The EngineeringResource ForAdvancing Mobility 400 COMMONWEALTH DRIVE WARRENDALE, PA 15096

850287

Full-Scale, On-Road Study of the Effect ofAutomobile Shape on its AerodynamicCharacteristics, and Comparison with

Small-Scale Wind Tunnel ResultsShimon Fanger-Vexler

and Joseph KatzFaculty of Mechanical Engineering

Automotive Program

Technion, I. I.T., Haifa, Israel

Ammon FouxDept. of Biomedical Engineering

Technion, I. I. T., Haifa, Israel

International Congress& Exposition

Detroit, MichiganFebruary 25 March 1, 1985

Systems Technical Support

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ISSN 0148 - 7191Copyright 1985 Society of Automotive Engineers, Inc.

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850287

Full-Scale, On-Road Study of the Effect ofAutomobile Shape on its AerodynamicCharacteristics, and Comparison with

Small-Scale Wind Tunnel Results

ABSTRACT

T h e d e s i g n o f p a s s e n g e r v e h i c l e s f o ri m p r o v e d 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 w i l l r e s u l ti n r e d u c e d f u e l c o n s u m p t i o n a n d b e t t e r r o a dh a n d l i n g d u r i n g h i g h - s p e e d d r i v i n g . I n t h i sr e s e a r c h , t e c h n i q u e s w e r e d e v e l o p e d t o m e a s u r et h e a e r o d y n a m i c d r a g a n d l i f t f o r c e s a c t i n g o n af u l l - s c a l e v e h i c l e u n d e r r o a d c o n d i t i o n s a n dt h e n w e r e c o m p a r e d w i t h r e s u l t s o b t a i n e d o nr e d u c e d - s c a l e m o d e l s i n a w i n d t u n n e l . A n u m b e ro f c o n f i g u r a t i o n s w h i c h c h a r a c t e r i z e c o m m o nv e h i c l e f o r m s w e r e i n v e s t i g a t e d f o r t h e i r e f f e c to n a e r o d y n a m i c e f f i c i e n c y a n d f u e l c o n s u m p -t i o n . E x p e r i m e n t a l s p e e d s w e r e b e t w e e n 7 0 a n d1 1 0 k m / h , t h e s e b e i n g r e p r e s e n t a t i v e o f h i g h w a yd r i v i n g c o n d i t i o n s .

A t y p i c a l p a s s e n g e r v e h i c l e o f t h e t h r e e -b o x t y p e w a s s e l e c t e d f o r t h e e x p e r i m e n t s , a n di t s e x t e r i o r f o r m w a s a l t e r e d b y m e a n s o fa t t a c h i n g v a r i o u s c o n f i g u r a t i o n s t o i t s f r o n t ,r e a r , a n d u n d e r b o d y p o r t i o n s . T h e s e a d d i t i o n st r a n s f o r m e d t h e o r i g i n a l v e h i c l e i n t o a f a s t -b a c k a n d " s t a t i o n w a g o n , " a n d w e r e u s e d i nc o m b i n a t i o n w i t h u n d e r b o d y a l t e r a t i o n s , s u c h a sf r o n t s p o i l e r , s i d e s k i r t s , a n d s m o o t h u n d e r -b o d y . D u r i n g r o a d e x p e r i m e n t s , d r a g f o r c e w a smeasu red by means o f a t e l eme t r i c sy s t em r ece iv -i n g d a t a o n d r i v e - s h a f t s t r a i n s , w h e r e a s l i f tf o r c e s w e r e m e a s u r e d b y r e l a t i v e v e r t i c a l d i s -p l a c e m e n t s i n t h e f r o n t a n d r e a r s u s p e n s i o n s .

S t a t i s t i c a l a n a l y s e s s h o w e d t h a t t h e d i f -f e r e n t c o n f i g u r a t i o n s h a d a s i g n i f i c a n t e f f e c to n t h e a e r o d y n a m i c f o r c e s . T h e c h a n g e i n c o n -f i g u r a t i o n s b r o u g h t a b o u t a m a x i m u m r e d u c t i o n i nd r a g c o e f f i c i e n t ( C D ) o f 5 1 % , r e l a t i v e t o t h eo r i g i n a l v e h i c l e . A s a r e s u l t , f u e l c o n s u m p t i o nw a s r e d u c e d b y 1 3 % ( a t 1 1 0 k m / h ) . L i f t f o r c e sd r o p p e d b y a s m u c h a 4 7 % . T h e m o s t e f f e c t i v ecomponents were a smooth underbody and a "fas t -

Shimon Fanger-Vexler Ammon Fouxand Joseph Katz

Faculty of Mechanical Engineering

Automotive Program

Technion, I. I. T., Haifa, lsrael

Technion, I. I. T., Haifa, Israel

b a c k " f o r m f o r d r a g , a n d a s m o o t h u n d e r b o d y a n df r o n t s p o i l e r f o r l o w l i f t . R e s u l t s o f t h e r o a dexpe r imen t s showed a r ea sonab l e co r r e l a t i on w i tht h o s e o b t a i n e d u s i n g r e d u c e d - s c a l e m o d e l s i n aw i n d t u n n e l .

INTRODUCTION

AUTOMOTIVE AERODYNAMICS has an important impacto n t h e h i g h - s p e e d s t a b i l i t y a n d f u e l e c o n o m y o fp a s s e n g e r c a r s ( 1 , 2 , 3 ) . * I t i s t h e r e f o r e i m p o r -t a n t , f r o m t h e a e r o d y n a m i c p o i n t o f v i e w , t oo p t i m i z e t h e v e h i c l e s s h a p e w h i l e i t i s s t i l li n t h e p r e l i m i n a r y d e s i g n s t a g e s . T h e m o s tc o m m o n , a n d p r o b a b l y t h e f a s t e s t , m e t h o d o fo b t a i n i n g t h e a e r o d y n a m i c c o e f f i c i e n t s f o rp r o d u c t i o n a u t o m o b i l e s i s v i a f u l l - s c a l e w i n dt u n n e l t e s t i n g . T h i s i s n e c e s s a r y b e c a u s ea n a l y t i c a l p r e d i c t i o n m e t h o d s f o r t h e c o m p l e xv i s c o u s f l o w a b o u t a p r a c t i c a l a u t o m o b i l e c o n -f i g u r a t i o n a r e s t i l l l i m i t e d ( 4 , 5 , 6 ) . I n a d d i -t i o n , w i n d t u n n e l t e s t i n g h a s t h e a d v a n t a g e o fc o n t r o l l e d a t m o s p h e r i c c o n d i t i o n s , F i x e d t e s t i n gf a c i l i t y , a n d ( f r e q u e n t l y ) a f a s t a n d r e l i a b l ed a t a - a c q u i s i t i o n s y s t e m . O n e o f t h e f e w d r a w -b a c k s o f f u l l - s c a l e w i n d t u n n e l s , a p a r t f r o mb e i n g q u i t e c o s t l y t o c o n s t r u c t , i s t h e a b s e n c eo f r e l a t i v e m o t i o n b e t w e e n t h e w i n d t u n n e l f l o o ra n d t h e t e s t e d o b j e c t . C o n s e q u e n t l y , t h e b o u n d -a r y l a y e r o n t h e f l o o r ( 7 ) b e n e a t h t h e m o d e l i sa c t u a l l y q u i t e d i f f e r e n t f r o m r e a l r o a d c o n d i -t i o n s ( 8 , 9 , 1 0 ) . M o v i n g w i n d t u n n e l f l o o ra r r a n g e m e n t s t o o v e r c o m e t h i s p r o b l e m h a v e b e e nt e s t e d i n s m a l l s c a l e ( 8 ) a n d d o p r o v i d e ap a r t i a l r e m e d y f o r s o m e o f t h e p r o b l e m s . T h eu s e o f t h i s m e t h o d i n f u l l - s c a l e t e s t s , h o w e v e r ,

* N u m b e r s i n p a r e n t h e s e s d e s i g n a t e r e f e r e n c e s a tend of paper.

0148-7191/85/0225-0287$02.50

Copyright 1985 Society of Automotive Engineers, Inc.

Dept. of Biomedical Engineering

SYSTEMS TECHNICAL SUPPORT

2 850287

i s b o t h c o m p l e x a n d e x p e n s i v e a n d t h e r e f o r e n o tu t i l i z e d .

F u l l - s c a l e r o a d t e s t i n g , o n t h e o t h e r h a n d ,m i g h t b e v e r y p r o m i s i n g . B u t a s a r e s u l t o f t h ea f o r e m e n t i o n e d d i f f i c u l t i e s ( d e p e n d e n c e o na t m o s p h e r i c c o n d i t i o n s a n d t h e r e q u i r e m e n t f o rm o b i l i t y o f e q u i p m e n t ) , t h e i r p r i m a r y i m p o r t a n c ei s i n p r o v i d i n g i n p u t f o r t h e d e v e l o p m e n t o fc o r r e c t i o n s , r e l a t i v e t o a c t u a l r o a d c o n d i t i o n s ,f o r w i n d t u n n e l f l o o r - b o u n d a r y - l a y e r a n d b l o c k -a g e e f f e c t s . M o s t o f t h e r o a d t e s t s r e p o r t e du p o n i n t h e l i t e r a t u r e f o r d e t e r m i n i n g t h ea e r o d y n a m i c c o e f f i c i e n t s , w e r e b a s e d o n t h ed e c e l e r a t i o n m e t h o d f o r o b t a i n i n g t h e d r a g f o r c e( 1 1 , 1 2 ) . T h e m e t h o d i s s u c h t h a t t h e t e s tv e h i c l e i s b r o u g h t t o a c e r t a i n i n i t i a l s p e e da n d t h e n t h e d r a g c o e f f i c i e n t i s d e t e r m i n e d f r o mr e c o r d s o f b o t h t h e r a t e o f d e c e l e r a t i o n a n d t h ed i s t a n c e t r a v e l e d b y t h e c o a s t i n g - d o w n v e h i c l e .A l t h o u g h t h e e x p e r i m e n t i n p r i n c i p l e i s s i m p l et o p e r f o r m , i t r e q u i r e s t h e e v a l u a t i o n o f i n e r -t i a l e f f e c t s . A s a r e s u l t , i t i s m o r e s e n s i t i v et o e x t e r n a l ( a t m o s p h e r i c ) d i s t u r b a n c e s t h a nw o u l d b e e x p e c t e d f r o m a c o n s t a n t - s p e e d t e s t ,a n d a l a r g e n u m b e r o f r e p e t i t i o n s f o r e a c h d a t ap o i n t a r e r e q u i r e d ( 1 2 ) . I t i s a l s o s u b j e c t t oc h a n g e s i n p i t c h a t t i t u d e , R e y n o l d s n u m b e r ,a i r s p e e d a n d d i r e c t i o n , a n d r o a d n o i s e d u r i n g at e s t . F u r t h e r m o r e , f o r t h e e x t r a c t i o n o f d r a gf r o m t h e t o t a l r e s i s t a n c e f o r c e , t h e t i r e -r o l l i n g r e s i s t a n c e n e e d s t o b e k n o w n a p r i o r i .

T h e p r i m a r y s c o p e o f t h e p r e s e n t r e s e a r c hw a s , t h e r e f o r e , t o d e v e l o p a m e t h o d f o r t h ec o n s t a n t - s p e e d a e r o d y n a m i c t e s t i n g o f a f u l l -s c a l e a u t o m o b i l e . D e t a i l s o n t h i s t e c h n i q u e a r eg i v e n i n t h e f o l l o w i n g t w o s e c t i o n s ( E x p e r i -m e n t a l S e t u p a n d V e h i c l e C o n f i g u r a t i o n s ) .

T h e s e c o n d o b j e c t i v e o f t h i s i n v e s t i g a t i o nw a s t o u t i l i z e t h i s m e t h o d f o r e x p l o r i n g t h ee f f ec t s o f some common geome t r i c a l changes i nm o d e l c o n f i g u r a t i o n o n t h e a e r o d y n a m i c c o e f f i -c i e n t s ; t h e s e r e s u l t s a r e p r e s e n t e d i n t h eR e s u l t s s e c t i o n .

T h e t h i r d o b j e c t i v e w a s t o c o m p a r e t h er e s u l t s o b t a i n e d b y t h i s m e t h o d w i t h t h e r e s u l t so f s m a l l - s c a l e w i n d t u n n e l t e s t i n g a n d f u l l -s c a l e t o w t e s t s . T h e o u t c o m e o f t h i s c o m p a r i s o nf o r s i x r e p r e s e n t a t i v e a u t o m o b i l e s h a p e s i sp r e s e n t e d i n t h e C o m p a r i s o n s e c t i o n .

EXPERIMENTAL SETUP

F o r t h e e x p e r i m e n t s , a t y p i c a l " t h r e e - b o xsedan" au tomob i l e was s e l ec t ed (Ope l Reco rd1 7 0 0 , m o d e l 1 9 7 3 , w i t h a n a u t o m a t i c t h r e e - s p e e dt r a n s m i s s i o n ) h a v i n g a n o v e r a l l l e n g t h o f4 . 5 6 7 m , a w i d t h o f 1 . 7 1 8 m , a h e i g h t o f1 . 4 0 5 m , a n d a f r o n t a l a r e a o f 1 . 8 1 3 m 2 ( f o r t h eb a s e l i n e c a r ) . T h e m e a s u r e d v a r i a b l e s i n c l u d e d

t h e a e r o d y n a m i c d r a g , t h e l i f t o f t h e f r o n t a n dr e a r a x l e s , a n d f u e l c o n s u m p t i o n . D r a g f o r c ew a s m e a s u r e d w i t h f u l l - b r i d g e s t r a i n g a u g e sl o c a t e d o n t h e v e h i c l e ' s d r i v e s h a f t ( F i g . 1 ) .F r o n t l i f t w a s m e a s u r e d b y t w o s e t s o f s t r a i ng a u g e s t h a t w e r e g l u e d t o t h e c o i l s p r i n g s o ft h e f r o n t s u s p e n s i o n a t a h a l f - b r i d g e c o n f i g u r a -t i o n o r i e n t e d a t 4 5 t o t h e c o i l a x i s . T h e r e a rl i f t w a s m e a s u r e d b y a l e a f s p r i n g d e v i c e w h i c hr e a c t e d t o t h e r e l a t i v e d i s p l a c e m e n t b e t w e e n t h ev e h i c l e a n d i t s r e a r a x l e ( F i g . 1 ) . I n t h i sc a s e a l s o , t w o s e t s o f s t r a i n g a u g e s , o n e a te a c h s i d e o f t h e l e a f s p r i n g , w e r e u s e d t om e a s u r e t h e r e a r b o d y d i s p l a c e m e n t ( w h i c h w a sd i r e c t l y c a l i b r a t e d t o m e a s u r e t h e a e r o d y n a m i cl i f t a t t h e r e a r a x l e ) . T h e r e s u l t s o b t a i n e df o r t h e l i f t f o r c e d o n o t i n c l u d e t h e l i f t o ft h e w h e e l s . T h e d i f f e r e n c e s i n t h e l i f t c o e f f i -c i e n t s c a u s e d b y t h e v a r i o u s v e h i c l e g e o m e t r i e sm u s t b e c o n s i d e r e d , t h e r e f o r e , o n l y i n a r e l a -t i v e s e n s e .

The measurement of the drag force wast r a n s m i t t e d f r o m t h e v e h i c l e s d r i v e s h a f t b yt h e s y s t e m s h o w n i n F i g . 1 ( i t e m s 4 , 5 , a n d 6 ) .B o t h t h e i n c o m i n g e l e c t r o m a g n e t i c c u r r e n t a n dt h e o u t g o i n g s t r a i n g a u g e i n f o r m a t i o n w e r et r a n s m i t t e d b y t h e f i x e d a n t e n n a ( F i g . 1 ,i t e m 4 ) t o t h e a m p l i f y i n g p r o c e s s o r ( A c u r e x ,Tors ion Measurement System, Model 1206) . Thet o t a l r e s i s t i n g f o r c e ( d r a g a n d w h e e l s r o l l i n gr e s i s t a n c e ) w a s d i r e c t l y o b t a i n e d b y m e a s u r i n gt h e s t r a i n ( d u e t o t o r q u e ) o n t h e d r i v e s h a f t .T h e d r i v e - s h a f t s t r a i n w a s c a l i b r a t e d t o t h et o t a l f o r c e , a c t i n g o n t h e v e h i c l e b o d y t h r o u g hi t s c e n t e r o f g r a v i t y , a s s h o w n i n F i g . 2 . T h ec a l i b r a t i o n l i n e i n F i g . 2 w a s o b t a i n e d b yr e p e t i t i v e a t t e m p t s t o d r i v e t h e v e h i c l e a g a i n s ta n a n c h o r p o s t ( w i t h o u t w h e e l s l i p p a g e ) a td i f f e r e n t e n g i n e t o r q u e l e v e l s , w h i l e r e c o r d i n gd r i v e - s h a f t s t r a i n a n d l o a d - c e l l t e n s i o n s i m u l -t a n e o u s l y . T h e r e s u l t s o f t h i s c a l i b r a t i o n t e s ti n d i c a t e d a h i g h l y l i n e a r r e g r e s s i o n c o r r e l a t i o nb e t w e e n t h e t o t a l f o r c e a n d d r i v e - s h a f t s t r a i n .

T h e s t a t i c c a l i b r a t i o n o f t h e l i f t f o r c e sw a s o b t a i n e d b y l o a d i n g a n d u n l o a d i n g t h e f r o n ta n d r e a r a x l e s w i t h e x t e r n a l w e i g h t s ( i n t h er a n g e o f l 0 0 k g , f r o n t a n d r e a r ) a n d t h e nr e c o r d i n g t h e c o r r e s p o n d i n g s t r a i n s . T o m i n i -m i z e t h e h y s t e r e s i s d u e t o t h e s h o c k a b s o r b e r so f t h e s u s p e n s i o n , s m a l l - a m p l i t u d e v e r t i c a lo s c i l l a t i o n s w e r e f o r c e d o n t h e v e h i c l e s b o d ya t e a c h d a t a p o i n t . I n t h i s c a s e o f t h e l i f tf o r c e t o o , h i g h l y l i n e a r c a l i b r a t i o n c u r v e s w e r eo b t a i n e d . M o r e c o m p l i c a t e d e f f e c t s d u e t od r i v e - t r a i n t o r q u e , s u c h a s c h a s s i s r o l l a n da t t i t u d e c h a n g e s d u r i n g t h e r o a d t e s t s , w e r e n o ti n v e s t i g a t e d h e r e ( b u t h t h e s e d a t a w i l l b en e e d e d f o r f u t u r e c o m p a r i s o n w i t h f u l l - s c a l ew i n d t u n n e l e x p e r i m e n t s ) .

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F i g . 1 . V e h i c l e d r a g a n d l i f t m e a s u r i n g s y s t e m : ( 1 ) v e h i c l e b o d y , ( 2 ) l e a f s p r i n g f o r m e a s u r i n g t h er e a r l i f t , ( 3 ) a t t a c h m e n t o f l e a f s p r i n g t o d i f f e r e n t i a l h o u s i n g ( l o n g i t u d i n a l l y i s f r e e t o s l i d e ) ,( 4 ) f i x e d a n t e n n a , ( 5 ) r o t a t i n g t r a n s m i t t e r o f d r i v e s h a f t s t r a i n s , ( 6 ) s i g n a l r e c e i v e r a n dp r e p r o c e s s o r , ( 7 ) s t r a i n g a u g e s f o r f r o n t l i f t m e a s u r e m e n t s ( o n b o t h s i d e s )

F i g . 2 . C a l i b r a t i o n m e t h o d a n d g r a p h f o r d r a g f o r c e : ( 1 ) a n c h o r a g e , ( 2 ) c a b l e ( p o i n t i n g a tv e h i c l e s c e n t e r o f g r a v i t y ) , ( 3 ) l o a d c e l l , ( 4 ) t o w p o i n t s

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T h r o u g h o u t t h e t e s t a l l d a t a w e r e r e c o r d e db y a c a s s e t t e d a t a r e c o r d e r ( K y o w a ,model RTP-501A/AL) which was located within thev e h i c l e .

T i r e s w e r e i n f l a t e d t o 3 0 p s i i n o r d e r t om i n i m i z e t h e i r r o l l i n g r e s i s t a n c e . T h e r o l l i n gr e s i s t a n c e o f e a c h t i r e w a s m e a s u r e d , o f f t h ev e h i c l e , on a commercial r ig (made by WerkNurnberg M.A.N.) . T h i s r i g i n c l u d e d a h o r i z o n -t a l l y r o t a t i n g c y l i n d e r w i t h a v e r y l a r g e d i a m -e t e r a n d s u r f a c e r o u g h n e s s s i m i l a r t o t h e c o n -c r e t e s u r f a c e o f t h e r u n w a y t h a t s e r v e d f o r t h er o a d t e s t s . I n t h e s e m e a s u r e m e n t s , t h e w h e e l sw e r e d r i v e n b y t h e r o t a t i n g c y l i n d e r , a n d b o t ht e s t s p e e d s a n d t i r e n o r m a l l o a d w e r e k e p t t h es a m e a s t h e y w e r e d u r i n g t h e r o a d t e s t s .B e c a u s e o f n u m e r o u s d i s c o n t i n u a t i o n s i n b o t h t h er o l l i n g r e s i s t a n c e a n d t h e r o a d t e s t , t h e l o a do n t h e t i r e s w a s s m a l l a n d n o t e m p e r a t u r e -r e l a t e d c o r r e c t i o n s w e r e m a d e t o t h e t i r er e s i s t a n c e .

T h e r e s u l t s f o r t h e d r a g f o r c e o b t a i n e d b yt h i s m e t h o d d o n o t i n c l u d e t h e d y n a m i c f r i c t i o nl o s s e s d u e t o t h e r o t a t i o n o f t h e d i f f e r e n t i a la s s e m b l y a n d t h e w h e e l b e a r i n g s . The magni tudeo f t h i s r e s i s t a n c e w a s m e a s u r e d i n s e p a r a t et o w i n g t e s t s ( i t s v a r i a t i o n t h r o u g h t h e t e s t w a sl e s s t h a n 0 . 5 k g , w h i c h i s a b o u t 0 . 7 % o f t h ed r a g a t 1 0 0 k m / h ) . T h i s r e s i s t a n c e , t o g e t h e rw i t h t h e t i r e r o l l i n g r e s i s t a n c e w a s l a t e rs u b t r a c t e d i n t h e p r o c e s s o f c o m p u t i n g t h eae rodynamic d rag .

The fuel-f low measurements were made by am a g n e t i c , p o s i t i v e - d i s p l a c e m e n t f l o w m e t e r(Brooks, model LS4150) . T h e v e h i c l e s p e e d o m e t e rw a s a l s o c a l i b r a t e d b y m e a n s o f a r a d a r s p e e dg u n ( C . M . I . I n c . , m o d e l J F l 0 0 ) .

VEHICLE CONFIGURATIONS AND TEST CONDITIONS

T h e m e a s u r i n g e q u i p m e n t , d e s c r i b e d i n t h ef o r m e r s e c t i o n , w a s u s e d t o m e a s u r e t h e v a r i o u sp a r a m e t e r s o f 1 6 v e h i c l e c o n f i g u r a t i o n s ( a l ls h a r i n g t h e s a m e r u n n i n g g e a r ) . T h e s e c o n f i g u r -a t i o n s w e r e o b t a i n e d b y a t t a c h i n g e i g h t d i f f e r -e n t c o m p o n e n t s ( a s s h o w n i n F i g s . 3 and 4 ) i nv a r i o u s c o m b i n a t i o n s t o t h e f r o n t , r e a r , a n dunde rbody po r t i ons :

1. Wedge-shaped front hood (which was madeo f p o l y u r e t h a n e f o a m c o v e r e d b y a f i b e r g l a s ss k i n ) . M o d i f i c a t i o n N o . 1 i n F i g . 3 .

2 . F a s t b a c k , M o d . N o . 2 .3. Sta t ion wagon, Mod. No. 3 .4. S m o o t h s h e e t m e t a l f l o o r ( r e a r s e c t i o n

o n l y ) , Mod. No. 4.5 . S m o o t h s h e e t m e t a l f l o o r ( c e n t r a l

s e c t i o n o n l y ) , M o d . N o . 5 .6 . S m o o t h s h e e t m e t a l f l o o r ( f r o n t s e c t i o n

o n l y ) , Mod. No. 6 ( s h o w n a l s o i n F i g . 4 ) .

F i g . 3 . C o n f i g u r a t i o n a l t e r a t i o n s f r o m t h eo r i g i n a l v e h i c l e : (1) wedge-shaped hood,( 2 ) f a s t b a c k , ( 3 ) s q u a r e b a c k , ( 4 , 5 ,and 6) smooth underbody panel e lements

F i g . 4 . O r i g i n a l v e h i c l e w i t h s i d e s k i r t s ( l ) ,f r o n t s p o i l e r s ( 2 ) , a n d s m o o t h u n d e r b o d y ( 3 )

7 . S i d e s k i r t s ( F i g . 4 ) .8 . F r o n t s p o i l e r ( F i g . 4 ) .

W i t h t h e s e m o d i f i c a t i o n s t h e 1 6 t e s t c o n f i g u r a -t i o n s s h o w n i n F i g . 5 w e r e o b t a i n e d .

T h e r o a d t e s t s w e r e c a r r i e d o u t o n a3 0 0 0 - m - l o n g , p l a n a r , smoo th a i r f i e ld runway .T h e r u n w a y w a s o r i e n t e d s u c h t h a t w i n d d i r e c t i o nc o i n c i d e d w i t h t h e p r e v a i l i n g w i n d . Since winde f f e c t d u r i n g s u c h e x p e r i m e n t s i s v e r y i m p o r t a n t( 1 3 ) , t h e d a t a u s u a l l y w e r e r e c o r d e d w i t h w i n dve loc i t y be low 0 .5 m/ s and neve r exceed ing av e l o c i t y o f 1 . 5 m / s . E a c h c o n f i g u r a t i o n w a st e s t e d a t a c o n s t a n t s p e e d f o r b o t h d i r e c t i o n so n t h e r u n w a y a t s p e e d s o f 7 0 - 1 1 0 k m / h a tl 0 - k m / h i n c r e m e n t s . D a t a w e r e o b t a i n e d f o r e a c hc o n f i g u r a t i o n a n d s p e e d b y a c o n t i n u o u s r e c o r d -i n g f o r 4 5 - 6 0 s . T h e s e d a t a p o i n t s w e r e t h e ndecoded using an X-Y recorder (Yokogawa 3078)u n d e r l a b o r a t o r y c o n d i t i o n s , t h u s p r o d u c i n gg r a p h s a s e x e m p l i f i e d b y F i g . 6 .

T h e c o n t i n u o u s d r a g a n d l i f t r e s u l t s w e r ec o n v e r t e d t o p o i n t s a m p l e s f o r s t a t i s t i c a lp u r p o s e s b y d e t e r m i n i n g f o r c e a t l - s i n t e r v a l sf o r e a c h g r a p h . T h u s a p p r o x i m a t e l y 5 0 d a t ap o i n t s w e r e d e t e r m i n e d f o r e a c h c o n f i g u r a t i o na n d s p e e d t e s t e d . R e s u l t s w e r e s t a t i s t i c a l l ya na l y z e d i n o r d e r t o o b t a i n t h e m e a n ( x ) , s t a n -d a r d d e v i a t i o n ( s ) , a n d t h e c o e f f i c i e n t o fv a r i a t i o n ( C V ) . T h e a v e r a g e c o e f f i c i e n t o f

SYSTEMS TECHNICAL SUPPORT

850287

Fig. 5. Definition of configurations investigated and their numerical label

Fig. 6 Example of t ime-dependent l if t and drag recordings

5

SYSTEMS TECHNICAL SUPPORT

SYSTEMS TECHNICAL SUPPORT

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v a r i a t i o n w a s l o w ( 5 % ) f o r t h e d r a g f o r c e ,w h e r e a s f o r t h e l i f t i t w a s l a r g e r ( u p t o 6 0 % )b e c a u s e o f t h e h i g h n o i s e - t o - s i g n a l r a t i o c a u s e db y s u s p e n s i o n o s c i l l a t i o n s . A l t h o u g h t h e o s c i l -l a t i o n s f o r l i f t f o r c e w e r e l a r g e , t h e y w e r ec e n t e r e d a b o u t a c o n s t a n t h o r i z o n t a l l i n e ;t h e r e f o r e t h e d i f f e r e n c e s i n t h e s e m e a n v a l u e sa s a r e s u l t o f c o n f i g u r a t i o n a n d s p e e d s h o w e dg o o d c o m p a r a t i v e r e s u l t s . I n o r d e r t o d e t e r m i n et h e s t a t i s t i c a l s i g n i f i c a n c e o f e a c h c o n f i g u r a -t i o n a l m o d i f i c a t i o n , a n u m b e r o f a n a l y s e s o fv a r i a n c e w e r e p e r f o r m e d . E a c h m o d i f i c a t i o n w a sa n a l y z e d i n c o m p a r i s o n t o t h e b a s i c c a r a n dfa s tback mode l s . S e p a r a t i o n o f t h e m e a n s w a sd e t e r m i n e d b y u s e o f a D u n c a n s m u l t i p l e r a n g et e s t ( 1 4 ) . T h i s i n f o r m a t i o n i s g i v e n i n m o r ed e t a i l i n R e f . 1 5 .

RESULTS

A t y p i c a l s e t o f e x p e r i m e n t a l r e s i s t i v ef o r c e r e s u l t s , v e r s u s t h e v e h i c l e s s p e e d i ss h o w n i n F i g . 7 . T h e t o t a l r e s i s t a n c e , a si n d i c a t e d , i n c l u d e s b o t h t h e a e r o d y n a m i c d r a ga n d t h e r o l l i n g r e s i s t a n c e o f t h e t i r e s . Thet i r e s ' r o l l i n g r e s i s t a n c e ( c o r r e c t e d f o r a c t u a lr o a d c o n d i t i o n s ; e . g . , b e a r i n g f r i c t i o n , l i f t ,e t c . ) s h o w s o n l y a s m a l l i n c r e m e n t w i t h i n c r e a s -i n g s p e e d .

T h e d r a g f o r c e r e s u l t s o f F i g . 7 c l e a r l yi n d i c a t e t h e p a r a b o l i c d e p e n d e n c e o f t h e a e r o d y -n a m i c d r a g o n t h e o n s e t a i r s p e e d . The sedan,w i t h i t s f o u r w i n d o w s o p e n ( c o n f i g u r a t i o n 1 * )

h a d t h e h i g h e s t d r a g , w h i l e t h e f a s t b a c k s h a p e( c o n f i g u r a t i o n 2 ) h a d f a r l e s s a i r r e s i s t a n c e .T h i s w a s s o s i n c e t h e h a t c h i n c l i n a t i o n a n g l e o fa b o u t 2 0 a l l o w e d a n a t t a c h e d f l o w r e g i m e a l o n gt h e r e a r w i n d o w ( o b s e r v e d b y m e a n s o f t u f t s f l o wv i s u a l i z a t i o n ) a n d t h e r e b y r e d u c e d d r a g w a sm e a s u r e d ( 1 6 , 1 7 , 1 8 ) . F u r t h e r m o r e , t h e c o m p l e xu n s t e a d y s e p a r a t i o n p a t t e r n b e h i n d t h e s e d a ncon f igu ra t i on i n t roduced h ighe r a e rodynamicn o i s e i n t o t h e r e s i s t a n c e f o r c e d a t a , c o m p a r e dt o c o n f i g u r a t i o n s 2 a n d 3 . T h i s w a s t h e r e a s o nt h a t t h e l e a s t - s q u a r e c u r v e f i t t i n g , s h o w n i nF i g . 7 , i m p r o v e d w i t h t h e i m p r o v e m e n t s i n t h eveh ic l e ae rodynamics .

T h e d r a g o f t h e s t a t i o n w a g o n ( c o n f i g u r a -t i o n 3 ) i n t h i s t e s t w a s f o u n d t o b e l o w e r t h a nt h e d r a g o f t h e b a s e l i n e c a r ( c o n f i g u r a t i o n l ) ,b u t i t w a s h i g h e r t h a n t h e d r a g o f t h e f a s t b a c ks h a p e . T h i s i s c o n s i s t e n t w i t h t h e f i n d i n g sr e p o r t e d b y o t h e r r e s e a r c h e r s ( e . g . , R e f . 1 9 ) .T h e d r a g c o e f f i c i e n t ( C D = D / 0 . 5 V

2 A ) , b a s e d o nt h e f r o n t a l a r e a A a n d w i n d s p e e d V , f o r e a c hv e h i c l e ' s c o n f i g u r a t i o n w a s o b t a i n e d a f t e rs u b t r a c t i n g t h e t i r e s ' r o l l i n g r e s i s t a n c e f r o mt h e t o t a l r e s i s t a n c e , w h i c h i s s h o w n i n F i g . 7 .T h e c o l l e c t i v e r e s u l t s f o r t h e d r a g c o e f f i c i e n t s( b a s e d o n t h e a v e r a g e o f a l l s p e e d t e s t s ) a r er e p r e s e n t e d b y t h e b a r d i a g r a m s o f F i g . 8 . I n as i m i l a r m a n n e r t h e l i f t v a r i a t i o n s w i t h t h ev e h i c l e s s p e e d w e r e m e a s u r e d , a n d a s a n e x a m p l et h e t e s t r e s u l t s f o r c o n f i g u r a t i o n 1 a r e p r e -s e n t e d i n F i g . 9 . H e r e L t o t r e p r e s e n t s t h et o t a l l i f t o f b o t h t h e f r o n t a n d r e a r a x l e s ,

F i g . 7 . V a r i a t i o n o f t o t a l d r a g f o r c e w i t h v e h i c l e s p e e d , a s o b t a i n e d b y d i r e c t r o a d t e s t s

850287

F i g . 8 . D iag ram o f compara t i ve d r ag coe f f i c i en t s

w h i l e L f i s t h e f r o n t - a n d Lr , i s t h e r e a r -a x l e l i f t c o m p o n e n t , r e s p e c t i v e l y . M o s t o f t h em e a s u r e d l i f t i s g e n e r a t e d a t t h e v e h i c l e sl e a d i n g e d g e , a n d t h e r e f o r e t h e l i f t a t t h ef r o n t a x l e ( w i t h o u t c o r r e c t i o n d u e t o d r i v e -t o r q u e - i n d u c e d c h a s s i s p i t c h u p ) i s c o n s i d e r a b l yh i g h e r t h a n t h e l i f t a t t h e r e a r a x 1 e . T h i s i sc o n s i s t e n t w i t h t h e h i g h s u c t i o n v a l u e s s h o w n b yt h e p r e s s u r e - d i s t r i b u t i o n d a t a p r o v i d e d i nR e f s . 19 and 20. T h i s s i t u a t i o n , t h a t m o s t o ft h e l i f t w a s g e n e r a t e d a t t h e f r o n t , w a s c o n s i s -t e n t f o r a l l t h e c o n f i g u r a t i o n s t e s t e d .

I n t h e s e e x p e r i m e n t s t h e l i f t w a s n o tm e a s u r e d f o r a l l t h e c o n f i g u r a t i o n s , b u t r a t h e ro n l y f o r t h o s e m o d i f i c a t i o n s w i t h a m a j o r e f f e c to n a e r o d y n a m i c l i f t f o r c e s ( j u d g e d o n g r o u n d s o fp r e v i o u s s m a l l - s c a l e w i n d t u n n e l t e s t s ( 2 1 ) ) .T h e l i f t - c o e f f i c i e n t r e s u l t s f o r t h e s e s i x t e s t sa r e shown g raph ica l l y by t he ba r d i ag rams o fF i g . 1 0 .

F i g . 9 . T h e e f f e c t o f v e h i c l e s p e e d o n l i f t( L r - r e a r a x l e , L f - f r o n t a x l e , L t o t - f r o n ta n d r e a r a x l e s )

B a s e d o n t h e d a t a p r e s e n t e d i n F i g s . 7 - 1 0 ,s e v e r a l c o n c l u s i o n s c a n b e d r a w n a b o u t t h ei n f l u e n c e o f t h i s v e h i c l e ' s e x t e r n a l g e o m e t r y o ni t s a e r o d y n a m i c d r a g a n d l i f t :

7

8 850287

F i g . 1 0 . D i a g r a m o f c o m p a r a t i v e l i f tc o e f f i c i e n t s

1. T h e e f f e c t o f t h e m o d i f i e d f r o n t ( w e d g es h a p e ) c a n b e e s t i m a t e d b y c o m p a r i n g t h e d r a gc o e f f i c i e n t s o f c o n f i g u r a t i o n s 1 a n d 3 w i t h t h ed r a g o f c o n f i g u r a t i o n s 4 a n d 6 , r e s p e c t i v e l y( w h i c h a r e e s s e n t i a l l y t h e s a m e c o n f i g u r a t i o n s ,b u t w i t h a m o d i f i e d w e d g e - s h a p e d f r o n t ) . I t i se v i d e n t t h a t t h e m o d i f i e d f r o n t r e d u c e d t h ed r a g . C o n s e q u e n t l y , t h e m o s t a e r o d y n a m i c c o n -F i g u r a t i o n a m o n g m o d e l s 1 t o 6 , w a s m o d e l 5 ,h a v i n g t h e w e d g e - s h a p e d f r o n t a n d f a s t b a c km o d i f i c a t i o n s . T h e r e d u c t i o n i n t h e a e r o d y n a m i cd r a g c o e f f i c i e n t d u e t o t h e s e m o d i f i c a t i o n s i s39%, as shown by Fig. 8 .

2 . T h e e f f e c t o f a s m o o t h u n d e r b o d y f a i r -i n g i s d e m o n s t r a t e d w h e n c o m p a r i n g t h e d r a g o ft h e o r i g i n a l s e d a n c a r ( c o n f i g u r a t i o n 1 ) w i t ht h e d r a g o f c o n f i g u r a t i o n 7 . H e r e t h e a d d i t i o no f t h e u n d e r b o d y f a i r i n g , b e t w e e n t h e c o o l i n gi n t a k e a n d t h e f r o n t a x l e ( s e e d e t a i l s i nF i g . 3 ) , r e s u l t e d i n a d r a g r e d u c t i o n o f 1 5 % .T h e e x t e n s i o n o f t h e f a i r i n g u p t o t h e r e a r a x l e( c o n f i g u r a t i o n 8 ) f u r t h e r r e d u c e d t h e d r a g ,w h e r e a s t h e c o m p l e t e f a i r i n g ( c o n f i g u r a t i o n 1 6 )d i d n o t r e s u l t i n a n y a d d i t i o n a l d r a g r e d u c -t i o n . T h i s i s p r o b a b l y d u e t o t h e s e p a r a t e df l o w , i n d i c a t e d b y t h e t u f t s m o t i o n i n t h a tr e g i o n . ( B u c h h e i m e t a l . i n R e f . 1 d i d f i n dt h a t a l o n g e r d i f f u s e r s e c t i o n a t t h i s r e g i o nr e d u c e d t h e v e h i c l e ' s d r a g . )

A s i m i l a r t r e n d w a s f o u n d w h e n t h e s e l o w e rp a n e l m o d i f i c a t i o n s w e r e a d a p t e d t o t h e a l r e a d ya e r o d y n a m i c a l l y e f f i c i e n t c o n f i g u r a t i o n 5 . Thea d d i t i o n o f t h e s m o o t h u n d e r b o d y p a n e l , e x t e n d e dt o t h e r e a r a x l e ( c o n f i g u r a t i o n 9 ) r e s u l t e d i n ad r a g r e d u c t i o n o f D C D = 0 . 0 5 6 , w h i c h w a s n o tf u r t h e r i m p r o v e d b y f u l l e x t e n s i o n o f t h e l o w e rf a i r i n g ( c o n f i g u r a t i o n 1 3 ) . A c t u a l l y t h e s e t w oc o n f i g u r a t i o n s ( 9 a n d 1 3 ) w e r e t h e m o s t a e r o d y -n a m i c t h r o u g h o u t t h e e x p e r i m e n t , w i t h a d r a gc o e f f i c i e n t o f C D = 0 . 2 3 1 , w h i c h i s 5 1 % l o w e rt h a n t h e a e r o d y n a m i c d r a g o f t h e b a s e l i n ev e h i c l e .

3. T h e e f f e c t o f t h e s e m o d i f i c a t i o n s o nt h e c o o l i n g a i r f l o w w a s n o t t h o r o u g h l y i n v e s t i -g a t e d i n t h i s s t u d y , and t he on ly conce rn i nt h a t r e g a r d w a s t h a t e n g i n e o v e r h e a t i n g s h o u l db e a v o i d e d ( d u r i n g t h e r o a d t e s t s , w i t h a m b i e n tt e m p e r a t u r e s o f 1 8 - 2 2 C ) . I t i s w o r t h w h i l e t op o i n t o u t t h a t c o o l i n g d r a g l o s s e s c a n b ereduced cons ide rab ly by ca re fu l ae rodynamicd e s i g n a s i t w a s s h o w n f o r l i g h t a i r c r a f t e n g i n ei n s t a l l a t i o n s ( 2 2 , 2 3 ) . T h e r e f o r e , a l a r g ep o r t i o n o f t h e d r a g - r e d u c t i o n p o t e n t i a l ( i f n o ta l l ) o f t h e m o d i f i c a t i o n s t e s t e d h e r e c a n f u l l yb e e x p l o i t e d .

O n e o f t h e p r o b l e m s t h a t c a n b e i n t r o d u c e db y a c o n t i n u o u s s m o o t h u n d e r b o d y p a n e l ( a p a r tf r o m a d d i t i o n a l w e i g h t ) i s t h e b l o c k a g e o f t h ee n g i n e - c o o l i n g a i r f l o w . T h e s i m p l e s t s o l u t i o nt h a t w a s t e s t e d i s t o d u c t t h e c o o l i n g a i rb e n e a t h t h e v e h i c l e ( a s i n c o n f i g u r a t i o n s 1 1a n d 1 2 ) a n d e j e c t i t i n t o t h e w a k e f l o w . Th i swas accompl i shed by cove r ing t he l ower su r f aceo f t h e c a r w i t h t h e f a i r i n g ( m o d i f i c a t i o n 5 i nF i g . 3 ) , and by a sma l l r ea rward d i sp l acemen t o ft h e r e a r b u m p e r , a s s h o w n i n F i g . 1 1 . I t w a sf o u n d t h a t s u f f i c i e n t a i r f l o w ( b a s e d o n s t a t i c

F i g . 1 1 . G e o m e t r y o f t h e a i r f l o w d u c t i n gb e n e a t h t h e v e h i c l e : ( 1 ) r e a r s e c t i o n o f d u c t ,( 2 ) s m o o t h u n d e r b o d y p a n e l , ( 3 ) o u t c o m i n ga i r f l o w

CONFIGURATION NUMBERS

9

p r e s s u r e d i f f e r e n c e ) w a s o b t a i n e d i n t h i s m a n n e rs o t h a t e n g i n e c o o l i n g c o u l d b e r e s o l v e d w i t h acomp le te underbody smooth pane l . The compar isono f t h e d r a g o f c o n f i g u r a t i o n s 1 1 a n d 1 2 ( h a v i n ga smooth underbody) w i th the d rag o f con f igu ra -t i o n s 5 a n d 2 , r e s p e c t i v e l y , s h o w t h a t t h e d r a g -reduc t ion e f fec t o f the smooth underbody wasp a r t i a l l y l o s t . I m p r o v e d c o o l i n g - a i r i n l e t a n de x i t , and smoother duc t ing (22 ,23) p robab ly w i l li nc rease the aerodynamic bene f i t s due to smoothu n d e r b o d y f a i r i n g s .

4 . T h e e f f e c t o f t h e u n d e r b o d y f a i r i n g c a nb e o b t a i n e d b y c o m p a r i n g t h e r e s u l t s o f t h esedan (con f igu ra t ion 1 ) and the same shape w i ththe smooth underbody pane l ( con f igu ra t ion 16 ) .The da ta o f F ig . 10 show a 12% reduc t i on o f t het o t a l l i f t d u e t o t h i s m o d i f i c a t i o n . T h e c o m b i -na t ion o f the lowes t d rag shape w i th the smoothu n d e r b o d y f a i r i n g ( c o n f i g u r a t i o n 1 3 ) h a s f u r t h e rr e d u c e d t h e l i f t t o a v a l u e w h i c h i s l o w e r b y3 7 % t h a n t h e l i f t o f t h e b a s e l i n e c a r ( s e eF i g . 1 0 ) .

5 . T h e e f f e c t s o f s o m e l i f t - r e d u c i n gm o d i f i c a t i o n s o n v e h i c l e d r a g c a n b e o b t a i n e d b yc o m p a r i n g t h e r e s u l t s o f c o n f i g u r a t i o n s 1 0 , 1 4 ,a n d 1 5 . T h e a d d i t i o n o f a f r o n t s p o i l e r ( s e eF i g . 4 ) t o c o n f i g u r a t i o n 1 1 ( w h i c h i s a d i r e c tm o d i f i c a t i o n t o c o n f i g u r a t i o n 1 0 ) r e s u l t e d i n a ni n c r e a s e o f t h e d r a g b y DCD = 0 . 0 5 4 . A s i m i l a rchange in drag (DCD = 0.048) was measured whent h e s p o i l e r w a s a d d e d t o t h e b a s e l i n e s h a p e w i t hthe smooth underbody pane l ( con f igu ra t ion 16 ) too b t a i n c o n f i g u r a t i o n 1 5 . T h e l i f t - r e d u c i n ge f f e c t o f t h e f r o n t s p o i l e r w a s e f f e c t i v e , a sc a n b e d e d u c e d f r o m F i g . 1 0 . A c t u a l l y , t h espo i le r and the smooth underbody in con f igu ra -t i o n 1 0 r e s u l t e d i n a 4 7 % l i f t r e d u c t i o n r e l a -t i v e t o t h e b a s e l i n e c o n f i g u r a t i o n . T h e n e te f f e c t o f t h e f r o n t s p o i l e r o n t h e l i f t c a n a l s ob e e s t i m a t e d b y c o m p a r i n g t h e l i f t o f c o n f i g u r a -t i o n s 1 5 a n d 1 6 , w h i c h s h o w e d a l i f t d i f f e r e n c eo f DCL = 0 . 0 5 1 . S i m i l a r l i f t - r e d u c i n g e f f e c t ,f o r t h e f r o n t s p o i l e r , w e r e r e p o r t e d i n R e f s . 1 6a n d 2 0 . B y f u r t h e r o p t i m i z a t i o n o f t h e s p o i l e rg e o m e t r y , d r a g r e d u c t i o n w a s a l s o o b t a i n e d .

T h e e f f e c t o f t h e s i d e s k i r t s ( s e e F i g . 4 )o n d r a g i s n e g l i g i b l e . T h i s c a n b e o b s e r v e dwhen compar ing the resu l t s o f con f igu ra t i on 14i n F i g . 8 w i t h t h e r e s u l t s o f c o n f i g u r a t i o n 1 6 .T h e r e d u c t i o n o f t h e l i f t o b t a i n e d b y t h i sdev ice i s abou t DCL = 0 .032 , as shown inF i g . 1 0 .

I n s u m m a r y , t h e b e s t c o n f i g u r a t i o n s , w i t hr e s p e c t t o d r a g , w e r e c o n f i g u r a t i o n s 9 a n d 1 3 ,b o t h w i t h a d r a g c o e f f i c i e n t o f 0 . 2 3 1 . T h i sv e h i c l e s t i l l h a d o n e s i d e m i r r o r ( t h e e f f e c t o fs i m i l a r e x t e r n a l m o d i f i c a t i o n s , s u c h a s m i r r o r s ,a r e g i v e n i n R e f . 2 4 ) a n d a f r o n t - p i l l a r - m o u n t e de x t e r n a l a n t e n n a . T h e i m p r o v e m e n t s r e s u l t e d i n

a n o v e r a l l d r a g c o e f f i c i e n t r e d u c t i o n o f 5 1 % ,t o g e t h e r w i t h a 3 7 % r e d u c t i o n o f t h e l i f t c o e f -f i c i e n t , c o m p a r e d t o t h e b a s e l i n e s h a p e . F u r -t h e r r e d u c t i o n o f t h e l i f t c o e f f i c i e n t w a so b t a i n e d w i t h t h e a d d i t i o n o f t h e f r o n t s p o i l e r( c o n f i g u r a t i o n 1 0 ) . H o w e v e r , t h i s m o d i f i c a t i o n ,w i t h t h e t e s t e d v e h i c l e g e o m e t r y , d i d i n c r e a s et h e d r a g f o r c e .

The ga in in fue l economy ow ing to the d ragd i f f e r e n c e b e t w e e n t h e b a s e l i n e a n d t h e l o w e s t -d r a g v e h i c l e i s d e m o n s t r a t e d i n F i g . 1 2 . T h e

F i g . 1 2 . Fuel consumption improvement by dragc o e f f i c i e n t r e d u c t i o n

f u e l f l o w , a t c o n s t a n t s p e e d , w a s n o t i c e a b l yreduced , and a t a speed o f 110 km/h a 12 .7%lower fuel consumpt ion was measured. T h i s f u e lconsumpt ion cou ld fu r the r be improved , in v iewof the above d rag reduc t ion , by mechan ica lm o d i f i c a t i o n s s u c h a s h i g h e r g e a r r a t i o o r as t a n d a r d c l u t c h i n s t e a d o f t h e t o r q u e c o n v e r t e r .Mechan ica l mod i f i ca t ions o f th i s t ype , however ,a f f e c t t h e o v e r a l l p e r f o r m a n c e o f t h e v e h i c l e( e . g . ,

l o w - s p e e d a c c e l e r a t i o n ) a n d t h e i r

10 850287

a p p l i c a t i o n r e q u i r e s s o m e a d d i t i o n a lcons ide ra t i ons apa r t f r om the ae rodynamicc o n s i d e r a t i o n .

COMPARISON WITH OTHER TESTING METHODS

The exper imental method which was presentedt h r o u g h o u t t h i s p a p e r h a s t h e a d v a n t a g e o f b e i n ga d i r e c t m e t h o d o f m e a s u r i n g a c t u a l r o a d r e s i s -t a n c e . O n t h e o t h e r h a n d , w i n d t u n n e l e x p e r i -m e n t s w i t h f u l l - o r s m a l l - s c a l e m o d e l s a r e f a rm o r e s i m p l e t o c o n d u c t a n d a r e l e s s d e p e n d e n t o ne x t e r n a l a t m o s p h e r i c c o n d i t i o n s . H o w e v e r , t h e ydo have p rob l ems such a s bounda ry l aye r i n t e r -f e r e n c e a t t h e t u n n e l f l o o r a n d w a l l , e f f e c t s o fb l o c k a g e a n d o f n o n r o t a t i n g w h e e l s , e t c . F o rt h i s r e a s o n i t i s e s s e n t i a l t o c o m p a r e w i n dt u n n e l d a t a w i t h o t h e r e x p e r i m e n t a l m e t h o d s , s ot h a t e i t h e r o f t h e s e t e c h n i q u e s c o u l d b e u s e dw h e n s u c h e n g i n e e r i n g d a t a a r e r e q u i r e d .

T h e c o m p a r i s o n w a s m a d e o n s i x b a s i c a u t o -m o t i v e s h a p e s ( c o n f i g u r a t i o n s 1 t o 6 ) , a n d t h er e s u l t s f o r t h e d r a g c o e f f i c i e n t a r e g i v e n i nF i g . 1 3 . I n a d d i t i o n t o t h e p r e s e n t m e a s u r e -ments , measurements were made by the towingm e t h o d a n d b y s m a l l - s c a l e ( 1 : 1 7 ) w i n d t u n n e lt e s t i n g ( r e s u l t s o f t h e s e e x p e r i m e n t s w e r er e p o r t e d i n R e f . 2 1 ) . T h e t o w i n g e x p e r i m e n t swere ac tua l l y pe r fo rmed on t he s ame veh i c l e w i tha 4 0 - m - l o n g t o w i n g c a b l e . T h e m e t h o d o f t e s t i n gw a s i d e n t i c a l t o t h e t e s t i n g r e p o r t e d h e r e , b u tt h e d r a g w a s m e a s u r e d a t c o n s t a n t s p e e d b ym e a s u r i n g t h e c a b l e t e n s i o n a t t h e t o w - c a re n d . S i n c e t h e t o w f o r c e a l s o i n c l u d e d t h ef o r c e r e q u i r e d f o r r o t a t i n g t h e d r i v e s h a f t a n dp a r t o f t h e t r a n s m i s s i o n , t h e d r a g w a s d e f i n e da c c u r a t e l y o n l y r e l a t i v e t o t h e b a s e l i n e s h a p e .O n c e t h e d r a g o f t h e b a s e l i n e c a r ( c o n f i g u r a -t i o n 1 ) w a s d e t e r m i n e d b y t h e c u r r e n t m e t h o d ,t h e p l o t t i n g o f F i g . 1 3 w a s m a d e p o s s i b l e . Mored e t a i l s o n t h e s e t e s t s a r e r e p o r t e d i n R e f . 2 5 .

I n t h e s m a l l - s c a l e w i n d t u n n e l e x p e r i m e n t s ,t h e m o d e l s w e r e m o u n t e d o n a v e r t i c a l s t i n gb a l a n c e i n a 1 - b y l - m ( t e s t s e c t i o n ) w i n dt u n n e l ( F i g . 1 4 ) . A n e l e v a t e d g r o u n d p l a n es e r v e d t o d i v i d e t h e w i n d t u n n e l b o u n d a r y l a y e r f r o m t h e m o d e l s ( f o r m o r e d e t a i l s r e f e r t oR e f . 2 1 ) . T h e a v e r a g e R e y n o l d s n u m b e r f o r t h e s et e s t s w a s a b o u t 2 . 5 x 1 05 ( b a s e d o n m o d e ll e n g t h ) c o m p a r e d w i t h t h e v a l u e o f 3 . 5 x 1 0 6 fo rt h e f u l l - s c a l e t e s t s . T h e m o d e l s h a d u n d e r b o d yd e t a i l s ( g a s t a n k , e x h a u s t s y s t e m , e t c . ) t h a tw e r e c a r v e d u p t o 3 m m d e e p i n t o t h e m o d e l , a n du p p e r b o d y d e t a i l s s u c h a s h e a d l i g h t s , s e p a r a t ew h e e l s , h o o d s , e t c . , b u t n o t c o o l i n g a i r p a s -s a g e s . S p e c i a l c a r e w a s t a k e n i n s h a r p e n i n g t h er e a r e n d a n d w i n d o w d e t a i l s i n o r d e r t o a c h i e v ea s e p a r a t i o n - l i n e l o c a t i o n s i m i l a r t o t h o s e o ft h e f u l l - s c a l e m o d e l . B l o c k a g e c o r r e c t i o n s w e r e

F i g . 1 3 . D i a g r a m o f t h e d r a g c o e f f i c i e n t s - - ac o m p a r i s o n o f d i f f e r e n t t e s t i n g m e t h o d s

n o t a p p l i e d i n t h e p r o c e s s o f d a t a r e d u c t i o n ,s i n c e t h e r a t i o b e t w e e n m o d e l c r o s s s e c t i o n t ow i n d t u n n e l t e s t s e c t i o n a r e a w a s l e s s t h a n 1 % .

T h e r e s u l t s o f t h e t o w i n g t e s t s , a s s h o w nb y F i g . 1 3 , c o m p a r e d w e l l w i t h t h e r e s u l t s o ft h e c u r r e n t r o a d t e s t s . T h e r e s u l t o f t h es m a l l - s c a l e w i n d t u n n e l e x p e r i m e n t s s h o w e d t h es a m e o r d e r o f d r a g d e c r e a s e a s t h e f u l l - s c a l et e s t s , b u t t h e i n c r e m e n t s w e r e s m a l l e r . T h el a r g e r v a l u e s o f t h e s e d r a g i n c r e m e n t s i n t h ef u l l - s c a l e t e s t s a r e p r o b a b l y a r e s u l t o f t h el a r g e r R e y n o l d s n u m b e r ( m o r e t u r b u l e n t b o u n d a r y

11

F i g . 1 4 . Sma l l - s ca l e w ind t unne l mode l and i t sm o u n t i n g i n t h e t e s t s e c t i o n

l a y e r ) , s o t h e t e n d e n c y o f t h e f l o w t o s t a ya t t a c h e d , b e c a u s e o f g e o m e t r i c a l m o d i f i c a t i o n s ,i s h i g h e r . F l o w v i s u a l i z a t i o n s b y t u f t s e n s u r e dt h i s , a n d t h e f l o w w a s f u l l y a t t a c h e d o n t h er e a r h a t c h ( c o n f i g u r a t i o n 5 ) i n f u l l - s c a l et e s t s , w h i l e i n t h e s m a l l - s c a l e t e s t s i t w a sp a r t i a l l y s e p a r a t e d . T h i s c a n p r o v i d e a ne x p l a n a t i o n t o w h y t h e f u l l - s c a l e d r a g r e d u c t i o ne f f e c t o f t h e f a s t b a c k c o n f i g u r a t i o n w a s l a r g e r .

A c o m p a r i s o n o f t h e l i f t c o e f f i c i e n t i s n o tr e p o r t e d h e r e , b u t t h e r e s u l t s f o r t h e b a s e l i n ec o n f i g u r a t i o n ( c o n f i g u r a t i o n 1 ) w e r e CL = 0.361i n f u l l s c a l e a n d 0 . 3 5 i n s m a l l s c a l e ( R e f . 2 1 ) .S i m i l a r l y , t h e l i f t o f t h e m o s t a e r o d y n a m i cs h a p e ( c o n f i g u r a t i o n 1 3 ) w a s 0 . 2 2 7 i n f u l ls c a l e , w h e r e a s i n t h e s m a l l s c a l e t h e v a l u e o f0 . 1 2 w a s o b t a i n e d . I n t h i s c a s e t h e l a r g e rd e v i a t i o n i s a g a i n d u e t o t h e f l o w b e i n g f u l l ya t t a c h e d i n t h e r e a r h a t c h r e g i o n i n f u l l s c a l e .T h i s c a n b e e x p l a i n e d b y t h e l a r g e r s u c t i o np e a k s t h a t a r e p r e s e n t o n t h e u p p e r s u r f a c e o ft h e v e h i c l e w h e n t h e a t t a c h e d s t r e a m l i n e s b e n dd o w n w a r d , a s i t w a s i n t h e l a r g e r R e y n o l d snumber exper iments .

CONCLUDING REMARKS

T h e r o a d - t e s t i n g t e c h n i q u e f o r t h e a e r o d y -n a m i c l i f t a n d d r a g m e a s u r e m e n t s w a s f o u n d t o b ev e r y p r a c t i c a l , a n d c o n f i g u r a t i o n c h a n g e s w e r ef a s t a n d s i m p l e t o p e r f o r m . T h i s m e t h o d i ss u i t a b l e f o r r e s e a r c h - o r i e n t e d e x p e r i m e n t s ,s i n c e t h e d e v e l o p m e n t o f a n i n s t r u m e n t e dr e s e a r c h v e h i c l e i s r e q u i r e d . T h e a d o p t i o n o ft h i s t e c h n i q u e f o r i n d u s t r i a l l y e f f i c i e n t m e a -s u r e m e n t s o f t h e a e r o d y n a m i c f o r c e s o n n u m e r o u sp r o d u c t i o n v e h i c l e s t h e r e f o r e s e e m s n o t t o b eeconomical.

T h e g e o m e t r i c a l c h a n g e s t h a t w e r e i n v e s t i -g a t e d h a d s i g n i f i c a n t e f f e c t s o n t h e a e r o d y n a m i c

f o r c e s . Whe ne ve r t he s e m od i f i c a t i ons a l l owed al a r g e r t r a n s f e r o f f r e e - s t r e a m m o m e n t u m t o t h er e a r s e c t i o n o f t h e v e h i c l e , a d r a g r e d u c t i o nw a s o b t a i n e d , s i n c e t h e a m o u n t o f r e a r - f l o ws e p a r a t i o n s w a s r e d u c e d . This was achieved bya d d i n g t h e s m o o t h u n d e r b o d y t o t h e b a s e l i n e ,r e s u l t i n g i n l e s s b l o c k a g e o f t h e f l o w b e n e a t ht h e v e h i c l e a n d i n a r e d u c t i o n o f t h e d r a gc o e f f i c i e n t b y a b o u t 0 . 2 . S i m i l a r l y , t h ei m p r o v e m e n t o f t h e u p p e r f l o w , w i t h t h e a i d o ft h e w e d g e - s h a p e d f r o n t a n d t h e f a s t b a c k c o n f i g -u r a t i o n , r e s u l t e d i n a r e d u c t i o n o f a b o u t 0 . 1 9i n t h e d r a g c o e f f i c i e n t . The combina t i on o f a l lt h e s e m o d i f i c a t i o n s , h o w e v e r , d i d n o t a d d u p i na l i n e a r m a n n e r ; bu t even so a ve ry l ow d ragc o e f f i c i e n t o f C = 0 . 2 3 1 o n a p r a c t i c a l r o a d -go ing ca r was ob t a ined . T h i s r e s u l t c o u l d b eeven fu r the r improved by de t a i l ed body work ,w h e e l f a i r i n g s , a n d t h e r e m o v a l o f t h e s i d em i r r o r a n d r a d i o a n t e n n a . T h e n e t d r a g - r e d u c i n ge f f e c t o f t h e w e d g e - s h a p e d f r o n t r a n g e d b e t w e e nD CD = 0 . 6 a n d D C = 0 . 3 , a n d o f t h e f a s t b a c km o d i f i c a t i o n w a s a b o u t CD = 0 . 1 3 t o

D = 0 . 1 6 , w h e n a d d e d t o t h e b a s e l i n e v e h i c l e .W i t h t h e a d d i t i o n o f t h e s m o o t h u n d e r b o d y , t h e s ea m o u n t s w e r e r e d u c e d , b u t n o t e l i m i n a t e d . I t i sc o n c l u d e d , t h e r e f o r e , t h a t t h e m o d i f i c a t i o n st e s t e d h e r e w i l l r e s u l t i n s i m i l a r t r e n d s w h e na p p l i e d t o a d i f f e r e n t t h r e e - b o x - t y p e a u t o m o -b i l e , b u t t h e l i n e a r b r e a k d o w n o f t h e c o n t r i b u -t i o n o f e a c h m o d i f i c a t i o n i s n o t e x a c t .

T h e c o m p a r i s o n o f t h e t o w i n g t e s t s ( w h i c ha r e c o n s i d e r e d t o b e l e s s a c c u r a t e ) w i t h t h ed i r ec t r oad - t e s t da t a showed good ag reemen t .The comparison with the low-Reynolds-number windt u n n e l d a t a s h o w e d g o o d q u a l i t a t i v e r e s u l t s , b u tb o t h d r a g a n d l i f t i n c r e m e n t s d u e t o g e o m e t r i cc h a n g e s w e r e l a r g e r i n t h e f u l l - s c a l e t e s t s .S m a l l - s c a l e t e s t s a r e t h e r e f o r e u s e f u l t o p r o -v i d e q u a l i t a t i v e i n f o r m a t i o n a b o u t t h e i n f l u e n c eo f geome t r i c a l changes . They cou ld be u se fu l i nd e v e l o p i n g g e n e r a l c o n f i g u r a t i o n s o r f o r c a l i -b r a t i n g a n a l y t i c a l p r e d i c t i o n m e t h o d s . Fori n f o r m a t i o n a b o u t f i n e d e t a i l s o f a p r o d u c t i o na u t o m o b i l e , h o w e v e r , f u l l - s c a l e ( w i n d t u n n e l )t e s t i n g i s s t i l l t h e b e s t w a y t o g o .

ACKNOWLEDGMENTS

T h e a u t h o r s w i s h t o a c k n o w l e d g e t h e i ra p p r e c i a t i o n t o t h e s t a f f o f t h e E n e r g y C e n t e ra n d t h e A e r o n a u t i c a l C e n t e r a t t h e T e c h n i o n ,I . I . T . f o r p r o v i d i n g t h e r e s o u r c e s a n d t h et e c h n i c a l s u p p o r t f o r t h i s r e s e a r c h . W e w i s h t oe x p r e s s o u r g r a t i t u d e t o L a u r a F a n g e r - V e x l e r f o rh e r a s s i s t a n c e i n m a n y p h a s e s o f t h i s r e s e a r c ha n d t o L e a h a n d A m i K r o n e n b e r g f o r t h e i r a s s i s -t a n c e i n d a t a c o l l e c t i o n . T h e a u t h o r s a l s o w i s h

85

D

D

CDD

12 850287

12. K o r s t , H . H . , a n d W h i t e , R . A . , " E v a l -u a t i o n o f V e h i c l e D r a g P a r a m e t e r s F r o m C o a s t -Down Experiments Conducted Under NonidealE n v i r o n m e n t a l C o n d i t i o n s , " J o u r n a l o f F l u i d sE n g i n e e r i n g , V o l . 103 , March 1981 , pp . 133-140 .

13. S o v r a n , G . , "The Effect of AmbientWind on a Road Vehicle 's Aerodynamic WorkRequirement and Fuel Consumption," SAE 84-0298,February 1984.

14. Duncan, D. B. , "Mult iple Range andM u l t i p l e F T e s t s , " B i o m e t r i c s , V o l . 1 1 , 1 9 5 5 ,p p . l - 4 2 .

15. F a n g e r - V e x l e r , S . , " E x p e r i m e n t a l S t u d yon t he E f f ec t o f Veh i c l e Shape on Ae rodynamicD r a g a n d L i f t F o r c e s , " M . S c . T h e s i s , F a c u l t y o fM e c h a n i c a l E n g i n e e r i n g , T e c h n i o n , I . I . T . , H a i f a ,I s r a e l , M a r c h 1 9 8 4 .

16. Hucho, W. H., "The Aerodynamic Drag ofC a r s , C u r r e n t U n d e r s t a n d i n g , U n r e s o l v e d P r o b l e m sand Future Prospects ," Symp. on Aerodynamic DragMechanisms of Bluff Bodies and Road Vehicles ,General Motors , Warren, Michigan, 1971; PlenumP r e s s , 1 9 7 8 , p p . 7 - 4 0 .

17. M o r e l , T . , "Aerodynamic Drag of BluffB o d y S h a p e s C h a r a c t e r i s t i c o f H a t c h - B a c k C a r s , "SAE 78-0267, 1978.

18. Ahmed, S. R. , " I n f l u e n c e o f B a s e S l a n ton the Wake Structure and Drag of Road Vehi-c l e s , " J o u r n a l o f F l u i d s E n g i n e e r i n g , V o l . 1 0 5 ,December 1983, pp . 429-434.

19. C o g o t t i , A . , B u c h h e i m , R . , G a r r o n e ,A. , and Kuhn, A. , "Comparison Tests Between SomeFull Scale European Automotive Wind Tunnels--P i n i n f a r i n a R e f e r e n c e C a r , " S A E 8 0 - 0 1 3 9 ,February 1980.

20. S c h e n k e l , F . K . , " T h e O r i g i n s o f D r a gand L i f t Reduc t i ons on Au tomob i l e s Wi th F ron tand Rear Spoi lers ," SAE 77-0389, 1977.

21 . K a t z , J . , " E v a l u a t i o n o f S m a l l - S c a l eA u t o m o t i v e W i n d T u n n e l T e s t i n g s , " t o b e p u b -l i s h e d b y I s r a e l J . o f T e c h n o l o g y , V o l . 2 1 ,1984.

22. C o r s i g l i a , V . R . , K a t z , J . , a n dK r o e g e r , R . A . , " F u l l - S c a l e S t u d y o f t h e E f f e c to f N a c e l l e S h a p e o n C o o l i n g D r a g , " J o u r n a l o fA i r c r a f t , V o l . 1 8 , N o . 2 , 1 9 8 1 , p p . 8 2 - 8 8 .

23. K a t z , J . , C o r s i g l i a , V . R . , a n dB a r l o w , P . R . , " C o o l i n g A i r I n l e t a n d E x i tG e o m e t r i e s o n A i r c r a f t E n g i n e I n s t a l l a t i o n s , "J o u r n a l o f A i r c r a f t , V o l . 1 9 , N o . 7 , 1 9 8 2 ,p p . 5 2 5 - 5 3 0 .

24 . M o r e l l i , A . , "Aerodynamic Basic BodiesS u i t a b l e f o r A u t o m o b i l e A p p l i c a t i o n s , " I n t e r n a -t i o n a l J o u r n a l o f V e h i c l e D e s i g n , T e c h n o l o g i c a lAdvances i n Veh i c l e Des ign Se r i e s , SP3 , Impac tof Aerodynamic on Vehicle Design, 1983,p p . 7 0 - 9 8 .

25. K a t z , J . , " E f f e c t o f E x t e r n a l G e o m e t r yon Automobile Aerodynamic Drag," E.E.C. ReportN o . 1 3 2 , T e c h n i o n , I . I . T . , H a i f a , I s r a e l , 1 9 8 2 .

to thank Dr. Gino Sovran from the G.M. ResearchL a b o r a t o r i e s f o r h i s m a n y u s e f u l c o m m e n t s o nthis manuscript.

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11. R o u s s i l l o n , G . , " C o n t r i b u t i o n t oAccurate Measurement of Aerodynamic Drag on aMoving Vehicle From Coast-Down Tests and Deter-m i n a t i o n o f A c t u a l R o l l i n g R e s i s t a n c e , " J o u r n a lo f Wind Eng inee r ing and Indus t r i a l Ae rodynamics ,V o l . 9 , 1 9 8 1 , p p . 3 3 - 4 8 .

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