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SAE TECHNICALPAPER SERIES 2000-01-0491

A New Aerodynamic Approach toAdvanced Automobile Basic Shapes

Alberto MorelliTorino Technical University

Reprinted From: Vehicle Aerodynamics(SP1524)

SAE 2000 World CongressDetroit, Michigan

March 6-9, 2000

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1 2000-01-0491

A New Aerodynamic Approach toAdvanced Automobile Basic Shapes

Alberto MorelliTorino Technical University

Copyright 2000 Society of Automotive Engineers, Inc.

ABSTRACT

Aerodynamic basic shapes are generally intended as nonwheeled bodies moving at a small distance from theground, effective and suitable for automobile applica-tions.The shape is furthermore designed to comply withrequirements other than aerodynamic accomodatingoccupants, luggage and mechanical parts within as smallas possible overall dimensions. However, even thoughthe basic body drag coefficient can be as low as 0.05, theaddition of wheels may increase the body drag, by two tothree times.

The new approach starts from the definition of aerody-namic criteria such as total lift close to zero, the pitchingmoment sign and value consistent with road holding andstability, a reduced sensitivity to side wind, gradual varia-tion of the cross sections, etc.. Then, the presence of thewheels is taken into account in order to reduce their aero-dynamic interference with the body, and to manage thewake mechanisms in order to recover the kinetic energyof the flow without fitting the body with a solid diffuser: infact, this would increase the car length without contribut-ing much to the usable space.Theoretical and experimental work leading to a newshape particularly short in the rear part and able toimprove both the accomodation of passengers in the rearseats and the visibility outside - at present a major draw-back of actual cars - are shown together with aerody-namic drag benefits.Other advantages resulting from the application of themethod are finally discussed.

1. INTRODUCTION

The attempt of introducing aerodynamic body shapesdates from the beginning of the automobile era (1890, /

1/ ), in those special cases where the goal was a speedrecord or a low energy consumption, etc., /2/. Otherwise,for about two decades the production motorcars main-tained the look of a horseless carriage, with the driver inopen air position. Their Cx exceeded the value of unity, /3/. In the 10s Renault, /4/, introduced a limousine withthe driver inside the passenger compartment and the Cxdropped down well below one. This good result, however,came as a fall-out of the new solution, aiming at a morecomfortable position for the driver.

2. SHORT HISTORICAL ACCOUNT ON AERODYNAMIC IMPROVEMENTS

It was only from the early thirties that Jaray introducedthe composite shape. Fig. 1, appearing in his patent(see also /3/, pag. 39-46), shows the method consistingof combining the elements taken from the typical bodiesused in aeronautics: the airship main body and theaeroplane wing section. Both the elements are cam-bered, following the previous experience carried out byKlemperer, /5/, aimed at demonstrating the beneficialeffect of a curved camberline on streamlined bodieswhen located close to the ground, as is the case of auto-mobiles disregarding the wheels.

Figure 1. The Jaray composite basic shape

2Figure 2. Cx vs. time. 1) FIAT Balilla 2) FIAT 1500 3) FIAT Topolino 4) Lancia Aprilia 5) FIAT 500 C 6) FIAT 600 7) FIAT 850 8) Alfa-Romeo Giulia 9) FIAT 124 10) FIAT 127 11) FIAT 132 12) FIAT Ritmo 13) FIAT Panda 14) FIAT Uno 15) FIAT Tipo 16) Lancia Thema 17) FIAT Tempra 18) FIAT Punto 19) FIAT Brava 20) Kamm car 21) FIAT Tipo3.

Fig. 2 shows Cx vs. time from 1930 to 2000 as measuredin the same full scale wind tunnel (except car n. 20). Mostof the models are FIAT Group cars having aerodynamiccharacteristics in line with the European production mod-els.

It can be seen that Cx dropped down by more than onehalf in the six decades considered. It can be alsoremarked that the progress was obtained by two steps.The first step occurred in the thirties by the introduction ofthe Jaray composite form, which, by the way, becamefashionable.Let us consider the Cx as the sum of two terms:

Cx = Cf + ( Ci + Cs ). (1)The first, Cf, friction drag is the viscous drag coefficient.The second, (Ci + Cs), is usually called the pressuredrag coefficient , being associated with the pressure, p,distribution on the body surface. From a conceptual pointof view, it is useful to subdivide the pressure drag into twocomponents: Ci and Cs. Ci, induced drag coeff., as it iswell known in aeronautics, is associated with the lift. Tobe precise, therefore, the word induced should be fol-lowed by the words: by the lift. However, as the lift, likeany transverse force, is generated by vortices adherent tothe surface and shedding longitudinally from the body, itis more common to associate the word induced to suchexplicative words as: by vortex shedding.Cs, determined by the difference:

Cs = Cx - Cf - Ci (2)

is conventionally called form drag coefficient.As supplementary information, we remind that Cf has asmall value ranging from 0.02 to 0.03, i.e., less than 10 %of the Cx of actual contemporary cars.Coming back to the application of the Jaray compositeform, we remark an average improvement of Cx of 0.15,i.e., more than 20 % less with respect to the 0.65 valuesof the early 30s.

Figure 3. The Kamm prototype

Having accomplished such an important improvement indrag, the manufacturers seemed to be skeptical aboutfurther reductions and Cx remained practicallyunchanged (Cx 0,5) for about three decades, eventhough prototypes like the Kamm car, Fig. 3, proved a Cx= 0.37 with a hatch-back rear end (K tail) much morepractical than the Jaray shape as far as the accomoda-tion of passengers and baggage in the rear is concerned.Kamm justified the benefit in drag by a better flow sepa-ration in the rear part of his body, not mentioning at all theabatement of lift, /6/.The weak point of the Jaray composite shape was proba-bly the lift, by both its influence on road holding qualitiesand on the induced drag coeff., Ci. Fig. 4 and 5 show thewake maps behind a FIAT 127 (1971, Cx = 0.48) and aFIAT UNO (1983, Cx=0.34) /7/. In the relevant period asecond step in drag reduction is achieved, accounting forCx 0.15 over an initial value of Cx 0.48 , i.e. about30 % : this was mostly obtained by the abatement of theinduced drag coefficient, Ci. /8/.

(19)(18)

(17)

(15)(16)

(14)

(1)

(4)(3)

(2)(5) (6) (7)

(8)

(9) (10)

(11)(12)

(13)(20)

(21)

0.00

0.10

0.20

0.30

0.40

0.50

0.60

0.70

1930 1940 1950 1960 1970 1980 1990 2000Year

Cx

3Figure 4. FIAT 127 (1971) wake maps

Keeping in mind that at present most of the Europeanproduction automobiles have drag coeff. ranging from aminimum of 0.26 to a max. of 0.36, the following Tablegives an approximate subdivision of the Cx components,/9/.

The form drag coeff., Cs, representing 75 to 85 % of thetotal drag, is by far the predominant part. Therefore,important reduction in drag can only be achieved by sub-stantial reductions of this component.

3. REDUCTION OF THE FORM DRAG COEFF. Cs

Two ways can be envisaged in principle:a. directb. indirect.

The first way is concerned with lower losses of energyalong the streamlines where the mutual exchange ofkinetic and pressure energies are the main cause of theanticipated flow separation from the body surface.

Figure 5. FIAT UNO wake maps

The second way that aims at reducing Cs throughout areduction of Cf looks very theoretical for the moment. Forexample, a reduction of viscosity by feeding a noble gasin the boundary layer through a porous surface wouldresult in a retarded flow separation but is very unpracti-cal. A more attractive way could be based on the use ofspecial (heavy) paints /10/. Anyway, no practical attemptshave been made to date in this direction as far as weknow, due to lack of knowledge of the physical process, /11/.Fig. 2, n. 21 represents the FIAT tipo 3 prototype (1984,Fig. 6) on which both a reduction of the induced drag, toalmost zero, and the form drag, led to a Cx lower than0.18. The reduction of form drag was obtained using thepromising technology that we propose to call fluid tailtechnique (FTT). The prototype had a notch-back rearend followed by a fluid- tail, /7/, as shown by the visual-ization of Fig. 7.

Coeff. Min. Max. % %Cx 0.26 100 0.36 100Cf 0.02 7.5 0.03 8Ci 0.02 7.5 0.06 17Cs 0.22 85 0.27 75

44. THE FTT TECHNIQUE

The direct way indicated in the previous 3, is usuallyrealised by streamlining the body down to its rear end.The body camber as indicated in ref. /12/, leads to a dragcoefficient of bodies moving close to the ground evenlower than 0.05, i.e. comparable to axially symmetricstreamlined bodies in free air. However, the car lengthwould result too long in the rear part, in contrast with theneed for the space required by passengers and baggage.In fact, at present the notch-back solution seems to beprogressively substituted by the hatch-back configura-tion.

Figure 6. The FIAT TIPO 3 prototype

The experimental observation of a wake behind someelementary body shapes, as bullets, for instance, showsthe formation of a ring vortex stationary with the bodysbase. This phenomenon can even be observed on a cir-cular flat plate, Fig. 8. The ring vortex acts like a diffuserprogressively reducing the core of the wake.

Figure 7. Flow visualization of the wake behind a notch- back

car (FIAT TIPO 3)

In order to reproduce this effect, the following conditionsshould apply, as shown by a number of experiments (seealso ref. /13/) at the appropriate Re numbers.

A. the flow separation line must be substantially trans-versal with respect to the body direction of motion.

B. the flow separation line must coincide or be veryclose to the perimeter of the base.

C. the perimeter of the base must be circular or ellipti-cal; in any case no angles must be present.

D. similar fluid dynamic conditions as pressure andvelocity vectors must apply at any point of the perim-eter.

Figure 8. Flow field of motion around a flat plate

Figure 9. FIAT PUNTO 75 wake maps

These conditions are not usually met on actual cars. Thedisturbance caused by the wheels do not allow the flowseparation lines to meet together from the underbody to

5both sides of the car: see for example the maps of Fig. 9,as measured in the wake 100 mm behind a FIAT PUNTO75 saloon car.

In order to overcome this drawback, a new device wasdesigned, tested and introduced on a similar car (FIATPUNTO 55). The detailed description of the device ispresented in ref. /21/. We give here only a short sum-mary.

Figure 10.

Figure 11.

5. AN EXAMPLE OF FTT. FIRST RESULTS.

Fig. 10 and 11 show a modified rear wheel and relevantwheel-arch of a FIAT PUNTO 55. The wheel is fittedwith a centrifugal fan addressing adequate flow ratesinto built-in ducts, D inside the wheel arch, A. The flowenters through the intake I and exits through the outlet Ualso visible in Fig. 12.

The shape of the outlets (one for each rear wheel) is 350mm width and 20 mm thick in order to cover a thin layer ofthe upper part of the wake caused by the wheels , soreproducing the continuity of the boundary layer all overthe perimeter of the base. After some adjustments, thedevice proved effective as shown by the maps of Fig. 13and 14.

Figure 12. FIAT PUNTO 55 fitted with modified aerodynamic device

Figure 13. Total velocity and micro drag maps in a transverse plane 270 mm behind the car of Fig. 12

6As a consequence, the Cx of the car modified as shownin Fig. 12, dropped down from 0.327 to 0.268, i.e. by 18% as measured in full scale wind tunnel. Coast-downroad tests indicated a benefit in drag by over 20 %. /21/.

Figure 14. Vorticity in the centre line plane and in an horizontal plane 500 mm over the ground, behind the car of Fig. 12

6. POTENTIAL OF THE FTT METHOD

The FTT method has been tested by fitting the describeddevice on a conventional Hatch-back car (FIAT PUNTO55). The research work is now continued with the objec-tive of optimising and finding the methods limits..

6.1. BODY DIMENSIONS For a given length c width band height a, the shape of max volume is, of course, thesquared box. The use of such a shape is subject to thefollowing limitations:1. necessity for a ground clearance, hG;2. non useable space due to:

2.1. anthropometric characteristics of the humanbody;

2.2. recommended positions of the occupants (driverand passengers);

3. compliance with visibility rules and recommenda-tions;

4. room for mechanical parts including the wheel loca-tion in seminternal compartments;

5. compliance with safety standards and recommenda-tions;

6. parking manoeuvrability;7. body struts encumbrance;8. other limitations due to functionality requirements

(doors opening, baggage access etc.).

Figure 15. T : transverse section; h : camberline.

Fig. 15 shows schematically the modifications of a boxshaped body in the case of a front drive transaxle salooncar taking into account the limitations listed above. It canbe remarked that the space required to accomodate thepayload

(occupants + baggage) is less 50 % of the over-all volume a x b x c of the box. However, Fig. 15 also indi-cates that the transverse area T of the body sections canvary gradually from the front to the rear, achieving itsmax. approx. at the rear end and the camberlineassumes the trend indicated h/2 in Fig. 15.

6.2. AERODYNAMIC CONDITIONS ADOPTED IN THENEW BODY SHAPE The possibility to realize a fluid tailoffers the opportunity to design a new body shape. Withref. to the papers /8/, /12/ and /14/, the aerodynamic cri-teria are confirmed or modified as follows.A. Zero lift (Z=0);B. Pitching moment coefficient, CM >or< 0 depending

on front or rear drive, respectively;C. Gradual variation of the cross section area vs. x; D. Low perimeter/area ratio of the cross sections;E. Gradual variation of the shape of the cross sectionsF. Low CN /CY ratio in order to reduce the side wind

sensitivity.The pitching moment coeff. should be positive in a frontdrive car (for example : CM = 0.1) in order to maintain thestability margin

(understeering degree) unchanged atvarying speed. In fact, a positive CM value will cause aload transfer from the rear to the front axle. The decreaseof the tire side stiffness in the front axle caused by thetractive thrust is thus counteracted. Both effects depend

c

b

b/2

h/2

T

hG

7approx. on the square of car velocity, resulting in a com-pensating effect at any speed. As a consequence,requirement A can be fulfilled with only two half waves inthe lift coeff. distribution vs. x, instead of three (see nextparagraph). This condition, together with points C and E,aims at reducing the pressure drag both in the induced(Ci) and the form (Cs) components.On the other hand, point D aims at reducing the wettedsurface, therefore the friction drag (Cf).In order to comply with point F, a progressively increaseof the car height vs. x (wedge shape), together with anappropriate cross section evolution, is adopted.

7. DETERMINATION OF THE NEW BODY SHAPE

The cross flow 2D method described in ref. /12/ was usedin the first stage. In fact, using the simple formula:

h = ho exp ( x Fx dx ) (3)where:

Fx = (4/b2) . x Cz dx (4)allows to determine directly the camberline h(x) of thebody, given its planview, b(x), and the lift distribution, Cz(x).Fig. 16 shows the reference system.The planview b(x), Fig. 17, was designed on the basis ofthe following considerations.

The mechanical components including the wheelsmust substantially stay inside its contour;

The base must be perpendicular to the x axis (direc-tion of motion);

The width b(x) must increase as gradually as possi-ble from the front to the rear, preferably without anyintermediate reduction.

Figure 16. Reference system

Figure 17. Basic body planview

The lift distribution, Fig. 18, was imposed in order to com-ply with points A and B of paragraph 6.2.

Figure 18. Lift distribution and camberline

Figure 19. Typical cross section shape

The shape of a cross section presented in Fig. 19, wasmainly designed taking into account the points 2 and 4 ofparagraph 6.1 and point F of paragraph 6.2. Locatingtheir centroids in the camberline, the basic body shape

8takes the form of Fig. 20. At the rear of this body, a shortsegment is added assuming the function of interfacebetween the body and the fluid tail ( see Fig. 16 ). Therole played by this segment is substantially thatdescribed in paragraph 4 from point A to D. In addition, itworks also as a first stage diffuser addressing the flowtowards the (virtual) tail camberline which is a straightsegment, Q, parallel to the ground plane. In fact, the fluidtail does not cause any lift, provided the base is normalto the cars its task being only that of reducing the basedrag

as much as possible.

Figure 20. Basic body shape

A second objective of the fluid tail is to minimise thepower needed to maintain the wake mechanism, i.e. thestationary ring vortex. Therefore, the ring vortex core vol-ume must be as small as possible, together with a highRankine radius. Otherwise, the effect would be as in theflat plate of Fig. 8 , causing a high drag (Cx=1,17) eventhough a fluid tail is present. This fact may be explainedby the remarkable dimension of the ring vortex as com-pared to the plate. Indeed, the separation line is the cir-cumference of the plate where the flow separates in aradial direction resulting in a mean diameter of the ringvortex almost equal to that of the plate.

8. AERODYNAMICS OF THE WHEEL AND WHEEL-ARCH (WWA)Whereas the aerodynamics of the wheel has been inves-tigated taking into account the presence of the groundplane /15/, /16/, /17/, /18/, the interference of the wheelwith the body received less attention. On the other hand,a simple addition of the wheels to a basic body proved tocause a high increase in drag, two to three times thedrag of the body alone.Cogotti /18/ and Mercker et al. /20/ showed that a rotatingwheel, in presence of the ground, originates a system ofthree pairs of counterrotating longitudinal vortices in thewake:

a pair of them sheds from the top, a second pair sheds from the wheel axis the third pair from the bottom, attached to the

ground.

Figure 21. Evolution of the jetting vortices. From the top: 1 - wheel rotating without fan; 2 - wheel fitted with centrifugal fan; 3 - combination of 1 and 2

The last pair, of higher intensity than the others, havebeen called jetting vortices.When the wheel is partially enclosed in the wheel-arch,the experimental research carried out at the Torino T.U.showed the following results:

three out of the six vortices practically disappear;those remaining are:

both the jetting vortices, even though their intensitymay not be equal, (Fig. 21).

the external vortex shedding from the wheel axis.Of the other three vortices, those from the top disappearbecause the wheel-arch hampers the external flow toattack the upper portion of the wheel. The axial internalvortex creates a vortex, of high Rankine core radius (i.e.of low intensity) inside the wheel-arch. Here the flow hasa total pressure coefficient much lower than outside. Aflow field of motion following the Coanda effect is, there-fore, possible.In fact, the research work also showed that it is possibleto reduce the intensity of the jetting vortices by fitting acentrifugal fan to the rim of the wheel.In order to obtain the flow mechanisms shown in Fig. 10and 22 both the rim and the wheel-arch must bedesigned accordingly.

'' 32 >>

'3

A

B

AB 0min1 =

A

B

1V

3V

2V

021

'max133 =

max122 21

' +=

Jetting

9In addition to the description of paragraph 5, it isobserved:

the flow remains attached to the inside of the tire, byCoanda effect, until the separation occurs in the pointS of the tread.

due to the shape of the wheel-arch, the flow isreversed and partially addressed to the intakes B vis-ible in Fig. 11.

approx., one third of the flow rate generated by thefan is sucked by B and conveyed by the duct D to theexits U.

the remaining 2/3 of the flow rate generated by thefan, originates a vortex of circuitation G1 whichremains attached (again by Coanda effect) to the tiretread, moving forward with its velocity until impact-ing the ground plane. Here the vortex splits into twobranches interfering with the two branches generatedby the jetting.

Fig. 21 shows the reconstruction of the phenomenon asobserved during the laboratory tests: a noticeable reduc-tion of the jetting vortices was obtained.The fans were fitted to the (rear) wheels in order to feed afluid power to the rear wake, here establishing the condi-tions A and B of paragraph 4, able to originate the sta-tionary ring vortex.It is worth mentioning that the air suction from the wheelintakes is also beneficial for reducing the thickness in theboundary layer on both sides of the car, and reducing thewidth of the wake behind the wheel axis. The flow rate per each fan is about 0.2 m3/s at 120 km/h.A 3-dimensional Navier-Stokes Computational FluidDynamics (CFD) investigation /21/ on the flow rate distri-bution at the fan outlets gave information on the effect ofthe radius R (see Fig. 10) of the leading edge of the airinlet. In order to direct the max rate of flow per degreeupwards, i.e. in the direction of the wheel arch inlets, B,the radius must be small:

R 3 mm

A small radius also reduces the wheels external width.

9. APPLICATION OF THE WWA LAYOUT TO THE BASIC SHAPE

As no automobile can run without wheels, some of thebasic bodies proposed in the past take into considerationtheir presence. However, as it can be noted for examplein ref. /3/ and /19/, the wheels are simply added to theunderbody and not rotating. As far as the drag is con-cerned, tests in scaled down models gave, in the bestcases:

Cx 0.15 .Taking into account that the frontal area of a wheel Sw, isapprox. one twentieth that of the body, and assuming

Cxw 0.5 the drag coefficient of an isolated wheel, theinterference coefficient, CxI, results:

CxI = Cx - CxB - Cxw (Sw / S) . (4/3) =

0,15 - 0.07 - 0.5 . (1/20) . (4/3) + 0.047 (5)The factor 4/3 stays for the number of wheels (4) multi-plied by the area ratio of the wheel not covered by thebody (1/3).Of course, a positive value means that adding the wheelsincreases the total drag. The possibility to obtain benefi-cial effects on the drag by adding rotating wheels to thebody, as discussed in paragraph 8, can be expressed inother words as achieving a negative interference coeffi-cient.

Summarising, fitting the wheels a centrifugal fan, againstthe additional power required to rotate the wheels (seeref. /21/), the following advantages are obtained:1. A narrower wake by air suction from the wheel inlets2. An important decrease in base drag by addressing a

part of the flow rate in the wake through inlets B,ducts D and outlets U (Fig. 10 and 11)

3. The remaining part of the fan flow rate induces byCoanda effect

a vortex over the tire tread interferingwith the jetting vortices of the tire. Their abatement isbeneficial in drag

4. Point 1 and 3 also apply to front wheels. Here, thewake can be deflected to a more longitudinal direc-tion, merging into the wake of the rear wheels.

In the case of the FIAT PUNTO 55 motorcar, only therear part was modified, as shown in Fig. 12. Tests andmeasurements both in full scale wind tunnel and on theroad, /21/, demonstrated that points 1 and 2 were cer-tainly achieved. Point 3 may also have been achieved.However, it was not possible to separate this contributefrom the others. The front wheels had no fans. As a con-sequence, point 4 was not checked. Nevertheless, theacquired benefits are in the order of 5 kW less power formotion to be compared with 240 W of extra powerrequired to drive the rear wheels. The ratio of power ben-efits over expenses is therefore about 20.

10. ADDITIONAL BENEFITS

The flow generated by the built-in fans of the wheels giveadditional benefits, some already obtained, others onlyexpected. They are as follows.

10.1. VISIBILITY We call passive visibility the qualityof a car to be seen from other vehicles. For active visibil-ity the quality to see outside the car.

10.1.1 passive visibility The width of the wake isreduced at the wheel level by the suction of the fans, asalready observed in paragraph 6.1. Therefore, the mistcaused by the water splash when raining, will remaincloser into the lane where the car is running.

10

Furthermore, the fluid-tail moving stationary with thevehicle creates a narrow wake behind it for the substan-tial absence of longitudinal vortices shedding from thebody.Both effects result in a better visibility for overtaking cars.

10.1.2 active visibility A major drawback of hatch-back cars is the spray of dirt on the rear window comingfrom the wheels wake requiring a frequent use of thewipers.The effect is due to particles lifted from the ground by thelower transverse vortex which is usually larger than theupper one albeit of lesser intensity.In the case of a fluid-tail, the transverse vortices havesimilar dimensions and energy, as it can be seen in Fig.14, thus performing no significant mutual mixture. On theother hand, the wakes behind the wheel and behind thebody are kept separate as much as possible by theactive flow blown from the slots (U in Fig. 8). Therefore,the content of dirt particles near the base of the body iskept to a minimum and the rear window expected toremain cleaner.

10.2 BRAKE COOLING With reference to Fig. 7, theflow generated by the fan cools the brake before beingdirected, with the aid of the wheel fairing W, into the archcavity.Wheels incorporating built-in fans for a similar use areclaimed by many patents, as it is known. In our case it isonly an additional quality of the device.

Figure 22. Approximate profile resulting from the adoption of the new basic body of Fig. 20. is a (solid) interface between the body and the fluid-tail.

10.3 TIRE TEMPERATURE Ventilation air tempera-ture may influence the tire temperature, especially whenthe wetted area is large. This is the case of air flowing outof the fan outlets and remaining attached to the tire sidesand track, as indicated in Fig. 7. On the other hand, theair temperature is dependent on the use of brakes, whichis more frequent when driving in town than on highways.

Therefore it is expected a quicker warming up of the tirein the city and a cooling effect in highway traffic.

11. CONCLUSION

The research work named 4A (Active AerodynamicsApplied to Automobile) began in 1993 at the T.U. ofTorino, Dept, of Energetics, in close cooperation withFIAT-AUTO - DT, Innovazione. The research aims at demonstrating the possibility ofreducing the form drag in automobiles having a largebase area (as hatch-back cars) by the formation of fluidtails obtainable by addressing small power air jets in thewake.

From a conceptual point of view, the result may be seenas an achievement based on the interference effectbetween the wheels and the body. To the purpose, a newlayout was adopted in the system wheel/wheel arch andbody base (Fig. 11).In order to determine the potential of the procedure, atheoretical method was adopted for the definition of thecamberline

of a new basic body. The rear part of the bodyis fluid acting as a diffuser, so improving the form dragcoefficient. In order to facilitate the formation of the fluid tail, an inter-face segment is added between the body and the fluidtail.

The resulting profile of the proposed new basic body isshown in Fig. 22. In order to substantiate the method leading to a rationaldefinition of the new basic body shape, further experi-mental work is presently carried out.

ACKNOWLEDGEMENTS

The author is indebted to Dr. Ing. Nevio Di Giusto, FIATAUTO SpA, for his important support and suggestions The author is also thankful to the following former stu-dents for the work done during the development of theirDegree Thesis

D. Barbero ; G. Ceruti ; A. Bertazzoni ; C. Giavani ;A. Bianchi dEspinosa ; G. L. Di Oto ; F. Patalacci.

For his contribute to the research work Dr. Andrea Tonoliof Torino T.U., Dept. of Energy.Special thanks are also due to:

Dr. Paola Bergamini and Dr. Ing. Bruno Bonis, FIAT-AUTO SpA.Dr. Ing. Antonello Cogotti, Pininfarina SpA, for WindTunnel testing.

11

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9. A. Morelli - Reduction of End-Use Loads - A GreatPotential for Raising Motor Car Fuel Economy -XXVI FISITA Congress, Prague, June 17- 21, 1996.

10. F.L. Galvao Materials for Friction Drag Reduction -Thin Films Congresso ATA Innovation and Reliabil-ity in Automotive Design and Testing - Firenze, Italy,April 8 -10 1992.

11. J. Krimm and C. Daly Electronic Contributions toSliding Friction - Phisics of Sliding Friction - byB.N.J. Persson and E. Tosatti - Kluver Academic Pub-lishers, Doodrecht, (NL) 1996.

12. A. Morelli - Low Drag Bodies Moving in Proximity ofthe Ground - ASME - Aerodynamics of Transporta-tion - Niagara Falls, June, 1979.

13. H. Werl - Le Tunnel Hydrodinamique au Service dela Recherche Arospatiale - Pubbl. ONERA n. 156,1974.

14. A. Morelli, L. Fioravanti, A. Cogotti -The Body Shapeof Minimum Drag SAE Congress, publ. n. 760186 -Detroit, February 23-27, 1976.

15. A. Morelli - Aerodinamica della ruota dautomobile ,Rivista ATA n. 6, giugno 1969.

16. W.R. Stapleford and G.W. Carr Aerodynamic Char-acteristics of Exposed Rotating Wheels - M.I.R.A.,Nuneaton (U.K.). Report No. 1970/2.

17. J.E. Fackerell and J.H. Harvey The Flow Field andPressure Distribution of an Isolated Road WheelBHRA Advances in Road Vehicle Aerodynamics ,1973, publ. No 10. (UK).

18. A. Cogotti Aerodynamic Characteristics of CarWheels - Int, Journal of Vehicle Design, Secial Publi-cation SP3, 1983, Printed in UK. ISBN 0 907776 019.

19. W.H. Hucho - Aerodynamik des Automobils -pag.158-161 -Vogel Verlag, 1981.(D).

20. E.Mercker and H. Berneburg On the Simulation ofRoad Driving of a Passenger Car in a Wind TunnelUsing a Moving Belt and Rotating Wheels Con-gresso ATA Innovation and Reliability in AutomotiveDesign and Testing - Firenze (Italy) April, 8-10,1992.

21. A. Morelli and N. Di Giusto - A New Step in Automo-bile Aerodynamics - Performance Improvements andDesign Implications - International ConferenceVehicles and Sistems Progress - Volgograd StateTecnical University.(Russia), Sept. 7-10, 1999.