Kelvin-Helmholtz instabilities occurring at a nacelle exhaust

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 and3) 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.