Investigation of Jet Noise Reduction by Plug Nozzle Porosity

This paper presents the results of a computational and experimental study on porous plug nozzles and the fluid mechanisms surrounding their observed noise reduction. The study explores a wide range of porous plug geometries, encompassing variations in hole size, plug length, and porosity. Computational fluid dynamics (CFD) analyses are employed in concert with an advanced automated mesh refinement scheme to investigate porous plug nozzle design strategies that may diminish shock strength, flow separation, and other undesirable characteristics associated with high noise levels. A selected subset of the porous plug nozzles that were analyzed computationally were fabricated at subscale and tested experimentally, obtaining far-field noise spectral data and schlieren flow visualizations. Detailed comparisons between CFD predictions and experimental results were focused on gaining an improved understanding of underlying physical mechanisms responsible for noise reduction associated with the porous plugs. A key observation is that the porosity relaxes the Coanda effect, thus reducing the strength of the shocks in the jet, which in turn alleviates shock-associated noise.