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NASA TECHNICAL MEYORANDUM NASA TM-76603 (BAsA-T,li-766d3) EiiiiiUCT;dti O F 3bi)klLh i i t i A G Y d 1 - i d 3 52 BY B L O Y i Y G OUT TuBOUGB P E G h O L & S I Y AiiEAS U C S F Y E A I L I I E S E P A R A T I O N bUtib~ES (#dtlokai Aero~autics and S ~ c e Baiaioistration) 1 9 p Uncias HC A G 2 / a P A31 C 3 C L 01A 3 3 / 0 1 Lo937 REDUCTION OF PROFILE DRAG SY BLCWING 03T THROUGH PEG HOLES I N AREAS O F STREAMLINE SEPARATION BUBBLES K.H. Horstmann and A . Q u a s t T r a n s l a t i o n of " V e r r i n g e r u n g d e s P r o f i l - w i d e r s t a n d e s dupch Ausb l a s e n a u s L o c h r e i h e n i m S e r e i c h l a m i n a r e r A b l o e s e b l a s e n n . (Deutsche Forschungs- i ~ n d B e r s u c h s a n s t a l t f u e r L u f t - 2nd Raumfahrt, I n s t i t u t f u e r Sntwurfs-Aerodynarnik, B ~ a u n s c h w e i ~ , West Germany), Deutscho G e s e l l s c h a f t f u e r L u f t - und Raumfahrt, Symposium u b e r Aerod:rnamischen W i d e r s t a n d , Cologne, West Gernany, Nov. 25-26, 1980, DGLR No. 80-103.
pp 1 - 18.
NATIONAL AERONAUTICS A N D SPACE ADMI!;ISTRATION WkSHIi2GTOL' D . C . 20546 JULY 1 9 8 1 W I N E SEPARATION BU
- T r a n s l a t i o n o f " ( V e r r i ~ ~ r u n g des P r o f i l w i d e t s t a n s
durch Ausblasen a u s Lochreihen im B e r e i c h Laminarer Abloseblase ), (Deutsclre Forschungs-und V e r s u c h s a n s t a l t f u e r Luft- und RamPahr ,
I
I n s t i t u t f u e r Entwurf s-Aerodynamik, Braunschweig, Yest Geman) ) Deutsche G e s e l l s c h a f t her Luft-und Raumfahrt, Symposium u t e r Aerodynamischen Widerstand, Cologne, West Ger~iany, Nov. 25-26, 1980,IXXJi Mo. 80-103. (A81-19392) pp 1-18.
- --- ---.- - a his paper inrestigates t h e problem o f s t r e a m l i n e separ-' a t i o n bubbles on a i r c r a f t p r o f i l e s and f u s e l a g e s . The a d d i t i o n d r a g is ex-ned i n r s l a t i o n t o i n c r e a s e d a n g l e o f i n c i d e n c e and unusual13 h i g h wall s h e e r stress. A r e d u c t i o n i n the s e p s r a t i o n bubble and a d e c r e a s e i n drag is o b t a i n e d w i t h pneumatic t u r b u l a t o r s t h a t blow ram a i r out of 0.6 mm p i l o t t u b e s a t a I d i s t a n c e of 16 mm. The pneumatic models c a n be implemented a t / v a r i a u s p o s i t i o n s and are a l s o round t o be e f f e c t i v e after t h e REDUCTION OF PROFILE DRAG BY BLOUTNG OUT ~ O U t 3 ~ PEG HOLES IN AREES OF STREAMLXNE SEPARATION BUBBLES 8 . H. Horsf mann and A. Q u a s t For R e numbers below about 5 x l o 5 , l a m i n a r s e p a r a t i o n b u b b l e s can o c c u r on a i r c r a f t p r o f i l e s and aircraf% bodies. F i g u r e 1 shows t y p i c a l p r e s s u r e d ~ s t r i b u t i o n s a l o n g t h e bottom side of a p r o f i l e .
This is laminar s e p a r a t i o n and subsequent t u r b u l e n t r e a t t a c h m e n t .
F i g u r e 2 gives a p r e l i m i n a r y drawing o f t h e flow c o n d i t i o n s i n a laminar s e p a r a t i o n bubble. Also F i g u r e 2 shows t h e p a i n t fig- u r e i n t h e r e g i o n o f a s e p a r a t i o n bubble.
Laminar s e p a r a t i o n bubbles a r e u n d e s i r a b l e because t h e y can i n c r e a s e t h e p r o f i l e d r a g b y means o f mechanisms which are not. y e t s u f f i c i e n t l y e x p l a i n e d , i n e i c a t e d i n F i g u r e s 3 and 4, According t o F i g u r e 5, due t o t h e l a m i n a r s e p a r a t i o n bubble, t h e r e is an a d d i t i o n a l u n d e r p r e s s u r e A c which Is p e r p e n d i c v l a r P ' t o t h e c o n t o u r and, t h e r e f o r e , has t h e component 5c s i n $+a) P i n t h e flow d i r e c t i o n . Accordingly, t h e a d d i t i o n a l d r a g o f 4 lam- i n a r separat.ion bubble would have t o I n c r e a s e w i t h a n g l e o f ' a t t a c k , A n a d d i t i o n a l e x p l a n a t f o n f o r t h e d r a g o f s e p a r a t i o n bubbles c c u l d be t h e f a c t that t h e t u r b u l e n t wall s h e a r s t r e s s is e x c e p t i o n a l l y l a r g e a f t e r r e a t t a c h m e n t , A combination o f both mechanisms is a l s o p o s s i b l e .
It i s n a t u r a l t o make t h e boundary l a y e r t u r b u l e n t a-lready ahead o f t h e s e p a r a t i o n p o i n t u s i n g t u r b u l a t o r s , T h i s method is known but has not. p e t found a p r a c t i c a l a p p l i c a t i o n . I n t-he case o f pneumatic t u r b u l a t o r s a c c o r d i n g t o F i g u r e 6 , ram a l r Is e x p e l l e d Numbers i n margin i n d i c a t - e p a g i n a t i o n o f f o r e i g n t e x t . , P i t 0 t t u b e s through 0.6 mm t u b e s separated by 1 6 nm*.
f m m Already with sum1 1 amount c o e f f i c i e n t s c on t h e order o f 10-
+, t h e 1 -
Q i n a r - -t urb u l e n t t r a n s i t i o n is brought a b o u t , t h e l a m i n a r s e p a r a t i o n bubble v a n i s h e s and t h e drag is reduced.
These bubble t u r b u l a t o r s , as t h e y w i l l be c a l l e d i n t h e f o l l o w i n g , have t h e f o l l o w i n g advan- tages compared w i t h mechanical t u r b u l a t o r s : - Amount c o e f f i c i e n t i s a d j u s t a b l e or can b e t u r n e d off.
- Blowing can o c c u r at v a r i o u s p o s i t i o n s .
- Blowing t u r b u l a t o r s are a l s o e f f e c t i v e , i f a f r is blown o u t
behind t h e s e p a r a t i o n p o i n t .
The e f f e c t of blowing o u t on t h e p r e s s u r e d l s t r i b u t l o n on the bottom side i s shown i n F i g u r e 7. The l a m i n a r s e p a r a t 2 o n bubble 3s e l i m i n a t e d f o r t h e most p a r t . F i g u r e 8 shows t h a t by u s i n g t h e bubble t u r b u l a t o r s , t h e d r a g o f t h e p r o f i l e shown here can b e reduced up t o 15%.
F i g u r e 9 shows t h e p r o f i l e p o l a r s f o r v a r i o u s blowing p o s i t i o n s , The most f a v o r a b l e l o c a t i o n is found t o be a t x l l = 0.76, F i g u r e 10 shows t h e d r a g v a r i a t i o n f o r v a r i o u s amount c o e f f i c i e n t s , Here w e have a f l a t optimum at c = 7 x 1 0 ~ ~ .
With i n c r e a s e i n Re num- B b e r , t h i s optimum v a l u e o f c becomes smaller and is about z e r o f o r 6 Q
Re = 3 x 10 . It seems t h a t t h e r e q u i r e d blowine volume f l u x p e r
wing a r e a has t o be c o n s t a n t . Blowing h o l e s e p a r a t i o n and d i a - meter have not y e t been v a r i e d . Up t o t h e p r e s e n t time, blowing was always p e r p e n d i c u l a r t o t h e c o n t o u r .
*
For a wing chord of 500 m,
+
The a d d i t i o n a l d r a g by momentum l o s s is t h e r e f o r e Acw = 2cQ = 2 x but t h e d r a g c o e f f i c i e n t o f 8 p r o f i l e is 5 x 1 0 ~ ~ .
F f g u r e 11 shows t h e d r a g v a r i a t i o n as a f u n c t i o n o f R e number of a modern p r o f i l e with d e a t a b i l l z a t l o n segments by means o f dash l i n e s . This touches t h e envelope o f o p t i m a l l y designed p r o f i l e s a t t h e design p o i n t . A t R e numbers above t h e deslgn p o i n t , t h e t r a n s i t i o n p o i n t then migrates f o m a r d s a l o n g t h e u n s u i t a b l e de- s t a b i l i z a t i o n path and t h e r e f o r e tbe d r a g becomes greater because o f t h e unnecessary s h o r t and laminar running l e n g t h s . F a r Re num- b e r s below t b e design p o i n t , laminar s e p a r a t i o n bubbles form because o f i n s u f f i c i e n t d e s t a b i l i z a t i o n , The s e p a r a t i o n bubbles are l a r g e r , t h e smaller t b e R e number. I n t h i s range, t h e Blowing t u r b u l a t o r s can be used. Figure 1 2 shows t b e measured d r a g v a r i a t i o n of a
p r o f i l e designed for R e - 3 x 606 (practically no destabilization
p a t h ) . One can c l e a r l y s e e t h a t by u s i n g bubble t u r b u l a t o r s , t h e mas+ favorable working range i n terms o f d r a g is s u b s t a n t i a l l y en- l a r g e d . By u s i n g bubble t u r b u l a t o r s , one approaches t h e envelope given i n Figure 11 f o r o p t i m a l l y designed conventional p r o f i l e s with d e s t a b i l i z a t i o n paths.
P r o f i l e s w i t h bubble t u r b u l a t o r s r e q u i r e s t a b l e p r e s s u r e d i s t r i - b u t i o n s , such as f o r example, t h a t o f t h e underside given f n Figure 1. It is important t h a t t h e d e s t a b i l i z a t i o n p a t h s which are d i f f i - c u l t t o c a l c c l a t e become unnecessary. Also, t h e y can only be c o r r e c t for t h e design p o i n t . I n a d d i t i o n , a p r o f t l e f o r bubble t u r b u l a t o r s is much l e s s s e n s i t i v e t o manufacturing a c c u r a c i e s than one with such d e s t a b i l i z a t i o n paths. Within c e r t a i n l i m i t s , it could a l s o be i n s e n s i t i v e t o s u r f a c e contamination.
A i r c r a f t a r e e a s i l y equipped w i t h bubble t u r b u l a t o r s , The g l i d e r SB-12 o f Akaflleg Bx~aunschweig has flown a l r e a d y f o r three- q u a r t e r s of a y e a r w i t h such t u r b u l a t o r s , Bubble t u r b u l a t o r s a r e i n s e n ? i t i v e t o r a i n and do not become n o t i c e a b l y contaminated, Figure 13 shows t h e p o l a r of t h e t u r b u l a t o r curved f l a p p r o f i l e f o r g l i d e r s (DFVLR-HQ 17/14,38 1, compared w i t h t h e prev2ously known b e s t p r o f i l e s , The r e p r e s e n t a t i o n Is f o r t h e prevailing R e numbers whicb a glider actually uses.
h e clearly sees the drag reduction which is especially Important for low lift coefficients.
Bubble turbulators can always be used whew the local Re num- ber is smaller than 3 r lo6, which l a for profile Re numbers below
5 x 10 . The Re numbers could even be greater if nose separations
were used as well. Bubble turbulators are especially effective bel~w Re = 2 x 10 .
Therefore, we have the following applications:
- aircraft isor general aviation
- gliders
- helicopter rotors
- propellers
- flow machines
- wind wheels
- model aircraft
Because of the fact that low Re numbers sometimes occur, it seems that the area of flow machines is very promising for this kind of application. For commercial aircraft, bubble turbulators are probably not of interest in the form described.
/ &
A t this point we would like to thank Professcr V, Ingen end his corarkers at the TH Delft for his very careful measurements and support.
F i g u r e 2, Diagrqin of the paint image aRd presumable flow conditions in the region of a laminar separation b u b b l e .
ORIGINAL PAGE Erj e p ( m O u - P ~ l q y 9f a p r ~ f t l e with l e f n a r s e p a r ~ t i o n Figure 3 , bubble on the topslde, Figure 4, Effects of ''minar separation bubbles on t h e profile pol .
Figure 8, Drag p o l w of the profile HQ 17/14.38 with and without blowing turbulators ~ ~ e a s u r e m e n t TH-Delft) Figure 9. Drag polar of the DPVLR profile CEiQ 17/14,38!
for a different b l ~ w a n g locatlon a l ~ n g the bottom side (Neasurement : TH D e l f t Figure 1 3 , Comparison of drag polars of ppeyZ~us profiles w i t h the profile DFVLR-HQ 17114.38 wfth blowing t u r b u l a t ~ r s along the bottom side