The reference pressures are determined in automotive wind tunnels by measurement of pressures and pressure differences at upstream positions along the wind tunnel nozzle. For closed wall wind tunnels usually the so called nozzle method is used, where the volume flux is calculated from a pressure difference measured at the nozzle contour and a calibration factor determined in the empty test section. For open jet wind tunnels a choice is available between nozzle and plenum method. For the plenum method the reference static pressure is taken from the plenum chamber and the dynamic pressure also refers to the plenum conditions. The static reference pressure in closed wall tunnels is calculated by subtracting the dynamic pressure from the total pressure in the settling chamber.
In this paper, the definitions and the differences between the two methods are discussed in detail. Possible sources of errors, such as velocity-dependent calibration factors, interference with boundary layer suction and empty test section velocity distribution are discussed. Special focus will be laid on the investigation of the interference of model displacement (blockage) and measured dynamic pressure. It will be shown, that local effects of the pressure build up in front of the model usually do not affect the accuracy of the measured dynamic pressure.
Nevertheless model blockage does affect the measured results for aerodynamic forces as well as for pressure distributions. A short discussion of the sources of errors and of possible theoretical corrections to force coefficients and pressure distributions will be given. In this case special focus will be laid on the influence of the distance between model and nozzle in an open jet wind tunnel. The measured effect on the pressure distribution of a vehicle will be analysed and it will be shown, that neither nozzle nor plenum method deliver correct pressure distribution results without additional correction, if the blockage level is high.