h a t r l p p e d boundary l a y e r agree w e l l I n the reg lon o f u = 8". w h l l e they d l f f e r from t h e case f o r n a t u r a l t r a n s l t l o n f o r nega t l ve =-values. Thls d l f f e r e n c e may be caused

F lg . 2. The l l f t c o e f f l c l e n t c as a f u n c t i o n o f the angle o f l nc ldence f o r d l t f e r e n t boundary- l a y e r t r a n s l t l o n c o n d l t l o n s .

by a lamlnar - bubble t ype - boundary- layer sepa- r a t l o n . For small p o s l t l v e angles o f lnc ldence. t h e momentum loss ln t roduced l n t o t h e boundary l a y e r by t r a n s l t l o n s t r l p s I s very small due t o the favorab le pressure g r a d l e n t . Thls 1s obv lous ly n o t t h e case I f the momentum loss I s l a r g e and growing r a p i d l y I n the f l o w f l e l d w l t h an adverse pressure g rad len t , as l n d l c a t e d by the reduced maxlmum l l f t w l t h a t r a n s l t l o n s t r i p o f 0.4 mn th lckness.

To u n d e r l l n e t h l s cons tdera t lon , a f l a t - p l a t e boundary- layer Flow u l l l be consldered. I n F lg . 3 the development o f a t u r b u l e n t boundary l a y e r on a f l a t p l a t e I s sketched. The momentum th lckness 62 f o r the f u l l y t u r b u l e n t boundary l a y e r f o r zero pressure g r a d l e n t (cu rve 1) can be descr lbed by t h e r e l a t l o n

0 . 0 3 6 ~ ~ A 2 = 1 / 5 ( 1 )

(U&/V)

A, ( rnrn

F l g . 3. The development o f the boundary- layer momentum th lckness a2 u l t h d l f f e r e n t pressure g rad len ts f o r a g lven a r t l f l c l a l roughness on a f l a t p l a t e .

I f - due t o an a r t l f l c l a l roughness - a momentum loss Ad2.0 I s In t roduced I n t h l s zero pressure g r a d l e n t t u r b u l e n t boundary l a y e r , t h l s momentum A62.0 remalns constant (curve 2) and 42 becomes

( n e g l e c t l n g the reduc t lon l n s k l n f r l c t l o n due t o Increased Umd2/v).

The p h y s l c a l e f f e c t o f a l o c a l roughness lmnersed I n a t u r b u l e n t boundary l a y e r can be I n t e r p r e t e d as a change o f the v l r t u a l o r l g l n x t o f the boundary l a y e r lnd lca ted I n curve 2. I n a boundary- layer f l o w w l t h a favorab le pressure g r a d l e n t . a l o c a l roughness causes a r a p l d Increase o f t h e momentum th lckness. The growth Ab2 can be est lmated by

and proposed by ~ r u c k e n b r o d t . ( 3,

For n = 6, the growth I s ma in ly dependent on t h e favorab le o r adverse pressure g rad len t g lven by the v e l o c l t y r a t l o u /ud. Based on t h l s r e l a - t l o n . I t can be d e m o n s h t e d t h a t a l o c a l d l s t u r b - ance lmnersed I n a t u r b u l e n t -boundary l a y e r devel - op lng I n a Favorable pressure g rad len t (cu rve 4) I s dy lng down w h l l e v l c e versa I n an adverse p res - sure g r a d l e n t boundary- layer f l o w (cu rve 3) the momentum th ickness I s r a p l d l y lnc reas lng . I n t h a t case a boundary l a y e r I s s lmulated whlch would develop a t a lower Reynolds number. Oue t o the change o f I t s correspondlng shape f a c t o r t h l s boundary l a y e r may separate e a r l l e r , a phenomenon

which was observed d u r l n g the h a l f model wlng l n v e s t l g a t l o n ( F l g . 2) .

To q u a n t l f y the e f f e c t o f the t r a n s l t l o n s t r l p s on t h e boundary layer developlng on a CASA h a l f wlng model, Preston tube measurements were performed I n order t o o b t a l n a rough est lmate o f the w a l l shear s t r e s s changes. For t h a t purpose a Preston tube and a s t a t l c pressure tube were pos l t l oned on the wlng upper sur face approxlmately I n the wlng mlddle (y /b = 0.44) and 50% chord l e n g t h (x /c = 0.5). For the I n v e s t l g a t l o n . t h e f l a p s were r e t r a c t e d and the englne n a c e l l e was removed. The Preston tube ( I n n e r dlameter d l = 0.3mn. ou te r dlameter da = 0.5 mn) and the s t a t l c pressure sur face tube (da = 0.2 mn) were n o t c a l t b r a t e d , b u t t h l s measurlng technlque per - m l t s a t l e a s t an es t lmat lon o f t h e q u a l l t a t l v e changes o f the w a l l shear s t ress . I n F lg . 4 t h e measured pressure c o e f f l c l e n t s .

a re p l o t t e d as func t lons o f t h e angle o f lnc ldence and f reest ream v e l o c l t y U, f o r d l f f e r e n t

boundary- layer t r a n s l t l o n cond l t l ons (pp = Preston tube pressure, p, = s t a t l c ressure, pS = measured s t a t l c pressure. qm = ~ / 2 U! = dynamic pressure) .

The r e s u l t s obta lned can be sumnarlzed as lows: f o l

I n the range - 5 " < a < 5 " . the n a t u r a l boundary- layer t r a n s l t l o n l n d l c a t e s a c l e a r Reynolds number dependence I n the v e l o c l t y range U, = 40 m/s t o 80 m/s. The r e l a t l v e l y h lgh w a l l shear s t ress values a t nega t l ve angles o f lnc ldence ( a = -5") and low veloc- l t l e s (U, = 40 m/s) may be caused by a l o c a l ,

F l g . 4. Sur face f l o w l n v e s t l g a t l o n s by means o f Preston tube measurements on the

3

CASA 3000 h a l f model.

7-

bubble- type separat lon. Thls e f f e c t dlsappears w l t h t r a n s l t l o n s t r l p s . I t i s l n t e r e s t l n g t o no te t h a t I n the range -5" < P < 5' the use o f d i f f e r e n t t r a n s l t l o n s t r l p thicknesses (0.25 mn, 0.4 mn. 0.75 mn) leads t o o n l y s l l g h t l y d l f f e r e n t r e s u l t s .

For h lgher angles o f lnc ldence. the boundary l a y e r changes d r a s t l c a l l y w l t h t h e use o f a 0.4 mn t h l c k (and l a r g e r ) t r a n s l t l o n s t r l p . A t a low Reynolds number (U, = 40 m/s) the th lckness r a t l o o f the boundary l a y e r and t h e t r a n s l t l o n s t r l p 1s more favorab le as f o r h l g h e r Reynolds numbers. Obvlous ly t h e p r l n - c l p a l conslderat lons over t h e f l a t - p l a t e boundary layer g lven below a r e conf lrmed by these r e s u l t s . I f a c r l t l c a l h e l g h t o f a l o c a l d l s tu rbance lmnersed I n a t u r b u l e n t boundary l a y e r I s reached, a favorab le pressure g r a d l e n t causes a d r a s t l c Increase o f the momentum th lckness . the w a l l shear s t r e s s l s decreaslng and t h e boundary- layer f l o w tends t o separate.

As a consequence o f t h l s experience we see the need f o r a t r l p p l n g devlce t h a t 1s c o n t r o l l a b l e and can be tnac t l va ted . e.g. f o r h l g h - l i f t I n v e s t l g a - t l o n s . The second p a r t o f the paper w l l l desc r lbe a p o s s l b l e t r a n s l t l o n procedure and experlences galned w l t h the a p p l l c a t l o n o f an e l e c t r o - a c o u s t i c generator .

3. Test o f E lect ro-Acoust ic T r a n s l t l o n Generator

3.1 Test Set-Up

The experlments were c a r r l e d ou t I n t h e 1:lO- sca le model wlnd tunnel o f the ONW. I n the 0.6m x 0.8m c losed t e s t sec t lon , a two-dlmenslonal t e s t model was mounted vertically. The model had a wedge-type (30 degree) leadlng-edge shape w l t h a span o f 600 mn and a leng th o f 1600 mn. The model was deslgned I n such a way t h a t a s l m l l a r l t y t ype o f lamlnar boundary-layer f l o w I n a favorab le pressure g rad ien t was obtalned, up t o a d l s t a n c e o f about 700 mn from the lead lng edge. Thls was v e r l f l e d f o r a maxlmum wlng tunne l f l o w speed o f about 100 m/s. The freestream tu rbu lence l n t e n s l t y was l e s s than 0.05%. Two e l e c t r o - a c o u s t i c gener- a t o r s were f l u s h mounted on the model a t d i s tances f rom the lead lng edge o f 250 mn ( f a v o r a b l e pressure g r a d l e n t ) and 930 mn (adverse pressure g r a d l e n t ) . r e s p e c t l v e l y . The e lec t ro -acous t l c generators e s s e n t l a l l y c o n s i s t o f a mlnl- loudspeaker and a meta l top cover w l t h th ree holes (d lameter 0.5 mn). The acous t l c waves emtt from t h e holes and then I n t e r a c t w l t h the boundary layers . The d e t a l l s o f the acous t l c generator a re g lven I n References 2 and 5. I n t h l s study. I f n o t o therw lse lnd lca ted , t h e acous t l c generators were d r l v e n by a s i n e wave w l t h frequency o f 2.2 kHz because t h e loudspeaker has I t s maxlmum e f f l c l e n c y a t t h l s frequency. A t a l e a d l n g edge d ls tance o f 300 mn. a s l n g l e c y l l n - d r l c a l roughness element w l t h a dlameter o f 1.0 mn cou ld be placed. The he lgh t o f the roughness e l e - ment cou ld be var led w l t h an accuracy o f 0.01 mn. At a l e a d l n g edge d ls tance o f 930 mn. another l n v a r l a b l e c y l l n d r l c a l rouqhness element was f l x e d (d iameter 1.25 mn) i n order t o establish a compar- l s o n w l t h the disturbances caused by the e l e c t r o - acous t l c generator I n an adverse pressure g r a d l e n t t r a n s l t l o n a l and/or tu rbu len t boundary- layer f l ow.

I n order t o measure the s t r u c t u r e o f the boundary layers , a s lng le -w l re ho t -w l re probe was

mounted on t h e t e s t model a t a d ls tance o f 500 mn from the lead lng edge ( F l g . 5 ) . The ho t -w l re probe can be t raversed across the boundary layer w l t h displacement r e s o l u t l o n o f 0.001 mn by a small t r a v e r s l n g mechanlsm l n s l d e t h e t e s t model. A DANTEC h o t - w l r e / f l l m anemometer (Br ldge 56C17) was used t o g e t the ou tpu t from the h o t - w l r e probe. The ho t -w l re probe was c a l l b r a t e d be fo re the a c t u a l measurements o f the boundary l a y e r were made. The OANTEC 56N21 l l n e a r l z e r and 56N20 s l g n a l condl - t l o n e r were used t o l l n e a r l z e t h e c a l l b r a t l o n curve and t o c o n d l t l o n the ho t -w l re s l g n a l s . The l l n e - a r l z e r ou tpu t was sent t o a mean vo l tage u n i t and a root-mean-square u n l t where the mean v e l o c l t l e s and the root-mean-square v e l o c l t l e s were measured. respec t l ve ly . The spectra, a u t o c o r r e l a t l o n s and t h e t lme s l g n a l s were measured and recorded by a dual channel FFT analyzer . The l o c a l sur face s t ress o f t h e t r a n s l t l o n a l and/or t u r b u l e n t bound- a r y layers was measured by a Preston tube. The Preston tube c o n s l s t s o f a 0.5 mn p l t o t tube and a separate 1.0 mn s t a t l c tube. The spaclng o f the p l t o t tube and t h e s t a t l c tube was about 8.5 mn. and they were taped on the model su r face a t t h e l o c a t i o n o f I n t e r e s t . I n t h l s study. on ly t h e c a l l b r a t l o n curve f o r t u r b u l e n t boundary- layer f l o w o f the Preston tube was used.

A l l da ta a c q u l s l t l o n was c o n t r o l l e d by a l o c a l data a c q u l s l t l o n system a t t h e model wlnd- tunnel l abora to ry . The row data were s to red on f loppy d lscs and d a t a processing was done a t t h e computer o f the LST 3m x 2.2m wlnd tunne l a t NLR (Nat lona l Aerospace Laboratory) and a personal computer a t t h e DNY.

d( 4- resto on tube

430

tY Acoustic generator A.G. Roughness element R.E.

F l g . 5. Sketch o f t h e t e s t model.

3.2 Resul ts and Dlscuss lon

Durlng t h e l n l t l a l phase o f the experlment, the efFect lveness o f the e l e c t r o - a c o u s t l c generator as an a r t l f l c l a l t r a n s l t l o n dev lce was q u a n t l f l e d agalnst a s l n g l e roughness element I n the lamlnar. favorable pressure g rad ien t boundary- layer f l o w (see Flg. 6 ) . The c r l t l c a l power o f t h e acous t l c generator and t h e c r l t l c a l h e l g h t o f the roughness element, a t whtch the l n t e r m l t t e n t (dashed l l n e s ) and f u l l y t u r b u l e n t ( s o l i d l l n e s ) s lgna ls were j u s t observed, were measured by a h o t - w l r e probe a t varlous u n l t l e n g t h Reynolds numbers. The r e s u l t s shown I n F l g . 6 l n d l c a t e t h a t t h e power needed t o t r l g g e r t r a n s l t l o n a t the lowest Reynolds number

0 In lcrmtl lcnl , w i lh A.G. Turbulenl. w i lh A.G. 1.0

0.80

0.60 ,

0.20

A.G. 0.00

0.

-- - _ Roughness clernenl

v Eleclr . acousl ic Genera,lor ,

F l g . 6. The c r l t l c a l powers of the a c o u s t l c gener- a t o r and the he lgh ts o f the roughness element t o g e t t h e l n t e r m l t t e n t and t u r b u l e n t s l g n a l s f rom the h o t - w l r e probe. The probe was a t (s,Y) = (430.0.5) mn.

(U = 15 m/s, Re/m = O.8E6) u l t h t h e acous t l c gener- a t o r about 1.0 mH. w h l l e the c r l t l c a l h e l g h t o f the s l n g l e roughness element was o f t h e order o f the l o c a l momentum thickness o f the lamlnar boundary l a y e r . As expected, both the c r l t l c a l powers and t h e c r l t l c a l he lgh ts dropped cons lderab ly when the Reynolds number was Increased, e s p e c l a l l y I n the low Reynolds number range.

The d e t a l l e d h o t - u l r e i n v e s t ~ ~ a t l o n s o f the tu rbu lence s t r u c t u r e I n the f a v o r a b l e pressure g r a d l e n t boundary l a y e r were c a r r l e d ou t . The measurements Inc luded the mean v e l o c l t y p r o f i l e s . t h e f l u c t u a t l n g v e l o c l t y d l s t r l b u t l o n . t h e spect ra and a u t o c o r r e l a t l o n func t lons . F igures 7a.b and 8a.b show us t h e comparlson o f t h e mean v e l o c l t y p r o f l l e s and t h e f l u c t u a t l n g v e l o c l t y p r o f l l e s whlch d l d n o t lead t o an unequlvocal r e s u l t f o r the d l f f e r e n t t r a n s l t l o n t r l p p l n g devlces.

The comparlson o f the I n t e g r a l l e n g t h scales o f the t u r b u l e n t boundary layers generated by the acous t l c generator and the s l n g l e roughness element I s g lven I n F lg . 9 . Usual ly , the l n t e g r a l l e n g t h sca le I s measured by an a u t o c o r r e l a t l o n f u n c t l o n l n v o l v l n g T a y l o r ' s hypothesls whtch I s v a l l d f o r homogeneous and Isotropic turbulence. I n our t e s t a s l l g h t v l b r a t l o n o f the w l r e o f t h e ho t -w i re probe made us unable t o o b t a i n an accurate, t n t e - g r a l t Ime scale from the a u t o c o r r e l a t l o n f u n c t l o n .

F l g . 7a. Mean v e l o c l t y p r o f l l e s o f t h e boundary layers w l t h severa l e x c l t a t l o n powers o f the f l r s t acous t l c generator . Re/m = 2.7E6. U, = 42.4 m/s, s = 430 mn.

F lg . 7b. Mean v e l o c l t y p r o f l l e s o f the boundary layers w l t h d l f f e r e n t he lgh ts o f the s l n g l e rough- ness element. Re/m - 2.7E6. U, = 42.4 m/s. s = 430 mn.

Flg. 8a. The d l s t r l b u t l o n s o f t h e ur , w l t h l n the boundary layers wl t h severa l e x c l t a t r o n powers o f the acous t l c generator . Re/m = 2.7E6, U, = 42.4 m/s. s = 430 m.

- kid = 0 75, d = 1 lnnt [ S = 300 s = 430 mm 1 . kid= 100,dz lmrn - "Itt+ .kld=l25,d=l1ntn fle/m = 2.7 x lo6 -

- , - kid z 1.50. d = I IIII~I U, = 12.4 ds

F lg . 8b. The d l s t r l b u t l o n s o f t h e u , l n s l d e t h e boundary layers w l t h d l f f e r e n t helghys o f the s l n g l e roughness element. Re/m = 2.7E6, U, = 42.4 m/s. s = 430 m.

Therefore. we used the spectrum d e n s l t y f u n c t l o n whlch I s t h e Four le r transformation p a l r o f the a u t o c o r r e l a t l o n func t lon . Uslng the d e f l n l t l o n o f t h e I n t e g r a l scales and the r e l a t l o n between t h e spectrum d e n s l t y f u n c t l o n and the a u t o c o r r e l a t l o n . we der ived

L = u(Y)E(O)

2 4erms

Flg. 9. The Integral length scale of the turbulent wedges generated by the acoustlc generator and the slngle roughness element. Re/m = 2.7E6, U, = 42.4 m/s. s = 4 3 0 mn.

where L 1s the lntegral length scale; u(Y) the local mean veloclty; erms the rms voltage of the hot-ulre probe; and E(0) the slngle-slded power spectrum denslty functlon at low fre ency range wlthln whlch lt 1s almost constant.@!) Flgures 10a.b show us an example of the spectrum and auto- correlatlon functions. The peak ln the spectrum was due to the vlbratlon of the wlre of the probe. Comlng back t o Flg. 9. the lntegral length scales

Flg. 10a. Spectrum from hot wlre (s = 430. Y = 0.5 mn) U, = 4 0 m/s.

Flg. lob. Autocorrelatlon functlon from hot wlre (s = 430. Y = 0.5 mn) U, = 4 0 m/s.

for the dlfferent trlpplng devlces made no slgnlfl- cant dlfference. The hot-wlre measurements told us that the turbulent structures generated by the acoustlc generator and the roughness element were almost the same at our measurlng posltlon. Due to the favorable pressure gradlent, all the dlffer- ences In the turbulent structure seem to damp out very fast.

However, when the acoustlc generator and the roughness element were mounted at a posltlon wlth an adverse pressure gradlent (s = 9 3 0 inn). wall shear stress measurements by means of a Preston tube led to more Interestlng results. Two cases were lnvestlgated:

Natural transltlon occurred at the locatlon of the trlpplng devlces. The boundary layer was fully turbulent at the trlpplng locatlon.

Flgure 11 shows the dlstrlbutlon of the local surface frlctlon coefflclents In the "transltlonal boundary layeru, whlch lndlcate that the local shear stress created by the electro-acoustlc gener- ator 1s smaller If the values are compared wlth those obtalned for the slngle roughness element. In prlnclple. thls 1s In agreement wlth the results of W. L. Goodman, already obtalned In Reference 7. Hls concluslon that the pulslng frequency Is the drlvlng parameter was not conftrmed In thls test. If the roughness element and the acoustlc generator are lmnersed ln a turbulent boundary layer. the local shear stress by means of the electro-acoustlc generator was found to be negllglbly small (Flg. 12). Thts lmplles that only In a transltlonal but not a fully turbulent boundary layer, a drag reduc- tlon by means of a n acoustlc excltatlon can be expected. The mechanism of thls drag reductlon is not clear yet and thus further lnvestlgatlons are requlred.

Concluslons

The experlmental results presented In thls paper lead to the followlng concluslons:

An artlflclal boundary--layer transltlon wlth local roughness strlps must be controlled care- fully In order to avold an unusual boundary- layer development for the Reynolds number Investlgated. For hlgh-llft condltlons, the momentum loss Introduced Into the boundary

- 0 1

Fig. 1 1 . Dlstrlbutlon of cf In the turbulent (transltlonal) boundary layer wlth adverse pressure gradlent. Re/m = 3.75E6, U, = 59.5 m/s.

o N a l u r a l transit ion C .- 1 v)

A with A.G. only, k2.2 kHz. W= 1.45 mW vcilh R.E. only, kid= 1.0, d= 1.25 rnm

0.2- I I I I I 900 950 1000 1050 11

s ( rnm

0 Turbulent boundary layer

1 cn

0.2 900 950 1000 1050 1100

s l mm

F lg . 12. D l s t r l b u t l o n o f c f I n t h e t u r b u l e n t boundary l a y e r w l t h adverse pressure g rad len t . Re/m = 4.88E6. U, = 75 m/s.

l a y e r by a l o c a l l y lmnersed roughness may grow so r a p l d l y I n an adverse pressure g r a d l e n t t h a t an e a r l l e r f l o w separat lon may occur. F l x l n g t h e boundary- layer t r a n s l t l o n a t & angles o f l nc ldence may e x h l b l t c h a r a c t e r l s t l c s o f h lgher Reynolds number f lows; however, t h e developlng t u r b u l e n t boundary l a y e r has the c h a r a c t e r l s t l c s o f lower Reynolds numbers,

I t was conf i rmed tha t . I n a systematical model experlment. u t l l l z a t l o n o f an e l e c t r o - a c o u s t l c generator leads t o no measurable change o f the l o c a l shear s t ress I f I t 1s app l ied l n a t u r b - u l e n t boundary layer , whereas a l o c a l roughness element shows a d r a s t l c lncrease I n t h e exper l - m e n t a l l y d e r l v e d s k l n - f r l c t l o n c o e f f l c l e n t s . When p o s l t l o n l n g the two d l f f e r e n t t r l p p l n g devlces I n t h e boundary- layer reg lon, where n a t u r a l t r a n s l t l o n occurs, the e l e c t r o - a c o u s t l c generator reduces the l o c a l shear s t ress .

As a conclus lon, no unequlvocal answer t o t h e quest lon. " I s a r t l f l c l a l t r a n s l t l o n a t o o l f o r h l g h

Reynolds number s lmu la t lon?" can be g lven. How- ever, I t l s proposed t o apply more soph ls t l ca ted methods, such as us lng an e lec t ro -acous t l c gener- a t o r , l ns tead o f l o c a l roughness elements. The problems dlscussed I n t h l s paper become even more ev lden t f o r lamlnar wlng l n v e s t l g a t l o n s c a r r l e d ou t I n wind tunnels .

Acknowledgments The authors would l l k e t o thank M r . A. V l v l e n t e (CASA) and Dr. D. Ecker t (DNW) f o r p r o v i d l n g t h e data f rom t h e CASA 3000 t e s t , and M r . H. Engel (NLR) f o r a s s l s t l n g M r . Teng d u r i n g t h e t e s t s I n the 1:10 DNW model tunne l .

5. References

1. Laster . M.L.. 'Boundary-Layer S lmu la t lon and Cont ro l l n Wlnd Tunnels." AGAR0 Advlsory Report AR-224. 1988.

2. Meler, H . U . , Zhou, M.-D., "The Development o f Acoust lc Generators and The l r A p p l l c a t l o n as a Boundary-Layer T r a n s l t l o n Contro l Devlce." E x ~ e r t m e n t s I n F lu ids . Vol. 11, pp. 93-104. 1991.

Sch l l ch t tng . H.. Boundary-Layer Theory, McGrau- Hl11, 7 t h Ed.. 1979

4. de Bru ln , A.C.. 'The E f f e c t o f S lng le Rough- ness Elements on Boundary-Layer T r a n s l t l o n I n a Favorable Pressure Gradlent , " NLR C R 89095L. 1989.

5. Meler, H.U. and Zhou, M.D.. "Boundary-Layer T r a n s l t l o n C o n t r o l l e d by Flush-Mounted E l e c t r o - Acoust lc Generator," AIAA Paper 89-1005, 1989.

6. Tennekes, H.and Lumley, J.L.. A F l r s t Course I n Turbulence. HIT Press. 1977.

7. Goodman. W.L.. "Emnons' Spot Forc lng f o r Turbu- l e n t Drag Reductton," AIAA Journal , Vol. 13. pp. 155-157, 1985.