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INVESTIGATION OF RACECAR REAR WING

Horace LAUSupervisor(s): A. Professor K. Hourigan

Department of Mechanical Engineering

Monash University, Melbourne, AUSTRALIA

ABSTRACT

In recent years, the objectives of most aerodynamicresearch are to minimize the drag, and at the same timeto maximize the negative li ft characteristics, in order toincrease the speed of the car and to improve thecornering performance. This report aims to investigatethe aerodynamic performance of a Joukowsky J815profil e using the computational fluid dynamic program[FLUENT]. The results of the calculations are presentedfor wings having both positi ve and negative incidences,as well as those with and without ground effect. Theseresults and patterns are then compared with previousexperimental and mathematical results and smokevisuali zation. Finall y the two elements wing withvarious flap deflection is also investigated.

LITERATURE SURVEYTypes of lift force generate on racecar

Airflow will diversify while it approaches on theracecar, there are two different superimposed patterns tobe considered that are the flow past over and the flowcirculating around the racecar. The flow inclined upwardover the racecar will cause the acceleration of air speedand decrease of pressure when comparing with theoutside boundary layer, therefore the suction zone wouldgenerate and the aerodynamic li ft would occur. Liftgenerated belongs to the vehicle shape and is producedby circulatory airflow, which is the flow circulatingaround the racecar and is also termed as aerodynamiccirculation (Houghton, 1993). This causes the flow fromunderside of the racecar to flow diagonall y towards thesides and to flow towards the roof, joining the fasterstream outside. This aerodynamic interference isrepresented by vortices and these vortices will continueas a traili ng vortex downstream.

Effect of lift force generation

One of the major limitations on the performance ofracecar is the slip limit of their tires. An increase of theaerodynamic li ft around the car would further reduce thelateral and longitudinal traction capabiliti es of the tires,which means reducing the tire and road adhesion.Moreover, the distribution of li ft forces acting on theracecar will determine the magnitude of the pitchingmoment, which tends to reduce the tire load on the roadand substantiall y lessen the grids when it arises. Thisaffective factor would highly abase the racecar

performance in several different ways such as itssteering properties, stabilit y and cornering speed.

Negative lift devicesBy optimizing the downforce and together with the

minimum aerodynamic drag force have become the mainobjectives of many racecar designers. But in fact, todesign a good performance racecar the exterior designershave to work together with mechanical engineersbecause the aerodynamic forces act externall y whichactuall y combine with other forces of mechanical nature,such as frictional force between tires and road.Therefore, it is unreasonable to consider aerodynamicsseparately from mechanical aspects (Woodbridge, 1996).By the reason of their common theme relied upon thediscipline of models, most of the development willconcern over the upper surface of the racecar (Scibor,1978). The improvement would perform based on eitherthe body shape or some add-on devices. By remodelingand integrating the body shape and surface, features suchas drooping nose, canopy and tail section, clearance andsmoothness of underside panel, front and rear windows,...etc. would be considered. Other than the influence ofshape, some add-on aerodynamic devices li ke wedgeshape body profil e and rear negative wing have beenused with varying degrees of success.

Negative rear wing was first introduced in the1960s. It is made of a short span wing usuall y li ke arectangular platform and has aeronautical cross-sectionalprofil e similar to those used on aircraft wings (Fenton,1996). The rear wing fixed at a certain angle ofincidence with respect to the oncoming airflow over therear axle just acts li ke a flap where the basic profil e isthe racecar. The rear wing is the most practical approachto adjust the down force and to perform the highesteff iciency comparing with other methods. Variableswhich will alter the level and eff iciency of down forceproduced by the rear wing include the number of wingelements, size, position, angle of attack, shape, aspectratio and other additional add-ons li ke end plates andGurney flap.

Area concerned on rear wing

Influence of shape and angle parametersThe shape and angle of the wing can be varied

widely, just a small geometrical or angle change of thewing would make a considerable difference to their

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aerodynamic characteristics (Riegels, 1961). Recently anultimate negative rear wing that can provide the bestperformance for all occasions is nowhere to be found,because the negative li ft force is inverse proportion tothe drag. By integrating the airfoil shape, heavil ycambered wing and high incidence will give a highnegative li ft coeff icient together with large drag penalty,which means that it will be more suitable for slow speedcircuits. With a li ghtly cambered wing and lowincidence, the result will be vice versa and it will bemore suitable for high speed circuits (Beccio, 1987).Therefore, the designers can only choose the wing shapeand level of incidence based on highest aerodynamiceff iciency with such requirement is taken intoconsiderations.

Influence of position parameterFrom the aerodynamic point of view, the rear wing

should be fixed comparatively high over the car to allowit to move in the undisturbed airstream. Yet in fact,when the wing mounts too high over the car, theaerodynamic drag force on the wing will producenegative pitching moment and will t end to rotate the cararound the rear wheels and to li ft the front wheel off theground (Katz, 1996). The pitching moment does actcontrary wise to the eff iciency of steering, especiall y athigh speed. However, the motion of the rear wing hasbeen limited by racecar regulations to a maximum heightof approximately the height of the roof, so that thedesign boundary is constrained. When the rear wing islowering graduall y towards the car body, the separatedairflow can only be practicall y reattached onto thesurface of the wing. If this happens, the rear wakebehind the racecar may extend up to the lower surface ofthe wing. This would affect the actual pressuredistribution on the wing and consequently itsaerodynamic li ft force. Furthermore, with wing atsuff icient low position and the wake behind the racecarapproaching each other, until a certain length, they willjoin together and this leads to sudden increase of li ft.This type of effect is called body-wing interference.

Influence by add-on devicesThe add-on devices of the rear wing can increase its

downforce within the racecar regulations applied. Theend plate has been used to hold the wing elements and tocut down the induced drag by impeding the flow roundthe wing tips so that the wing assumes a new effectiveaspect ratio, which is greater than the geometrical value.The end plate also creates an airflow pattern that isapproximately two-dimensional, in order to suff icientlyincrease the aerodynamic li ft. Another common device isthe Gurney flap, which is attached perpendicular to thetraili ng edge of the rear wing (Katz, 1987). The Gurneyflap can give a substantial rise in negative li ft that isgenerated by the wing, but a large drag force penaltywould also be introduced simultaneously. Multipleelements of rear wings have also become famili ar inrecent years, they are very similar to the airfoil i naircraft but this supplementary wing does not retract intothe main wing (Katz, 1989).

The development of the position and arrangement ofrear wing and the corresponding aerodynamic

information can be obtained by using analytical,computational or experimental techniques (Dominy,1992). Because of the enormous cost of equipment,models and manpower incurred by wind tunnel testing,Computational Fluid Dynamics has been widelydeveloped due to the progress in super computer powerand computational schemes (Hinemo et al, 1992).

In this study the computational technique is used forexamining the performance of a typical rear wing modelof the racecar. With this highly cambered airfoil modelthe effects of the angle of attack and wing location areinvestigated. Also the comparison of the computationalaerodynamic data with the experimental results areincluded. Furthermore, the two elements wing is alsoinvestigated in this paper.

MODEL DESCRIPTION

The shape of the wing being chosen for thisinvestigation is the Joukowsky J815 profil e, which is atypical highly cambered rear wing of racecar (Riegels,1961). The coordinate of the wing section has beentraced around to obtain 116 points, concentrated near theleading and traili ng edges. These points were thenentered into the geometry program (DDN) and gridgeneration program (P-Cube) in GeoMesh, which is aprepocessor for CFD solver to reproduce the wingsection. A single element wing with chord of 235mmand wing span of 1000mm has been produced in Figure1. For the two elements wing, the additional wingsection with the same shape but in ¼ scale of size wasused. The operating boundary of the model with a crosssection of 1200mmx1700mm is suitable when accuracyand time have been taken into consideration.Comparisons of result accuracy are discussed in the nextsection of this paper.

Figure 1 : Joukowsky J815 profil e with grid.

The grid model with finer resolution inside theleading and traili ng domains of the wing can providemore detailed and accurate results within these criti calareas. The general size of the grid will li e between40000 to 95000 cell s as in Figure 1. The determinationof the size actuall y depends on the requirement ofpositions, arrangement and different incidences. Forinstance, some special models will require more than150000 cell s to provide reliable results.

The CFD solver FLUENT is the computer code inwhich the setting of the model and the adjustment of

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operating condition are made. The equation thatFLUENT applies to solve the fluid flow problem is theNavier-Stoke Equation� �� �

VpVVV o 2Re

1t

U ��� !"#$%&&and the Quadratic Upwind Interpolation (QUICK)scheme is used to provide higher order accuracy. Theviscous model used for this investigation is the twodimensional, directional positioned, segregated,renormali zation group (RNG) k-epsilon turbulencemodel, which is also discussed in greater depth in thenext section. The airflow velocities are 45ms-1 and 75ms-

1, which correspond to a range of Reynolds numbers(based on the wing chord) of 7.2x105 to 1.2x106, withthe atmosphere temperature of 298K. The operatingpressure was set at 101325Pa atmosphere pressure,roughness constant at 0.5, density of air at 1.225kg/m3

and residual criterion at 1x10-7.

COMPARISON OF RESULT ACCURACY

In order to aff irm the accuracy of the aerodynamicresults that were attained from the computationalprogram, several comparative methods had beenemployed, such as comparing different resolutions,velocities, turbulent models, boundary sizes, andpressure distribution plots, and comparing with priormathematical and experimental results. First of all , theaccuracy of the results is found by comparing theexperimental research result of NACA 642-415 wingsection conducted by the National Advisory Committeefor Aeronautics NACA (Abbott, 1959) with thecomputational result conducted by the use of designatedoperating condition and setting performed in existinginvestigation of Joukowsky J815 racecar rear wing. Fromthe comparison of the gradient and data as shown inFigure 2, the computational results gives a reasonablematch with the experimental research results.

Furthermore, by increasing the boundary size to(2000mmx6000mm) of the operating area and a finerresolution to 160000 cell s would only affect the result by5% to 8%, but it prolongs the computational time bymore than 10 to 15 times. By comparing the resultsobtained in different viscous models, the results ofstandard k-epsilon turbulence model being less accuratethan that of the RNG k-epsilon turbulence model, alsoagrees with the background theory based (Wilcox,1993). Finall y, the prior mathematical and experimentalresults of a similar racecar rear wing profil e, WortmannFx-72 MS150A was selected to compare with thecomputational result in this investigation. Thiscomparison is shown again in Figure 2, bothexperimental and mathematical results do give anobvious similarity of aerodynamic characteristics whencompared with the result given by the Joukowsky J815model. There are several reasons for the possibilit y ofthe slight differences though, such as the uncertainty ofReynolds number used by the prior testing of WortmannFx-72 rear wing, and that the operating condition set forthe model and for the test airfoil model are not identical.The relations between the li ft and drag characteristicscan also be explained by the results obtained from this

examination. When Reynolds number is increased, thecoeff icient of li ft and drag will also be increased. Thisrelation can be formulated further by the equation

Reynolds Number '( c2V Re )

to explain the variance of velocity (V), chord length (c)and viscosity (µ).

Figure 2 : Comparison of CFD and EXP. result(FX-72 & NACA642-415).

Overall the operating boundary and condition thatwere chosen for the investigation were predictablyaccurate enough to compare with actual wind tunnelresults of the two-dimensional real scale model. Smokevisuali zation will also be compared in later section.

COMPUTATIONAL RESULTS

The computational results will be demonstrated bysolving three main physical situations of interest. Thefirst case being the effect of the wing with differentangle of incidences within the non-boundary (i.e.isolated) conditions. In the second case, the effect ofground proximity on the wing and the body winginterference are to be considered. The third situation tobe investigated is the effect on the wing aerodynamiccharacteristic by adding a supplementary element withvariations of its angle of deflection and groundclearance.

Effect on Angle of attack

The resulting data were compared with the rearwing having different velocities 45ms-1 and 75ms-1, anddifferent viscous models. Figure 3 demonstrates thecoeff icient of li ft versus drag with the angle of attackvarying from –8o to 24o. With the angle of attack beingequal, it was found that the change of velocity would notaffect the aerodynamic characteristic so much. With the

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lower speed, the li ft and drag coeff icient would onlyreduce slightly. But with the use of standard k-epil sonturbulence model, the result was observed to be fairlyinconsistent and inaccurate, as well as obtaining loweraerodynamic eff iciency. The li ft curve has shown thisparticular highly cambered airfoil , which would reach itsspecific stalli ng point at approximately S =20o. Thecircumstance of stall can be explained by the fact thatthe separated flow at the leading edge will not reattachto the lower surface. When this occurs, the largeseparated region of unordered flow on the lower surfaceproduce an increase in pressure on that surface andconsequently, a sudden loss in negative li ft (downforce).Moreover, with negative incidence, for example, S =-4°,S =-8°, even it gave a good value of drag coeff icient, butit would not generate enough negative li ft required ofrear wing character. Therefore, the angle of attackbetween 0o to 16o is the most appropriate working rangeto be focused on for further investigation.

Figure 3 : Effect of angle of attack compared with-CL and CD.

Figure 4 : Aerodynamic eff iciency compared withdifferent Angle of attack.

The aerodynamic eff iciency is the ratio betweenvertical aerodynamic force acting on the car and itsdrag. Obviously, as the car performance strictly dependson vertical loads, the latter should be increased as far as

possible without affecting the vehicle drag. Thus thegreater the aerodynamic eff iciency:

D

L TT

D

L

C

CE

the better will generall y be the car performance.Nevertheless, there are cases where instead ofoptimizing the aerodynamic eff iciency, either the termCD or the term CL tends to be optimized even at theexpense of eff iciency. The aerodynamic eff iciencypresented in Figure 4 demonstrates the angle of attackS =0o with the maximum eff iciency Emax whichexponentiall y decreases while angle of attack increases.As the aerodynamic eff iciency curves also show, theangle of attack S =–8o resulted in rapid deterioration ofthe negative wing li ft due to the drop of li ft coeff icientand increases in drag. Again with Figure 3, the drag wasreduced when the angle of attack of the wing wasreduced, and it obtained the minimum coeff icient of dragCDmin at incidence of S =-4° and increased thereafter.This occurrence was agreed with all different viscousmodels and velocities.

Figure 5 : Pressure coeff icient distributed on thewing with different angles of attack.

Other than looking at the li ft coeff icient of the rearwing characteristic, the aerodynamic characteristic canalso be explained by analysis of the pressure distributionaround the wing. Figure 5 shows some typical pressuredistributions for the wing at various angles of incidence.It is convenient to deal with non-dimensional pressuredifferences with pU , the pressure for upstream, beingused as the datum. Thus the coeff icient of pressure isintroduced below

V

)(2

21 V WXY pp

CP

Looking at the plot for zero angle of attack Z =0°, it isseen that there are small regions at the nose and tail ofthe lower surface where Cp is positi ve, but that overmost of the section Cp is negative. The reduced pressureon the lower surface is tending to draw the sectiondownwards while that on the upper surface has theopposite effect. The pressure distribution on the uppersurface is positi ve and there is a resultant downward

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force on the section, which is the negative li ft (Houghtonand Carpenter, 1993).

As incidence is increased from zero, the pressurereduction on the lower surface increases both in intensityand extent until , at large incidence, it actuall yencroaches on a small part of the front upper surface.Moreover, the stagnation point moves progressivelyfurther back on the upper surface, and the increasedpressure on the upper surface covers a greater proportionof the surface.

The large negative values of Cp reached on thelower surface at large incidences, e.g. p =16°, are alsonoteworthy. In some cases values of –7 are found as inp =24°. By an equation

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1 qrstuvwx yqCP

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Effect of Ground proximity

The ground interference plays an important role inli ft generations. Thus, to simulate a ground clearance ofhc, a wall boundary with non-slip condition (i.e. accountof shear stress on the wall that is specified) wasdesignated in the computational program at the height ofh under baseline of the wing profil e, resulting in a solidsurface (ground). Three angles of attack, { =0°, { =4° and{ =8° with various height h=60mm, h=90mm, h=135mm,h=200mm, h=300mm and h=500mm from wingreference point were chosen for examination. Thiscorresponded to the lower surface of the wing being30mm, 60mm, 105mm 170mm 270mm and 470mm fromthe car body (ground) respectively. In general, the resultspresented in Figure 6 showed an increase of li ft whenthe wing was lowered towards the car body. Thisbehavior can be justified by the fact that the airflowbetween them was accelerating and thus creating a lowpressure region on the lower surface of the wing, whichalso created a suction on the ground (upper surface ofthe car body) presented in Figure 7. A high pressureregion (shown in red color) that can be visuali zeddeveloped on the upper surface of the wing. Therefore,an excessive downforce was generated by thiscircumstance.

Theoreticall y, for a non-viscous flow decreasing thearea between the wing and the ground causes thevelocity to increase as shown in Figure 8 and, thereforenegative li ft to occur. This negative li ft, in theory, tendsto increase infinitely when this area tends to zero, Figure9. But the air is a viscous medium and the phenomenonof viscosity creates a boundary layer around the wingwhich interferes with the ground when the distance h is

suff iciently small . This in turn slows down the flow ofair between the ground and the wing, instead ofspeeding it up, as is the case for non-viscous flow. Thetheoretical assumption of an ideal non-viscous airflow isno longer valid (Scibor, 1975). Furthermore, because ofthe highly compressed flow under the wing, somebackward flow and local vortices have also been

generated below the wing.

Figure 6 : Effect of ground proximity comparedwith –CL and CD.

Figure 7 : Pressure coeff icient distribution on4o of incidence and h=60mm above ground.

Figure 8 : Velocity magnitude on 4° of incidenceand h=135 above ground.

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The flow in this region was observed to be unordered asshown in Figure 10, which represented a height of30mm corresponding to 0.55mm clearance. While thesewake regions will extend and find a criti cal unstablephenomenon behind the racecar, the wake can suddenlyspread itself out during fast cornering in relation to thechange of relative wind direction. These effectivephenomenons only occur when the height clearance issuff iciently small as in the example shown in Figure 10.At the same time the slowing down of the flowunderside leads to an increase in the flow rate aroundthe upper surface of the wing which causes an increaseof the aerodynamic force. The effect of this would be toreverse the sign of the aerodynamic li ft from negative topositi ve. This sudden variation of aerodynamiccharacteristic and wake shape can obviously affect theeff iciency of the rear wing.

Figure 9 : Lift coeff icient variance of the wing graduall y towards the ground.

Figure 10 : Vorticity magnitude occur on the � =4°and 0.55mm ground clearance.

As described earlier, the use of pressure distributionto analyse the wing aerodynamic characteristics can alsobe used to explain the effect of ground proximity aspresented in Figure 11. With the wing closer to theground, the CP on the lower surface reduced in both theintensity and extent (increase of negative pressure) andpressure increase on the upper surface simultaneously,

which produces a suction effect on the lower surface andhighly increases its downforce (McCormick, 1995).

Back again to Figure 6, the drag is also reducedwhen the wing is moved closer the ground, and it obtainsthe minimum coeff icient of drag CDmin at the height of135mm (i.e. 105mm clearance) and the regain of dragafter that criti cal height.

Figure 11 : Pressure coeff icient distributed on thewing with different height.

Effect of two elements wing

The geometry and grid distribution of the twoelements wing section used in the investigation is shownin Figure 12, the supplementary wing with the scale of ¼of main wing. This shape of the airfoil sli ghtly differsfrom that of multi -element airfoil s used on aircraft, atthe gap region between the two elements, since thesecond element does not retract into the main wing (Katz& Largman, 1989). These two elements wing utili zedthe modified wing with increased camber in an attemptto maximize downforce potential and the wing assemblyto the deflection of the supplementary

Figure 12 : Two elements wing geometry and grid

distribution.

element, which is an ideal way to adjust rear downforcefor different circuits requirement. As the resultspresented in Figure 13 shows clearly that the twoelements wing has a much higher li ft than the wingwithout second element. The reason can be explained byvector diagram shown in Figure 14. The vector diagramindicates that the flow is always separated and theformation of wake region on the suction side (lowpressure region), behind the second element wing which

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is shown in blue color. Furthermore, on the pressure sideand above the wing, a small flow-recirculation area isobserved. Therefore the li ft increase can be attributed tothe additional twisting of the streamlines at the vicinityof the wing traili ng edge.

Figure 13 : Comparison of CD and -CL between oneand two element rear wing also with different angleof deflected flap.

When the gap between the two wing elements wasclosed, the li ft dropped further down together with areduction of aerodynamic eff iciency and increase of dragpenalty, which showed the positi ve interaction betweenthe elements and the advantage of the multi -elementsdesign (Duncan, 1990).

Figure 14 : Vector diagram of two element wingwith 45o deflection.

Figure 15 : Comparison of CD and –CL between oneand two element rear wing with effect of groundproximity.By altering the deflection of the second element

which is also being investigated and demonstrated inFigure 13, the 30o deflected flap has generated higheraerodynamic eff iciency compared with 45o and 60o

deflection. Further on the investigation of groundproximity of two elements wing, the magnitude of theli ft and drag coeff icients was demonstrated with thesame increased ratio, by comparing the results presentedin Figure 15 and Figure 13. According to the theory ofground effect that was described in previous section, theeffect of ground proximity with a single element wingcan also be hold for two elements wing.

Flow Visualization

Flow visuali zation techniques, such as the tuftmethod, oil film method, streak li ne method, hydrogenbubble method, Schlieren method and smoke technique...etc., play an important role in understanding therelationship between the body configuration of a vehicleand its aerodynamic characteristics (Tsuyoshi, 1979).Smoke technique mainly contributes to the observationof the external airflow around the vehicle. Theproduction of smoke lines in air is used for the basicinvestigation of boundary layers or shear flows as wellas for the solution of technical flow problems. This kindof visuali zation generall y yields qualitative results, andtherefore, is applied often in addition to othercomputational and/or experimental techniques. Thesmoke flow visuali zation in this investigation enablesthe recognition of a certain structure and behavior of thewake region, flow separation, separation bubble,turbulent reattachment, stagnation point ...etc. and howit is affected by different angle of attack.

The smoke tunnel used in this investigationbasicall y consists of two parts, namely a two-dimensional small -scale wind tunnel and an oil smokegenerator. This smoke generator was developed byapplying the principle that the smoke is generated by thevaporization of a mineral oil , and the oil vapor thendistributes and travels through a row of 29 nozzles (i.e.2mm dia.) into the test section.

A wooden-made half scale (chord) Joukowsky J815profil e was located in the center of the test section, withan angle rotator attached to it. The smoke was jet athorizontal incidence with low speed allowing for traitvisuali zation. Their smoke visuali zations were picturedat 1/60 second together with strobe light. Some samplepictures presented in Figure 16 to 19 show the flowpattern around the wing at various angles of attack. InFigure 16, negative angle of attack � =-8o was presented,which shows the flow separation development on theupper surface which provides low negative li ftcoeff icient. Two regions of separation are clearly shown

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Figure 16 : Flow visuali zation from Smoke Tunnelwith negative angle of attack « =-8° (1/60 shuttlespeed with strobe light).

Figure 17 : Flow visuali zation from Smoke Tunnelwith positi ve angle of attack « =0° (1/60 shuttlespeed without strobe light).

Figure 18 : Flow visuali zation from Smoke Tunnelwith positi ve angle of attack « =4° (1/4 shuttlespeed without strobe light).

in Figure 17 with « =0o. The region on the upper surfacewas shown by a small black patch just behind theleading edge and it reattaches on the surface. The otherseparation region is about 40% chord at the lowersurface. As the angle of attack flowing on the uppersurface is graduall y increased, it starts to separate at thetraili ng edge and just behind the leading edge on thelower surface as in Figure 18. For further increase ofangle of attack until it reaches its maximum negative li ftcoeff icient, the wing will stall and the separation on thelower surface is full y developed as in the case shown inFigure 19.

Figure 19 : Flow visuali zation from Smoke Tunnelwith angle of attack after stall « =24° (1/60 shuttlespeed with strobe light).

Most pictures also show that the boundary layerseparates from the wing, a wake forms which containsthe energy being introduced into the flow by the dragforce against the wing. As seen in those positi veincidence models, the wake consists of a “dead airregion” in which the pressure is nearly constant. A freeshear layer bounding this region is present between thewake and the potential flow. This shear layer is muchmore unstable than boundary layer (wall shear layer)because the wall has a damping effect (Eppler, 1990).Back on the observation of the pictures, in thedownstream the free shear layer collapses and, in furtherdownstream, behind the wing, the wake consists of avortex motion which may have a certain structure li ke aKarman street as in Figure 18. The wing having negativeangle of attack « =-8°, behind the leading edge of theupper surface or the vortex structure may be moreirregular as the suction area behind the wing with angleof attack « =24°.

In the experiment using smoke in the tunnel, theflow was laminar because of its lower velocity and thatReynolds number different to the turbulent flow used toperform in computational program. The laminarboundary layer that was found in the smoke tunnelaround the wing needs much less adverse pressuregradient for separation to occur than the turbulent one.Due to the greater extent of a positi ve pressure gradienton such a layer, a much more rapid separation of theflow would be caused than if the layer were turbulent. Alaminar layer is said to be away from the surface andcauses a larger wake than turbulent one, but which canprovide a better visuali zation of wake region such asKarman vortex as shown in the pictures.

This highly cambered wing model with relativelylarge curvatures, and high local curvature over theforward part of the chord may initi ate a laminarseparation when the wing is at a moderate angle ofattack. Small disturbances grow much more readil y andat lower Reynolds numbers in separation, as comparedto attached boundary layers. Consequently the separatedlaminar layer may well undergo transition to turbulencewith characteristic rapid thickening. This rapidthickening may be suff icient for the lower edge of the,now turbulent, shear layer to come back into contact

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with the surface and reattach as a turbulent boundarylayer on the surface. In this way a bubble of fluid istrapped under the separated shear layer between theseparation and re-attachment points. Within the bubble,the boundary is usuall y taken to be the streamline whichleaves the surface at the separation point, two regimesexist. In the upstream region, a pocket of stagnant fluidat constant pressure extends back some way and behindthis a circulatory motion develops, the static pressure inthis latter region increasing rapidly towards thereattachment point. This can be referred to Figure 6(Houghton & Carpenter, 1993).

Figure 20 : Vorticity contour diagram with 60¬deflection two elements wing.

In general, the velocity vector diagrams of variousangle of attack generated by computational programwere remarkably close to the flow patterns visuali zedaround the wing model tested in the smoke tunnel. Andthe vorticity contour diagram with the unsteady statetime dependant condition also indicate some similarityof vorticies behind the wing as in Figure 20. Theseindicated that the results generated by computationalprogram were fairly consistent with the experimentalresults excluding the dissimilarity of velocity andReynolds number.

CONCLUSIONThe present investigations focus on understanding

of the behavior of a racecar rear wing having effect ofboth positi ve and negative angle of incidences, with andwithout the ground effect and an additional wingelement added on, with its effect to the aerodynamiccharacteristic including the effect of ground proximity.

The following conclusions are obtained from theinvestigation:

1) (i) The Computational Fluid Dynamic program waswell suited to develop and was used to analyze theaerodynamic characteristics of most automobilesand its components. The CFD program can providea noticeably accurate result, as seen in thecomparison with the experimental data provided byNACA.(ii ) The RNG k-epsilon turbulence model gave amore accurate solution than standard k-epsilonturbulence model.

(iii ) Variance of Reynolds number will considerablychange the aerodynamics characteristics of the rearwing extended to the full vehicle.

2) This typical rear wing stalled at ­ s=20° and theworking section between ­ =0° through 16° angle ofattack were most appropriate to be used on racecar.With the various types of racecar circuits, wing withdifferent angles will require different categoriesstated as follows:

-Fast speed circuit: Wing with ­ =0° to 4° incidencewould give a highest aerodynamic eff iciency E,which means a lowest drag penalty together with afair negative li ft force.-Medium speed circuit: Wing with ­ =4° to 8°incidence achieved a moderate range of negative li ftforce between others, which could provide betterhandling and cornering abilit y than the one used infast speed circuit.-Low speed circuit: Wing with ­ =8° to 12° thatshows an increase of drag for the high angle ofattack, but for its performance at high attitudeswhich makes it suitable for slow speed circuitwhere high level of vertical load are needed.

3) The influence of the ground proximity clearlyshows that the li ft coeff icient increasedexponentiall y with the rear wing graduall y loweringtowards the car body, and there was only a slightchange on the drag coeff icient. When the heightclearance between the wing and car body wassuff iciently small , it would slow down the airflowbetween them as the results gave for the heightlarger than 36mm. Also some local vortex andbackward flow were found under the lower surface,which would largely interfere with the wingaerodynamic characteristic as compared with thereport of Scibor-Rylski, 1975.

4) The results for the two elements wing shown by anadditional wing element added on can highlyincrease the downforce coeff icient, but in the meantime, a larger drag penalty would be obtained. Withthe ground effect of the two elements wing, the li ftand drag coeff icient increase proportionall y as thecase in single element wing. By varying thedeflection of second element angle, it was an idealway to adjust rear downforce for different circuitneeded. The results also proved that the secondelement with a 30° of deflection would give amoutstanding aerodynamics eff iciency ahead from allthe others.

5) The smoke visuali zation has given a betterinterpretation of flow behavior around the rearwing, with the influence of angle of attack. Thevisuali zation has clearly shown the structures of theflow such as, wake region, traili ng vortex, Karmanvortex street ...etc, and combining with thecomputational results obtained together, anexcellent understanding of the aerodynamicscharacteristics and behavior of this particular rearwing model are provided.

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Further recommendations of this investigation includecarrying on a series of experimental tests on the full -scale rear wing being isolated as well as with the wingbeing attached to the full car body. This type ofexperiment can properly validate the result of thepresent one.

Moreover, a three dimensional model should beconstructed into the program together with other add ondevices such as Gurney-flap, end plates, and may be amulti -elements wing used in the Formula I car racing.Some conditional effects li ke side wind is also importantto consider and simulate during further investigation.

ACKNOWLEDGEMENTS

This investigation was supported by MonashUniversity, Department of Mechanical Engineering. Theauthor also gratefull y acknowledges Associate ProfessorK. Hourigan for his valuable information and assistancethroughout the whole investigation. Special thanks aregiven to P. Reichl and M. Tan for their assistance andadvice on computational program. The author wouldwish to thank Mr. Long (Monash Workshop) for hissupport and assistance on smoke tunnel andexperimental apparatus.

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