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.

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