A numerical investigation of shock-associated noise

Under imperfectly expanded conditions, a supersonic jet emits shock associated noise. If not controlled, it may lead to significant cabin noise during cruise and structural fatigue in aircraft components. The present research is focused on a fundamental understanding of the generation mechanism of shock associated noise with the goal of improving the currently available models.; A model problem is formulated around the interaction region formed by the jet shear layer and an isolated shock cell, here idealized as an oblique compression-expansion wave. Both the shock-disturbance interaction and its resultant acoustic field are captured by direct numerical simulation using a highly optimized numerical method. Two different types of three-dimensional, perturbed shear layers are studied: a transitional and a turbulent layer. In both types of shear layer, a base-line simulation without any shock and at least one compression case are conducted.; Results in the transitional and the turbulent layers are similar. In the near field, only small-amplitude oscillations of the shock tip is observed. Local enhancement of the Reynolds normal stresses and thermodynamic fluctuations, and local suppression of the Reynolds shear stress occur on the compressive side of the shock cell. However, all turbulence intensities return to their upstream level immediately after the interaction, indicating minimal global effect of compression on the turbulence. In the acoustic field, shock noise appears as cylindrical waves emanating from the localized shock-disturbance interaction region, and exhibits an omni-directional character. The acoustic amplitude of shock noise scales with the near-field pressure fluctuation at the shock tip while its spectral peak agrees well with that of the near-field velocity fluctuations.; Acoustic emission is observed when the streamwise velocity fluctuation of the interacting flow structure changes from positive to negative values as it convects past the shock tip. In the light of this finding, the two-dimensional shock-vortex problem is revisited. Both the shock motion and the acoustic emission are shown to be strongly connected to the streamwise velocity fluctuation field. The sound generation mechanism is interpreted as scattering of the shock wave by the energetic velocity fluctuations in the shear layer.