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Application of computers to automobile aerodynamics
Journal of Wind Engineering and Industrial Aerodynamics, 33 (1990) 419-428
419
Elsevier Science Publishers B.V., Amsterdam - - Printed in The Netherlands
Invited Paper
APPLICATION OF COMPUTERS TO AUTOMOBILE AERODYNAMICS M. TAKAGI
Vehicle Research Laboratory, Nissan Motor Co., Ltd., Yokosuka 237 ('Japan) Summary Computers are recently applied to aerodynamics in two ways. One is computer aided flow visualization (CAFV) and the other is computational fluid dynamics (CFD). In the field of automobile engineering, both are very popular today. There arc many varieties in CAFV, that is, mere computer graphics of velocity and pressure measurements, image processing of visualized flow fields, computed tomography and so on. The CAFV results give us more quantitative information than conventional flow visualization results. It means that CAFV enables us to understand flow fields more precisely. The flow fields around vehicle bodies can be solved numerically using several methods. These are panel methods, k-¢ methods and direct simulation methods. The panel methods are suitable for the potential flow making it very useful to analyze attached flows. The latter two methods can deal with separated flows exactly, and they help us compute, for example, separated wake regions. The CFD results are inevitably combined with computer graphics and show us very realistic flow images. That is another advantage of CFD.
Keywords Automobile aerodynamics, computer aided flow visualization, computational fluid dynamics
1. I N T R O D U C T I O N T h e r e are m a n y k i n d s o f a e r o d y n a m i c p r o b l e m s in a u t o m o t i v e e n g i n e e r i n g . T h e y are u s u a l l y c l a s s i f i e d into 5 kinds as listed in T a b l e 1, that is, 4 a r o u n d v e h i c l e b o d i e s as is seen in H u c h o [1981] and one related to engines. Table 1 Automobile aerodynamic problems 1. 2. 3. 4. 5.
A e r o d y n a m i c characteristics D e t a i l s o f f l o w adjacent to b o d y surface E n g i n e c o o l i n g air f l o w s V e n t i l a t i o n and air c o n d i t i o n i n g f l o w s in p a s s e n g e r c o m p a r t m e n t s F l o w s in e n g i n e c y li n d e r s and intake and e x h a u s t m a n i f o l d s
T h e a e r o d y n a m i c c h a r a c t e r i s t i c s a f f e c t v e h i c l e d r i v i n g p e r f o r m a n c e s s u c h as m a x i m a l speed, cross w i n d stability, a c c e l e r a t i o n , f u e l c o n s u m p t i o n and so on. T h e a e r o d y n a m i c characteristics are usually referred to as six c o m p o n e n t s o f a e r o d y n a m i c force, o f w h i c h the m o s t f a m o u s is the drag.
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420 T h e f l o w s a r o u n d the v e h i c l e b o d y do n o t o n l y d e c i d e the a e r o d y n a m i c characteristics, b u t also b e c o m e t u r b u l e n t and generate various vortices adjacent to the b o d y surface. The vortices there i n f l u e n c e some kinds of p e r f o r m a n c e s , that is, the w i n d s h i e l d w i p e r p e r f o r m a n c e , the w i n d n o i s e level a n d the b a c k light contamination. The m o t o r vehicles utilize the internal c o m b u s t i o n e n g i n e s and it is necessary to radiate the g e n e r a t e d heat into the air. Thus, a big flow q u a n t i t y is f a v o r a b l e with respect to the radiator and cooler c o n d e n s o r cooling. H o w e v e r , a small flow quantity is preferable as far as the heater p e r f o r m a n c e and drag p e n a l t y are concerned. That is w h y the flow in the e n g i n e c o m p a r t m e n t should be optimized. T h e f l o w i n the p a s s e n g e r c o m p a r t m e n t , that is, for v e n t i l a t i o n a n d air c o n d i t i o n i n g , is not o n l y a e r o d y n a m i c but also c o u p l e d with the heat. F u r t h e r m o r e , the h u m a n f e e l i n g should be taken into account, w h i c h m a k e s the p r o b l e m more difficult. The flows related to e n g i n e s are c o m p r e s s i b l e and divided into three categories. O n e is i n the i n t a k e m a n i f o l d , where the c h a r g i n g e f f i c i e n c y is i m p o r t a n t . The s e c o n d is f r o m the exhaust m a n i f o l d through the muffler, w h i c h has a temperature difference. T h e last is in the e n g i n e cylinder, w h i c h includes the c o m b u s t i o n and is very difficult to deal with strictly. T h e a b o v e - m e n t i o n e d a e r o d y n a m i c p r o b l e m s are u s u a l l y treated b y v a r i o u s methods. The m e a s u r e m e n t s of physical properties are m o s t popular, w h i c h are the forces, the pressures, the velocities and the t e m p e r a t u r e s . T h e flow v i s u a l i z a t i o n t e c h n i q u e s are also v a l u a b l e to u n d e r s t a n d the p h e n o m e n a taking place in the flow field. R e c e n t l y , c o m p u t e r aided flow v i s u a l i z a t i o n ( C A F V ) m e t h o d s have b e e n e m p l o y e d a n d e n a b l e d us to c o m p r e h e n d the flow quantitatively. A n o t h e r usage of c o m p u t e r s is c o m p u t a t i o n a l fluid d y n a m i c s (CFD). A c c o r d i n g to the d e v e l o p m e n t of s u p e r c o m p u t e r s a n d efficient algorithms today, the N a v i e r - S t o k e s e q u a t i o n s c a n be solved at relatively high R e y n o l d s n u m b e r regions. T h e e x p e r i m e n t a l a n d c o m p u t a t i o n a l m e t h o d s in a u t o m o t i v e a e r o d y n a m i c s are very s i m i l a r to those in a e r o n a u t i c a l a e r o d y n a m i c s . H o w e v e r , the a u t o m o b i l e s are u s u a l l y less a e r o d y n a m i c c o m p a r e d with the aeroplane, w h i c h m a k e s the p r o b l e m s m o r e difficult. The application of computers; that is C A F V and C F D , to a u t o m o t i v e a e r o d y n a m i c s is r e v i e w e d and the results o b t a i n e d by the author and his co-workers are described here. 2. C A F V T h e flow v i s u a l i z a t i o n is u s u a l l y carried out to u n d e r s t a n d the flow field at a g l a n c e , w h i c h is i m p o s s i b l e b y the o r d i n a r y m e a s u r e m e n t s o f p h y s i c a l properties. The flow v i s u a l i z a t i o n t e c h n i q u e s used i n a u t o m o t i v e a e r o d y n a m i c s field are divided into two types as s h o w n in Carr et. al. [1986]. O n e is simple c o n v e n t i o n a l methods, such as oil flows, tufts, smokes and h e l i u m b u b b l e s . T h e y are v a l u a b l e b e c a u s e they c a n be u s e d b y a n y b o d y at a n y time. T h e other is C A F V , w h i c h is m o r e difficult a n d t i m e c o n s u m i n g t h a n the c o n v e n t i o n a l m e t h o d s b u t gives us the q u a n t i t a t i v e insight - of the flow field . . . . The simplest C A F V is the usage of the c o m p u t e r graphics as the tool of the display " ~ ~ o f the e x p e r i m e n t a l data. A series of its a p p l i c a t i o n to the w a k e s of m o t o r vehicles are c a r r i e d o n b y Cogotti. A t first [1985], ,~ he utilized the c o m p u t e r graphics to display ,- .~ the total p r e s s u r e loss m e a s u r e d b y a Kiel p r o b e i n the w a k e . Formally, a light e m i t t i n g diode ( L E D ) set co-axially with the ,_ . ~ ~. ~ , p r o b e had b e e n u s e d as the display, w h i c h c a n be traced b a c k to C r o w d e r [1980]. His efforts have g o n e to the a p p l i c a t i o n o f n e w .................................. p r o b e s , that is, a s e v e n - h o l e p r o b e [1986] Fig. 1 Cross flow vectors in wake a n d a f o u r t e e n - h o l e p r o b e [1987]. T h e y of fastback vehicle ( C A F V )
421 e n a b l e us to u n d e r s t a n d the m o r e p r e c i s e f l o w fields, for e x a m p l e the r e v e r s e f l o w in the w a k e . 2,1 A~r0~lynamic Characteristics T h e s t r u c t u r e o f the w a k e o f a v e h i c l e m o d e l is v e r y i m p o r t a n t , b e c a u s e it is c l o s e l y r e l a t e d to its a e r o d y n a m i c c h a r a c t e r i s t i c s , as H a c k e t t et al. [1987] s h o w e d t h r o u g h the w a k e i n t e g r a t i o n . F i g . 1 s h o w s the e x a m p l e o f the c r o s s f l o w v e c t o r d i s t r i b u t i o n in the w a k e o f a fast b a c k vehicle. T h e d a t a w e r e m e a s u r e d b y a threec o m p o n e n t C T A ( C o n s t a n t T e m p e r a t u r e A n e m o m e t e r ) o f w h i c h the s e n s o r was a t t a c h e d to t h e t r a v e r s e r h e a d . T h e d a t a w e r e p r o c e s s e d b y the d a t a a c q u i s i t i o n c o m p u t e r e x p l a i n e d b y O g a t a et al. [1987] to m a k e out the figure. T h e r e c a n b e seen the e x i s t e n c e o f a p a i r o f l o n g i t u d i n a l vortices, w h i c h d o e s not e x i s t in c a s e o f s e d a n o r s q u a r e b a c k s h a p e . It is w e l l k n o w n that this t y p e o f v o r t e x p a i r s i g n i f i c a n t l y i n c r e a s e s the d r a g a n d lift forces. 2.2 F10w in D e t a i l The fundamental techniques of image p r o c e s s i n g , s u c h as f e a t u r e e x t r a c t i o n a n d t h i n n i n g are o f t e n h e l p f u l to d e a l w i t h the flow visualization data. F i g . 2 is the e x a m p l e o f the t h i n n i n g a p p l i e d to the f l o w t o w a r d an e n g i n e c o m p a r t m e n t , w h e r e the center line extraction of streamlines v i s u a l i z e d b y the s m o k e m a d e f r o m dry ice a r e c a r r i e d out. T h e c o o r d i n a t e s o f the s t r e a m l i n e s a r e e a s i l y d e p i c t e d b y this p r o c e s s i n g , t h o u g h it is v e r y simple. In this c a s e , t h e f l o w q u a n t i t y i n t o the e n g i n e c o m p a r t m e n t c a n be o b t a i n e d b y counting the n u m b e r o f i n c o m i n g s t r e a m l i n e s , as the a v e r a g i n g o f the fluctuating s m o k e lines a n d the i n t e r p o l a t i o n b e t w e e n the a d j a c e n t s m o k e lines are easily made. T h e m e t h o d is e x p e c t e d to b e a p p l i e d to the f l o w m o r e a d j a c e n t to the b o d y , b e c a u s e the t u r b u l e n c e Fig. 2 there is r e l a t e d to the w i n d n o i s e , the w i p e r p e r f o r m a n c e a n d so on.
T h i n n i n g o f s m o k e lines t o w a r d v e h i c l e front e n d ( C A F V )
2.3 F l o w in E n e i n e C o m p a r t m e n t T h e m e t h o d in c a l c u l a t i n g the t w o dimensional v e l o c i t y v e c t o r s is o f t e n necessary. It c a n b e c a r r i e d o u t b y m e a s u r i n g the d i r e c t i o n s a n d e x t e n t o f the m o t i o n s o f f i n e t r a c e r s ( K o b a y a s h i et al. [1987]). In this m e t h o d , s e v e r a l p r o b l e m s s h o u l d be s o l v e d , w h i c h are the r e c o g n i t i o n o f the i n i t i a l a n d t e r m i n a l p o s i t i o n s o f e a c h s t r e a m line, the s e p a r a t i o n o f i n t e r s e c t i n g p a t h l i n e s , the e l i m i n a t i o n o f ill c o n d i t i o n e d p a t h l i n e s and so on. T h e r e f o r e , it is m a i n l y a p p l i e d to f l o w f i e l d s w i t h c o m p a r a t i v e l y slow speeds using relatively small amount of t r a c e r s . H o w e v e r , this m e t h o d g i v e s us a l a r g e a m o u n t o f i n f o r m a t i o n a n d then can b e a p p l i e d to m a n y p r o b l e m s . Fig. 3 is an e x a m p l e w h e r e the m e t h o d i s a p p l i e d t o t h e f l o w in a n e n g i n e c o m p a r t m e n t o f a m o t o r vehicle using h e l i u m b u b b l e s as the tracers. T h e figure s h o w s the
Fig. 3
V e l o c i t y v e c t o r s in e n g i n e compartment (CAFV)
422 p l a n v i e w a n d the hood was replaced by a transparent one. The flow in the e n g i n e c o m p a r t m e n t o f the car with F F ( F r o n t - w h e e l - F r o n t - d r i v e ) l a y o u t is seen in the figure, where the front e n d of the vehicle is located d o w n w a r d . It flows not only simply d o w n s t r e a m but also l o n g i t u d i n a l l y along the e n g i n e block. Furthermore, the soap b u b b l e s generated at the e n g i n e b l o c k head are partly f l o w n u p s t r e a m i n d u c e d b y the c o o l i n g fan located b e h i n d the radiator. Such a flow pattern varies according to the vehicle speed and the c o o l i n g fan speed. As this t e c h n i q u e has b e e n extended to t h r e e - d i m e n s i o n a l flow cases, the resolution along the optical axes of the cameras is not adequate b e c a u s e of the unsatisfactory n u m b e r of pixels in the computer. 2.4 F l o w in P a s s e n g e r C o m o a r t m e n t Fig. 4 shows the velocity vectors of the flows in a t w o - d i m e n s i o n a l p a s s e n g e r c o m p a r t m e n t model. The flow comes from the top of the dash board and goes out of the rear parcel shelf. This figure was m a d e b y a s i m i l a r m e t h o d to that o f Fig. 1, t h o u g h the scale is very small. Three vortices are clearly o b s e r v e d in front of front and rear seat backs a n d above the front seat back.
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Velocity vectors in 2-dim. p a s s e n g e r compartment model (CAFV)
2.5 F l o w in E n g i n e C y l i n d e r T h e c o m p u t e r graphic t e c h n i q u e is also applicable to the m o t i o n picture. The data o b t a i n e d e x p e r i m e n t a l l y are stored in a c o m p u t e r and then displayed o n a CRT. Afterwards, a cine- or T V c a m e r a takes photographs of the display frame b y frame. A n o t h e r a l t e r n a t i v e is the direct c o n v e r s i o n , w h i c h records the graphic data into a V T R m a k i n g use o f the N T S C signals. Fig. 5 is an e x a m p l e of the swift flows in a reciprocal e n g i n e . T h e flow in the e n g i n e c y l i n d e r is m e a s u r e d b y a t w o - c o m p o n e n t L D V (Laser D o p p l e r Velocimeter). T h e s w i r l f l o w s b e f o r e a n d at the c o m p r e s s i o n top d e a d c e n t e r are s h o w n c o r r e s p o n d i n g to the four shapes of the intake port. The swirl flow is affected b y the intake port shape a n d is seen to be strongest in the rightmost case i n the figure. The p h e n o m e n a taking place in the e n g i n e c y l i n d e r are easily u n d e r s t o o d b y the a n i m a t i o n m a d e of these data. 2.6 A n o t h e r E x a m p l e C o m p u t e d t o m o g r a p h y (CT) technology makes the t w o - d i m e n s i o n a l transparency d i s t r i b u t i o n figure from the data of the transmitted light m a g n i t u d e w h e n the object u n d e r test is irradiated from m a n y directions. It c a n be traced b a c k to m e d i c a l X-ray CTs a n d n o w is w i d e l y applied to e n g i n e e r i n g problems. As the a p p l i c a t i o n to fluid d y n a m i c s , the temperature distribution i n a flame and the c o n c e n t r a t i o n m e a s u r e m e n t i n the spray ( N a k a y a m a et al. [1985]) are reported. Fig. 6 is the e x a m p l e which visualizes the cross sectional view of the diesel fuel spray g e n e r a t e d from the i n j e c t o r of the e n g i n e for m o t o r vehicles u t i l i z i n g H e - N e laser as the light source. The spray has a d o u g h n u t shape j u s t d o w n s t r e a m of the injector, but as it goes d o w n s t r e a m , the hole in the d o u g h n u t disappears. The weak point o f this m e t h o d is that it takes a long time to obtain the data o f t r a n s m i t t e d light and the d e v e l o p m e n t of a high speed data acquisition m e t h o d is required.
423 / I n t a k e Port
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Fig. 6
Swirl flow in e n g i n e c y l i n d e r ( C A F V )
C T of diesel fuel spray ( C A F V )
424 3. C O M P U T A T I O N A L FLUID D Y N A M I C S The application of C F D to a u t o m o b i l e e n g i n e e r i n g was l a u n c h e d about 10 years ago. T h e C F D works a r o u n d vehicle bodies so far were e x c e l l e n t l y r e v i e w e d in the two s y m p o s i a o r g a n i z e d b y the I n t e r n a t i o n a l A s s o c i a t i o n for V e h i c l e D e s i g n and V o l k s w a g e n w e r k A G in 1982. At the early stage of the C F D a p p l i c a t i o n to a u t o m o b i l e s , the p a n e l m e t h o d is w i d e l y used b e c a u s e it is suitable for t h r e e - d i m e n s i o n a l flows as is s h o w n in A h m e d et al. [1977]. T h e p a n e l m e t h o d is i m p r o v e d in a f e w ways later. O n e is u s i n g h i g h e r order i n t e g r a t i o n s b y Stafford [1982] and a n o t h e r is c o m b i n e d with vortex m e t h o d b y C h o m e t o n [1982], and the other is the c o m b i n a t i o n with b o u n d a r y layer theory b y Hirschel et al. [1982]. H o w e v e r , the basic l i m i t a t i o n of the p a n e l m e t h o d is not r e m o v e d , w h i c h can n o t deal with viscous effects correctly. As the m e t h o d to treat v i s c o u s flows c o r r e c t l y , finite d i f f e r e n c e and finite v o l u m e m e t h o d s are u s u a l l y used i n the a u t o m o b i l e field. A n o t h e r p r o b l e m is how to c a l c u l a t e the effect o f the t u r b u l e n c e . The a e r o d y n a m i c p r o b l e m s e n c o u n t e r e d in a u t o m o b i l e s h a v e rather high R e y n o l d s n u m b e r and the flow s h o u l d be t u r b u l e n t there. The m a j o r way to simulate the t u r b u l e n c e is k-e m o d e l today. This m e t h o d is w i d e l y used i n the world and m a n y e x a m p l e s applied to the flows a r o u n d vehicle bodies. T h e y are b y D e m u r e n et al. [ 1 9 8 2 ] , H a r n m o n d et al. [1985], and R a w n s l e y et al. [1986] to n a m e a few. W i l l o u g h b y et al. [1985] applied this m e t h o d to the flow toward the radiator. The review o f the c o m p u t a t i o n of the flow in the e n g i n e c y l i n d e r is m a d e by, for e x a m p l e , G o s m a n [1985]. As an alternative approach, the direct s i m u l a t i o n m e t h o d is often used. It does n o t require a n y t u r b u l e n c e m o d e l but utilizes a t h i r d - o r d e r - u p w i n d finite difference s c h e m e and c a n solve both l a m i n a r and turbulent flows. That m e a n s the m e t h o d can c o v e r all a e r o d y n a m i c p r o b l e m s o f the automobiles. 3.1 A e r o d y n a m i c Characteristics The flow field a r o u n d the vehicle b o d y is once computed, the a e r o d y n a m i c force exerted onto it is easily evaluated, one of w h i c h is due to the pressure difference and a n o t h e r is the skin friction. Thus, the p r o b l e m is reduced to h o w to solve the flow field exactly. Fig.7 is an e x a m p l e of the velocity vectors o n a vehicle, w h i c h was c o m p u t e d by the p a n e l method. As the p a n e l m e t h o d is based o n the potential theory, the skin friction is n u l l and the i n t e g r a t i o n of the pressure a r o u n d the w h o l e surface vanishes on a c c o u n t of d ' A l e m b e r t ' s paradox. T h u s , its n o use e v a l u a t i n g the a e r o d y n a m i c characteristics by the p a n e l method.
Fig. 7
Flow a r o u n d vehicle by panel m e t h o d ( C F D )
425 Fig. 8 is the s i m i l a r e x a m p l e which is c o m p u t e d by direct simulation. An e x a m p l e of the pressure distribution in the central plane is shown in Fig.9 and the a e r o d y n a m i c c h a r a c t e r i s t i c s are in T a b l e 2, c o m p a r e d with the c o r r e s p o n d i n g experirnental values. The results show that the c o m p u t a t i o n is a p o w e r f u l tool to evaluate the a e r o d y n a m i c drag of the vehicle. On the other hand, the lift is difficult to infer, because the underfloor flow can not be simulated correctly.
Fig. 8
Flow around vehicle by direct simulation (CFD)
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Pressure distribution (CFD)
Table 2 Computed and experimental aerodynamic characteristics
cL
Experiment Computation
0.226 0.232
0.024 0.228
t
426 3,2 F l o w in D e t a i l T h e p a n e l m e t h o d is v e r y u s e f u l b e c a u s e o f its short c o m p u t i n g time, if it is a p p l i e d to the a t t a c h e d f l o w region. F o r e x a m p l e , the v e l o c i t y v e c t o r s s h o w n in Fig. 7 are v e r y v a l u a b l e , w h e n the w i p e r p e r f o r m a n c e is predicted. A n o t h e r a p p l i c a t i o n is h o w to l o c a t e the o p e n i n g s to i n t r o d u c e the e n g i n e c o o l i n g and v e n t i l a t i n g air flows f r o m the p r e s s u r e d i s t r i b u t i o n v i e w point. T h e f l o w w i t h s e p a r a t i o n s h o u l d be s o l v e d b y the finite d i f f e r e n c e m e t h o d . T h e c o m p u t e d r e s u l t s c o v e r a l l the f l o w f i e l d a n d t h e n the d e s i r e d f i g u r e is e a s i l y o b t a i n e d . Thus, f o r e x a m p l e , the vortices b e h i n d the A - p i l l a r and in the w a k e can be o b s e r v e d , w h i c h is s e e n in Fig. 8 in part. Fig. 10 is a n o t h e r e x a m p l e w h i c h is the f l o w a r o u n d a t w o - d i m e n s i o n a l w i p e r blade. T h e a e r o d y n a m i c force, e s p e c i a l l y the lift, c a n be c a l c u l a t e d b y i n t e g r a t i n g the p r e s s u r e d i s t r i b u t i o n , w h i c h d e c i d e s the m a x i m a l s p e e d that the b l a d e w o r k s out to w i p e o f f the rain d r o p s . T h e c o m p a r i s o n o f the results o b t a i n e d with d i f f e r e n t b l a d e shapes e n a b l e s us to select the f a v o r a b l e b l a d e shapes.
~--~ - ~ ~ : ~ _ . - - : . i ~
-" Fig. 10
~ ~ ~ ~ _ - ~ - ~
F l o w a r o u n d 2-dim. w i p e r b l a d e m o d e l ( C F D )
3.3 F l ~ w in E n e i n e Comt~artment Fig. 11 is the e x a m p l e o f the f l o w a r o u n d a t w o - d i m e n s i o n a l front end. Here the b u m p e r , the r a d i a t o r grille, the apron, the radiator, a n d the u n d e r c o v e r are t a k e n into a c c o u n t , w h e r e the total p r e s s u r e losses d o w n s t r e a m o f the r a d i a t o r are s u m m e d u p to the radiator. A s the s t r e a m l i n e s are d r a w n in the figure, the f l o w q u a n t i t y into the e n g i n e c o m p a r t m e n t is e a s i l y m e a s u r e d s i m p l y b y c o u n t i n g t h e i r n u m b e r . T h e e f f e c t o f the f r o n t e n d s h a p e on the f l o w q u a n t i t y is e v a l u a t e d b y c o m p a r i n g the c o m p u t e d r e s u l t s w i t h d i f f e r e n t s h a p e s as is s h o w n b y K a w a s h i m a et al. [1988]. T h o u g h the c o m p u t a t i o n is c o n d u c t e d t w o - d i m e n s i o n a l l y and the a c t u a l f l o w is threed i m e n s i o n a l , the c o m p u t a t i o n is v e r y u s e f u l w h e n the f l o w q u a n t i t y r a t i o to the o r i g i n a l c a s e is c o n c e r n e d . N o w , t h r e e - d i m e n s i o n a l c o m p u t a t i o n is also c a r r i e d out.
Fig. 11
F l o w a r o u n d 2 - d i m . front e n d m o d e l ( C F D )
427 3.4 F l o w in P a s s e n g e r Comt~artment F i g . 12 is the C F D e x a m p l e o f the f l o w in the t w o - d i m e n s i o n a l p a s s e n g e r c o m p a r t m e n t , w h i c h is the s a m e as in F i g . 4 . T h e c o m p u t e d f l o w is v e r y s i m i l a r to the e x p e r i m e n t a l one, that is, the f l o w m a k e s t w o v o r t i c e s in front o f the front a n d r e a r seat b a c k s a n d a n o t h e r in r e v e r s e a b o v e the front seat b a c k . A s a m a t t e r o f fact, the e x p e r i m e n t in F i g . 4 is c a r r i e d out in o r d e r to v e r i f y this c o m p u t a t i o n . T h e a g r e e m e n t is satisfactory.
Fig. 12
F l o w in 2-dim. p a s s e n g e r c o m p a r t m e n t m o d e l ( C F D )
3.5 F l o w in E n g i n e C y l i n d e r A n e x a r n p l e o f the f l o w in an e n g i n e c y l i n d e r is s h o w n in F i g . 13, w h i c h is computed by a program suitable for compressible and viscous flows with a SGS m o d e l . T h e r e s u l t s s h o w the s i m i l a r i t y w i t h the e x p e r i m e n t a l o n e s as is s h o w n in F i g . 5. T h e s w i r l f l o w in the c y l i n d e r v a r i e s s i g n i f i c a n t l y w h e n the s h a p e o f the c o m b u s t i o n c h a m b e r is altered. 3.6 A n o t h e r E x a m p l ~ F i g . 14 s h o w s a f l o w f i e l d a r o u n d c i r c u l a r c y l i n d e r s s i d e - b y - s i d e , w h i c h is c o m p u t e d to i n v e s t i g a t e the b a s i c p e r f o r m a n c e o f the radiator. T h i s is the p r e s s u r e c o n t o u r m a p a n d the w h i t e r e g i o n s h a v e r e l a t i v e l y h i g h air speed. It is seen that the w a k e o f e a c h c y l i n d e r v a r i e s f r o m l o c a t i o n to l o c a t i o n . T h o u g h a w i d e w a k e n e i g h b o r s o n a n a r r o w one a n d vice v e r s a in this case, the w a k e p a t t e r n m a y c h a n g e d r a s t i c a l l y w h e n the s e p a r a t i o n b e t w e e n the c y l i n d e r s is i n c r e a s e d . Thus, it is e a s y to d e v e l o p the r a d i a t o r w i t h l o w resistance and h i g h h e a t e x c h a n g e e f f i c i e n c y .
Fig. 13
F l o w in e n g i n e cylinder (CFD)
Fig. 14
Flow around cylinders side-by-side (CFD)
428 4. CONCLUSIONS The application of computers to automobile aerodynamics is reviewed. One is C A F V and the other is CFD. Both are c o m m o n l y used in the whole range of automotive engineering, as follows. 1) Computer graphic display of experimental data is the typical CAFV. 2) Wake survey is popular and helps us improve aerodynamic characteristics. 3) Image processing is very promising, though the application is scarce today. 4) CT is useful when the structure of the spray is examined. 5) Panel method is suitable for three-dimensional attached flows. 6) Direct simulation method can treat the all flows related to automobiles. 7) CFD can be the tool to infer the aerodynamic characteristics. REFERENCES Ahmed, S. R. et al. 1977, "The Calculation of the Flow Field past a Van with the Aid of a Panel Method", SAE Paoer 770390. Carr, G. W. et al. 1986, " A e r o d y n a m i c Flow Visualization Techniques and Procedures", SAE Information Reoort. HSJ1566. Chometon, F 1982, "Calculating Three-Dimensional Separated Flow Around Road Vehicles", Int. J. of Vehicle Design, SP3, Impact of Aerodynamics on Vehicle Design, pp. 374-386. Cogotti, A. 1985, "Pininfarina - New Measurement Techniques in the Wind Tunnel", Proc. Autotechnolo~,ies Montecarlo 85. Cogotti, A. 1986, "Car-Wake Imaging Using a Seven-Hole Probe", S A E Paoer 860214. Cogotti, A. 1987, "Flow Field Surveys behind Three Squareback Car Models Using a Fourteen-Hole Probe", SAE Patmr 870243. Crowder, J. P. 1980, "Quick and t~asy Flow-Field Surveys", Astronautics and Aeronautics, Vol. 18, No. 10, pp. 38-39, 45. Demuren, A. O. et al. 1982, "Calculation of Three-Dimensional Turbulent Flow around Car Bodies", Svmn. on Vehicle Aerodynamics. Wolfsburg. Gosman, A . D . 1985, "Computer Modeling of Flow and Heat Transfer in Engines, Progress and Prospects", Proc. C O M O D I A 85, Tokyo, pp. 15-26. Hackett, J. E. et al. 1987, "On the Influence of Ground M o v e m e n t and Wheel Rotation in Tests on M o d e m Car Shapes", ~ 870245. Hammond, D. Jr. 1985, "Use of a Supercomputer in Aerodynamics Computations at General Motors Research Laboratories", ~ 850473. Hirschel, E. H. et al. 1982, "Theoretical and Experimental Boundary-Layer Studies on Car Bodies", Syrup. on Vehicle Aerodynamics, Wolfsburg. Hucho, W-. H. 1981, Aerodynamik des Automobils, Vogel-Verlag, Wurzburg, p. 355. Kawashima,K et al. 1988, "Front -End Air Flow Rate Simulation", ~ to be published. Kobayashi, T. et al. 1987, "Some Considerations on Automated Image Processing of Pathline Photographs", Flow Visualization IV, Hemisphere Publishing, pp. 241-246. Lawnsley, S. M. et al. 1986, "Application o f the P H O E N I C S Code to the Computation of the Flow Around Automobiles", ~ 860217. Nakayama, M e t al. 1985, "Visualization of Spray Structure by Means of Computed Tomography", Proc. C O M O D I A 85, Tokyo, pp. 131-139. Ogata, N. et al. 1987, "Nissan's Low-Noise Full-Scale Wind Tunnel", S A E Paper 870250. Stafford, L . G . 1982, "A High-Order Boundary Integral Equation Technique for the Computation of Vehicle Flow Fields", Int. J. of Vehicle Design, SP3, Impact of Aerodynamics on Vehicle Design, pp. 401-428. Willoughby, D. A. et al. 1985, "A Quasi-Three-Dimensional Computational Procedure for Prediction o f Turbulent Flow T h r o u g h the Front-End of Vehicles", ~ 850282.
419
Elsevier Science Publishers B.V., Amsterdam - - Printed in The Netherlands
Invited Paper
APPLICATION OF COMPUTERS TO AUTOMOBILE AERODYNAMICS M. TAKAGI
Vehicle Research Laboratory, Nissan Motor Co., Ltd., Yokosuka 237 ('Japan) Summary Computers are recently applied to aerodynamics in two ways. One is computer aided flow visualization (CAFV) and the other is computational fluid dynamics (CFD). In the field of automobile engineering, both are very popular today. There arc many varieties in CAFV, that is, mere computer graphics of velocity and pressure measurements, image processing of visualized flow fields, computed tomography and so on. The CAFV results give us more quantitative information than conventional flow visualization results. It means that CAFV enables us to understand flow fields more precisely. The flow fields around vehicle bodies can be solved numerically using several methods. These are panel methods, k-¢ methods and direct simulation methods. The panel methods are suitable for the potential flow making it very useful to analyze attached flows. The latter two methods can deal with separated flows exactly, and they help us compute, for example, separated wake regions. The CFD results are inevitably combined with computer graphics and show us very realistic flow images. That is another advantage of CFD.
Keywords Automobile aerodynamics, computer aided flow visualization, computational fluid dynamics
1. I N T R O D U C T I O N T h e r e are m a n y k i n d s o f a e r o d y n a m i c p r o b l e m s in a u t o m o t i v e e n g i n e e r i n g . T h e y are u s u a l l y c l a s s i f i e d into 5 kinds as listed in T a b l e 1, that is, 4 a r o u n d v e h i c l e b o d i e s as is seen in H u c h o [1981] and one related to engines. Table 1 Automobile aerodynamic problems 1. 2. 3. 4. 5.
A e r o d y n a m i c characteristics D e t a i l s o f f l o w adjacent to b o d y surface E n g i n e c o o l i n g air f l o w s V e n t i l a t i o n and air c o n d i t i o n i n g f l o w s in p a s s e n g e r c o m p a r t m e n t s F l o w s in e n g i n e c y li n d e r s and intake and e x h a u s t m a n i f o l d s
T h e a e r o d y n a m i c c h a r a c t e r i s t i c s a f f e c t v e h i c l e d r i v i n g p e r f o r m a n c e s s u c h as m a x i m a l speed, cross w i n d stability, a c c e l e r a t i o n , f u e l c o n s u m p t i o n and so on. T h e a e r o d y n a m i c characteristics are usually referred to as six c o m p o n e n t s o f a e r o d y n a m i c force, o f w h i c h the m o s t f a m o u s is the drag.
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420 T h e f l o w s a r o u n d the v e h i c l e b o d y do n o t o n l y d e c i d e the a e r o d y n a m i c characteristics, b u t also b e c o m e t u r b u l e n t and generate various vortices adjacent to the b o d y surface. The vortices there i n f l u e n c e some kinds of p e r f o r m a n c e s , that is, the w i n d s h i e l d w i p e r p e r f o r m a n c e , the w i n d n o i s e level a n d the b a c k light contamination. The m o t o r vehicles utilize the internal c o m b u s t i o n e n g i n e s and it is necessary to radiate the g e n e r a t e d heat into the air. Thus, a big flow q u a n t i t y is f a v o r a b l e with respect to the radiator and cooler c o n d e n s o r cooling. H o w e v e r , a small flow quantity is preferable as far as the heater p e r f o r m a n c e and drag p e n a l t y are concerned. That is w h y the flow in the e n g i n e c o m p a r t m e n t should be optimized. T h e f l o w i n the p a s s e n g e r c o m p a r t m e n t , that is, for v e n t i l a t i o n a n d air c o n d i t i o n i n g , is not o n l y a e r o d y n a m i c but also c o u p l e d with the heat. F u r t h e r m o r e , the h u m a n f e e l i n g should be taken into account, w h i c h m a k e s the p r o b l e m more difficult. The flows related to e n g i n e s are c o m p r e s s i b l e and divided into three categories. O n e is i n the i n t a k e m a n i f o l d , where the c h a r g i n g e f f i c i e n c y is i m p o r t a n t . The s e c o n d is f r o m the exhaust m a n i f o l d through the muffler, w h i c h has a temperature difference. T h e last is in the e n g i n e cylinder, w h i c h includes the c o m b u s t i o n and is very difficult to deal with strictly. T h e a b o v e - m e n t i o n e d a e r o d y n a m i c p r o b l e m s are u s u a l l y treated b y v a r i o u s methods. The m e a s u r e m e n t s of physical properties are m o s t popular, w h i c h are the forces, the pressures, the velocities and the t e m p e r a t u r e s . T h e flow v i s u a l i z a t i o n t e c h n i q u e s are also v a l u a b l e to u n d e r s t a n d the p h e n o m e n a taking place in the flow field. R e c e n t l y , c o m p u t e r aided flow v i s u a l i z a t i o n ( C A F V ) m e t h o d s have b e e n e m p l o y e d a n d e n a b l e d us to c o m p r e h e n d the flow quantitatively. A n o t h e r usage of c o m p u t e r s is c o m p u t a t i o n a l fluid d y n a m i c s (CFD). A c c o r d i n g to the d e v e l o p m e n t of s u p e r c o m p u t e r s a n d efficient algorithms today, the N a v i e r - S t o k e s e q u a t i o n s c a n be solved at relatively high R e y n o l d s n u m b e r regions. T h e e x p e r i m e n t a l a n d c o m p u t a t i o n a l m e t h o d s in a u t o m o t i v e a e r o d y n a m i c s are very s i m i l a r to those in a e r o n a u t i c a l a e r o d y n a m i c s . H o w e v e r , the a u t o m o b i l e s are u s u a l l y less a e r o d y n a m i c c o m p a r e d with the aeroplane, w h i c h m a k e s the p r o b l e m s m o r e difficult. The application of computers; that is C A F V and C F D , to a u t o m o t i v e a e r o d y n a m i c s is r e v i e w e d and the results o b t a i n e d by the author and his co-workers are described here. 2. C A F V T h e flow v i s u a l i z a t i o n is u s u a l l y carried out to u n d e r s t a n d the flow field at a g l a n c e , w h i c h is i m p o s s i b l e b y the o r d i n a r y m e a s u r e m e n t s o f p h y s i c a l properties. The flow v i s u a l i z a t i o n t e c h n i q u e s used i n a u t o m o t i v e a e r o d y n a m i c s field are divided into two types as s h o w n in Carr et. al. [1986]. O n e is simple c o n v e n t i o n a l methods, such as oil flows, tufts, smokes and h e l i u m b u b b l e s . T h e y are v a l u a b l e b e c a u s e they c a n be u s e d b y a n y b o d y at a n y time. T h e other is C A F V , w h i c h is m o r e difficult a n d t i m e c o n s u m i n g t h a n the c o n v e n t i o n a l m e t h o d s b u t gives us the q u a n t i t a t i v e insight - of the flow field . . . . The simplest C A F V is the usage of the c o m p u t e r graphics as the tool of the display " ~ ~ o f the e x p e r i m e n t a l data. A series of its a p p l i c a t i o n to the w a k e s of m o t o r vehicles are c a r r i e d o n b y Cogotti. A t first [1985], ,~ he utilized the c o m p u t e r graphics to display ,- .~ the total p r e s s u r e loss m e a s u r e d b y a Kiel p r o b e i n the w a k e . Formally, a light e m i t t i n g diode ( L E D ) set co-axially with the ,_ . ~ ~. ~ , p r o b e had b e e n u s e d as the display, w h i c h c a n be traced b a c k to C r o w d e r [1980]. His efforts have g o n e to the a p p l i c a t i o n o f n e w .................................. p r o b e s , that is, a s e v e n - h o l e p r o b e [1986] Fig. 1 Cross flow vectors in wake a n d a f o u r t e e n - h o l e p r o b e [1987]. T h e y of fastback vehicle ( C A F V )
421 e n a b l e us to u n d e r s t a n d the m o r e p r e c i s e f l o w fields, for e x a m p l e the r e v e r s e f l o w in the w a k e . 2,1 A~r0~lynamic Characteristics T h e s t r u c t u r e o f the w a k e o f a v e h i c l e m o d e l is v e r y i m p o r t a n t , b e c a u s e it is c l o s e l y r e l a t e d to its a e r o d y n a m i c c h a r a c t e r i s t i c s , as H a c k e t t et al. [1987] s h o w e d t h r o u g h the w a k e i n t e g r a t i o n . F i g . 1 s h o w s the e x a m p l e o f the c r o s s f l o w v e c t o r d i s t r i b u t i o n in the w a k e o f a fast b a c k vehicle. T h e d a t a w e r e m e a s u r e d b y a threec o m p o n e n t C T A ( C o n s t a n t T e m p e r a t u r e A n e m o m e t e r ) o f w h i c h the s e n s o r was a t t a c h e d to t h e t r a v e r s e r h e a d . T h e d a t a w e r e p r o c e s s e d b y the d a t a a c q u i s i t i o n c o m p u t e r e x p l a i n e d b y O g a t a et al. [1987] to m a k e out the figure. T h e r e c a n b e seen the e x i s t e n c e o f a p a i r o f l o n g i t u d i n a l vortices, w h i c h d o e s not e x i s t in c a s e o f s e d a n o r s q u a r e b a c k s h a p e . It is w e l l k n o w n that this t y p e o f v o r t e x p a i r s i g n i f i c a n t l y i n c r e a s e s the d r a g a n d lift forces. 2.2 F10w in D e t a i l The fundamental techniques of image p r o c e s s i n g , s u c h as f e a t u r e e x t r a c t i o n a n d t h i n n i n g are o f t e n h e l p f u l to d e a l w i t h the flow visualization data. F i g . 2 is the e x a m p l e o f the t h i n n i n g a p p l i e d to the f l o w t o w a r d an e n g i n e c o m p a r t m e n t , w h e r e the center line extraction of streamlines v i s u a l i z e d b y the s m o k e m a d e f r o m dry ice a r e c a r r i e d out. T h e c o o r d i n a t e s o f the s t r e a m l i n e s a r e e a s i l y d e p i c t e d b y this p r o c e s s i n g , t h o u g h it is v e r y simple. In this c a s e , t h e f l o w q u a n t i t y i n t o the e n g i n e c o m p a r t m e n t c a n be o b t a i n e d b y counting the n u m b e r o f i n c o m i n g s t r e a m l i n e s , as the a v e r a g i n g o f the fluctuating s m o k e lines a n d the i n t e r p o l a t i o n b e t w e e n the a d j a c e n t s m o k e lines are easily made. T h e m e t h o d is e x p e c t e d to b e a p p l i e d to the f l o w m o r e a d j a c e n t to the b o d y , b e c a u s e the t u r b u l e n c e Fig. 2 there is r e l a t e d to the w i n d n o i s e , the w i p e r p e r f o r m a n c e a n d so on.
T h i n n i n g o f s m o k e lines t o w a r d v e h i c l e front e n d ( C A F V )
2.3 F l o w in E n e i n e C o m p a r t m e n t T h e m e t h o d in c a l c u l a t i n g the t w o dimensional v e l o c i t y v e c t o r s is o f t e n necessary. It c a n b e c a r r i e d o u t b y m e a s u r i n g the d i r e c t i o n s a n d e x t e n t o f the m o t i o n s o f f i n e t r a c e r s ( K o b a y a s h i et al. [1987]). In this m e t h o d , s e v e r a l p r o b l e m s s h o u l d be s o l v e d , w h i c h are the r e c o g n i t i o n o f the i n i t i a l a n d t e r m i n a l p o s i t i o n s o f e a c h s t r e a m line, the s e p a r a t i o n o f i n t e r s e c t i n g p a t h l i n e s , the e l i m i n a t i o n o f ill c o n d i t i o n e d p a t h l i n e s and so on. T h e r e f o r e , it is m a i n l y a p p l i e d to f l o w f i e l d s w i t h c o m p a r a t i v e l y slow speeds using relatively small amount of t r a c e r s . H o w e v e r , this m e t h o d g i v e s us a l a r g e a m o u n t o f i n f o r m a t i o n a n d then can b e a p p l i e d to m a n y p r o b l e m s . Fig. 3 is an e x a m p l e w h e r e the m e t h o d i s a p p l i e d t o t h e f l o w in a n e n g i n e c o m p a r t m e n t o f a m o t o r vehicle using h e l i u m b u b b l e s as the tracers. T h e figure s h o w s the
Fig. 3
V e l o c i t y v e c t o r s in e n g i n e compartment (CAFV)
422 p l a n v i e w a n d the hood was replaced by a transparent one. The flow in the e n g i n e c o m p a r t m e n t o f the car with F F ( F r o n t - w h e e l - F r o n t - d r i v e ) l a y o u t is seen in the figure, where the front e n d of the vehicle is located d o w n w a r d . It flows not only simply d o w n s t r e a m but also l o n g i t u d i n a l l y along the e n g i n e block. Furthermore, the soap b u b b l e s generated at the e n g i n e b l o c k head are partly f l o w n u p s t r e a m i n d u c e d b y the c o o l i n g fan located b e h i n d the radiator. Such a flow pattern varies according to the vehicle speed and the c o o l i n g fan speed. As this t e c h n i q u e has b e e n extended to t h r e e - d i m e n s i o n a l flow cases, the resolution along the optical axes of the cameras is not adequate b e c a u s e of the unsatisfactory n u m b e r of pixels in the computer. 2.4 F l o w in P a s s e n g e r C o m o a r t m e n t Fig. 4 shows the velocity vectors of the flows in a t w o - d i m e n s i o n a l p a s s e n g e r c o m p a r t m e n t model. The flow comes from the top of the dash board and goes out of the rear parcel shelf. This figure was m a d e b y a s i m i l a r m e t h o d to that o f Fig. 1, t h o u g h the scale is very small. Three vortices are clearly o b s e r v e d in front of front and rear seat backs a n d above the front seat back.
t._--
~/7--
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17 I
e e, it \ , , , ,
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\ Fig. 4
Velocity vectors in 2-dim. p a s s e n g e r compartment model (CAFV)
2.5 F l o w in E n g i n e C y l i n d e r T h e c o m p u t e r graphic t e c h n i q u e is also applicable to the m o t i o n picture. The data o b t a i n e d e x p e r i m e n t a l l y are stored in a c o m p u t e r and then displayed o n a CRT. Afterwards, a cine- or T V c a m e r a takes photographs of the display frame b y frame. A n o t h e r a l t e r n a t i v e is the direct c o n v e r s i o n , w h i c h records the graphic data into a V T R m a k i n g use o f the N T S C signals. Fig. 5 is an e x a m p l e of the swift flows in a reciprocal e n g i n e . T h e flow in the e n g i n e c y l i n d e r is m e a s u r e d b y a t w o - c o m p o n e n t L D V (Laser D o p p l e r Velocimeter). T h e s w i r l f l o w s b e f o r e a n d at the c o m p r e s s i o n top d e a d c e n t e r are s h o w n c o r r e s p o n d i n g to the four shapes of the intake port. The swirl flow is affected b y the intake port shape a n d is seen to be strongest in the rightmost case i n the figure. The p h e n o m e n a taking place in the e n g i n e c y l i n d e r are easily u n d e r s t o o d b y the a n i m a t i o n m a d e of these data. 2.6 A n o t h e r E x a m p l e C o m p u t e d t o m o g r a p h y (CT) technology makes the t w o - d i m e n s i o n a l transparency d i s t r i b u t i o n figure from the data of the transmitted light m a g n i t u d e w h e n the object u n d e r test is irradiated from m a n y directions. It c a n be traced b a c k to m e d i c a l X-ray CTs a n d n o w is w i d e l y applied to e n g i n e e r i n g problems. As the a p p l i c a t i o n to fluid d y n a m i c s , the temperature distribution i n a flame and the c o n c e n t r a t i o n m e a s u r e m e n t i n the spray ( N a k a y a m a et al. [1985]) are reported. Fig. 6 is the e x a m p l e which visualizes the cross sectional view of the diesel fuel spray g e n e r a t e d from the i n j e c t o r of the e n g i n e for m o t o r vehicles u t i l i z i n g H e - N e laser as the light source. The spray has a d o u g h n u t shape j u s t d o w n s t r e a m of the injector, but as it goes d o w n s t r e a m , the hole in the d o u g h n u t disappears. The weak point o f this m e t h o d is that it takes a long time to obtain the data o f t r a n s m i t t e d light and the d e v e l o p m e n t of a high speed data acquisition m e t h o d is required.
423 / I n t a k e Port
a a
b
c
b
b + Shroud
c
b + Shroud
(a) 30* C A BTDC a
b
c
b + Shroud
Co) T D C Fig. 5
Fig. 6
Swirl flow in e n g i n e c y l i n d e r ( C A F V )
C T of diesel fuel spray ( C A F V )
424 3. C O M P U T A T I O N A L FLUID D Y N A M I C S The application of C F D to a u t o m o b i l e e n g i n e e r i n g was l a u n c h e d about 10 years ago. T h e C F D works a r o u n d vehicle bodies so far were e x c e l l e n t l y r e v i e w e d in the two s y m p o s i a o r g a n i z e d b y the I n t e r n a t i o n a l A s s o c i a t i o n for V e h i c l e D e s i g n and V o l k s w a g e n w e r k A G in 1982. At the early stage of the C F D a p p l i c a t i o n to a u t o m o b i l e s , the p a n e l m e t h o d is w i d e l y used b e c a u s e it is suitable for t h r e e - d i m e n s i o n a l flows as is s h o w n in A h m e d et al. [1977]. T h e p a n e l m e t h o d is i m p r o v e d in a f e w ways later. O n e is u s i n g h i g h e r order i n t e g r a t i o n s b y Stafford [1982] and a n o t h e r is c o m b i n e d with vortex m e t h o d b y C h o m e t o n [1982], and the other is the c o m b i n a t i o n with b o u n d a r y layer theory b y Hirschel et al. [1982]. H o w e v e r , the basic l i m i t a t i o n of the p a n e l m e t h o d is not r e m o v e d , w h i c h can n o t deal with viscous effects correctly. As the m e t h o d to treat v i s c o u s flows c o r r e c t l y , finite d i f f e r e n c e and finite v o l u m e m e t h o d s are u s u a l l y used i n the a u t o m o b i l e field. A n o t h e r p r o b l e m is how to c a l c u l a t e the effect o f the t u r b u l e n c e . The a e r o d y n a m i c p r o b l e m s e n c o u n t e r e d in a u t o m o b i l e s h a v e rather high R e y n o l d s n u m b e r and the flow s h o u l d be t u r b u l e n t there. The m a j o r way to simulate the t u r b u l e n c e is k-e m o d e l today. This m e t h o d is w i d e l y used i n the world and m a n y e x a m p l e s applied to the flows a r o u n d vehicle bodies. T h e y are b y D e m u r e n et al. [ 1 9 8 2 ] , H a r n m o n d et al. [1985], and R a w n s l e y et al. [1986] to n a m e a few. W i l l o u g h b y et al. [1985] applied this m e t h o d to the flow toward the radiator. The review o f the c o m p u t a t i o n of the flow in the e n g i n e c y l i n d e r is m a d e by, for e x a m p l e , G o s m a n [1985]. As an alternative approach, the direct s i m u l a t i o n m e t h o d is often used. It does n o t require a n y t u r b u l e n c e m o d e l but utilizes a t h i r d - o r d e r - u p w i n d finite difference s c h e m e and c a n solve both l a m i n a r and turbulent flows. That m e a n s the m e t h o d can c o v e r all a e r o d y n a m i c p r o b l e m s o f the automobiles. 3.1 A e r o d y n a m i c Characteristics The flow field a r o u n d the vehicle b o d y is once computed, the a e r o d y n a m i c force exerted onto it is easily evaluated, one of w h i c h is due to the pressure difference and a n o t h e r is the skin friction. Thus, the p r o b l e m is reduced to h o w to solve the flow field exactly. Fig.7 is an e x a m p l e of the velocity vectors o n a vehicle, w h i c h was c o m p u t e d by the p a n e l method. As the p a n e l m e t h o d is based o n the potential theory, the skin friction is n u l l and the i n t e g r a t i o n of the pressure a r o u n d the w h o l e surface vanishes on a c c o u n t of d ' A l e m b e r t ' s paradox. T h u s , its n o use e v a l u a t i n g the a e r o d y n a m i c characteristics by the p a n e l method.
Fig. 7
Flow a r o u n d vehicle by panel m e t h o d ( C F D )
425 Fig. 8 is the s i m i l a r e x a m p l e which is c o m p u t e d by direct simulation. An e x a m p l e of the pressure distribution in the central plane is shown in Fig.9 and the a e r o d y n a m i c c h a r a c t e r i s t i c s are in T a b l e 2, c o m p a r e d with the c o r r e s p o n d i n g experirnental values. The results show that the c o m p u t a t i o n is a p o w e r f u l tool to evaluate the a e r o d y n a m i c drag of the vehicle. On the other hand, the lift is difficult to infer, because the underfloor flow can not be simulated correctly.
Fig. 8
Flow around vehicle by direct simulation (CFD)
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Fig. 9
Pressure distribution (CFD)
Table 2 Computed and experimental aerodynamic characteristics
cL
Experiment Computation
0.226 0.232
0.024 0.228
t
426 3,2 F l o w in D e t a i l T h e p a n e l m e t h o d is v e r y u s e f u l b e c a u s e o f its short c o m p u t i n g time, if it is a p p l i e d to the a t t a c h e d f l o w region. F o r e x a m p l e , the v e l o c i t y v e c t o r s s h o w n in Fig. 7 are v e r y v a l u a b l e , w h e n the w i p e r p e r f o r m a n c e is predicted. A n o t h e r a p p l i c a t i o n is h o w to l o c a t e the o p e n i n g s to i n t r o d u c e the e n g i n e c o o l i n g and v e n t i l a t i n g air flows f r o m the p r e s s u r e d i s t r i b u t i o n v i e w point. T h e f l o w w i t h s e p a r a t i o n s h o u l d be s o l v e d b y the finite d i f f e r e n c e m e t h o d . T h e c o m p u t e d r e s u l t s c o v e r a l l the f l o w f i e l d a n d t h e n the d e s i r e d f i g u r e is e a s i l y o b t a i n e d . Thus, f o r e x a m p l e , the vortices b e h i n d the A - p i l l a r and in the w a k e can be o b s e r v e d , w h i c h is s e e n in Fig. 8 in part. Fig. 10 is a n o t h e r e x a m p l e w h i c h is the f l o w a r o u n d a t w o - d i m e n s i o n a l w i p e r blade. T h e a e r o d y n a m i c force, e s p e c i a l l y the lift, c a n be c a l c u l a t e d b y i n t e g r a t i n g the p r e s s u r e d i s t r i b u t i o n , w h i c h d e c i d e s the m a x i m a l s p e e d that the b l a d e w o r k s out to w i p e o f f the rain d r o p s . T h e c o m p a r i s o n o f the results o b t a i n e d with d i f f e r e n t b l a d e shapes e n a b l e s us to select the f a v o r a b l e b l a d e shapes.
~--~ - ~ ~ : ~ _ . - - : . i ~
-" Fig. 10
~ ~ ~ ~ _ - ~ - ~
F l o w a r o u n d 2-dim. w i p e r b l a d e m o d e l ( C F D )
3.3 F l ~ w in E n e i n e Comt~artment Fig. 11 is the e x a m p l e o f the f l o w a r o u n d a t w o - d i m e n s i o n a l front end. Here the b u m p e r , the r a d i a t o r grille, the apron, the radiator, a n d the u n d e r c o v e r are t a k e n into a c c o u n t , w h e r e the total p r e s s u r e losses d o w n s t r e a m o f the r a d i a t o r are s u m m e d u p to the radiator. A s the s t r e a m l i n e s are d r a w n in the figure, the f l o w q u a n t i t y into the e n g i n e c o m p a r t m e n t is e a s i l y m e a s u r e d s i m p l y b y c o u n t i n g t h e i r n u m b e r . T h e e f f e c t o f the f r o n t e n d s h a p e on the f l o w q u a n t i t y is e v a l u a t e d b y c o m p a r i n g the c o m p u t e d r e s u l t s w i t h d i f f e r e n t s h a p e s as is s h o w n b y K a w a s h i m a et al. [1988]. T h o u g h the c o m p u t a t i o n is c o n d u c t e d t w o - d i m e n s i o n a l l y and the a c t u a l f l o w is threed i m e n s i o n a l , the c o m p u t a t i o n is v e r y u s e f u l w h e n the f l o w q u a n t i t y r a t i o to the o r i g i n a l c a s e is c o n c e r n e d . N o w , t h r e e - d i m e n s i o n a l c o m p u t a t i o n is also c a r r i e d out.
Fig. 11
F l o w a r o u n d 2 - d i m . front e n d m o d e l ( C F D )
427 3.4 F l o w in P a s s e n g e r Comt~artment F i g . 12 is the C F D e x a m p l e o f the f l o w in the t w o - d i m e n s i o n a l p a s s e n g e r c o m p a r t m e n t , w h i c h is the s a m e as in F i g . 4 . T h e c o m p u t e d f l o w is v e r y s i m i l a r to the e x p e r i m e n t a l one, that is, the f l o w m a k e s t w o v o r t i c e s in front o f the front a n d r e a r seat b a c k s a n d a n o t h e r in r e v e r s e a b o v e the front seat b a c k . A s a m a t t e r o f fact, the e x p e r i m e n t in F i g . 4 is c a r r i e d out in o r d e r to v e r i f y this c o m p u t a t i o n . T h e a g r e e m e n t is satisfactory.
Fig. 12
F l o w in 2-dim. p a s s e n g e r c o m p a r t m e n t m o d e l ( C F D )
3.5 F l o w in E n g i n e C y l i n d e r A n e x a r n p l e o f the f l o w in an e n g i n e c y l i n d e r is s h o w n in F i g . 13, w h i c h is computed by a program suitable for compressible and viscous flows with a SGS m o d e l . T h e r e s u l t s s h o w the s i m i l a r i t y w i t h the e x p e r i m e n t a l o n e s as is s h o w n in F i g . 5. T h e s w i r l f l o w in the c y l i n d e r v a r i e s s i g n i f i c a n t l y w h e n the s h a p e o f the c o m b u s t i o n c h a m b e r is altered. 3.6 A n o t h e r E x a m p l ~ F i g . 14 s h o w s a f l o w f i e l d a r o u n d c i r c u l a r c y l i n d e r s s i d e - b y - s i d e , w h i c h is c o m p u t e d to i n v e s t i g a t e the b a s i c p e r f o r m a n c e o f the radiator. T h i s is the p r e s s u r e c o n t o u r m a p a n d the w h i t e r e g i o n s h a v e r e l a t i v e l y h i g h air speed. It is seen that the w a k e o f e a c h c y l i n d e r v a r i e s f r o m l o c a t i o n to l o c a t i o n . T h o u g h a w i d e w a k e n e i g h b o r s o n a n a r r o w one a n d vice v e r s a in this case, the w a k e p a t t e r n m a y c h a n g e d r a s t i c a l l y w h e n the s e p a r a t i o n b e t w e e n the c y l i n d e r s is i n c r e a s e d . Thus, it is e a s y to d e v e l o p the r a d i a t o r w i t h l o w resistance and h i g h h e a t e x c h a n g e e f f i c i e n c y .
Fig. 13
F l o w in e n g i n e cylinder (CFD)
Fig. 14
Flow around cylinders side-by-side (CFD)
428 4. CONCLUSIONS The application of computers to automobile aerodynamics is reviewed. One is C A F V and the other is CFD. Both are c o m m o n l y used in the whole range of automotive engineering, as follows. 1) Computer graphic display of experimental data is the typical CAFV. 2) Wake survey is popular and helps us improve aerodynamic characteristics. 3) Image processing is very promising, though the application is scarce today. 4) CT is useful when the structure of the spray is examined. 5) Panel method is suitable for three-dimensional attached flows. 6) Direct simulation method can treat the all flows related to automobiles. 7) CFD can be the tool to infer the aerodynamic characteristics. REFERENCES Ahmed, S. R. et al. 1977, "The Calculation of the Flow Field past a Van with the Aid of a Panel Method", SAE Paoer 770390. Carr, G. W. et al. 1986, " A e r o d y n a m i c Flow Visualization Techniques and Procedures", SAE Information Reoort. HSJ1566. Chometon, F 1982, "Calculating Three-Dimensional Separated Flow Around Road Vehicles", Int. J. of Vehicle Design, SP3, Impact of Aerodynamics on Vehicle Design, pp. 374-386. Cogotti, A. 1985, "Pininfarina - New Measurement Techniques in the Wind Tunnel", Proc. Autotechnolo~,ies Montecarlo 85. Cogotti, A. 1986, "Car-Wake Imaging Using a Seven-Hole Probe", S A E Paoer 860214. Cogotti, A. 1987, "Flow Field Surveys behind Three Squareback Car Models Using a Fourteen-Hole Probe", SAE Patmr 870243. Crowder, J. P. 1980, "Quick and t~asy Flow-Field Surveys", Astronautics and Aeronautics, Vol. 18, No. 10, pp. 38-39, 45. Demuren, A. O. et al. 1982, "Calculation of Three-Dimensional Turbulent Flow around Car Bodies", Svmn. on Vehicle Aerodynamics. Wolfsburg. Gosman, A . D . 1985, "Computer Modeling of Flow and Heat Transfer in Engines, Progress and Prospects", Proc. C O M O D I A 85, Tokyo, pp. 15-26. Hackett, J. E. et al. 1987, "On the Influence of Ground M o v e m e n t and Wheel Rotation in Tests on M o d e m Car Shapes", ~ 870245. Hammond, D. Jr. 1985, "Use of a Supercomputer in Aerodynamics Computations at General Motors Research Laboratories", ~ 850473. Hirschel, E. H. et al. 1982, "Theoretical and Experimental Boundary-Layer Studies on Car Bodies", Syrup. on Vehicle Aerodynamics, Wolfsburg. Hucho, W-. H. 1981, Aerodynamik des Automobils, Vogel-Verlag, Wurzburg, p. 355. Kawashima,K et al. 1988, "Front -End Air Flow Rate Simulation", ~ to be published. Kobayashi, T. et al. 1987, "Some Considerations on Automated Image Processing of Pathline Photographs", Flow Visualization IV, Hemisphere Publishing, pp. 241-246. Lawnsley, S. M. et al. 1986, "Application o f the P H O E N I C S Code to the Computation of the Flow Around Automobiles", ~ 860217. Nakayama, M e t al. 1985, "Visualization of Spray Structure by Means of Computed Tomography", Proc. C O M O D I A 85, Tokyo, pp. 131-139. Ogata, N. et al. 1987, "Nissan's Low-Noise Full-Scale Wind Tunnel", S A E Paper 870250. Stafford, L . G . 1982, "A High-Order Boundary Integral Equation Technique for the Computation of Vehicle Flow Fields", Int. J. of Vehicle Design, SP3, Impact of Aerodynamics on Vehicle Design, pp. 401-428. Willoughby, D. A. et al. 1985, "A Quasi-Three-Dimensional Computational Procedure for Prediction o f Turbulent Flow T h r o u g h the Front-End of Vehicles", ~ 850282.