Linear Stability Implications of Mean Flow Variations in Turbulent Jets Issuing from Serrated Nozzles

Nozzle serrations or chevrons are being deployed for reducing the noise from jet engines. The turbulent mean flow field of such jets takes on a serrated character, and the linear stability eigenspectrum for such serrated mean flows determines the evolution of the coherent wavepackets that are linked to the aft angle noise radiated. In particular, the lower the growth rate and phase speed of the instability, the lower is the expected noise radiation. Here we identify four parameters of the mean flow serrations – viz. number of lobes, their protrusion, their width relative of overall circumference, and the average thickness of the shear layer. These four parameters are systematically varied to synthesize a family of mean flow profiles. The corresponding stability analyses indicate the following trends. As expected from results for round jets, the average shear layer thickness has an inverse effect on both growth rate and phase speed. Deeper penetration and higher number of lobes reduce the growth rate of the relevant instability while mildly enhancing its phase speed. The relative width of the lobes do not appear to be a relevant parameter. These theoretical trends are supported by noise measurements in the parametric study on chevron nozzles performed at NASA.