Page 51 - Computational Fluid Dynamics for Engineers

36 1. Introduction

and down load on the front and local flow angularity with respect to the front
wheels and wheel wells, which results in a high drag unless this flow is downed
or diverted. Drag considerations also apply to the internal flow system such as
the radiator and air conditioning unit; this will require more attention as the
automobiles become more compact.
Under actual driving conditions, one encounters natural wind, that is not
usually in the direction of the vehicle motion. This results in an asymmetric
flow with respect to the vehicle. Viewed from above, the relative wind angle
corresponds to what is known in aerodynamics as the angle of sideslip; this
angle results in force components in the direction of the wing and normal to
it. Resolution of these components in the car axis system results in a drag
component which usually increases with the yaw angle and a side force. The side
force manifests itself in a negative yawing moment about the center of gravity
of the car, which tends to increase the yaw angle. The cross-flow sensitivity
caused by the yawing moment is somewhat reduced by the forward movement
of the center of gravity in current front-wheel-drive models. However, as vehicles
become lighter to obtain better fuel economy, the problem is bound to reemerge.
Simulation of the natural wind profile in a wind tunnel is extremely difficult
because the wind speed most likely is not constant and the wind profile is
affected by terrain features or buildings.

1.5.1 Applications of CFD to Automobiles
As discussed in [29], there is a large effort underway in applying CFD to road
vehicles with different degrees of sophistication. The simplest approach is to use
panel methods (Section 6.4, [5]) and calculate the inviscid flow around the body.
Even though this approach does not provide flow separation, vortex flow and
drag prediction, it can be useful for pin-pointing possible trouble areas such as
strong pressure gradients and ground effects on the velocity field. This approach
is more suitable to study generic models than to obtain detailed information
on a given design. The next degree of sophistification of the CFD approach
is to perform inviscid flow calculations with vortex wakes added to the panel
method. However, in order to predict flow separation locations and the initial
vortex strength, boundary-layer calculations (Chapter 7) must be performed.
Provided that the body is relatively smooth, that is, local protuberances and
gaps are suppressed, this improved panel-boundary-layer approach shows defi-
nite promise [29].
The next degree of sophistication of the CFD approach is to solve the
Reynolds-time-averaged Navier-Stokes (RANS) equations. The success of this
approach varies; for example, sometimes drag is predicted accurately but the
pressure distribution does not agree well with experiments or vice-versa. Some
of the discrepancies are due to inadequate meshing which results from efforts
to reduce computing time. However, in general they are due to the selection