Correction of Nozzle Gradient Effects in Open Jet Wind Tunnels

In open jet wind tunnels with high blockage ratios a sharp rise in drag is observed for models approaching the nozzle exit plane. The physical background for this rise in drag will be analyzed in the paper. Starting with a basic analysis of the dependencies of the effect on model and wind tunnel properties, the key parameters of the problem will be identified. It will be shown using a momentum balance and potential flow theory that interaction between model and nozzle exit can result in significant tunnel-induced gradients at the model position.

In a second step, a CFD-based investigation is used to show the interaction between nozzle exit and a bluff body. The results cover the whole range between open jet and closed wall test section interaction. The model starts at a large distance from the nozzle, then moves towards the nozzle, enters the nozzle and is finally completely inside the nozzle. The results for the open jet and closed wall cases correspond to the results expected by classical wind tunnel correction theory. The drag level for the model entering the nozzle exit plane is shown to be significantly higher than the drag observed for closed walls. This highlights the existence of an additional drag mechanism not related to the dynamic pressure.

In a third step, the CFD results and additional experimental results are used to validate the newly derived correction formula for the effect of the nozzle gradient. Using well-known benchmark cases from the literature it will be shown, that the proposed correction eliminates the effects of nozzle proximity almost completely.