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Modelling and assembly of the full vehicle 349
anti-roll bars form more than about one third of the overall roll stiffness – in
other words if K Tr is greater than 0.5 K Ts .
6.8 Aerodynamic effects
Some treatment of aerodynamics is generally given in existing text books
(Milliken and Milliken, 1995; Gillespie, 1992) dealing with vehicle dynamics.
Other textbooks are dedicated to the subject (Hucho and Ahmed, 1998).
The flow of air over the body of a vehicle produces forces and moments acting
on the body resulting from the pressure distribution (form) and friction
between the air and surface of the body. The forces and moments are considered
using a body centred reference frame where longitudinal forces
(drag), lateral forces, and vertical forces (lift or down thrust) will arise. The
aerodynamic moments will be associated with roll, pitch and yaw rotations
about the corresponding axes.
Current practice is generally to ignore aerodynamic forces for the simulation
of most proving ground manoeuvres but for some applications and
classes of vehicle this is clearly not representative of the vehicle dynamics
in the real world, for example winged vehicles. It is often said that for some
vehicles of this type the down thrust is so great that this could overcome the
weight of a vehicle, allowing it, for example, to drive upside-down through
a tunnel, although this has never been demonstrated.
The lack of speed limits on certain autobahns in Germany also means that
a vehicle manufacturer selling a high performance vehicle to that market
will need to test the vehicle at speeds well over twice the legal UK limit.
The possibility of aerodynamic forces at these high speeds destabilizing the
vehicle needs to be investigated and where physical testing is to be done,
equivalent computer simulation is also desirable. Other effects such as side
gusting are also tested for and have been simulated by vehicle dynamicists
in the past.
An approach that has been commonly used is to apply forces and moments
to the vehicle body using measured results, look-up tables, from wind tunnel
testing. As the vehicle speed and the attitude of the body change during the
simulation the forces and moments are interpolated from the measured data
and applied to the vehicle body. A difficulty with such an approach is that the
measured results are for steady state in each condition and that transient
effects are not included in the simulation. Consideration has been given to the
use of a computational fluid dynamics (CFD) program to calculate aerodynamic
forces and moments in parallel with (co-simulation) an MBS program
solving the vehicle equations of motion. The problem at the current time with
this approach is the mismatch in the computation time for both methods.
MBS models of a complete vehicle can simulate vehicle handling manoeuvres
in seconds, or even real time, whereas complex CFD models can
involve simulation times running into days. Current CFD methods also have
difficulty with aerodynamic transient effects (e.g. vortex shedding) although
an emerging group of ‘multi-physics’ codes look set to address these problems.
Thus there is no realistic prospect of the practical use of transient aerodynamics
effects being modelled in the near future. However, genuine
transient aerodynamic effects, such as those involved in so-called ‘aeroelastic