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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
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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.
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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.
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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.
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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
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5Figure 2. Aerodynamic features of the OPEL Calibra ITC racing
car
Figure 3. Impact of different wind-tunnel setups on the drag
coefficient
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6Figure 4. Computational mesh at the surface of the vehicle
Figure 5. Calculated flow velocities near surface(Rotating
wheels and moving windtunnel ground)
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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
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8Figure 8. Impact on different internal flows on the pressure
distribution at the front-end
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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
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10
Figure 11. Calculated diffuser flow field for two underbody
designsa) Baseline diffuser, strakes and splitter configuration
b) Optimized diffuser, strakes and splitter configuration