8.6 Automotive aerodynamics
An example of flow around a road vehicle was
used to discuss some boundary conditions in Chapter 4
and to illustrate
the cost of simulating turbulence in Sec. 6.8
. An
aerodynamics simulation was
undertaken to capture the air flow around the vehicle, described by
a CAD model. The aim was to calculate the
drag coefficient at a speed of 
.
A mesh of 20 million cells was generated, with
the vehicle facing a freestream flow velocity 
. The vehicle and
ground formed solid boundaries, with far-field boundaries positioned
upstream and 
downstream of the vehicle.
Along the elevated sections of the far-field
boundary, the cell length was 
, reducing to
towards the vehicle by splitting within specified regions.
Additional cell layers along the vehicle surface resulted in a
near-wall cell height of 
.
The simulation used the steady-state algorithm in
Sec. 5.12
, with an incompressible fluid
with uniform 
.
The freestream boundary conditions from
Sec. 4.16
were applied to 
and
at
the far-field boundaries, with reference values 
, 
and 
. The condition
was a applied at solid boundaries, with 
applied to the vehicle
and 
on the ground to emulate their relative motion.
Turbulence was modelled using the 
SST model
described in Sec. 7.11
. Turbulence levels
of 
and 
were applied at the freestream boundaries and the standard
wall function from Sec. 7.5
was applied at the
vehicle and ground.
The simulation ran for 3000 iterations using
numerical schemes recommended in Sec. 3.23
. The drag coefficient
was
calculated from the projected frontal area 
and the 
-component
of
the force 
on the vehicle using Eq. (8.1
).
The flow in the wake of the vehicle is naturally
unsteady, which prevents convergence to a steady-state solution.
Beyond 1500 iterations, however, the solution oscillates around an
estimated mean 
.
