Effects of rear spoilers on groundvehicle aerodynamic dragHalil Sadettin Hamut and Rami Salah El-Emam

Faculty of Engineering and Applied Science,University of Ontario Institute of Technology (UOIT), Oshawa, Canada

Murat AydinFaculty of Engineering and Applied Science,

University of Ontario Institute of Technology (UOIT), Oshawa, Canada andInstitute of Energy, Istanbul Technical University, Istanbul, Turkey, and

Ibrahim DincerFaculty of Engineering and Applied Science,

University of Ontario Institute of Technology (UOIT), Oshawa, Canada

AbstractPurpose The purpose of this paper is to examine the aerodynamic effects of rear spoiler geometryon a sports car. Today, due to economical, safety and even environmental concerns, vehicle aerodynamicsplay a much more significant role in design considerations and rear spoilers play a major role in this area.Design/methodology/approach A 2-D vehicle geometry of a race car is created and solved usingthe computational fluid dynamics (CFD) solver FLUENT version 6.3. The aerodynamic effects areanalyzed under various vehicle speeds with and without a rear spoiler. The main results are comparedto a wind tunnel experiment conducted with 1/18 replica of a Nascar.Findings By the CFD analysis, the drag coefficient without the spoiler is calculated to be 0.31.When the spoiler is added to the geometry, the drag coefficient increases to 0.36. The computationalresults with the spoiler are compared with the experimental data, and a good agreement is obtainedwithin a 5.8 percent error band. The uncertainty associated with the experimental results of the dragcoefficient is calculated to be 6.1 percent for the wind tunnel testing. The sources of discrepanciesbetween the experimental and numerical results are identified and potential improvements on themodel and experiments are provided in the paper. Furthermore, in the CFD model, it is found thatthe addition of the spoiler caused a decrease in the lift coefficient from 0.26 to 0.05.Originality/value This paper examines the effects of rear spoiler geometry on vehicle aerodynamicdrag by comparing the CFD analysis with wind tunnel experimentation and conducting an uncertaintyanalysis to assess the reliability of the obtained results.Keywords Aerodynamic drag, Drag and lift coefficient, Rear spoiler, Wind tunnelPaper type Research paper

The current issue and full text archive of this journal is available atwww.emeraldinsight.com/0961-5539.htm

Received 20 March 2012Revised 20 September 2012Accepted 12 October 2012

International Journal of NumericalMethods for Heat & Fluid Flow

Vol. 24 No. 3, 2014pp. 627-642

r Emerald Group Publishing Limited0961-5539

DOI 10.1108/HFF-03-2012-0068

NomenclatureA area (m2)b standard systematic uncertaintyCD drag coefficientCL lift coefficientFD drag force (N)FL lift force (N)L length (m)

M number of measured data pointsP pressure (kPa)r experimental resultsRe Reynolds numbers standard deviationU overall uncertaintyV velocity (m/s)

The authors would like to thank Dr Akif Ezan for all his help and support with modeling thevehicle geometry and UOIT Engineering Lab Technician Qi Shi for helping them with theexperiments.

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1. IntroductionToday, due to economical, safety and even environmental concerns, vehicleaerodynamics play a much more significant role in design considerations than it didbefore. It is estimated that the aerodynamic drag is the governing form of resistancewhen vehicles reach speeds of 80 km/h or greater, especially considering the factthat 65 percent of the power required at 110 km/h is consumed due to overcomingaerodynamic drag (Leduc, 2009; Diamond, 2004). Therefore the improvements inaerodynamic characteristics can result in significant decrease in driving stability,handling, fuel consumption and overall efficiency. Thus, it is no surprise that rearspoilers are becoming more widely used in the auto industry. With increasing in the oilprices and more stringent legal regulations, more research is conducted on accessoriesthat can improve the vehicles fuel efficiency. Among these accessories, rear spoilersare one of the major ones due to being relatively inexpensive, easy to install andaesthetically appealing. Furthermore, it can reduce the drag coefficient and increasethe fuel efficiency significantly for most passenger vehicles, while decreasing the liftcoefficient in the expense of the drag coefficient in most race cars. However, the benefitsof the rear spoiler are not limited only to fuel cost. By also deflecting the traveling airupwards by increasing the pressure rear deck, the rear spoiler can provide bettertraction, faster turning, acceleration and brake as well as increasing the vehicle safety.

Even though the rear spoiler is one of the key players in vehicle aerodynamics, thereare only limited amount of associated studies in the literate that incorporates boththe computational analysis and experimental verification. The studied vehicles varyfrom mini-vans to modified race cars and are analyzed for various purposes such asimproving aerodynamic and aero-acoustic characteristics, preventing exhaust smokepatterns and even reducing the dirt and/or snow accumulating on the rear surface ofthe vehicle (Kiyoshi et al., 1997; Parihar et al., 2006).

Tsai et al. (2009) analyzed the effects of five different rear spoilers on the aerodynamicsand aero-acuostics of a Honda S2000 under 180 km/h. The authors constructed the modelin ICEM/CFD and used FLUENT as the CFD solver with k-A model. They havedetermined the sound pressure levels for each case and also calculated drag coefficientsranging from 0.4 to 0.51 and lift coefficients ranging from!0.001 to 0.06. The lowest dragcoefficient is calculated in the case without the spoiler and it increases in different spoilerdesigns in the attempts of reducing the lift coefficient. Zake (2008) analyzed BLM carwith spoiler and touring wings in order to understand their aerodynamic effects on thevehicle between 70 and 150 km/h and determined that the lift coefficient is reduced inthe expense of the drag coefficient. Kim (2004) studied the effects of rear spoilers on wakeflow characteristics and drag for large-sized buses. The author assumed symmetry andcreated half of the 3-D model of the vehicle from the centerline, without the side mirrors.The author used RNG k-A model in order to incorporate the secondary straining effectthat is not considered with the standard k-A model and set the Reynolds number to5.27 " 106 based on the body height and inlet velocity. The author calculated the dragcoefficient for the vehicle to be 0.518, which is within 4 percent of the experimental valuedetermined in a wind tunnel by the 1/16-scale of a model commercial bus in the reference.By adding a rear spoiler to the vehicle, the initial extreme vertex produced on thecross-car axis at the rear upper body was removed, in the expense of creating a new

Xi measurements of the ith variableGreek symbolsr density (kg/m3)

m viscosity (kg/m-s)tij shear-stress tensor (N/m2)P dependent variable

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vertex on the rear end surface and reduced the drag coefficient by more than 12 percent.Kim et al. (2008) also developed a rear spoiler for a mini-van and conducted analysis usingCFD standard and RNG k-A models at Reynolds number 2.72 " 106 with and withoutthe spoiler and found reduction in drag and lift by 5 and 100 percent, respectively.

As can be seen from the literature, there is a significant lack of studies thatexamines the effects of rear spoilers on vehicles, that combines computational analysisand experimental testing. Moreover, the authors do not consider the error associatedwith the uncertainties throughout the experimental process. Thus, the purpose ofthis study is to examine the aerodynamic effects of rear spoiler geometry in on a racecar under various wind speeds using a CFD solver FLUENT. The results are comparedto the wind tunnel testing of a scaled down vehicle model and uncertainty analysis isconducted to assess the reliability of the results.

2. Experimental setupThe 1/18-scale Nascar model was tested in a Flotek wind tunnel designed by the GDJInc as seen in Figure 1. The wind tunnel has exceptionally consistent velocityprofiles across the test section, with turbulence measures of less than 0.2 percent, and

(4,202 mm)TOTAL LENGTH

(3,977 mm)STAND LENGTH

(3,847 mm)TUNNEL LENGTH

(1,600 mm)TOTAL HEIGHT

Steel Contraction cone Steel Diffuser SectionClear Acrylic Test SectionMounted on 1.5 in. Steel Angel

PVC Shelf

1.25 x 1.25Steel Supports

Control Box Mount2 x 2 Steel Frame

Fan Guard

2 HP DC Motor

Source: Per GDJ Inc.

(861 mm)FRONT STAND

WIDTH(1,067 mm)FRONT TUNNEL

WIDTH

(731 mm)REAR STAND

WIDTH

Figure 1.Flotek 1440 wind tunnelused in the experiment

and its dimensions

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allows speeds up to 40m/s. It has a 4200 " 4200 inlet and 140 overall length with 1200 "1200 " 3600 visible test section.

For the experiment, a 1/18-scale model of a Nascar is used with nine pitot tubesconnected across the top body geometry in the same plane (except for the additionalspoiler measurement), in order to get the velocity and pressure distribution of the windflowing over the vehicle. A strain gauge is connected at the bottom in order to calculatethe drag force of the vehicle under the velocities tested. The pressure/velocity anddrag force measurements are monitored and calculated through a software, while thepressure measurements are also calculated through manometers on the wind tunnel.The scaled model with the sensors can be seen in Figure 2.

2.1 Buckingham-Pi verificationThe wind tunnel experiment was conducted using a 1/18-scale of a Nascar in a windtunnel with 4200 " 4200 inlet and 140 overall length. In order to confirm that the scaledmodel would be a good representation of the actual prototype, dimensionless analysisis conducted, ensuring that the geometric, kinematic and dynamic similarities areachieved. To be able to make this comparison, Reynolds number is selected as theindependent dimensionless parameter and a variation of the standard drag coefficientis selected as the dependent parameter as seen in Equation (3) (compressibilityparameters are neglected due to lowwind velocities, Mach number iso0.1). Buckingham-Pi theory reveals that when the chosen independent variable (Reynolds number) betweenthe model and the prototype is determined to be the same, then the dependent variable inthe prototype (drag coefficient) is guaranteed to be equal to the dependent variable in themodel (Cengel and Cimbala, 2006). Thus, the drag coefficient for the actual vehicle can bedetermined as long as the Reynolds number between the model and the prototype aredetermined to be similar:

P1 f P2 ; where P1 FDrV 2L2 and P2 rVLm

1Here, air density for the measured wind tunnel temperature is determined to be1.184 kg/m3 along with a dynamic viscosity of 1.849 " 10!5 kg/m-s. The velocity ofthe actual prototype is taken as 30m/s and the characteristic length of 2.4m is used

123

456

78

9

Figure 2.1/18-scale Nascarmodel along with 13measurement points

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for the car. The actual length is used for the characteristic length based on the sameassumption used in the vast majority of the studies conducted (Macciacchera and Ruck,2001; Caldichoury et al., 2011). With the aforementioned parameters, the Reynoldsnumber is calculated to be 4.6 " 106.

If the temperatures are taken to be the same for the prototype and model testing,then the density and viscosity parameters will be similar for both cases. Hence thesignificant reduction is size (1/18 scale) requires the wind velocity to be 18 times higherto achieve similarity. Since the prototype speed is usually already 30m/s, the model isrequired to be exposed to 540m/s, which is not feasible with the wind tunnel used.In order to work around this limitation, correlation between drag coefficient andReynolds number is developed for practical use based on the experimental data. FromFigure 4, it can be seen that around Reynolds number of 2.9 " 104, the drag coefficientbecomes independent of the Reynolds number. By using this correlation, it can be assumedthat the drag coefficient will not differ significantly between the highest achieved windvelocity and the required wind velocity to achieve dynamic symmetry. Therefore dragcoefficient value is calculated using the aerodynamic drag force at this velocity.

2.2 ExperimentIn order to verify the accuracy of the Fluent model and compare it to real life case, theexperiment is conducted in the wind tunnel. Special attention has been given to haveto match the CFD model parameters (such as temperature and wind speed) and well asmaking it as realistic as possible. The vehicle is placed in the middle of the test area ofthe wind tunnel facing the incoming wind and secured tightly to reduce any potentialdiscrepancy in measurements. The data for the pressure and velocities on the vehicleupper body based on different wind speeds are retrieved.

The vehicle grill (in point 1, Figure 3) has the highest pressure and zero velocitysince it is the stagnation point of the flow. As the wind travels over the vehicle, itspressure initially is reduced, causing an increase in the velocity, and increases againas it finally reached to the rear of the vehicle, reducing its associated velocity, but stillbeing less than the initial due to frictional forces. The comparison of pressure andvelocity distributions is illustrated in Figure 3.

The measurements from the strain gauge are displayed in the drag force sectionof the software and provided the respective force in grams force. By knowing theassociated air density, vehicle frontal area and wind velocity, the drag coefficient iscalculated using the drag force equation:

FD 12rV 2ACD 2

Based on the air density at 251C and frontal area of 31.5 cm2, the drag coefficient of thevehicle can be calculated with respect to any tested wind velocity. The correlationbetween the wind velocity and drag coefficient can be seen in Figure 4. The pointsare best fitted with respect to available data.

It should be noted that, however, there will be some additional error associatedwith the experimental calculations. The source of this error comes from the fact thatthe cross-sectional area of the wind tunnel is relatively small, which can havesignificant impact on the flow velocities. If the cross section of the wind tunnel is notlarge enough, the effective freestream wind passing the vehicle can acceleratesignificantly which can increase the aerodynamic drag and therefore prevent thesimilarity conditions to be achieved. In order to determine the impact of this effect,

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wind tunnel blockage can be calculated based on the ratios of the model frontal areaand the cross-sectional area of the wind tunnel test section.

The scaled model frontal area is measured to be 0.00315m2 and the manufacturerstunnel specifications is used for the respective cross section (0.0929m2), creating a

Grill

Top Surface Pressurecentimeters of water

Top Surface Velocity(m/s)

0.07 0

5

1015

2025

50

40

30

20

10

0123456789

Convergingaccelerating flow

(7)

Convergingaccelerating flow

(2-4)

Low PressureHigh Speed

(5-6)High Pressure

Low Speed(1)

High PressureLow Speed

(8-9)

1

0252423

231120

2626

23456789

3.893.523.204.264.103.170.782.49

HoodHoodHoodRoofRoof

TrunkTrunk

Spoiler

GrillHoodHoodHoodRoofRoof

TrunkTrunk

Spoiler

Figure 3.Pressure and velocitydistribution overthe vehicle

0.38

0.37

0.365

0.36

0.355

0.35

0.345

0.34

0.335

0.33

0.375

2 2.2 2.4 2.6 2.8 3Re(104)

C D

y = 2E10x 3 2E06x 2 + 0.0041x 2.9816R 2= 0.97

Figure 4.Drag coefficientas a function ofReynolds number

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blockage ratio of 3.3 percent. Since the acceptable blockage ratio in the literatureis determined to be up to 7.5 percent (Holmes, 2001), this value will not have muchsignificant impact on the drag coefficient determined by the experiment. Furthermore,the flow profile in the wind tunnel will be slightly different than the actual scenario dueto the vehicle being stationary during testing. This will result in a boundary later buildup on the wind tunnel floor, which changes the underbody flow from the actual caseand hence prevent kinematic similarity.

The lift coefficient on the other hand (shown in Equation (3)) was not been able to becalculated due to the limitations of the experimental setup. Because of the spaceconstraints inside the model vehicle, the pitot tubes were placed only on the upperportion of the vehicle, therefore aerodynamic effects of the underbody were not beenable to be captured properly. Moreover, the available software was not capable ofcalculating the lift force associated with the vehicle. Hence only drag force measurementsare used for the analysis:

FL 2rV 2AFl 3

3. Uncertainty analysisIn order to assess the reliability of the results from the wind tunnel experiments,one should consider the error associated with systematic and random uncertaintiesthroughout the experimental process. For the study, single-sample experimentuncertainty will be analyzed due to relatively low number of independent data pointstaken at each test point as well as the having relatively small fixed error. In order todetermine the overall uncertainty of the experiment, the contributors of uncertaintyassociated with the capability of the instrument and the instability of the processes aremerged with the help of Taylor series expansion and root-sum-square methods.

Initially the fixed and variable errors associated with the wind tunnel systemare determined by not varying the experimental process (zeroth-order level). Next, theprocess instability and random instrument error in the process is determined runningdifferent processes using the same equipment, procedures and instrumentation(first-order level). Finally the overall uncertainty in the experiments, including theeffects of process unsteadiness, is estimated by taking the root-sum square of the fixederrors due to instrumentation and first-order uncertainty (Nth-order level) (Moffat, 1988).

3.1 Zeroth-order analysisThe drag coefficient of the vehicle is determined from the data obtained through theexperiments using the modified version of Equation (4). The uncertainties associatedto each term in the equation are determined with respect to the measurement errors foreach element. The zeroth-order estimates of the systematic and random standarduncertainties for the aforementioned variables can be seen in Table I.

Variable Value Systematic standard uncertainty Random standard uncertainty

FD 1.2152Na 0.02 N

r 1.184 kg/m3 0.004 kg/m3 V 35.41m/s 0.3m/sA 0.0035m2 0.004m2

Note: aDrag force with respect to the highest wind velocity of 35.41m/s

Table I.Zeroth-order estimates ofsystematic and random

standard uncertainties forvariable in drag coefficient

determination

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The uncertainties in the measured variables cause the uncertainty in the results (r inthis case) and hence is often modeled using a propagation equation based on Taylorseries expansion to find the systematic standard uncertainty (br) is given below(Coleman and Steele, 2009):

b2r XJi1

qrqXi

! "2b2xi

XJi1

qrqXi

! "2s2xi for r r X1;X2 . . .XJ

# $ 4where bxi is the standard systematic uncertainty and sxi is the standard deviation formeasurement of each variable Xi. The partial derivatives needed for the aboveequation are given below:

qCDqFD

2rV 2A

5

qCDqr

!2FDr2V 2A

6

qCDqV

6FDrV 3A

7

qCDqA

2FDrV 2A2

8

By substituting the partial derivatives into the Taylor series expansion equation, thesystematic uncertainty (with respect to air density and vehicle cross-sectional area) iscalculated as follows:

b2CD qCDqr

br

! "2 qCD

qAbA

! "29

which results in bCD 0:0014 after substituting the values in the above equation.By using the same method, the random uncertainty associated with the expression

(with respect to drag force and velocity) is given below:

s2CD qCDqFD

bFD

! "2 qCD

qVbv

!2 10

which results in sCD 0:0205 after substituting the values in the above equation.Thus the large-sample expression for the combined standard uncertainty UCD, with

a 95 percent confidence, can be calculated as shown in the equation below:

UCD 2 b2CD s2CD% &1

2 0:0412 11This uncertainty, which shows the suitability of the wind tunnel, corresponds to 12.1percent of the calculated drag coefficient.

3.2 First-order analysisBy having ten repeated measurement values of the variable CD, the sCDvalue is knownfrom previous experiments as 0.02. If the first-order random standard uncertainty is

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divided into two categories, namely zeroth-order and sample-to-sample variation, therandom standard uncertainty due to sample-to-sample variation can be calculated asgiven below:

sCD# $

SAMPLE VAR sCD

# $21ST

! sCD# $2

ZEROTH

h i12 0:021 12

This uncertainty, which estimates the scatter in the results of different trials,corresponds to 6.9 percent of the calculated drag coefficient.

3.3 Nth-order analysisFinally the overall uncertainty of the mean result (with an uncertainty if 95 percent) isobtained from the following equation:

UCD 2 b2CD s2CD

! "12

0:0192 where sCD sCDffiffiffiffiffi

Mp 0:0095 13

where the bar symbol above the drag coefficient represents the mean value and M isthe number of measured data points. This uncertainty, which represents the overalluncertainty composed of fixed errors due to instrumentation and first-orderuncertainty, corresponds to 6.1 percent of the calculated drag coefficient.

4. Numerical analysisWhen a vehicle is driving on the road, as the air flows through the vehicle, it movestowards the point A in Figure 5 (so called stagnation point), where the static and totalpressures become equal. From this point, the flow divides into, above and below thevehicle. Determining the flow pattern underneath the vehicle can be very complex anddepends on various factors such as the height of the underbody and the presence offairings. Thus raising the vehicle underbody and/or having smooth fairings that coverthe mechanical elements underneath the vehicle can lower the air drag significantly.However, these options are usually ignored when compared against other issues thismay causes such as more costly maintenance (Genta, 1997).

In point B of Figure 5, the flow velocity increases by the pressure being less thetotal (or even less than the ambient) pressure. Between points C and D, the flowdetached and attached back again due to the sharp change in the vehicle geometry.The pressure distribution between points E and F primarily depends on the shape ofthe vehicle roof, but will still be relatively low. At the end of the roof the velocity slows

AB C

D E F

Source: Adapted from Genta (1997)

Figure 5.Streamlines around a

passenger vehicle in thesymmetry plane

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down as the pressure rises and the flow detaches after point F. This phenomenon canbe seen in Figure 5.

This phenomenon is demonstrated by cutting the vehicle geometry in rubber andplacing it in a Hele-Shaw-type fluid visualization apparatus, as well as placing theactual scaled replica in the wind tunnel with a piece of string on the upper body, whichin both cases, give an idea on the boundary streamlines, separation and wake regionsin the rear portion of the vehicle. The initial one is done with the help of water andred dye between two glass sheets where the vehicle geometry is placed downstream ofthe water flow. Even though the streamlines look non-ideal due to the limitations of theexperiment, the flow separation in the rear portion of the vehicle and respective wakeregions can be clearly seen, as shown in Figure 6.

The latter one is demonstrated in the wind tunnel where the flow of the stringguides as streamlines over the vehicle. It can be seen that, with the rear spoiler, the flowseparation occurred further behind the vehicle which affects the drag force at thislocation. This is shown in Figure 7.

4.1 Governing equationsThe external flow around the car can be considered laminar or turbulent according to thespeed of the car. Two-dimensional incompressible steady-state Navier-Stokes equationswere used to predict the associated aerodynamic features. The continuity andmomentumequations, that are seen below, are solved numerically with k-A turbulence model:

quqx qvqy 0 14

r uquqx v qu

qy

! " ! qP

qx m q

2u

qx2 q

2u

qy2

!15a

r uqvqx v qv

qy

! " ! qP

qy m q

2v

qx2 q

2v

qy2

!15b

Figure 6.Streamlines over thevehicle geometry in theHele-Shaw-type fluidvisualization apparatus

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The standard k-A model is a semi-empirical model that is based on model transportequations for the turbulence kinetic energy, k which have been driven from the exactequation, and the other transport equation represents the dissipation rate and it wasobtained from the physical reasoning and bears little resemblance to its mathematicallyexact counterpart. This model has been used extensively during the last two decadesand has been widely accepted for engineering applications. While the standard k-Aturbulence model was being the focus of turbulence modeling, another turbulencemodeling techniques were also intensively developed by researchers. May be some ofthem are more superior and more user-friendly for general CFD users than the standardk-A turbulence model. However, the standard k-A turbulence model performs better asfar as the computational time and cost are considered. In this study, standard k-Aturbulence model is chosen to predict the flow field around the vehicle.

4.2 CFD analysisThe computational domain of the model is prepared on ICEM Ansys software. Themodel represents the external flow over a 2-D model of the car which resides in a windtunnel. Two different cases are studied for the same car geometry with andwithout spoiler.The car model length is 2.56m and its height is 0.7m. Also the computational domainrepresenting the tunnel walls are considered as 13m long and 5m high. The mesh iscreated for the whole computational domain. For model a, Figure 8(a) which represents themodel of the car without spoiler, an orthogonal mesh is created, while for the second case,with the spoiler is meshed using tri-mesh trend, Figure 8(b) which is recommended formeshing models dealing with aerodynamic studies. The meshes are refined until meshindependence is reached; mesh refining is performed in the area around the car where theflow characteristics are predicted to be affected significantly. Table II shows the meshfining steps that are taken. The computational mesh is shown in Figure 8. The uniformvelocity is specified as the inlet boundary condition, outlet flow is specified for pressureoutlet. The top and down boundaries of the computational domain are selected aswall where we are modeling the flow of the air around the car body in the wind tunnel.

Figure 7.The streamline

representation withrespect to the string

placed over the vehiclein the wind tunnel

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Inlet velocity applied as 30m/s (Re 1 " 106). The drag coefficient and lift coefficient aredimensionless forms of drag and lift. They are defined with respect to Equations (4) and(5), where the area A is defined as the characteristic area of the body, which is consideredas the frontal area (the projected area seenwhen looking to the object in a direction parallelto the upstream velocity) for the drag coefficient calculations. For the lift coefficientcalculations, the planform area (which is represented by the projected area observedby looking towards the object in a direction normal to the upstream flow) is considered.

4.3 Numerical resultsFinite volume method is used through which the conservation principles wereapplied to the model. The governing differential equations are integrated to yeild a setof algebric equations to ensure all the quantities are conserved. Next, these algebricequations are solved through numerical means to obtain the unknown quantities.The standard SIMPLE scheme was utilized to solve the pressure velocity couplingdiscretized equation. First-order upwind scheme was used for discretization ofthe turbulence kinetic energy and the turbulence dissipation rate. The solver usesunder relaxation to control the update of computed values at each iteration. The underrelaxation factor applied to this study were 0.1, 0.3, 0.8 and 0.8 for pressure, momentum,turbulence kinetic energy, turbulence dissipation rate, respectively.

The drag coefficient calculated for the car without the spoiler as 0.31 and for the casewith spoiler 0.36 while the lift coefficient decreased from 0.26 to 0.05. The spoilers areused for decreasing the drag coefficient, however, for most sports cars; the lift force is

Without spoiler

With spoiler

Figure 8.The computationalmeshes used for the carwithout and with thespoiler

Number of cells Drag coeff. (without spoiler) Number of cells Drag coeff. (with spoiler)

26,058 0.29 36,841 0.3675,474 0.31 84,345 0.36120,436 0.31

Table II.Mesh independency

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more important as it controls the car stability, thus the drag coefficient can be sacrificedin order to achieve the desired lift coefficient. Figure 9 represents the shape of the streamlines of the external flow around the car body with and without the spoiler at the sameoperating conditions. It is seen that the spoiler caused a deflection in the streamlines nearthe car body which affected its shape in the rear area after the car body.

The static pressure contours are represented for both with and without spoiler casesin Figure 10, as it is clear that the pressure over the rearend of the car with the spoiler ishigher, which is going to increase the drag force across the spoiler and hence for thebody of the vehicle. It can be also concluded that a sudden deceleration of the flowoccurs in the front bumper which creates an increase in the static pressure.

As calculated by the experiments, the front part of the vehicle is simulated to havezero velocity and highest pressure, shown both through the CFD analysis and windtunnel experiment (Figures 10 and 11, along with experimental point number 1 inFigure 3, respectively). Subsequently, the flow accelerates in a rapid trend over the topof the body causing a pressure suction zone, which is indicated by the blue region insimulation results provided in Figure 10, along with the high-velocity region in theexperimental points 5 and 6. Finally, the pressure gradually increased as the flowvelocity decreased and the flow eventually detached from the vehicle, which is alsocaptured through these figures.

Even though the CFD model creates significant value in terms of predicting theaerodynamic drag and lift associated with the vehicle under predetermined conditions,verifies the experiment outcomes and the associated literature values within smallerror margin; there are some shortcomings/potential improvements associated with themodel. Due to the complicated geometry and unavailability of data of the tested vehiclesexterior geometry, the model is based on a more generic passenger vehicle geometry.

Furthermore, in order to save computational time, the model is constructed in 2-Dwhich was unable to consider some of the real life effects associated with the cross-carair flow distribution. It should be expected that, due to having no runoff area for the airflow that flow over or under the vehicle, some of the flow that travels from the top orbottom of the vehicle would actually separate from the side and travel through thevehicle side. Since this phenomenon is not taking into consideration, the accelerationover and under the body could be over predicted, resulting in larger velocities in theseregions. Moreover, the 2-D model did not incorporate the wheels in the geometry which

Without spoiler

With spoiler

Figure 9.Stream lines of the air

flow around the car body

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reduces the drag force calculated in the simulation, due to reducing the frontal area ofthe vehicle. Finally, the separate flows inside the actual vehicles, namely the coolingflow inside the engine department and the passenger cabin, are also ignored in themodel. Even though the passenger cabin effects are usually negligible, the air flowingthrough the engine department can affect the overall drag coefficient significantly(Launder and Spalding, 1974; Theera-apisakkul and Kittichaikarn, 2005).

5. ConclusionsThis paper examines the aerodynamic effects of rear spoiler geometry on a race car. 2-Dvehicle geometry is constructed using ICEM commercial package and the results arecompared with a wind tunnel experiment of a 1/16 scale of the actual vehicle.Through the CFD analysis, the drag coefficient without the spoiler is calculated to be0.31. When the spoiler is added to the geometry, the drag coefficient increases to 0.36.

6.48e+025.69e+024.90e=02

3.31e+022.51e+02

9.23e+011.28e+01

6.66e+011.46e+022.25e+023.05e+023.84e+024.64e+025.43e+026.23e+027.02e+027.82e+028.61e+029.41e+02

1.72e+02

4.10e=02

6.19e+025.23e+024.26e+023.29e+022.32e+021.35e+023.84e+01

5.85e+011.55e+022.52e+023.49e+024.46e+025.43e+026.40e+027.36e+028.33e+029.30e+021.03e+031.12e+031.22e+031.32e+03

Without spoiler

With spoiler

Figure 10.Contours of the staticpressure (Pa)

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The computational results with the spoiler are compared with the experimental data,and a good agreement is obtained within a 5.8 percent error band. The sources of errorsare identified along and potential improvements on the model and experiments areprovided in the paper. Furthermore, in the CFD model, it is found that the additionof the spoiler caused a decrease in the lift coefficient from 0.26 to 0.05.

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4.38e+014.23e+014.09e+013.94e+013.79e+013.65e+013.50e+013.36e+013.21e+013.06e+012.92e+012.77e+012.63e+012.48e+012.33e+012.19e+012.04e+011.90e+011.75e+011.61e+011.46e+011.31e+011.17e+011.02e+017.30e+005.84e+004.38e+002.92e+001.46e+000.00e+00

8.76e+00

4.82e+01

4.34e+014.10e+013.86e+013.61e+013.37e+013.13e+012.89e+012.65e+012.41e+012.17e+011.93e+011.69e+011.45e+011.20e+019.64e+007.23e+004.82e+002.41e+000.00e+00

4.58e+01

Without spoiler

With spoiler

Figure 11.Velocity contours (m/s)

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Macciacchera, I.U. and Ruck, B. (2001), Pressure fluctuations induced by road vehicles inambient air a model study, Proceedings of the Workshops on physical modelling ofenvironmental flow and dispersion, University of Hamburg. Hamburg, September 3-5.

Moffat, R.J. (1988), Describing the uncertainties in experimental results, Experimental Thermaland Fluid Science, Vol. 1 No. 1, pp. 3-17.

Parihar, A., Kulkarni, A., Stern, F., Xing, T. and Moeykens, S. (2006), Using FlowLab, aneducational computational fluid dynamics tool, to perform a comparative study of turbulencemodels, Computational Fluid Dynamics Journal, Vol. 15 No. 1, pp. 175-182.

Theera-apisakkul, K. and Kittichaikarn, C. (2005), Numerical Analysis of Flow Over Car Spoiler,Computational Mechanical Laboratory, Department of Mechanical Engineering, Faculty ofEngineering, Kasetsart University, Bangkok.

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Zake, R. (2008), Aerodynamics of aftermarket rear spoiler, BS thesis, University of Malaysia, Pahang.

Corresponding authorHalil Sadettin Hamut can be contacted at: Halil.Hamut@uoit.ca

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