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 .