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SAE TECHNICALPAPER SERIES 2000-01-3549

Advances in Wind Tunnel Aerodynamics forMotorsport Testing

Steve Arnette and Bill MartindaleSverdrup Technology, Inc.

Reprinted From: Proceedings of the 2000 SAE MotorsportsEngineering Conference & Exposition

(P-361)

Motorsports Engineering Conference & ExpositionDearborn, Michigan

November 13-16, 2000

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2000-01-3549

Advances in Wind Tunnel Aerodynamics for Motorsport Testing

Steve Arnette and Bill MartindaleSverdrup Technology, Inc.

Copyright 2000 Society of Automotive Engineers, Inc.

ABSTRACT

As the popularity of motorsport continues to surgethroughout the world, so to does the level of competition inthe motorsport community. Participants work to achievea performance edge through superior engineering. As anenabling tool, the wind tunnel has become a focus forenhancing performance. This is evidenced by theincreasing interest among motorsport teams in dedicatedwind tunnel facilities, as best exemplified by the FormulaOne community. Part of the reason for this increasingfocus on wind tunnels is the availability of breakthroughtechnologies that better simulate on-track conditions,providing new opportunities to enhance performance. Twoareas that are the subject of strong current interest are (i)test section configurations that eliminate wind tunnelinterference effects to provide the highest possibleaerodynamic simulation fidelity and (ii) high speed rollingroad systems with integrated force measurement systemsthat provide high fidelity simulation of underbody effects.This paper presents an overview of these technologies,including selected computational and experimental resultsthat illustrate the simulation advantages obtained withthese new wind tunnel technologies.

INTRODUCTION

The increasing emphasis of wind tunnel usage in themotorsport community is well exemplified by the FormulaOne community. Initially, the wind tunnels employedwere typically of the open-return type designed for testingof models at 25% scale. Over time, standard practiceevolved to testing at 40% scale. Many of the facilities inuse were converted aeronautical wind tunnels. Theseearly facilities contained some of the first rolling roadsystems to simulate underbody aerodynamics in the windtunnel, although the rolling road systems were confined tolow speeds relative to those of actual competition.

A snapshot of the wind tunnels currently being used inthis community shows that the majority of the testing isoccurring at model scales of either 40% or 50%. The

increasing model scale allows better geometry fidelity ofthe model to the actual vehicle. The typical configurationfor these wind tunnels is a solid wall test section with across-sectional area of 7-14m2. The maximum windspeed capability of these facilities is typically 50 m/s, butranges as high as 70m/s in some cases. Note that thesespeeds represent the limiting capability of traditionalrolling road systems, where concern for the life of therolling road belt severely limits high speed operation.

The wind tunnels currently being built or planned for theFormula One community represent yet another incrementin simulation capability. Testing is planned for a typicalmodel scale of 60%, with the added requirement to obtainquality simulation for full-scale testing. The top speed ofthese facilities is at least 70m/s, with most beingdesigned for an 80m/s capability. Most of these facilitiesare being designed with non-traditional test sectionconfigurations, driven by the needs for (i) superioraerodynamic simulation at 60% model scale and (ii) highquality simulation fidelity for full-scale testing. The rollingroad systems of these facilities represent a dramaticincrement in top speed capability, with systems capableof routine operation at speeds up to 100m/s if required.

The purpose of this paper is to provide an overview ofemerging technologies that are prominent in the newgeneration of wind tunnels dedicated to motorsporttesting. Chief among these are (i) test sectionsemploying contoured wall technology to eliminateboundary interference effects which degrade aerodynamicsimulation quality and (ii) next-generation rolling roadsystems which provide a dramatic increase in top speedcapability and the possibility of measuring aerodynamicforces transmitted to the rolling road through the rotatingtires. A discussion of these technologies is timely giventhe increasing emphasis being given to wind tunnel testingfor both stock car and open wheel classifications. Thebasis for this invited paper was an invited SAE TOPTECpresentation given in October 1999.1

OVERVIEW OF TEST SECTION TECHNOLOGY

Of the wind tunnels currently used for motorsportdevelopment, the vast majority possess either solid wall orsemi-open jet test sections. For both types, boundaryinterference causes the external aerodynamic simulationin the wind tunnel to differ from the actual situation on theroad.

SOLID WALL TEST SECTION For the solid wall testsection, boundary interference is fundamentally caused bythe confinement of the flow between the test model (orvehicle) and the solid test section boundary. Thisunavoidable effect results in over-acceleration of the flowover the model, as compared to the actual situation on theroad. To minimize these effects, it is advisable to hold thevehicle blockage ratio (ratio of model frontal area to crosssectional area of test section) to no more than about 5%.This is why solid wall wind tunnels devoted to full-scaleautomotive development have large test sections, e.g. theGeneral Motors Aerodynamic Laboratory Wind Tunnel2

has a test section area of approximately 610 ft2 (57m2).Because of the bulk of work done in solid wall windtunnels, spanning both automotive and aerospace productdevelopment, the topic of solid wall boundary interferencehas been investigated extensively.

Seven different wall boundary correction methods werecompared in an SAE-sponsored exercise to assess theaccuracy of correction methods.3 Of the seven, three werejudged to be acceptable. Even the acceptable methodsfailed to provide an adequate correction at blockage ratiosof 10%. Of the three acceptable methods, the pressuresignature correction method represents the most rigorousaerodynamic development. Its use, however, has tendedto be limited by the requirement for wall pressuresignature measurements and lengthy calculations tocomplete the correction procedure. Even if the one of thepreferred correction methods is applied, the resultsindicate that testing must be carried out at blockageratios much less than 10% to produce accurate testdata.3

It is important to note that sources of degradation otherthan boundary interference are possible in the solid walltest section. For example, the combination of largemodel blockage and model location in close proximity tothe contraction section can result in model influence onthe wind tunnels dynamic pressure measurement. Ifpresent, the result will be a facility-indicated dynamicpressure different than the actual dynamic pressureincident on the vehicle. Another potential source ofdegradation is static pressure influence from thedownstream diffuser. As shown by Garry et al.,4 if thebase of the model is too close to the diffuser at thedownstream end of the test section, the interaction of thediffuser and the model wake will cause the static pressureat the base of the model to be artificially elevated, therebyaltering the pressure forces on the vehicle. Note that both

of these potential sources of simulation error can beminimized through proper facility design. Boundaryinterference, however, can be minimized only throughincreasing the test section area, which translates directlyto increased capital cost of the wind tunnel facility.

Although correction procedures such as the pressuresignature method help close the gap between the windtunnel and actual on-road aerodynamic performance, itwould obviously be preferable to measure actual on-roadperformance in the wind tunnel. This is increasingly trueas motorsport becomes more competitive. With racecardevelopers looking for aerodynamic advantage with thedesign of essentially every element on the vehicle,situations will arise where the performance incrementsbetween various design options are much smaller than thecorrection increment needed to translate the wind tunnelmeasurements to on-road performance. Along a similarline of thought, the need for correction arisesfundamentally from improper flow simulation over themodel. As advanced diagnostic tools such as PressureSensitive Paint,5 Particle Image Velocimetry,6 and PlanarDoppler Velocimetry7,8 which provide detailed, spatially-distributed mappings of local flow structure becomemore commonplace in the wind tunnel, proper flowsimulation at localized positions on the vehicle isbecoming increasingly important. Although post-testcorrections are suitable for force and moment coefficients,they are not capable of improving flow simulation in thewind tunnel.

OPEN JET TEST SECTION Many wind tunnels usedfor motorsport development possess an open jet testsection. For this configuration, the test model is placed ina large plenum chamber and the nozzle flow entering theplenum occupies only a modest portion of the plenumscross-section. After flowing over the model, the flow isthen collected at the rear of the plenum. Historically, thistype of test section has been preferred in Europe, basedon the idea that aerodynamic simulation fidelity to theopen road is less degraded by increasing model blockagethan it is for a solid wall test section. For the open jettest section, model blockage ratio is defined as the ratioof the model frontal area to the cross sectional area of thenozzle. In the semi-open jet test section (where the modelblockage is typically 10% or more), the main source ofdegraded aerodynamic simulation is the finite dimensionof the jet exiting the nozzle. On the road, the vehicle isimmersed in a semi-infinite flow.

Blockage corrections for the semi-open jet test sectionare less mature than those for the solid wall test section.9

The work of Mercker and Wiedemann10 is the mostcomprehensive work to date. As a result, it is difficult tocomment definitively on the success of open jet correctionmethods. It is noted, however, that the recentexperimental investigation of Hoffman et al.11 indicatesthat the aerodynamic simulation obtained from open jettest sections is superior to that obtained from slotted walltest sections for model blockages ranging from 7% to

25%. The lack of knowledge regarding quantitativeperformance is the driving reason that essentially all newwind tunnels planned for the near future that will bededicated to motorsport testing will not have an open jettest section.

Open jet test sections are universally beset by an adversestatic pressure gradient in the downstream portion of thetest section.9,12 The common result is that test vehiclesinstalled in the test section are subjected to an elevatedstatic pressure in the base region. This horizontalbuoyancy effect is exactly analogous to that encounteredin a solid wall test section if the test vehicle is located tooclose to the downstream diffuser. Although the obviousanswer is to increase the length of the open jet, Arnette etal.12 have shown that open jet test sections exhibit anincreasing tendency towards low-frequency unsteadinessas the length of the open jet increases. The result is atrade-off situation for the wind tunnel designer. Alsosimilar to the solid wall test section, locating the testvehicle or model too close to the nozzle can lead to modelinfluence on the facility measurement of dynamicpressure, with similar negative effects.

CONTOURED WALL TEST SECTION Becausecontoured wall technology has its origin in adaptive walltechnology, a review of adaptive wall test sections ispresented prior to discussing the details of contoured walltest sections.

Adaptive Wall Test Section The idea of an adaptive walltest section is simply to shape the side and top walls ofthe test section such that they correspond to externalstreamlines that would be present over the vehicle on theopen road. More formally stated, if the flow angularitydistribution on a control surface surrounding a bodycoincides with the distribution that would be present atthat location for the body located in an infinite domain, thebody will experience no interference effects.13 For theadaptive wall wind tunnel, the side and top walls of thetest section become the control surface. Shaping thesewalls to correspond to streamlines on the open roadmeans that there will necessarily be no interferenceeffects to degrade the external aerodynamic simulation.This implies that no correction of force coefficients isneeded and that the local aerodynamics on the vehicle areproperly simulated.

The concept of an adaptive wall test section actuallyoriginated in aeronautical testing circles, and wasdeveloped for automotive applications by SverdrupTechnology in the 1980s, resulting in a patent.14 Thetheoretical background, principle of operation, and resultsfrom validation experiments for the adaptive wall testsection are presented elsewhere,1.13,14 and are notrepeated here. The result of this internal developmentwork, which spanned more than a decade, is the ability toachieve interference-free external aerodynamic simulationfor model blockages of at least 30%. Over the past

decade, the technology has been successfully applied inthe motorsport community.

A cross-sectional schematic of the adaptive wallconfiguration employed in Sverdrups sub-scale windtunnel laboratory is presented in Figure 1. The slats onthe top and side walls run the length of the test sectionand are shaped by actuators spaced evenly along thelength of each slat. For the adaptive wall concept, theactuators are part of a closed loop system that uses:

The streamwise distribution of static pressuresmeasured at the test section walls (which capturesthe influence of the model)

A potential flow algorithm (which is independent of themodel geometry) that uses the pressuremeasurements and a convergence rate parameter topredict new wall positions

A control system that translates the algorithm outputto commands for the wall actuators

A tolerance that defines convergence, which istypically based on the convergence of the wallpressure measurements for the final wall shape.

It is important to note that no portion of the adaptive wallalgorithm is dependent on the geometry of the model orvehicle being tested. As a result, the technology is notone where one must first know the answer to achieve itin the wind tunnel, which is a common misperception.As typically implemented, no more than 6 iterations areusually required to achieve the final wall position. For afully-automated system, the entire wall shaping processoccurs while the wind tunnel is running, with only a fewseconds required for each iteration.

Previous results from Sverdrups sub-scale wind tunnelhave demonstrated the ability to achieve interference-freeresults for model blockages up to 30%.13 Recent resultsobtained for the standard MIRA fastback model3 in thesub-scale adaptive wall test section are presented inFigure 2.11 The drag coefficient indicated at 11% blockagein Fig. 2 was obtained in the adaptive wall test sectionprior to any wall deflection (i.e. straight walls). Thecircular points were obtained in independent tests inother European wind tunnels.11 Note that, for all datapoints, the ratio of the boundary layer displacementthickness to the model underbody clearance is constant.This implies consistent underbody effects for all data inthe plot. The faired curve through the data was based onthe data points at 1.7%, 2.7%, 4.8%, and 11% blockage.The curve intersects the drag coefficient axis atapproximately 0.223. The square data point indicated at0% blockage in the figure (CD = 0.223) is the resultobtained from the adaptive wall test section with a modelblockage of 11%. Its exact coincidence with theextrapolation to zero blockage (indicated by the fairedcurve) illustrates the simulation quality attainable withadaptive wall technology. Similar results are attained inthe adaptive wall test section for model blockagesapproaching 30%.

Contoured Wall Test Section With an eye towardmotorsport applications, Sverdrup has investigateddifferent levels of wall adaptation to determinerequirements for different levels of simulation accuracy.This has taken the form of both experimental andcomputational investigations. The result of this work isthe contoured wall test section, in which both side wallsand the top wall of the test section each assume a two-dimensional contour (i.e. there is only a single slat in theside walls and top wall). The wall shapes can still bemodified, but the shape is modified via manual actuatorsin an open loop configuration. This concept is similar tothe streamline wall configuration described by Hucho.15

Both adaptive and contoured wall test sections enjoy adecided advantage relative to solid wall, semi-open jet, orslotted wall test sections. Because of their ability togenerate interference-free results at blockage ratiosapproaching 30%, the size of the test section (and windtunnel) can be much smaller than if a traditional testsection configuration is employed. This represents animportant reduction in both capital cost and operatingcost for the facility, for superior simulation quality.

There are two main advantages of the contoured wall testsection versus the adaptive wall test section. The first isthat the contoured wall test section does not include theautomated, closed loop control system that shapes thewalls of the test section. This plus the reduced numberof wall slats creates a much simpler system, which alsorepresents an increment in reduced capital cost. Thebasic principle of the contoured wall test section is toobtain most of the simulation benefit of adaptive walltechnology in a simpler configuration.

The absence of an automated control system is mostideally suited to motorsport applications. This isbecause a given team tests essentially the same modelfor an entire season, if not multiple seasons. The samewould hold true for various motorsport teams within agiven classification using the same wind tunnel facility.Hence the requirement for wall re-shaping is greatlydiminished relative to that which would be present in anOEM contoured wall wind tunnel devoted to passengercar development. In summary, the contoured wall testsection is a good solution for testing a single type ofvehicle where infrequent wall reshaping is required.When an occasional wall redefinition is required, it isachieved via manual actuation.

Simulation Quality in the Contoured Wall Test SectionComputational simulations have been carried out toinvestigate the simulation quality attainable with thecontoured wall test section. A single sedan shape with afrontal area of 23.0ft2 (2.14m2) was employed as the testarticle for all of the simulations. All simulations werecarried out with the PMARC potential flow code using aparameter-based wake model to represent the vehiclewake. The simulations were carried out for the full vehicleoriented at 0 yaw and 7 yaw, and for the half-vehicle

oriented at 0 yaw, for each of the following threegeometries:

The open-road condition The model in an 11.6ft x 23.2ft (3.54m x 7.07m) solid

wall wind tunnel (test section area of 269ft2),representing a model blockage of 8.5%

The model in a 9.0ft x 14.0ft (2.74m x 4.27m)contoured wall test section (test section area of126ft2), representing a model blockage of 18.3%

Note that the solid wall test section area is 210% largerthan the contoured wall test section area. Figure 3presents an illustration of the zero yaw simulation resultsfor the open road condition.

Table 1 presents the cumulative results of thesimulations. No attempt has been made to correct theforce and moment coefficients from the solid wall testsection for blockage effects. Examining Table 1, thecorrespondence of the contoured wall test section to theopen road test section is very good, with deviationsranging from 0.0% to approximately 3.5%. Comparingthe solid wall test section to the open road, the deviationsrange from approximately 15% to more than 100%. Thecontoured wall test section clearly provides superioraerodynamic simulation quality to the solid wall testsectioneven though the latter is more than twice aslarge!

Just as important, it should be noted that the contouredwall simulations for the vehicle yawed at 7 were run withthe test section wall shape that was optimized for 0model yaw. Despite not having optimized the wallshapes for the 7 yaw condition, Table 1 shows that thereis essentially zero degradation of the lift and dragcoefficients, and only very minimal degradation for thepitching moment coefficient. This illustrates that wallreshaping is not required for the range of yaw anglesexpected for motorsport testing. Further, it directlysupports the fact that, for a given model geometry, wallreshaping is required only infrequently for the contouredwall test section.

In the final analysis, it is useful to compare the quality ofsimulation attainable from adaptive wall wind tunnels andcontoured wall wind tunnels. Our internal computationaland experimental work, including the results presented inTable 1, shows that the adaptive wall test section offersthe best possible aerodynamic simulation. The adaptivewall test section has been proven capable of providingforce and moment coefficients with errors of no more than0.5% to 1.5%, with no data correction required. Theseresults can be achieved for model blockages of at least30%. The contoured wall test section provides force andmoment coefficients with errors of 1% to 5%, with nodata correction required. This level of performance canbe achieved for model blockages in excess of 20%with20% blockage suggested as a comfortable design point.As illustrated by the computational exercise presented

here, these simulation accuracies are far superior to thatwhich can be attained with traditional test sectionconfigurations.

The ability to achieve quality aerodynamic simulation formodel blockages in excess of 20% with no need for datacorrection has opened new possibilities for motorsportdevelopment. For instance, this type of facility makes itpossible to do both model-scale and full-scale testing inthe same moderately-sized wind tunnel. The ability to domeaningful full-scale testing in the wind tunnel, even if itis possible only intermittently (e.g. during the racing off-season), could represent a major enhancement to ateams ability to achieve performance on the track.

OVERVIEW OF MODERN ROLLING ROADTECHNOLOGY

Rolling road systems are a critical system for windtunnels dedicated to motorsport testing, and represent asecond area where modern wind tunnels are achievingdramatic gains in simulation capability. As discussed inthe introduction, traditional systems were limited to a topspeed of 40-50m/s. This is fundamentally a limitation ofelastomer belt systems, which typically experienceseverely degraded belt life for even intermittent operationat high speeds.

The advanced rolling road systems of today have brokenthrough this limitation, offering a top speed capability of upto 100m/s if required. This has been achieved by using astainless steel belt in place of the traditional elastomerbelt, a technology pioneered by MTS Systems throughtheir experience with tire testing. For the stainless steelbelt, routine operation at high speeds does not translateto degraded belt life. A typical belt lifetime of 2000 testinghours per unit is achieved with these high-speed systems.

The other primary breakthrough of modern rolling roadsystems is the ability to measure forces transmittedthrough the belt by the rotating tires. This evolution hasbeen driven by motorsport applications.

Two primary configurations for these high-speed rollingroad systems have emerged: a single, wide belt systemand a five belt system including narrow center belt.

Single, wide belt system Figure 4 presents aphotograph of an MTS Flat-Trac rolling road system.The belt is fabricated of stainless steel. Belt widths canrange up to 10.5ft (3.2m). The flat length of the rollingroad can range up to 33.0ft (10.0m). The systems arecapable of top speeds up to 100m/s.

Figure 5 presents a photograph of an MTS Flat-Trac

downforce measurement system. These modules arelocated under the belt directly beneath a rotating tire onthe model. The presence of four downforce measurementsystems at the locations of the rotating tires on the model

is illustrated in Figure 6a. Using load cell technology,each downforce measurement system measures the fullvertical load at its respective tire, including both the weightof the model and the aerodynamic downforce. As shownin the figure, the force measurement modules and rollingroad system are typically integrated into a turntable toallow the complete system to be yawed. Measurementof side and axial forces is achieved through independentmeans. The measurement systems for these forcecomponents are integrated into the vehicle restraintsystem.

As shown in Figure 6b, modern rolling road systems arenow commonly being designed to handle both scalemodel testing as well as full-scale testing. Vertical forcemeasurement through the belt is possible for both.

Five Belt System The other primary configuration formodern rolling road systems is the five belt system.Figure 7 presents a schematic of the MTS Flat-Trac fivebelt system. As shown in the figure, this system is idealfor use with a traditional external force balance, whichmakes it an attractive option for upgrading existing windtunnels. The system consists of four mini-belts, onebeneath each tire, plus a narrow central belt that runsfrom upstream of the vehicle to some downstream of thevehicle. As suggested by its integration with an externalbalance, these systems have so far been intended mainlyfor full-scale vehicle testing.

As shown in Figure 8, the vehicle is actually supported byfour rocker panel support struts with integrated actuation.These struts are connected directly to the vehiclechassis, and provide automatic control of vehicle rideheight during testing. Both the vehicle support struts andthe mini-belts are supported by an intermediate supportframe which is connected to the external balance. Thenet result is that all forces are transmitted to the externalbalance. Thus, unlike the single belt system, the five beltsystem does not alter the method with which forces aremeasured.

The narrow center belts typically range in width from 1.0mto 1.4m. The mini-belts beneath each tire typically rangein width from approximately 240mm to 410mm. Similar tothe single belt system, the five belt system is capable oftop speeds of up to 100m/s with excellent belt lifecharacteristics.

Single belt versus five belt Organizations developingnew motorsport wind tunnels have to this point generallypreferred the single belt system. For example, this isespecially true in the Formula One community. However,the five belt system is ideal for upgrading a wind tunnelthat has an existing external balance. For example, asimilar upgrade solution was successfully implemented atthe Pininfarina Wind Tunnel.16 Even for a new wind tunnel,the traditional force measurement configuration mayrepresent an advantage.

It is interesting to compare these advanced rolling roadsystems to the common underbody simulation of aboundary layer treatment system with blowing ordistributed suction, but no rolling roadthe conditionsunder which a large portion of the wind tunnel testing forstock car racing in North America occurs. Compared tothis, the five belt system provides the overwhelmingmajority of the underbody simulation benefit to be gainedfrom a single-belt system. The five belt configuration isalso attractive for passenger car testing, providing dualuse possibilities that can be important for facilities thatexecute both OEM testing and motorsport testing. Again,this type of situation is most logical for upgrading a windtunnel with an existing external balance.

CONCLUSION

As the popularity of motorsport continues to grow, so todoes the effort spent on wind tunnel testing to gaincompetitive advantage. The purpose of this paper is toprovide an overview of two areas of technology that arehaving a major impact on wind tunnel testing dedicated tomotorsport.

Contoured wall test section technology has come to berecognized as a cost-effective means of achieving qualitysimulation. The technique is grounded in adaptive walltechnology, and provides the bulk of the advantagesassociated with adaptive wall technology in a simpler,less costly system. By enabling accurate aerodynamicsimulation at large model blockage, the technology allowstest objectives to be met in a much smaller facility thanwould be required for traditional test sectionconfigurations. These emerging test objectives includefull-scale testing in a moderately-sized facility, which canbe achieved. The results presented here demonstrate thesimulation advantage to be gained from a contoured walltest section. Because the test section area drives boththe size and power consumption of a wind tunnel,contoured wall technology also provides a gain in facilitycost effectiveness (capital and operational).

The other major impact area regarding wind tunnel testingis rolling road systems. Single belt and five belt systemsare now available that substantially enhance theunderbody simulation capability of wind tunnel facilities,including routine operation at speeds up to 100m/s. Forthe single belt system, vertical forces can be measureddirectly through the rolling road, for both model scale andfull scale testing. The five belt system can be integrateddirectly into a traditional external balance, providing anenhanced ability to simulate underbody effects whilemaintaining the traditional force measurement system.

Both of these test section-focused technologies directlyenhance wind tunnel simulation quality, and therefore dataquality. As a result, the contoured wall test sectionconfiguration and high-speed rolling road systems with

integrated force measurement are both gaining wideacceptance in the motorsport community

ACKNOWLEDGMENTS

The authors would like to acknowledge the substantialcontributions of David Meier of MTS Systems(David.Meier@MTS.com) regarding advanced rolling roadsystems.

REFERENCES

1. Arnette, S.A., Martindale, W.R., Meredith, W.S., andHoffman, J.H., Advances in Wind TunnelAerodynamics for Motorsport Testing, SAE TOPTEC,October 1999.

2. Kelly, K. B., Provencher, L. G., and Schenkel, F. K.,"The General Motors Aerodynamic Laboratory - AFull-Scale Automotive Wind Tunnel," SAE Paper820371, 1982.

3. SAE Standards Committee on Closed Test SectionWind Tunnel Boundary Corrections, "Closed-Test-Section Wind Tunnel Blockage Corrections for RoadVehicles," SAE SP-1176, 1996.

4. Garry, K. P., Cooper, K. R., Fediw, A., Wallis, S. B.,and Wilsden, D. J., "The Effect on Aerodynamic Dragof the Longitudinal Position of a Road Vehicle Modelin a Wind Tunnel Test Section," SAE Paper 940414,1994.

5. Duell, E., Everstine, D., Mehta, R., Bell, J., andPerry, M., "Pressure Sensitive Paint TechnologyApplied to Low Speed Automotive Testing," to bepresented at the 2001 SAE Congress, March 2001.

6. Hoffman, J.M., Arnette, S.A., Porter, C.B., Sung, B.,and Arik, B.E., Application of Particle ImageVelocimetry in the Korea Aerospace ResearchInstitute Low Speed Wind Tunnel, AIAA-2000-0411,2000.

7. Elliott, G.S., and Beutner, T.J., "Molecular FilterBased Planar Doppler Velocimetry," Progress inAerospace Sciences, 35(1999), pp. 799-845.

8. Arnette, S.A., Elliott, G.S., Mosedale, A.D., andCarter, C.D., "A Two-Color Approach to PlanarDoppler Velicimetry," AIAA-98-0507, 1998.

9. SAE Standards Committee on Open Jet Wind TunnelAdjustments, Aerodynamic Testing of Road Vehiclesin Open Jet Wind Tunnels, SP-1465, 1999.

10. Mercker, E., and Wiedemann, J., "On InterferenceEffects in Open Jet Wind Tunnels," SAE Paper960671, 1996.

11. Hoffman, J., Martindale, W., Arnette, S., Williams, J.,and Wallis, S., "Effect of Test Section Configurationon Aerodynamic Drag Measurements," to bepresented at the 2001 SAE Congress, March 2001.

12. Arnette, S.A., Buchanan, T.D., and Zabat, M., "OnLow-Frequency Pressure Pulsations and staticPressure Distribution in Open Jet Automotive WindTunnels," SAE 1999-01-0813, 1999.

13. Whitfield, J.D., Jacocks, J.L., Dietz, W.E., and Pate,S.R., "Demonstration of the Adaptive-Wall ConceptApplied to an Automotive Wind Tunnel," AIAA 82-0584, 1982.

14. Whitfield, J.D., Jacocks, J.L., Dietz, W.E., and Pate,S.R., "Demonstration of the Adaptive-Wall ConceptApplied to an Automotive Wind Tunnel," SAE Paper820373, 1982.

15. Hucho, W.-H., ed. Aerodynamics of Road Vehicles,4th edition, SAE, 1998, p. 641.

16. Cogotti, A., Ground Effect Simulation for Full-ScaleCars in the Pininfarina Wind Tunnel, SAE Paper960996, 1996.

CONTACT

The primary contact for this paper is Stephen A. Arnette,Ph.D., Vice President, Sverdrup Technology, Inc.(arnettsa@sverdrup.com).

Table 1. Results of computational investigation of contoured wall simulation quality.

Complete Vehicle 0 Yaw

CL (lift force) CD (drag force) Cm (pitching moment)

Open Road -0.2518 0.3680 0.1001

Contoured Wall Tunnel (18.3% blockage) -0.2558 0.3654 0.1014

Solid Wall Tunnel (8.5%blockage) -0.3048 0.2918 0.0852

(Cont Wall Open Road) / Open Road 1.59% -0.71% 1.30%

(Solid Wall Open Road) / Open Road 21.05% -20.71% -14.89%

Half Vehicle 0 Yaw

CY (side force) Cn (yawing moment) Cl (rolling moment)

Open Road 0.3852 -0.2896 -0.3371

Contoured Wall Tunnel (9.2% blockage) 0.3854 -0.2976 -0.3369

Solid Wall Tunnel (4.3% blockage) 0.8986 -0.1256 -0.746

(Cont. Wall Open Road) / Open Road 0.05% 2.76% -0.06%

(Solid WallOpen Road) / Open Road 133.28% -56.63% 121.30%

Complete Vehicle 7 Yaw

CL (lift force) CD (drag force) Cm (pitching moment)

Open Road -0.2536 0.3768 0.1008

Contoured Wall Tunnel (18.3% blockage) -0.2497 0.3792 0.1043

Solid Wall Tunnel (8.5% blockage) -0.3088 0.3019 0.0853

(Cont Wall Open Road) / Open Road -1.54% 0.64% 3.47%

(Solid Wall Open Road) / Open Road 21.77% -19.88% -15.38%

Figure 1. Schematic of Sverdrups sub-scale adaptive wall test section.

Figure 2. Vehicle surface pressure distribution obtained from CFD fora sedan shape oriented at zero yaw for interference-free conditions.

Figure 3. Drag coefficients for the MIRA fastback model from various wind tunnels. The regressed curveserves as an extrapolation to zero blockage for the four points obtained at various blockages. The solidpoint at 11% blockage was obtained in the adaptive wall test section with no wall shaping (i.e. straightwalls). The solid point at 0% blockage was also obtained in the adaptive wall test section at a modelblockage of 11%, but after optimizing the wall contours.

Figure 4. Photograph of an MTS Flat-Trac rolling road system.

0.20

0.21

0.22

0.23

0.24

0.25

0.26

0.27

0.28

0.29

0.30

0.00 0.01 0.02 0.03 0.04 0.05 0.06 0.07 0.08 0.09 0.10 0.11 0.12

Blockage Ratio

Cd

College Of AeronauticsCranfield Institute of Technology

Straight Wall Test Sectionreported in SAE SP-1176

d*/H = 0.16Sverdrup Adaptive Wall Tunnel

Adapted WallGeometric Blockage = 0.11

d*/H = 0.15

Trendline

Sverdrup Adaptive Wall TunnelStraight Wall

Geometric Blockage = 0.11d*/H = 0.15

Passenger Car Shape

Figure 5. Photograph of an MTS Flat-Trac vertical force measurement system.

Figure 6. Scale model (top) and full scale (bottom) installations on a rolling road system, with verticalforce measurement systems located beneath the rolling road. (illustration courtesy of MTS).

Figure 7. Schematic of a car installed on an MTS Flat-Trac five belt rolling road system that hasbeen integrated with an external force balance (illlustration courtesy of MTS).

Figure 8. Schematic of the MTS Flat-Trac five belt system integrated with an external forcebalance (illustration courtesy of MTS).

5-BELT CONCEPTMOVING BELT SYSTEM

ADD-ONAIR BEARINGPANELS

NARROW(CENTER) BELT

DISTRIBUTED SUCTIONFLOOR PANELS

ROTATING WHEELMINI-BELT

FORCEBALANCE

ROCKER PANELSUPPORT ACTUATORS

MINI-BELTS

NARROWBELT

TURNTABLE

FORCEBALANCE