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211 Virtual Car and Motorcycle Aerodynamics*
Norbert GRUEN 1)
, Rainer DEMUTH 2)
, Gerhard HOFER 3)
, Hans KERSCHBAUM 4)
This paper describes the status of CFD usage in the aerodynamic development of passenger cars and motorcycles at the BMW Group.
After an explanation of the significance and time frame of aerodynamics in the overall development process the questions are discussed
which can be addressed via simulation today. Considering the simulation process it gets clear that the bottleneck is the effort needed to
prepare the geometry. For this reason the primary tool at BMW is a Lattice-Boltzmann code which allows the easy handling of complex
configurations. The achievable accuracy to date is sufficient for a productive use and is shown on selected examples. Finally some
practical applications demonstrate the benefit of simulation for the aerodynamicist.
Keywords: Aerodynamics, Development Process, Simulation, CFD, Lattice-Boltzmann
1. INTRODUCTION
Due to the level of maturity that simulation methods
have reached today, CFD tools are more and more
employed in a productive manner in the aerodynamic
development process. The objective is twofold. First one
tries to reduce the number of cost and time intensive
experiments in the early phase and on the other hand CFD
enables a deeper understanding of the flow phenomena
around and through detailed vehicle models in the later
phase.
This paper presents the progress of CFD usage during
the aerodynamic development of passenger cars and
motorcycles at the BMW Group since a previous
presentation at JSAE 2001 [1].
First an overview is given of the development process
and the questions that can be tackled by simulation in the
different phases. Then the simulation process is explained
from geometry preparation to results analysis. Some
selected validation cases demonstrate the accuracy that
can be achieved at the moment. Finally a number of
practical examples is used to show the capabilities for in
detail analysis of the flow field and the benefit for the
aerodynamicist.
2. AERODYNAMICS IN THE OVERALL
DEVELOPMENT PROCESS
Aerodynamics is one of the few disciplines to get
involved in vehicle development at the very beginning. In
the course of the overall development process more and
more details of the vehicle become available and
accordingly the questions become more detailed (Fig.1).
Already in the initial phase the first styling ideas have to
be assessed with respect to the impact of proportions on
aerodynamic forces and driving stability. At this point
only virtual models exist and hence simulation is the tool
of choice.
At the transition to the concept phase simplified
underhood and underbody details are included, often taken
from the predecessor. This allows to address thermal
management questions as well. However, the investigation
of soiling and snow deposition has to wait until
roadworthy prototypes are available for physical testing.
CFD tools are not yet sophisticated enough to be applied
to these problems in depth.
Fig.1: Aerodynamic Questions and Tools [2]
No matter whether simulation or physical testing is
employed [3], the time frame for aerodynamics in the
overall development process is quite short and requires
efficient approaches to have an impact on the product.
Simulation is almost the only way to identify problems
early enough to avoid cost and time intensive measures if
detected in later phases.
____________________________________________________
*Presented at 2007 JSAE Annual Congress.
1), 2), 3), 4) BMW Group, Center for Innovation and R&D,
Knorrstr. 147, D-80788 Muenchen, Germany

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3. SIMULATION PROCESS
Regardless of the particular tool, each CAE simulation
process starts with the collection of geometry input data.
In case of aerodynamics we distinguish between the
exterior skin, created by the stylist either virtually or in
clay, and the underhood and underbody components
(Fig.2).
Preparing the geometry as input for the CFD tool means
to create a facetized representation of the individual
components. For the Lattice-Boltzmann code
(PowerFLOW, [4]) used at BMW it is not necessary to
generate a single connected surface mesh. The entire
configuration is composed of any number of individual
solids and/or zero-thickness shells which may intersect
each other.
Fig.2: Simulation Process
Another advantage of the tool is the automatic creation
of the fluid mesh which is a lattice of rectangular cells as
depicted in Fig.3.
Fig.3: Detailed Motorcycle Model in the Lattice
Their intersection with the geometry is discretized
automatically as part of the simulation. The user just
defines the resolution level by simple geometries or
offsets for the near surface layers. Together with the
efficient parallelization of the code a total turnaround
from the start of the geometry preparation to first results
of two days to two weeks is possible, depending on the
complexity and quality of the geometry data.
The simulation itself always runs in transient mode. If
available, initial conditions from a similar case can be
used to shorten the time needed to reach a settled quasi
steady-state. For external aerodynamics the typical
monitoring quantities are drag and lift to decide when to
stop the simulation and which period should be used to
obtain time-averaged results (Fig. 4).
Fig.4: History of Transient Forces
There is a vast manifold of options to visualize the
flow field by surface images, cutting planes, streamlines,
isosurfaces etc., examples are shown in Figs. 5 and 6. For
a first quick impression standardized analysis scripts are
used to generate images, animations and report files.
Fig.5: External Flow Field Visualization (Near-Surface
Velocity, Streamlines and Total Pressure)

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Fig.6: Underhood Flow Field Visualization
(Streamlines colored by Temperature)
4. VALIDATION EXAMPLES
Simulation results will only be accepted if there is a
sound confidence in their accuracy and reliability, proven
by validation with experimental data. Fig.7 shows the
comparison of drag and rear axle lift coefficients for an
array of different vehicles. It is obvious that there are
differences in the absolute values but also that the trend
correlation is mostly acceptable for productive work.
-0,050
0,000
0,050
0,100
0,150
0,200
0,250
0,300
0,350
0,400
0,450
DragandLiftCoefficients
Cx PowerFLOW
Cx Wind Tunnel
Cz-rear PowerFLOW
Cz-rear Windtunnel
Fig.7: Comparison of Drag and Rear Lift Coefficients
for an Array of different Vehicles
To impose exaggerated demands on the congruence is
not reasonable because the wind tunnel is also only a
simulation of the real conditions on the road and it is
known that different tunnels will produce different results.
Especially in the early phase relative predictions are most
important to rule out low potential variants.
Predicting lift and in particular its balance on front and
rear axle correctly is more difficult than drag prediction
but at the same time extremely important for driving
stability. Fig.8 shows an example for good correlation
and demonstrates how simulation can help to gain insight
in the mechanism of lift generation. The bars represent the
lift contribution of slices through the vehicle and the curve
depicts their integral in streamwise direction.
Fig.8: Validation of Lift Distribution in Detail
After an initial lift increase on the nose due to more
suction on the hood than on the underbody a large region
of downforce follows, caused by the decelerated flow
towards the cowl. The resulting high rear axle lift is
generated primarily by the low pressure over the entire
passenger compartment and not by lift on the rear end
itself (a car with such a lift balance as this validation
model would never make it into production).
External aerodynamics is closely linked to underhood
and underbody flow. Therefore even the early models in
the initial phase are mostly investigated including at least
simplified flow through the engine compartment and a
rough underbody to account for the contribution of the
cooling package to aerodynamic properties. On the other
hand, once engine types have been finalized the
aerodynamicist has to deliver appropriate cooling air mass
flow rates to ensure proper thermal management.
The cooling package is represented by porous media for
radiators and the fan is calculated in a rotating frame of
reference. With this model an accuracy of about 5% is
achieved for the cooling air mass flow rates.
Advanced simulations beyond aerodynamics include the
coolant side to obtain the heat flow rates and radiation
from high temperature surfaces, for instance on exhaust
systems and turbochargers. We have just started to
validate PowerFLOW in this respect and do not have a
final idea about the accuracy of rejected heat and surface
temperatures when exploiting all these features.
5. APPLICATION EXAMPLES
In view of the broad spectrum of questions tackled by
simulation beyond the classical determination of forces
and moments

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• Aerodynamic Loads on Structures
• Passenger Comfort in Open Convertibles
• Unsteady Aerodynamics / Stability
• Thermal Management
• Brake Cooling
• Soiling and Gravel Impact
only a few selected examples can be discussed here.
Already standard is the handover of aerodynamic forces
on parts like hoods and windows to the structures groups.
In addition to simply deliver the force vector and its point
of incidence (Fig.9) it is possible to map the 3D load
distribution directly onto the structure model.
Fig.9: Transfer of Aero-Loads (Force Vector and 3D
Distribution) to Structure Models
Almost like on motorcycles, the passengers of an open
convertible are exposed to wind and weather. The primary
criterion to assess their comfort is the near surface
velocity distribution, shown in Fig.10.
Fig.10: Passenger Comfort in Open Convertibles
(Near-Surface Velocity Magnitude)
The demand for highly dynamic but still safe handling
properties of BMW cars and motorcycles up to top speed
has led to an increased interest in unsteady aerodynamics.
Fig. 11 compares the transient yawing moment of two cars
when a sinusoidal side wind gust is traveling downstream
[5]. Apart from the peak value when the gust hits the front
end, the major difference, marked by a circle, occurs in
the way the yawing moment is dying out. In particular this
distinction is perceived by the driver as a more or less
insecure sensation.
Fig.11: Evaluation of Unsteady Results for Stability
6. CONCLUSION
Since the previous presentation of aerodynamic
simulation at the BMW Group [1] a considerable progress
has been made. The level of detail that can be handled
today and the achieved accuracy permit a productive
usage as a complementary tool to wind tunnel and road
tests. In particular the possible field of application has
broadened significantly. Next challenges are, besides the
everlasting request for more accuracy and reliability, the
speedup of the simulation process and the smooth
interfacing to other CAE disciplines.
REFERENCES
[1] Validation and Application of CFD to Vehicle
Aerodynamics,
Wolf Bartelheimer
JSAE Paper 20015332, Yokohama, Japan, 2001
[2] Hans Kerschbaum, Norbert Gruen
“Complementary Usage of Simulation, Wind
Tunnel and Road Tests during the Aerodynamic
Development of a new BMW SUV”, FISITA Paper
F2006M035, Yokohama, Japan, 2006
[3] Hans Kerschbaum, Norbert Gruen, Peter Hoff,
Holger Winkelmann, “On Various Aspects of
Testing Methods in Vehicle Aerodynamics”, JSAE
Paper 20045445, Yokohama, Japan, 2004
[4] Norbert Gruen, “Application of a Lattice-
Boltzmann Code in Vehicle Aerodynamics”, von
Karman Institute for Fluid Dynamics, Brussels,
Belgium, Lecture Series 2005-05, 2005
[5] Numerical Investigations on the Unsteady
Aerodynamics of Road Vehicles under Gusty
Weather Conditions
Rainer Demuth, Philipp Buck
5th
MIRA International Vehicle Aerodynamics
Conference, 2006, Gaydon, UK