Séminaire de Mécanique d'Orsay
Jeudi 10 février à 14h au FAST
Génération de bruit de jet par les grandes structures d'instabilité-approches linéaires et non-linéaires
Lutz Lesshaft
Ladhyx, École Polytechnique - CNRS
 

Jet noise is in large part generated by large-scale vortical structures that develop due to the instability of the jet shear layer. The noise radiated by these structures is tonal, i.e. it appears in the form of discrete peaks in the acoustic far-field spectrum. Numerical simulation results will be presented for the particular case of very hot jets: such jets are found to display self-excited oscillations in the form of a regularly shedding of ring vortices. Comparison with theory demonstrates that this nonlinear phenomenon ("nonlinear global mode") is caused by a linear, local absolute instability of the unperturbed jet profile. Analysis of the acoustic field, based on the Lighthill equation, characterizes enthalpy fluctuations in the jet as the dominant noise source mechanism. Some recent results, from the ongoing PhD work by Xavier Garnaud, on linear global instability modes of compressible jets will also be presented. This approach takes into account the presence of a nozzle, and the non-parallelism of the unperturbed jet flow. A new numerical procedure for the computation of global modes has been developed for this study, based on the concept of selected frequency damping. This method presents the advantage of very low memory requirements when compared to common techniques like the shift-invert transformation.