400 Commonwealth Drive, Warrendale, PA 15096-0001 U.S.A. Tel: (724) 776-4841 Fax: (724) 776-5760

SAE TECHNICALPAPER SERIES 980040

Aerodynamic Optimization of the Opel CalibraITC Racing Car Using Experiments and

Computational Fluid Dynamics

Frank Werner and Steffen FrikAdam Opel AG

Josef Schulze

Reprinted From: Developments in Vehicle Aerodynamics(SP-1318)

International Congress and ExpositionDetroit, Michigan

February 23-26, 1998

The appearance of this ISSN code at the bottom of this page indicates SAEs consent that copies of thepaper may be made for personal or internal use of specific clients. This consent is given on the condition,however, that the copier pay a $7.00 per article copy fee through the Copyright Clearance Center, Inc.Operations Center, 222 Rosewood Drive, Danvers, MA 01923 for copying beyond that permitted by Sec-tions 107 or 108 of the U.S. Copyright Law. This consent does not extend to other kinds of copying such ascopying for general distribution, for advertising or promotional purposes, for creating new collective works,or for resale.

SAE routinely stocks printed papers for a period of three years following date of publication. Direct yourorders to SAE Customer Sales and Satisfaction Department.

Quantity reprint rates can be obtained from the Customer Sales and Satisfaction Department.

To request permission to reprint a technical paper or permission to use copyrighted SAE publications inother works, contact the SAE Publications Group.

No part of this publication may be reproduced in any form, in an electronic retrieval system or otherwise, without the prior writtenpermission of the publisher.

ISSN 0148-7191Copyright 1998 Society of Automotive Engineers, Inc.

Positions and opinions advanced in this paper are those of the author(s) and not necessarily those of SAE. The author is solelyresponsible for the content of the paper. A process is available by which discussions will be printed with the paper if it is published inSAE Transactions. For permission to publish this paper in full or in part, contact the SAE Publications Group.

Persons wishing to submit papers to be considered for presentation or publication through SAE should send the manuscript or a 300word abstract of a proposed manuscript to: Secretary, Engineering Meetings Board, SAE.

Printed in USA

All SAE papers, standards, and selectedbooks are abstracted and indexed in theGlobal Mobility Database

1980040

Aerodynamic Optimization of the Opel Calibra ITC Racing CarUsing Experiments and Computational Fluid Dynamics

Frank Werner and Steffen FrikAdam Opel AG

Josef Schulze

Copyright 1998 Society of Automotive Engineers, Inc.

ABSTRACT

The requirements for racing car aerodynamics are farmore extensive and demanding than those for passengercars. Since many of the relevant aerodynamic featurescannot be measured easily, if at all, Computational FluidDynamics (CFD) provides a detailed insight into the flowphenomena and helps in understanding the underlyingphysics. This paper summarizes some aspects of the aerody-namic optimization process for the Opel Calibra ITC rac-ing car, starting from the production car design andincluding exterior and interior aerodynamic computations,together with wind tunnel experiments.

INTRODUCTION

The design regulations for the Class 1 Racing Car Cham-pionship ITC, as for its predecessor DTM, have becomeincreasingly more liberal. It has proved necessary to addhighly sophisticated aerodynamic features, in order toimprove the aerodynamics of the racing cars and henceachieve competitiveness. This has resulted in greatertechnical differences between racing cars and their corre-sponding production vehicles.Fig.1 explains the enormous significance of drag optimi-zation for racing cars. Due to the high speeds, a smallincrease in drag leads to a substantial rise in the requiredengine power needed to overcome this drag. For exam-ple, the Calibra racing car has a drag coefficient (CD) ofabout 0.36 for a particular setup, whereas the value forthe equivalent Calibra production vehicle is only 0.26,which is the lowest for all production passenger cars. Thehigher drag coefficient of the Calibra racing car is mainlycaused by the rear wing, needed to provide the desireddownward force. All values in Fig.1 concerning therequired additional engine power are given with respectto this particular setup.

The other drag coefficients mentioned in Fig.1 describethe range of other setups and the estimated values ofsome competitor cars. It is obvious that these differenceshave a decisive effect on possible accelerations andspeeds, and hence may decide the race.Since Class 1 racing cars are so powerful (approx. 380kW), many other factors must also be carefully consid-ered. For example, the required downward forces andbalancing must be provided without significantly impair-ing the drag coefficient. In addition, the cooling systemsfor the engine, brakes, and electronic devices and theefficiency of the engine intake system are of vital impor-tance.

To fulfill these tasks, the following aerodynamic featureswere developed and applied to the Opel ITC racing carCalibra (Fig.2):

rear wing with gurney (1) underbody almost completely covered (2) diffuser channels separated by strakes (3) wooden bars at each side of the vehicle to reduce the

underbody flow leakage (4) wheel-house ventilation (5) variable device using a set of flaps to shut the inlet of

the engine cooling duct (6) brake cooling duct (7) front splitter (8)

AERODYNAMICS DEVELOPMENT

The development of racing cars is characterized byextremely short design cycles. Hence, to achieve all therequired objectives, there must be a very close interac-tion between experimental and computational activities.In addition, each engineering discipline must focus on istown particular strengths in order to maximize effective-ness.

2In the past, aerodynamic development was mainly per-formed in wind tunnels, ignoring the relative movementbetween the vehicle and the ground. However, our pre-liminary wind tunnel tests, performed with rotating as wellas non-rotating wheels, showed that the relative move-ment of the vehicle with respect to the ground has a sig-nificant effect on the underbody flow (Fig.3). All values inFig.3 are denoted with respect to the drag coefficient forthe baseline racing car with fixed wheels and ground. The importance of more sophisticated tests can be illus-trated as follows: measurements for cars with non-rotat-ing wheels indicated that only one modification -completely covering the underbody - leads to animproved drag coefficient, whereas all other optionsincrease the drag. However, in contrast to these results,the realistic setup, which included rotating wheels andmovement relative to the ground, showed improvementsfor all cases. A general rule is that a test gives higherdrag coefficients when performed with rotating wheelsthan with non-rotating wheels. Considering the very lowground clearance of a racing car (approx. 30 mm) thiseffect could be predicted. Consequently, all wind tunneltests and computational models, especially those used tosimulate underbody flow phenomena, must include rotat-ing wheels and movement relative to the ground.The following paragraphs describe some of the aerody-namic features mentioned above, in more detail:

COMPUTATIONAL MODEL

In addition to numerous wind tunnel tests, the develop-ment of most of the aerodynamic features was supportedextensively by Computational Fluid Dynamics (CFD). Athree-dimensional wind tunnel model was created, con-taining 3.6 million fluid cells. The fine detail of this com-putational mesh can be seen in Fig.4, which displays thesurface grid for the vehicle. The turbulent flow is calcu-lated with the CFD code STAR-CD [3], solving Reynolds-averaged Navier-Stokes equations with a RNG k-e turbu-lence model [4]. The computational model included allrelevant aerodynamic features mentioned above (Fig.2),and assumed the racing car to be symmetric. All maininterior ducts (engine cooling, brake cooling, airboxintake) had to be modelled, as the interaction of theexternal and internal flows was part of the investigation.Rotating wheels and movement relative to the groundwere included, in order to simulate realistic road condi-tions.

The calculated flow velocities shown in Fig.5 and the cp-distribution near the surface of the car, see Fig.6, give anoverall view of the surface flow characteristics of the ITCCalibra racing car. Due to the huge size of the computational grid, the aero-dynamics simulation of the complete racing car took toomuch modelling and computing time for setup optimiza-tion or sensitivity studies to be performed. Consequently,the aim of the CFD work was to predict trends and toachieve a better understanding of qualitative flow charac-

teristics, rather than to calculate values such as drag orlift coefficients. This approach seems to be the only feasi-ble one as current computer codes are not able to predictdrag and lift with the required accuracy [5]. Certain taskssuch as setup optimizations, which are characterized byvery small design changes e.g. an inclination or offset ofthe rear wing, can be performed much more quickly bymeans of experimental devices than by computationalanalyses. Some simulations were performed using simplified sub-models, which included all relevant aerodynamic fea-tures, in order to save time.

AERODYNAMICS OPTIMIZATION

ENGINE COOLING DUCTS The main tasks were toachieve a uniform flow through the radiator and to mini-mize the interaction of the external flow and the flow leav-ing the duct. Here, a simplified model of the front bodywas used to enable easier geometry modifications andfaster turnaround. The vehicle was assumed to be sym-metric, so that a half model of the front body, from thefront splitter to the B-pillar, was employed. The under-body flow and the wheel rotation were not included. The baseline geometry led to a relatively strong interac-tion between the external flow and the flow exiting thecooling duct, which generated an extended flow separa-tion near the front wheel (Fig.7a). This large recirculationzone widened the vehicles aerodynamic effective cross-section, so that the drag increases.Several duct shapes were analyzed in order to minimizethis effect. Due to package restrictions for the wheelhouse and front fender regions, nearly all the modifica-tions had to be carried out within the envelope of thebaseline duct. An additional constraint was that the airflow through the radiator must remain uniform. All theserequirements made the use of CFD essential for a sys-tematic optimization strategy. The interaction among the two flow streams was consid-erably reduced (Fig.7b) by shape modifications betweenthe radiator and the outlet and by the introduction ofvanes. These vanes were inserted at the duct exit inorder to deflect the air flow leaving the cooling duct, sothat it became nearly tangential to the external flow.Thus, the wake next to the front wheels almost disap-peared.A device to shut the cooling duct helped to completelyavoid the interaction of the two flows. This device con-sisted of several flaps, activated automatically by vehiclespeed and coolant temperature. The orientation of theflaps at the fully open position was defined by the calcu-lated flow velocities at the inlet plane of the duct. Thus,the effect of the flaps on the flow for the fully open posi-tion could be minimized. With this device, when the flapsare closed the drag coefficient can be reduced by up toapproximately 0.02.

3FRONT END AERODYNAMICS Various front end con-figurations were simulated, in order to determine how theexternal flow was affected by the internal flows throughthe airbox, brake and engine cooling systems. Differentinternal flows were considered, starting with a completelyclosed front, which of course is not feasible. It wasassumed that the vehicle moved at 250 km/h and that theengine was running at maximum rpm (approx. 12.000rpm). Fig.8 displays the normalized pressure distributionin the symmetry plane. Clearly, the internal flows led toconsiderable changes in the front end pressure distribu-tion. In particular, the low pressure region at the front ofthe hood (case 1) almost disappeared when the internalducts were open (cases 3 and 4). This effect is moreimportant for racing cars than for ordinary passengercars, as racing car engines operate at higher rpms for agiven vehicle speed so that the ratio of the air flowthrough the airbox to the external flow is much higher. These results demonstrate that the internal flows must betaken into account for an accurate prediction of the frontend flow.

REAR WING Since the Calibra is shaped like a coup,the rear wing configuration was of major concern. Sys-tematic experimental and computational studies wereperformed with different multi-component airfoil configu-rations and gurney lengths, in order to achieve both lowdrag and well balanced maximum downward forces onthe axles.

These studies revealed an important rear design consid-eration: Fig.9 shows the computed air flow velocity vec-tors near the rear wing for the early Calibra styling body(production car) quantifying the flow angle close to theleading edge of the rear wing. It was found that the inflowvelocity vector changed its orientation from middle to sidebody section. The rear spoiler in the current vehicle wasredesigned to perform in this way and was then mea-sured in the wind tunnel. The new design led to an 8%increase in downward force, with the same drag force asa conventional design with constant wing angles. Fig.10displays the calculated flow field and pressure distributionfor this optimized multi-component airfoil. Overall, theexperimental and computational optimization indicatedthat coup-styled racing cars need to be treated differ-ently to notchback cars.

DIFFUSER The diffuser is an important means ofincreasing the downward force at the rear axle. The opti-mum diffuser angle for lift and drag was determined by anextensive series of wind tunnel tests. These tests wereperformed in the wind tunnel in Emmen, Switzerland,which has facilities to simulate rotating wheels and move-ment relative to the ground. The results of the three-dimensional simulation of thecomplete racing car showed that the flow around the rearwheels disturbed the flow at the diffuser intake, so thatflow separations occured inside the diffuser channels(Fig.11a).

In order to improve the diffuser performance, firstly thediffuser angle was optimized. Additionally, strakes wereused to subdivide this region into several separate chan-nels, reducing leakage of the underbody flow. Further-more the front splitter and the position of the strakes weremodified to increase and direct the underbody flow andhence reduce the influence of the flow around thewheels. Due to these design modifications, the flow wasthen completely attached to the upper side of the diffuser(Fig.11b), giving a low pressure level at the rear under-body. The higher underbody flow rate additionallydecreased the static pressure and thereby led to a higherdownward force.

CONCLUSION

Aerodynamics optimization is of vital importance inachieving competitiveness for racing cars, because oftheir very high performance levels. The extremely shortdesign cycle for such a vehicle requires very close coop-eration between experimental and computational devel-opment work.Experimental data and computational results togethershow that the simulation of racing car aerodynamics mustinclude the modelling of all aerodynamic car features.The optimization strategy for the Opel Calibra racing carinvolved the following:

Internal flows (airbox, engine and brake cooling duct)must be taken into account to simulate the correctfront end body flow. The re-entering of these flowsinto the external flow is of major importance, becausethey can have a major effect on external flow charac-teristics.

The very low ground clearance of racing cars meansthat rotating wheels and movement relative to theground must be modelled in order to accurately pre-dict the underbody flow. Measured data show thatneglecting the relative movement of the vehicle andthe wheels with respect to the ground may some-times lead to completely wrong results when deter-mining the influence of geometry modifications onthe lift and drag coefficients. This means that a modelwith non-rotating wheels and fixed with respect to theground cannot even predict trends reliably.

To achieve the required results in time, some analy-ses had to be performed using simplified computa-tional models. However, even these modelscontained all significant local aerodynamic features.

This aerodynamic optimization, characterized by thesimultaneous application of experimental and computa-tional tools, together with the close cooperation of allengineers involved, was part of the success of the OpelCalibra Class 1 racing car, winning the ITC Champion-ship in 1996.

4ACKNOWLEDGMENTS

The authors thank F. Ross, adapco, who performed someof the flow calculations.

REFERENCES

1. H. Emmelmann, H. Berneburg, J. Schulze, The Aerody-namic Development of the Opel Calibra, SAE Paper900317

2. F. Indra, Welche Vorteile bringt der Motorsport fr dieSerienentwicklung?, Automobil Revue 16, 1994

3. Computational Dynamics Ltd., STAR-CD Manual Version3.0, 1997

4. V. Yakhot, S.A. Orszag, Renormalization group analysisof turbulence. J. Scientific Computing, 1:1-51, 1992

5. M. Ramnefors et al., Accuracy of Drag Predictions onCars Using CFD - Effect of Grid Refinement and Turbu-lence Models, SAE Paper 960681

Figure 1. Required additional engine power for different drag coefficients

5Figure 2. Aerodynamic features of the OPEL Calibra ITC racing car

Figure 3. Impact of different wind-tunnel setups on the drag coefficient

6Figure 4. Computational mesh at the surface of the vehicle

Figure 5. Calculated flow velocities near surface(Rotating wheels and moving windtunnel ground)

7Figure 6. Calculated pressure distribution on the surface(Rotating wheels and moving ground)

Figure 7. Horizontal section through the cooling duct (simplified model)a) Baseline design

b) Optimized design

8Figure 8. Impact on different internal flows on the pressure distribution at the front-end

9Figure 9. Inflow vector orientation near leading edge of rear wing(Calculated vector field for Calibra styling model)

Figure 10. Calculated flow field and pressure distribution at rear wing of the Opel Calibra ITC racing car

10

Figure 11. Calculated diffuser flow field for two underbody designsa) Baseline diffuser, strakes and splitter configuration

b) Optimized diffuser, strakes and splitter configuration