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13 On Various Aspects of Testing Methods
in Vehicle Aerodynamics*
Hans KERSCHBAUM 1)
, Norbert GRUEN 2)
, Peter HOFF 3)
, Holger WINKELMANN 4)
This paper addresses various aspects of testing methods in aerodynamics and heat management of passenger cars, motorsports vehicles
and motorcycles. BMW group testing is comprised of track tests (heat management and soiling), physical simulation (wind tunnel) and
numerical simulation (Computational Fluid Dynamics). These different methods of assessing vehicle properties are not considered as
competing tools, rather they are utilized in a complementary fashion. The integration of the different approaches in the aerodynamic
development process will be discussed in detail.
Keywords: Vehicle Aerodynamics, Development Process, Wind Tunnel Testing, CFD
1. INTRODUCTION
The start of thorough aerodynamic development at the
BMW group dates back to the early 80s, when the current
full scale wind tunnel was established. At that time only
full scale clay or foam models were used. Since
approximately 1990 the early phase employed 40% scale
clay models, but did not include the analysis of flow
through the engine bay and detailed underbodies. At that
time, state of the art testing was at stationary conditions,
i.e. without moving floor and with non-rotating wheels.
Occasionally potential flow and boundary layer methods
were used to simulate basic vehicle shapes. Around the
year 2000 the wind tunnel models were equipped with a
detailed underbody and a traversing gear was used to
measure off-surface flow field characteristics. At the same
time the BMW group began to utilize more sophisticated
CFD (Computational Fluid Dynamics) analysis methods.
However, it was soon realized that aerodynamic
optimization, especially on the front end, may be
misleading if the underhood flow, which changes the flow
characteristics, is not included. Further, comparisons of
different wind tunnel experiments and CFD validation
results made obvious that testing under stationary
conditions was not sufficient. The removal of these
deficits led to the development process in place today.
2. CURRENT DEVELOPMENT PROCESS
2.1 Basic Considerations
An advanced vehicle development process must ensure
consistent and coherent research and analysis concepts in
the initial design steps. During the subsequent phase these
concepts are finalized and made ready for production
(Fig.1). This implies that all essential parameters need to
be investigated during the concept phase without having
physical prototypes at hand for the proof of concept.
Fig.1: The Aerodynamic Development Process
To achieve consistent concepts in the aerodynamic
development, high quality testing is required for all
relevant parameters, because vehicle proportions and the
styling are based upon these. Further crucial boundary
conditions for the aerodynamic layout are the flow
through the cooling package and engine compartment as
well as underbody concepts.
Apart from product related concepts also the impact of
simulation technology must be considered, in particular
the impact of ground effect, wheel rotation, wind tunnel
interference and the interaction between the flow through
and around the vehicle.
During this initial development phase the aerodynamic
engineer is offered a variety of virtual and experimental
tools. Each of these has its own strengths and weaknesses
concerning quality, speed and complexity of usage. The
successful integration of aerodynamic development in the
overall development process depends on the intelligent
combination of all available tools. In the following chapter
various aspects of the concept phase are discussed.
____________________________________________________
*Presented at 2004 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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2.2 Experimental Simulation Technology
(1) Process Requirements. The prediction of integral
aerodynamic vehicle properties in the early phase with
focus on drag, lift and cooling airflow rates is crucial for
consistent concepts. Beyond the current status the
potential of various optimization measures has to be
reported.
The acceptable uncertainty for predicting the final
production drag value is roughly ∆Cx = ±0.005, i.e. the
fidelity requirement for ranking different concepts or
styling themes is even higher. It must be ensured that the
effect of individual optimization measures on the entire
vehicle may be accurately transferred to the final vehicle
configuration. For example, the assessment of cooling
airflow rates requires a correct coupling of intake
openings. The goal is an absolute accuracy of ±10% for
the mass flow rate but the focus is on the correct
prediction of trends with different optimization measures.
Short loops are typical for the proportional design
stage. Timeframes in the order of weeks for the different
styling cycles impose a high challenge on the aerodyamics
process. Only a narrow time frame remains after the
delivery of geometry data to the aerodynamics group for
model generation and optimization before it must be
returned to the styling department. During this period
multiple proportion and styling variants have to be
assessed and optimized simultaneously. If the
aerodynamic process fails to keep pace, the design cycles
cannot be accompanied and optimizations do not feed
back into the loop.
These time constraints and the multitude of models led
to the usage of simplified and scaled wind tunnel models.
In order to save time and cut model building cost, a model
scale of 40% proved to be sensible at BMW because it
still allows the representation of geometry details with
sufficient accuracy and also ensures a realistic visual
impression. A further reduction of model size would cause
problems with small details and the portability of
aerodynamic effects to full size cars. For instance a
change in trunk height of 25mm, which is a massive
modification from styling point of view, translates into
only 10mm on the 40% model. To represent the
equivalent 6.25mm on a 25% model would push the limits
of model building.
(2) Boundary Conditions. The deficit of conventional
wind tunnels with stationary floor and wheels to simulate
road conditions has been documented in numerous
publications. In [1] it has been demonstrated that the
entire flow field changes if the floor is moving and the
wheels are rotating. As a consequence the potential and
results of optimization are different between the two
experimental techniques.
This applies not only to aerodynamic fairings on the
underbody but also to exterior skin parts like front end,
trunk and sidewalls.
To account for these effects already in the concept
phase where proportions and styling are fixed, the model
must be prepared for testing under both conditions.
Another parameter with a strong influence on the
result of optimization is the interaction between the flow
through and around the car. In [2] it is shown that exterior
shape modifications can increase the cooling air mass
flow rate up to 15%, a potential that must not be ignored
during the proportion finding phase. On the other hand,
[2] also shows that including the cooling air mass flow in
experiment yields different optimal configurations than
with a closed front end. Hence, the transfer of results from
the model phase to the final full scale car is corrupted with
an incomplete wind tunnel simulation or ignoring cooling
air.
(3) Model Concepts. The concept phase at BMW is
characterized by multiple iteration loops between styling
and aerodynamics. The necessity for rapid adaptation of
changes pushes the limits of current model building
technology. Comprehensive validation efforts imposed
further requirements for the necessary level of detail
which will be discussed in the following.
The quality of (experimental) simulation in the early
phase, using 40% models , depends on the level of detail.
During the past few years BMW has moved from low
level models with smooth underbody to highly detailed
underbody representations (s. Fig.2).
Fig.2: Levels of Detail on the Underbody of 40% Models
State-of-the-art rapid construction technologies like
stereo-lithography (STL), milling, laminating, deep-
drawing and the availability of flexible synthetic
materials (glass- and carbon-fiber reinforced plastics)
allow any level of detail (LOD) today. Limiting factors
are only cost constraints and the time available for model
production.
Fig.3: Process Capability vs. Level of Detail
The dependency of process capability, i.e. the ability
to keep pace, from the level of detail and hence the quality
of conclusions is sketched in Fig.3. An optimal process

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would be located in the upper right corner, i.e. high
quality and high process capability simultaneously. With a
given testing technology a change in the level of detail
moves the process along the depicted lines. For a process
approaching the optimum, the process capability will
remain high with increasing level of detail, so this is the
main focus when optimizing the process.
Extensive validation work and also the results of CFD
simulations yielded essential input for the evolution of a
model system with the following properties:
o Largely automatic manufacturing
o Representation of underhood flow
o Rapid exchangebility of modules
o Supports moving floor and rotating wheels
o Usability in different wind tunnels
(mountings)
o Relief milling of underbody details
According to the above requirements the 1:2.5 scale
models are composed of PU-foam on a steel frame with
additional STL or laminated parts.
Fig.4: Exploded View of a Modular 40% Scale Model
For measurements in wind tunnels with moving floor
separate wheels are made of carbon fiber reinforced
plastic. The engine bay is manufactured by sophisticated
milling techniques and filled with rapid modeled engine
and cooling package components. The modular assembly
is made clear in Fig.4.
For the detail optimization in the later phase of
aerodynamics/styling coordination full scale PU-foam
models are fabricated which fulfill the same requirements
as the scale models, and also roadworthy prototypes are
used for the proof of aerodynamic concepts.
2.3 Virtual Simulation Technology
In recent years computational fluid dynamics (CFD)
methods have reached such a level of maturity that they
could be moved more and more into productive use [3,4].
(1) Process Requirements. However, it is not enough to
achieve a sufficient accuracy of the results in terms of
absolute values (±2% for drag and lift and ±10% for
cooling air mass flow) and more importantly to predict
the correct trends of modifications. A CFD tool must also
feature a turnaround time from CAD data to results which
is compatible with the overall process schedule. This does
not only include the preparation of CFD input data from
CAD, CAS (Computer Aided Styling) or laser scanned
clay model geometry but also the effort needed to set up
simulation cases, run the simulation and analyze/visualize
the results.
In particular during the early phase of proportion
investigations it is mandatory to have tools for rapid
morphing of vehicle shapes available as well.
(2) The CFD tool employed in aerodynamics at BMW is
PowerFLOW, a Lattice-Boltzmann code. Its advantage
compared to traditional Navier-Stokes methods is the
automatic grid generation, i.e. no manual work is
necessary to discretize the fluid domain around the car.
Fig.5: CFD Model Details (BMW X5)
The geometry input consists of the surface
facetizations of any number of objects. In the case setup
these objects can be arranged in arbitrary fashion and even

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intersect each other. Thus it is possible, if necessary, to
run simulations at very high levels of detail (Fig.5).
(3) Benefits Due to the ease-of-use it does not require a
numerical specialist to apply PowerFLOW. This enables
the aerodynamic engineers to employ CFD as a
complementary tool to physical testing. Therefore at
BMW there is no separate assignment of personnel for
experiment and simulation.
Beyond the calculation of integral quantities like drag,
lift, mass flow rates etc. the advanced visualization and
analysis tools enable insight into flow field details (Fig.6)
which are not accessible by experiment or require an
effort which is not justifiable and cannot keep pace with
the development process.
Fig.6 CFD Simulation incl.Underhood Flow (BMW X5)
Moreover, a thorough analysis of simulation results
can deepen the understanding of the flow field and thus
give valuable hints for optimization work in the wind
tunnel. The final goal is to improve the simulation process
so far that initial selections of different styling variants
can be made solely as a virtual process.
3. CONCLUSION
The current shortening of design cycles is pushing the
limits of the aerodynamic development process.
Experimental and virtual simulation technology must be
optimized in order to deliver feedback in the narrow time
frame remaining for aerodynamic optimization.
In the recent past it became clear that features like
moving ground, rotating wheels and cooling airflow must
be included in the initial phase of proportion finding.
Otherwise it is possible to miss opportunities for
improvements being transferred to the final phase. This
implies that scale models must have a certain level of
detail to account for all crucial phenomena. To maintain
process capability this requires advanced technology for
the manufacturing of wind tunnel models like
stereolithography, sophisticated milling and a modular
composition of the whole assembly.
Due to the progress in computational fluid dynamics
(CFD) in the recent years, it has become possible to utilize
virtual testing in the early phase. However, like with
physical models, process capability is only guaranteed if a
rapid turnaround can be achieved in terms of model
generation, ease-of-use, simulation time, results analysis
and also morphing tools to create modifications.
The key to an advanced aerodynamic simulation
process is the intelligent usage of experimental and virtual
technologies as complementary tools.
REFERENCES
[1] The New 5-Belt Road Simulation System of the
IVK Wind Tunnels
Jochen Wiedemann, Juergen Pothoff
SAE Paper 03B-102, 2003
[2] Interaction between Underhood and External
Flow
Marco Brümmer
Unpublished Internal BMW Report, 2003
[3] Validation and Application of CFD to Vehicle
Aerodynamics,
Wolf Bartelheimer
JSAE Paper 20015332, 2001
[4] CFD Simulation in Motorcycle Aerodynamics at
the BMW Group,
Christian Kleiner, Norbert Grün
HdT Conference on Motorcycle Development,
2003, Munich, Germany