Parabolized Stability Analysis of Jets Issuing from Serrated Nozzles

The mixing noise in jet engine exhaust is one of the main noise sources in aviation. Nozzle lip serrations, commonly called chevrons, are one of the technologies deployed to address the issue in modern civilian aircraft. These nozzle serrations impinge on the shear layer and change its dynamics; the jet plume becomes lobed or serrated near the nozzle in a time-averaged sense, but reverts to a round profile further downstream. A parametric study of subsonic (Mach 0.9) turbulent jets issuing from serrated nozzles was conducted at the NASA SHJAR facility [1]. It demonstrated that such a device reduces the low frequency mixing noise aft of the jet (the peak noise component), while increasing the high frequency omnidirectional noise. The latter component is commonly attributed to enhancement of small-scale turbulence by the impingement of the chevron tips on the shear layer. The low-frequency louder component is less well understood, but is usually linked to the large-scale coherent structures present within the turbulent shear layer. Here we propose a model for these coherent structures in a serrated jet, based on the theory of parabolized stability equations (PSE).