Blockage Effects in Automotive Wind-Tunnel Testing

A series of geometrically similar, different-sized, car-shaped models of differing sizes have been tested in closed and open-jet wind tunnels at MIRA and DNW to determine the blockage effects due to cars and to assess various blockage correction methods. The blockage ratios represented in the test programme ranged from 2 to 10 per cent in the closed-jet tunnels and from 2 to 22 per cent in the open-jet facilities.

Despite attempts to minimise the variations in Reynolds number and to allow for the effects of differing relative thicknesses of tunnel floor boundary layers, the scatter in the experimental data was often large in relation to the effects being studied. Moreover, the open-jet blockage study was mostly inconclusive because of larger effects which were probably due to other causes than tunnel blockage.

The blockage effects in the closed-jet appeared to be generally closer in magnitude to those implied by the area-ratio method (based on continuity), rather than the Hackett and Wilsden pressure-signature method or the recent method by Mercker.