ABSTRACTState of the art aerodynamic research of vehicles oftenemploys strongly simplified car models, such as the Ahmedand the SAE body, to gain general insights. As these modelsexhibit a high degree of abstraction, the obtained results canonly partly be used for the aerodynamic optimization ofproduction vehicles. Aerodynamic research performed onspecific vehicles is on the other hand often limited due totheir short life span and restricted access. A new realisticgeneric car model for aerodynamic research - the DrivAerbody - is therefore proposed to close this gap. This paperfocuses on the development of the model and the firstexperimental results, namely force and pressuremeasurements of the different configurations. Theexperiments were performed in the recently updated WindTunnel A of the Institute of Aerodynamics and FluidMechanics at the Technische Universität München.

INTRODUCTIONDue to growing customer consciousness and various nationaland international agreements, the reduction of CO2 emissionshas become increasingly important. As the car sector is oneof the big contributors to the overall CO2 emission, it isnecessary to lower the fuel consumption of contemporarycars. Aerodynamic optimization of cars still offers big savingopportunities, especially as its importance increases with theuse of recuperation systems (see Hucho [1]).

To further optimize the car geometry, it is important tounderstand the occurring aerodynamic phenomena. So fartwo basic approaches to investigate these aspects of the flow

exist (see G.M. Le Good [2]): the use of strongly simplifiedcar models and that of production vehicles.

Generic car models, such as the SAE model and the Ahmedbody, make it easy to relate the observed phenomena tospecific areas and thus help to understand basic flowstructures. At the same time, more complex flow phenomena,e.g. at the underbody and the wheels/wheelhouses, cannot bereproduced due to the oversimplification of these geometries.On the other hand, it is usually not feasible to investigatethese phenomena on a specific production vehicle, as, due toits short life span and restricted access, typically littlevalidation data is available. Recognizing the need for a modelcombining the strengths of both approaches, various more orless generic models, such as the VW reference car and theMIRA reference car, have been proposed (cf. G.M. Le Good[2]). However, while these reference cars mark a step in theright direction, these models are still too generic tocompletely understand the complex phenomena occurring atrealistic vehicles.

To close this gap, the Institute of Aerodynamics and FluidMechanics of the Technische Universität München (TUM), incooperation with two major car companies, the Audi AG andthe BMW Group, therefore, proposes a new realistic genericcar model. The body is based on two typical medium-classvehicles and includes three interchangeable tops and twodifferent underbody geometries to allow for a highuniversality. To encourage the use of the DrivAer model inindependent research projects, the geometry and acomprehensive database with both numerical andexperimental results will be published on the website of theinstitute1.

Introduction of a New Realistic Generic Car Modelfor Aerodynamic Investigations

2012-01-0168Published

04/16/2012

Angelina I. Heft, Thomas Indinger and Nikolaus A. AdamsTechnische Universität München

Copyright © 2012 SAE International

doi:10.4271/2012-01-0168

1http://www.aer.mw.tum.de/en/research-groups/automotive/drivaer

The aim of this paper is to present this new geometry to abroad audience and to provide first experimental results. Inthe beginning the necessity of a new realistic generic cargeometry will be discussed, followed by a short summary ofthe development of the DrivAer model. In the next section,the experimental wind tunnel setup will be presented and thegeneral approach and the data processing will be explained.This passage will be followed by the discussion of theexperimental results categorized into force and pressuremeasurements. Concluding the paper, the experimentalresults will be summarized and a short outlook on futureinvestigations of the DrivAer model will be given.

DEVELOPMENT OF THE DRIVAERMODELBACKGROUNDThe flow around a bluff body moving in proximity of a staticground is governed by the interaction with the ground andhighly turbulent separation and reattachment. Both theexperimental reproduction of the relative movement betweenvehicle and ground and unsteady investigations are highlycomplex and, therefore, associated with high experimentaleffort. Yet, to be able to further optimize road vehicles it isnecessary to completely understand these phenomena.

The improvement of wind tunnel facilities, especially as tothe introduction of ground effect simulation through movingbelts, allows a more precise experimental investigation oftime-accurate flow content (see Janssen [3], Cogotti [4]).Another important advance is the development of smaller andmore accurate pressure transducers that can easily be fittedinto the surface of wind tunnel models.

Categorizing the existing aerodynamic approaches in recentpapers on the subject of vehicle aerodynamics, G.M. Le Good[2] identifies two main classes: the investigations performedon strongly simplified generic models and those carried outon real production cars.

Commonly, especially time-accurate investigations resort tothe use of strongly simplified bodies, such as the SAE body,as presented by Cogotti [5], see Figure 1, and the Ahmedbody, as described by Ahmed [6], see Figure 2. These modelsoffer the advantage of a reasonable computational andexperimental effort compared to production vehicles and thepossibility to examine the flow effects of different parts of thevehicle with limited interference effects. Furthermore, incontrast to specific production vehicles, the simplified modelsoffer a broad spectrum of both numerical and experimentalvalidation data and are accordingly well suited for validationpurposes.

Figure 1. Dimensions of the SAE model.

Figure 2. Dimensions of the Ahmed body.

On the other hand, as their shapes are very unlike actual cargeometries, these insights cannot be readily applied in thedevelopment of production vehicles. Complex areas of thecar geometry, such as the A- and C-pillars, the highly curvedrear end, and the wheelhouse region, are especially impaired.Therefore, during the actual optimization process, often realproduction car geometries are employed. As these are usuallyonly accessible to a limited group of people, they are rarelyfeatured in more than one published work and, thus, cannotserve for validation purposes.

The gap between these two approaches makes a new realisticgeneric car model that combines the advantages of bothmodel types desirable. While several attempts to satisfy thisneed have been made, for example by introducing the MIRAreference car or the VW reference car (see G.M. Le Good[2]), the authors of this paper think that these models are stilltoo generic to allow for detailed investigations of complicatedflow phenomena. To this end, the DrivAer model is proposedand will be - along with numerical and experimental results -made available to the public. The computer-aided design(CAD) geometry will be published on the homepage of theInstitute of Aerodynamics and Fluid Mechanics at the TUMand independent experimental and numerical studies usingthe geometry are strongly encouraged.

DEVELOPMENTThe geometry of the DrivAer model is based on thegeometries of two medium sized cars, the Audi A4 and theBMW 3 Series (see Heft et al. [7]). Audi AG and the BMWGroup generously provided the CAD data of the differentconfigurations of the original vehicles. The original CADsurfaces were simplified and approximated by characteristic

curves. In the next step, the curves of both original cars weremerged to generate the new CAD geometry.

Figure 3 shows a sketch of the fastback configuration of the1:2.5 DrivAer model measured in the Wind Tunnel A of theInstitute of Aerodynamics and Fluid Mechanics.

Figure 3. Main Dimensions of the 1:2.5 DrivAer Model.

The flow phenomena at the backlight of contemporaryproduction vehicles can be divided into three main groupswith distinct aerodynamic behavior (see Hucho [8]): estateback vehicles, fastback vehicles, and notchback vehicles.While the flow over an estate back vehicle detaches at theend of the roof thus creating a big wake region, the flow overa fastback or a notchback vehicle is strongly influenced bythe angle of inclination of the backlight. For fastback vehiclesthe strong vortices emanating from the C-pillars typicallyinduce a downwash region on the backlight, thus, inhibitingthe detachment of the flow at the end of the roof or forcingthe flow to reattach on the backlight depending on the angleof inclination. These C-pillar vortices are less pronounced fornotchback vehicles. Therefore, the flow detaches at the end ofthe roof at smaller angles of inclination, the reattachmentdepends, amongst other things, on the length and height ofthe trunk. To allow for a thorough investigation of thesedifferent behaviors, the DrivAer model was developed as amodular concept that comprises three interchangeable tops, asshown in Figure 4 - just as for example the SAE body and theMIRA reference car (see G.M. Le Good [2]). Furthermore, asshown in Figure 5, two different underbody geometries areprovided: a smooth underbody for symmetrical investigationsand a detailed underbody based on the simplified underbodygeometry of the Audi A4.

Figure 4. DrivAer body with different tops.

(a). Detailed Underbody

(b). Smooth UnderbodyFigure 5. Different underbody configurations: (a)

detailed underbody and (b) smooth underbody.

The results presented in this paper were obtained using themock-up configuration of a scaled 1:2.5 DrivAer model, i.e.without considering a cooling flow. However, futureinvestigations will concentrate on cooling configurations forboth conventional and electric cars.

EXPERIMENTAL SETUPThe DrivAer model was measured in the recently updatedWind Tunnel A of the Institute of Aerodynamics and FluidMechanics at the Technische Universität München, ahorizontal Göttingen type wind tunnel. The open test sectionof the Wind Tunnel A has a length LT=4.8m, the nozzle has aheight of HN=1.8m and a width of WN=2.4m. Vortexgenerators are installed at the nozzle exit to reduce thepressure fluctuations induced by the developing shear layers.To allow for ground simulation (GS) with rotating wheels,the wind tunnel has been equipped with a moving belt system(see Mack et al. [9]). As the moving belt lies 60mm higherthan the static ground configuration, the effective nozzleheight is reduced to HN,eff=1.74m. Especially for theoptimization of underbody geometries and the wheelhouseregion, it is essential to simulate the relative motion betweenthe vehicle and the ground [3]. The vehicle body is held fromabove by a central strut while the wheels are supportedseparately by four horizontal struts from outside of the testsection. In the measured configuration, there is no physicalconnection between the body and the wheels. The model isplaced over a polyester-based belt of 1.39m width and adistance of 4.53m. The basic configuration can be seen inFigure 6.

Figure 6. Picture of the experimental setup in the WindTunnel A.

The moving belt system can operate up to a velocity of 50m/s. The boundary layer is reduced using a passive boundarylayer scoop. For a more detailed description of the windtunnel setup, its characteristics, such as the static pressuredistribution and the boundary layer profile, be kindly referredto Mack et al. [9].

The decision on the size of the wind tunnel model is based onvarious factors. On the one hand, it is important to take theeffects of wind tunnel blockage on the aerodynamic results

into account. To obtain physically accurate results, theblockage ratio

(1)

should be as small as possible (see Hucho [8]). At the sametime, it is desirable to satisfy the Reynolds number similarityto ensure the physical similarity of the flow structures:

(2)

If measuring a 1:2.5 model, the free stream wind speedshould be 2.5 times higher than for the 1:1 vehicle. In vehicleaerodynamics it is common to perform the measurements atthe wind speed u=140km/h which would correspond to anecessary free stream velocity of almost u=100 m/s for a1:2.5 model. On the other hand, it has been observed that thedrag coefficient reaches a relatively stable level for higherReynolds numbers. At the beginning of the DrivAermeasurement cycle, therefore, it has to be verified that thedrag coefficient reaches a constant level for the chosenReynolds number. To that end, the drag coefficient atdifferent Reynolds numbers will be examined.

Taking the dimensions of the Wind Tunnel A and its capacityinto consideration, the chosen 1:2.5 model with a blockageratio φ=8% seems to be an adequate compromise between thetwo requirements.

To facilitate the instrumentation of the wind tunnel modelwith pressure taps, the top part of the model was laminated,while the underbody geometry was cut out of high-densityfoam. The force measurements were obtained using a maininternal 6-component force balance that was placed betweenthe top strut and the model and four separate 1-componentforce balances attached to the wheels. The forces of allbalances were added up and averaged over threemeasurement intervals of 10s each.

The time-averaged pressure measurements were conductedusing a multiport pressure measurement system. The systemhas a full scale range FS=17kPa and an overall accuracy of±0.15%FS and is connected to the surface pressure tapsthrough flexible tubing (see Vogel [10]). Up to 192 ports canbe measured successively in one measurement cycle. For thetime-averaged measurements a sampling rate of 20 Hz and anaveraging period of 10s were chosen.

During one measurement cycle, 188 locations distributedover the surface of the model were measured. In this paper,only some of the most relevant regions will be discussed,namely the symmetry plane that was equipped with 61 probes(in the configuration with detailed underbody), the z=60mm

plane which runs approximately through the stagnation pointwith 21 measurement locations (see Figure 7), the windshieldwith 16 pressure taps (see Figure 21), the side windowdirectly behind the A-pillar where 11 probes were located(see Figure 24), and finally the rear windows of the estateback and the notchback configuration with 32 probes each(see Figures 25 and 26).

Figure 7. Distribution of pressure taps in the z=60mmplane of the 1:2.5 model.

Additionally, time-accurate surface pressures wereinvestigated at the backlight of the fastback model. Areas ofperiodic detachment and reattachment can be identified withspectral estimates. For this purpose 40 miniature pressuretransducers of the type HCL12X5P were placed on the rearslant of the fastback (see Figure 8). These pressuretransducers have a pressure range of ±12.5mbar and feature atypical error of ±0.05%FS and a maximum error of±0.25%FS for combined non-linearity and hysteresis [11].The instantaneous data was obtained during a samplinginterval of 120s at a sampling frequency of 2000Hz. Themeasurements were filtered with a low pass filter of 1000 Hz.

Figure 8. Distribution of pressure taps at the backlight ofthe fastback top.

APPROACH AND DATAPROCESSINGAs the Wind Tunnel A of the Institute of Aerodynamics andFluid Mechanics is not temperature and humidity controlled,the measuring conditions can change quite drasticallybetween summer and winter. To maintain comparability, the

measurements were not conducted at fixed velocities, butrather at fixed Reynolds numbers. The Reynolds numberswere determined as the equivalent of 10m/s, 20m/s, 30m/sand 40 m/s in an air-conditioned dry air environment at 20°Cand at sea level (see Table 1). The chosen reference lengthlref=1.84m corresponds to the length of the scaled DrivAermodel.

Table 1. Correlation between the Reynolds number andthe velocity at standard conditions.

When working with ground simulation, it is furthermoreimportant to separate the aerodynamic from the frictionalforces between the moving belt and the wheels. To isolate theaerodynamic forces, a measurement series with operatingmoving belt and without wind was conducted and a quadraticequation for the frictional forces was derived by curve fitting(see Figure 9).

Figure 9. Rolling resistance over the moving beltvelocity.

To obtain the correct aerodynamic forces, the rollingresistance is calculated for the current moving belt velocityand subtracted from the measured forces.

The measured forces are depicted as non-dimensional forcecoefficients. The drag coefficient can be calculated as:

(3)

The pressure samples presented throughout this paper are alsonon-dimensioned, i.e. calculated as the ratio of the

differential pressure measured at the surface p and thepressure in the plenum p∞ and the dynamic pressure:

(4)

For more convenience a simple categorizing system will beused in the figures and their captions to describe the modelconfigurations. This system will be quickly introduced here,for a complete scheme please be referred to the definitionssection at the end of this paper. The acronymE_S_woM_wW, for example, describes the estate backvehicle with smooth underbody, without mirrors, and withwheels. The first part of the acronym stands for the chosenrear end geometry (E: estate back, F: fastback, and N:notchback), the second represents the underbody geometry(D: detailed and S: smooth), while the third and fourth grouprefer to the presence of the mirrors (wM: with mirrors, woM:without mirrors) and the wheels (wW: with wheels, woW:without wheels).

EXPERIMENTAL RESULTSFORCE MEASUREMENTSCorrelation between the drag coefficient and theReynolds numberAt first, the correlation between the force coefficients and theReynolds number with and without ground simulation will bediscussed. As mentioned before, it is not possible to carry outthe experiments at the correct Reynolds number in the windtunnel facilities at the Institute of Aerodynamics and FluidMechanics. Therefore, it is important to demonstrate that theeffects of Reynolds number changes are insignificant at theoperating Reynolds number, before conducting furthermeasurements. To obtain these results, force measurements atfour different Reynolds numbers (see Table 1) wereconducted for each configuration.

As the results are quite similar for all configurations, theywill be discussed exemplarily for the fastback configuration(F_D_wM_wW, see Figure 10). The drag coefficient cxconverges with growing Reynolds numbers towards the valuecx=0.278 (with moving belt) and cx=0.284 (without movingbelt). A further increase in Reynolds number is not expectedto change the results significantly.

The examination of the 1:2.5 DrivAer model at the reducedReynolds number of 4.87E+6 consequently seems to be avalid approach.

Figure 10. Correlation between the total drag coefficientand the Reynolds number for F_D_wM_wW.

The drag coefficient without ground simulation is by trendhigher than with ground simulation, although the differencelessens with growing Reynolds numbers.

Drag coefficients of the different configurations.Table 2 shows a comparison of the values of the dragcoefficients for the mock-up configuration of the originalvehicles, i.e. with closed cooling ducts, and the DrivAermodel. The comparative data was provided by Audi AG [13]and the BMW Group [14-15].

Table 2. Comparison of the drag coefficients of thedifferent configurations of the DrivAer model without

ground simulation with the drag values of the mock-upAudi A4 and the BMW 3 series geometries.

Similar trends can be observed for the original vehicles andthe DrivAer model. The estate back has the highest dragcoefficient, while the difference between the fastback and thenotchback configuration is very small. The drag values of theDrivAer model are distinctly higher than the drag coefficientsof the original vehicles. This is due to the fact, that theaveraged geometries of the DrivAer model have not been inany way optimized aerodynamically. However, they lie well

2As the Audi A5 Sportback has a wider wheel track and wheels with a bigger section width than the DrivAer model, the drag coefficient presented here has been corrected to compensate thesedifferences.3At the time of the development, of the DrivAer model, the portfolio of the BMW Group did not include a 3 Series Coupé. The geometric data used for the derivation of the DrivAer modelwas obtained adapting the BMW 5 Series Coupé. Therefore, a comparison of the aerodynamic data would not offer further insights.

in the range of other mid-sized cars. With appropriateoptimization measurements lower levels of drag are possible.

Figures 11,12,13 show the drag coefficients for the differentconfigurations comparing the results obtained with (white)and without (grey) ground simulation.

For the fastback configuration (see Figure 11), we can clearlydistinguish between the trends of the experiments withwheels and those with closed wheelhouses. Considering theresults of the experiments with wheels, the drag coefficientmeasured without moving belt is higher (8-15 counts) than inthe cases with ground simulation. This tendency coincideswith the findings of Elofsson and Bannister [16]. Thedifferences observed for the moving belt might be due to theinteraction of the C-pillar vortices with vortices emanatingfrom the rotating rear wheels. With closed wheelhouses, wecannot observe this trend; in fact, the difference betweenground simulation and no ground simulation seems to benegligible (approximately 1 count).

If we compare the configuration with detailed underbody andsmooth underbody, we see that the underbody geometryaccounts for a significant difference of 32-34 counts (withGS), while the mirrors make a difference for 14-16 counts(with GS).

Removing the wheels shows a significant reduction of thetotal drag coefficient of 102 counts (with GS).

Figure 11. Drag coefficient for different configurationsof fastback top.

The notchback configuration exhibits similar behavior to thefastback configuration with a clear distinction between thecases with closed wheelhouses where the drag coefficient isequal or rather slightly higher for the cases with groundsimulation, and the cases with wheels where the moving beltlessens the drag coefficient by 9-12 counts (see Figure 12).As the rear end geometry is quite similar to that of thefastback model, similar phenomena should be responsible forthe structure of the flow.

The differences induced by the underbody geometry accountfor 30-31 counts, the mirrors add up to 14-15 counts and thewheels amount to 96-99 counts.

Figure 12. Drag coefficient for different configurationsof notchback top.

For the estate back geometry, the differences of the dragcoefficient between the experiments with and without groundsimulation are a lot smaller than those observed for thefastback and the notchback configuration (see Figure 13).This could indicate that the detachment at the rear of theestate back geometry is determined only by the form of therear end itself and, thus, not that susceptible to slightlychanged flow structures. A different behavior betweenconfigurations with and without wheels cannot be seen. Thedifference in drag coefficient between measurements withand without moving belt lies in a range of 0-4 counts whichcannot be seen as aerodynamically significant.

The underbody geometry accounts for 27 counts, the mirrorsmake a difference of 12 counts while the wheels only add upto 83-85 counts.

Figure 13. Drag coefficient for different configurationsof Estate back top.

PRESSURE MEASUREMENTSPressure coefficient distribution in selected planesIn the following section, the distribution of the pressurecoefficient in different planes will be discussed. If not noteddifferently, the shown distributions correspond to themeasurements of the detailed underbody with mirrors andwith wheels.

Comparison of the different rear end and underbodyconfigurations

Figure 14 shows the distribution of the pressure coefficient inthe symmetry plane at the top of the vehicle for the differentrear end forms (F_D_wM_wW, E_D_wM_wW, andN_D_wM_wW) as a function of the x coordinate. Themeasurements were performed using ground simulation. Thearea formed by the x=0 axis and the cp-curve can be used as ameasure for the vertical lift forces acting on the upper andlower surface of the car (see Ahmed et al. [16]).

The values of the three configurations coincide well at thefront of the vehicles. Starting at the exchangeable top,significant differences occur.

Figure 14. Distribution of the pressure coefficient in they=0mm plane at the top of the vehicle for the Estate back(E_D_wM_wW), the Notchback (N_D_wM_wW) and the

Fastback (F_D_wM_wW) configuration using groundsimulation.

From the stagnation point at the front of the vehicle, thepressure drops swiftly as the flow accelerates over theradiator grill and gains steadily over the bonnet of the car. Asthe flow is backed up at the junction between bonnet andwindshield, the pressure coefficient reaches a local maximumafter which the pressure decreases again over the windshield.Depending on the top fixated on the body, a differentpressure recovery behavior can be observed. While the estateback recovers the pressure faster, the flow detaches at theedge between the roof and the backlight at approximatelyx=1300mm and, therefore, does not reach the ambientpressure behind the car. The notchback configuration behaves

like the fastback geometry up to approximately x=700mmwhere the flow accelerates again including an additionalpressure loss, approximately at the height of the rear wheels,followed by a steep pressure recovery over the backlight anda small drop at the end of the vehicle. The pressuredistribution of the fastback configuration lies between that ofthe estate back and the notchback configuration. Both thefastback and the notchback configuration almost reachambient pressure behind the vehicle.

The pressure distribution in the midplane of the detailedunderbody is very similar for all three configurations (seeFigure 15). Beginning approximately at the rear wheels(x=1100mm), the estate back configuration recovers lesspressure than the other two configurations and, therefore,does not meet the ambient pressure behind the vehicle.Between the fastback and the notchback configurationvirtually no differences exist.

Figure 15. Distribution of the pressure coefficient in they=0mm plane at the bottom of the vehicle for the Estateback (E_D_wM_wW), the Notchback (N_D_wM_wW)and the Fastback (F_D_wM_wW) configuration using

ground simulation.

After a strong acceleration of the flow at the front end, thepressure begins to recover and reaches a local level of highpressure at the height of the mirrors. At the height of the rearwheels, another short pressure drop followed by a steeppressure recovery at the end of the vehicle takes place.

Just as the behavior at the bottom of the vehicle, the pressuredistribution in the horizontal plane through z=60mm (locationmarked by the black line) does not show great discrepanciesbetween the different configurations. Again the fastback andthe notchback version are almost identical (see Figure 16).After the acceleration at the radiator grill, the pressure risessteadily over the side of the vehicle. The flow detaches bothat the front and the rear wheels. As the pressure recoversbehind the wheels, it can be assumed that the flow reattachesat both locations.

Figure 16. Distribution of the pressure coefficient in thez=60mm plane for the Estate back (E_D_wM_wW), the

Notchback (N_D_wM_wW) and the Fastback(F_D_wM_wW) configuration using ground simulation.

Figure 17 shows the influence of the choice of underbodygeometry on the pressure distribution at the top of thefastback configuration. While the overall distribution doesnot change significantly, the pressure drop at the front ismore pronounced for the smooth underbody. The differencesat the rear end are most likely due to measurement errors.This will be verified during the next measurement cycle.

Figure 17. Influence of the underbody geometry on thedistribution of the pressure coefficient in the y=0mm

plane at the top of the fastback vehicle (F_S_wM_wWand F_D_wM_wW) using ground simulation.

Influence of ground simulation on the distribution of thepressure coefficient

Furthermore, the influence of the moving belt on thedevelopment of the pressure coefficient in the selected planeswas investigated. As the tendencies for the threeconfigurations are very similar, only the fastbackconfiguration (FS_wM_wW) will be discussed further(Figures 18,19,20). On the upper part of the vehicle, virtuallyno differences between the two cases exist (see Figure 18).The only deviations appear at the rear end where the pressureis slightly higher for the moving belt case. The slightly higher

pressure at the base can be partly responsible for the lowerdrag coefficient.

Figure 18. Influence of ground simulation on thedistribution of the pressure coefficient in the y=0mmplane at the top of the fastback vehicle with a smooth

underbody (F_S_wM_wW).

The different underbody geometries are only slightlyinfluenced by the moving belt, the flow is accelerated morestrongly and the cp- values are therefore slightly lower,whereas the detailed underbody is influenced more stronglythan the smooth version (see Figure 19). The smoothunderbody only shows small deviations in the wheel area.Over the detailed underbody, the flow accelerates at the frontwheels and recovers the pressure more slowly with groundsimulation; the deviations at the rear wheels are smaller.

For the smooth geometry the pressure coefficient is negativeover the whole underbody, the underbody works as a diffusorgenerating negative lift. The detailed underbody on the otherhand shows a short region of positive pressure between thewheels.

Figure 19. Influence of ground simulation on thedistribution of the pressure coefficient in the y=0mmplane at the underbody of the Fastback vehicle for

different underbody configurations (F_S_wM_wW andF_D_wM_wW).

In the horizontal z=60mm plane, the deviations are mainlylocated behind the wheels where the flow is acceleratedthrough the rotation of the wheels (see Figure 20). Still, thesedifferences are swiftly compensated, so, the overall flowdistribution does not change significantly.

Figure 20. Influence of ground simulation on thedistribution of the pressure coefficient in the z=60mmplane of the Fastback vehicle with smooth underbody(F_S_wM_wW) with and without ground simulation.

Pressure coefficient distribution on the surfaceIn the following, the distribution of the pressure coefficienton different surfaces will be examined and the differencesbetween the individual configurations will be discussed.

WindshieldIn Figure 21 the distribution of the non-dimensional pressurecoefficient cp on the wind shield is shown. The pressure wasmeasured at 16 locations and the results were afterwardsinterpolated by a MATLAB routine. At the base of the windshield, a stagnation area with moderately high pressure can beidentified. The flow accelerates towards the roof and towardsthe A-pillar.

Figure 21. Pressure distribution on the windshield(F_D_woM_wW).

Comparisons of the distribution of the pressure coefficientswith and without moving belt show almost no difference (the

total differences are less than 0.5% of the maximum pressuremeasured on the windshield, Figure 22 (a)). In Figure 22 (b)the differences between the configuration with and withoutmirrors are compared. The deltas were calculated bysubtracting the measured values with mirrors from the valuesobtained without mirrors. At the bottom of the windshield,the flow accelerates at the height of the side mirrors, as theflow goes around the mirrors, after slightly slowing downbefore. The changes are about 10% of the maximumpressures measured on the windshield.

Figure 22. pressure differences on the windshield for (a)measurements with and without ground simulation(F_D_woM_wW) and (b) measurements with and

without side mirrors (F_D_wM_wW andF_D_woM_wW).

Side WindowIn Figure 23 the pressure distribution on the side windowwithout mirrors is presented. The whole area is a lowpressure zone. At the A-pillar a region of very low pressure islocated which indicates the location of the A-pillar vortex.The A-pillar vortex develops close to the root of the A-pillarand detaches before reaching the roof thus creating the locallow pressure zone seen in Figure 23. Figure 24 shows thesame surface pressure distribution for the configuration withattached mirrors. While the flow is slowed down in the wakeof the mirror the A-pillar vortex grows stronger. The pressurefurther downstream of the mirror is not affected significantlyby the mirrors.

Figure 23. Distribution of the pressure coefficient at theside window without mirror (F_D_woM_wW).

Figure 24. Distribution of the pressure coefficient at theside window with mirror (F_D_wM_wW).

BacklightAt the entire rear window of the estate back configuration azone of moderately low pressure has developed with cp-values ranging from −0.08 to approximately −0.05 (seeFigure 25). This distribution shows that the pressure couldnot be completely compensated before the flow detaches atthe backlight. The deltas between the measurements with andwithout ground simulation and the configuration with andwithout mirrors are very small and range between −0.005 and+0.005.

Figure 25. Distribution of the pressure coefficient at therear window of the estate back configuration

(E_D_wM_wW).

For the notchback configuration, the flow is accelerated overthe roof causing a low pressure zone at the beginning of thebacklight (see Figure 26). On the backlight itself, the flowbegins to slow down which can be seen from the augmentingpressure. The zone with the lowest pressure is located in thetop corner close to the C-pillar. This is due to the C-pillarvortex that develops when the flow is accelerated over the C-pillar.

Figure 26. Distribution of the pressure coefficient at therear window of the notchback configuration

(N_D_wM_wW).

As described before, the fastback top was equipped with 40time-accurate pressure transducers (see Figure 8). This allowsthe evaluation of unsteady flow phenomena at the backlightof the fastback top. As it was found that the groundsimulation and the presence of the mirrors do not influencethe pressure distribution on the backlight significantly, in thefollowing, only the configuration with detailed underbody,without mirrors and with wheels (F_D_woM_wW) withground simulation will be discussed.

To obtain the averaged pressure distribution at the backlightof the fastback configuration, the instantaneous values wereaveraged. Figure 27 shows that at the C-pillar a large lowpressure zone exists which coincides with the C-pillar vortex.The high pressure zone at the lower part of the backlightshows the reattachment of the separation bubble.

Figure 27. Distribution of the pressure coefficient at therear window of the fastback configuration

(F_D_wM_wW).

The NACA profile of the top strut is believed to influence theflow structures at the backlight significantly, as described byHetherington and Sims-Williams [18]. To identifyinterference frequencies and the influence on the drag and liftof the vehicle, further measurements without the moving beltsystem and the mounting struts will be performed.

SUMMARYIn this paper a new realistic generic car geometry - theDrivAer model - has been introduced. The model was testedin the Wind Tunnel A of the Institute of Aerodynamics andFluid Mechanics at the TUM.

It has been demonstrated that the influence of the Reynoldsnumber on the drag coefficient lessens with augmentingReynolds numbers and that the investigation of the DrivAerbody at the reduced Reynolds number of Re=4.87E+6 istherefore feasible.

Examining the drag coefficients of the differentconfigurations, two interesting observations can be made:both the fastback and the notchback DrivAer model show asignificant impact of the operating moving belt on the resultsfor attached rotating wheels. These differences do not occurwhen the wheelhouses are closed. Therefore, it is reasonableto deduce that the vortices emanating from the rotating rearwheels interact with the C-pillar vortices at the backlight ofthe fastback and the notchback geometry. The estate backconfiguration, on the other hand, is neither for attachedwheels nor for closed wheelhouses influenced significantlyby ground simulation.

Additionally the distributions of the pressure coefficient indifferent planes were discussed. Again, apart from thebacklight itself, the fastback and the notchback geometryshow very similar behavior, while the distribution at theestate back geometry shows different results. Groundsimulation does not change the pressure distribution at the topof the vehicles. Small deviations exist in the z=60mm planedirectly behind the wheels and along the underbody.

Furthermore, the authors evaluated the pressure distributionon different parts of the vehicle surface. It has been shownthat the moving belt does not influence the distribution of thepressure coefficient at the top of the vehicle significantly. Themirrors have a substantial influence on the distribution on thewindshield and the side window as the flow accelerates overthe A-pillar and the A-pillar vortex grows stronger.

FUTURE WORKIn a next step the time-accurate measurements will beexamined and compared to numerical simulations. Theidentification of the corresponding vortices and the thorough

interpretation of the instantaneous results will be presented ina supplementary paper.

The results show that the flow around the upper part of themodel is not significantly altered by the use of groundsimulation. On the other hand, the top sting that is used tohold the model distorts the flow at the rear windowdecisively. To evaluate the magnitude of this error it isplanned to conduct further experiments with a fixed groundwithout the struts.

In addition, measurements in different wind tunnels will beperformed to estimate the effects of blockage and theReynolds number dependency.

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8. Hucho, W.-H. (ed.), “Aerodynamik des Automobils,”Vieweg+Teubner, Wiesbaden, ISBN 0-13-978-3528039592,2005.

9. Mack, S., Indinger, T., Adams, N. A., and Unterlechner,P., “The Ground-Simulation Upgrade of the TUM WindTunnel,” SAE Technical Paper 2012-01-0299, 2012, doi:10.4271/2012-01-0299.

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15. Elofsson, P. and Bannister, M., “Drag ReductionMechanisms Due to Moving Ground and Wheel Rotation inPassenger Cars,” SAE Technical Paper 2002-01-0531, 2002,doi:10.4271/2002-01-0531.

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CONTACT INFORMATIONAngelina HeftInstitute of Aerodynamics and Fluid MechanicsTechnische Universität MünchenBoltzmannstr. 1585748 Garchingangelina.heft@aer.mw.tum.de

ACKNOWLEDGMENTSThe authors would like to thank the technical staff at theInstitute of Aerodynamics and Fluid Mechanics (TUM),especially W. Lützenburg and H.-G. Frimberger.Furthermore, we would like to express our thanks to S. Mackfor the implementation of the ground simulation in the WindTunnel A and the instrumentation of the DrivAer model. Wesincerely thank both the Audi AG and the BMW Group fortheir invaluable support in the development process of theDrivAer body.

DEFINITIONS/ABBREVIATIONSCAD

Computer Aided Design

CFDComputational Fluid Dynamics

FFTFast Fourier Transformation

FSFull Scale Range

GSGround Simulation

PSDPower Spectral Density

TUMTechnische Universität München

To describe the different configurations of the DrivAer modela simple categorizing system has been derived. In thefollowing short paragraph/passage the system usedthroughout the paper will be presented.

First letter: describes the rear end configuration

FFastback

EEstate back

NNotchback

Second letter: describes the underbody configuration

DDetailed Underbody

SSmooth Underbody

Third block of letters: describes the mirror configuration

wMWith Mirrors

woMWithout Mirrors

Fourth block of letters: describes the wheel configuration

wWWith Wheels

woWClosed Wheel Houses

E_S_woM_wW therefore describes the configuration Estateback with smooth underbody, without mirrors and withwheels.

NOMENCLATUREAref

Reference Area

cpPressure coefficient

cxDrag coefficient

FRRolling Resistance

FxDrag Force

HNHeight of the Nozzle

WNWidth of the Nozzle

lrefReference Length

LTLength of the wind tunnel test section

pPressure

p∞Plenum pressure

ReReynolds number

uWind speed

νKinematic viscosity

ρDensity

φWind tunnel blockage ratio

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