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 eqn.

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A mesh of 20 million cells was generated, with the vehicle facing a freestream flow velocity eqn. The vehicle and ground formed solid boundaries, with far-field boundaries positioned eqn upstream and eqn downstream of the vehicle.

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Along the elevated sections of the far-field boundary, the cell length was eqn, reducing to eqn towards the vehicle by splitting within specified regions. Additional cell layers along the vehicle surface resulted in a near-wall cell height of eqn.

The simulation used the steady-state algorithm in Sec. 5.12 , with an incompressible fluid with uniform eqn .

The freestream boundary conditions from Sec. 4.16 were applied to eqn and eqn at the far-field boundaries, with reference values eqn, eqn and eqn. The condition eqn was a applied at solid boundaries, with eqn applied to the vehicle and eqn on the ground to emulate their relative motion.

Turbulence was modelled using the eqn SST model described in Sec. 7.11 . Turbulence levels of eqn and eqn were applied at the freestream boundaries and the standard wall function from Sec. 7.5 was applied at the vehicle and ground.

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The simulation ran for 3000 iterations using numerical schemes recommended in Sec. 3.23 . The drag coefficient eqn was calculated from the projected frontal area eqn and the eqn-component eqn of the force eqn on the vehicle using Eq. (8.1 ).

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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 eqn.

Notes on CFD: General Principles - 8.6 Automotive aerodynamics