On the Correction of Interference Effects in Open Jet Wind Tunnels

This paper deals with the correction for interference effects on the flow about bluff bodies in open-jet wind tunnels. Altogether, five different interference effects in open-jet tunnels are identified and described by physical models. Besides the classical jet-expansion correction which has been treated repeatedly by various authors throughout the last 60 years, the new correction method also includes the effect of jet deflection due to the proximity of a vehicle to the nozzle. Furthermore, far-field interference effects originating from the nozzle and the collector are described. For this purpose nozzle and collector effects are treated as solid- wall blockage phenomena, and with the aid of Biot-Savart principles the induced velocity at the model position is calculated. Finally, the static- pressure gradient in an empty test section generates a horizontal buoyancy force when a model is placed in a wind tunnel stream. This buoyancy force is considered in the correction procedure by averaging the streamwise pressure gradient over the front and rear parts of a vehicle. Validating the correction method, it is further shown that the interference terms can be used also in an inverted sense by defining lay-out parameters for a correction-free wind tunnel.