An experimental study of the aeroacoustics of a subsonic jet impinging normal to a large rigid surface

This paper presents results from an experimental study of unsteady surface pressures and far-field noise produced by a subsonic jet impinging normal to a large, rigid, flat surface. The tests were performed in an anechoic room for jet Mach numbers from 0.54 to 0.85, and for jet-to-surface heights of from 5 to 10 jet diameters. Results showed that the root-mean-square surface pressure levels were proportional to jet dynamic pressure and were independent of jet height for radial distances from the stagnation point greater than 3 jet diameters. Far-field measurements indicated a significant increase in noise over that of a free jet for all cases of impingement. Cross spectral calculations between the surface and the far field suggested that at a Mach number of 0.70 and a height of 5 jet diameters the additional noise originated mainly in the outer edge of the impingement region between 1 and 3 jet diameters from the stagnation point.