Aeroacoustic characterization of acoustically-treated turbulence grids in axisymmetric contraction for experiments in open-jet wind tunnels

The application of an acoustically absorbent material (melamine foam) is investigated for the treatment of turbulence grids in an anechoic open-jet wind tunnel facility featuring an axisymmetric contraction. A comparative study of both the generated turbulence and the grids’ self-noise is performed. It is found that the application of melamine foam on the downstream side of the grids marginally affects the produced turbulence, while providing an efficient suppression of tonal peaks in the grids’ self-noise spectrum. Broadband noise levels instead show opposing trends depending on the frequency range considered. On the one hand, a general decrease due to the acoustic treatment is observed for Strouhal numbers lower than unity. On the other, an increase, in the form of broad peaks, is seen to occur over certain higher frequency ranges.