Kelvin-Helmholtz Instabilities Occurring at a Nacelle Exhaust

Abstract

The difference of flow velocity between the free stream and the jet stream at a nacelle exhaust generates a shear layer where some hydrodynamic instabilities, called Kelvin-Helmholtz instabilities, can occur. Some instabilities occur in the shear layer in the computations with Actran/DGM a time domain code which solves the LEE equations (see, 1 , 2 and 3 ) but it is often difficult to conclude whether they are physical or not. This document recalls the theory on the Kelvin-Helmholtz instabilities and gives guidelines to sort out the physical and numerical instabilities. This methodology is put into practice on concrete cases studied in the frame of the TURNEX project.

Single velocity-component modeling of leading edge turbulence interaction noise

“…Additionally this type of mean flow, containing a strong velocity shear layer, can generate instabilities in LEE simulations. Methods exist to obtain stable solutions (see Zhang and Chen, 44 Manera et al, 45 and Ewert and Shr€ oder, 46 for example), but their use is beyond the scope of this work. Therefore, inviscid mean flow solutions are used here to avoid these potential issues.…”

An efficient discontinuous Galerkin method for aeroacoustic propagation

“…it follows that the forcing frequency is of the same order of magnitude of the momentum thickness of the mean shear flow. Under these conditions it can be shown that the vortical mode is excited and hydrodynamic flow oscillations, induced by the associated Kelvin–Helmholtz instability, occur. Figures (a) and (b) show the instantaneous LEE and APE solutions at t = 16, respectively.…”

A review of CAA for fan duct propagation and radiation, with application to liner optimisation

“…In the physical flow, such modes are controlled by viscous and non-linear effects, absent in an LEE model. In some instances the mixing layers grow sufficiently rapidly for the growth of the KH instabilities to be controlled by the spreading effect [18]. It is difficult however to guarantee that an LEE solution will not be destabilized at any point in the solution domain.…”