Effect of a Chevron Nozzle on Noise Radiation from a Compressible Jet

Abstract

The effect of a chevron on a radiated (acoustic) component of near-field pressure fluctuations in a high-speed compressible jet is experimentally investigated for a nozzle exit Mach number of 0.8. The main focus in the present experimental study is to examine the effect of a chevron on noise sources, their location in the jet flowfield, and the noise radiation pattern in the far-field through simultaneous measurement of near-field and far-field pressure fluctuations using a cross-correlation technique. The formation of lobes in the mixing layer close to the chevron nozzle exit alters the near-field and far-field noise characteristics as compared to the base nozzle. The overall sound pressure level closer to the chevron nozzle exit increases, whereas downstream of the potential core, it reduces in comparison with the base nozzle. Furthermore, the position of the near-field/far-field cross-correlation coefficient peak suggests that the interaction of chevrons with the jet flow moves the dominant noise source upstream and closer to the nozzle exit. The chevron nozzle disintegrates large-scale flow structures, thereby reducing the noise levels radiating at a lower polar angle in the far-field. The near-field/far-field cross-correlations suggest that an acoustic component of the near-field pressure fluctuations, propagating from the jet to the higher polar angles in the far-field, is generated by fine-scale turbulent flow structures in the region close to the nozzle exit, and the chevron nozzle is found to strongly support this mechanism. An increase in the noise levels closer to the nozzle exit in the near-field and at a higher polar angle in the far-field are due to high-frequency noise sources generated by the chevron nozzle.