Mitigation strategies for acoustic reflections generated by an axisymmetric wind tunnel nozzle

The issue of acoustic reflections caused by an axisymmetric nozzle is addressed both numerically and experimentally. The contraction (with an exit diameter of 420 mm and a contraction ratio of 2) is representative of typical open-jet wind tunnel exits. Such a category of wind tunnels are extensively used in aeroacoustic studies due to the possibility of acoustic treatment of the test chamber (plenum) walls and the placement of acoustic sensors outside of the flow. The distance between the wind tunnel’s nozzle exit and the model being tested is usually limited to around one hydraulic diameter in order for the model to be fully contained within the exit jet’s core. Partly for this reason, the wind tunnel nozzle normally protrudes inside the anechoic plenum to distance the tested model from the (acoustically-treated) walls. This effectively creates a cavity, which is in communication with the test section. Acoustic simulations through a commercial finite-element code (COMSOL Multiphysics) show that the presence of the nozzle leads to interference patterns within the test section and a substantial modification of a source’s measured directivity pattern. Experimental measurements in a fully anechoic chamber confirm these results. Melamine foam inserts on both the exit flange and part of the inner walls of the contraction are shown to mitigate the issue partly.