SUPERSONIC JET MIXING ENHANCEMENT USING IMPINGEMENT TONES FROM OBSTACLES OF VARIOUS GEOMETRIES

This paper reports the results of an experiment that investigated the effect of impingement tones, generated by obstacles of various geometr ies, on the spreading of a supersonic jet flow. A rectangular superson ic jet was produced using a convergent-divergent nozzle that was opera ted near its design point (with shocks minimized). Immersing obstacles in the flow produced an intense impingement tone that then propagated upstream (as feedback) to the jet lip and excited the antisymmetric h ydrodynamic mode in the jet, thus setting up a resonant self-sustainin g loop. The violent flapping motion of the jet due to excitation of th e antisymmetric mode, combined with the unsteady wakes of the obstacle s, produced large changes in jet mixing. The experiment controlled the frequency and amplitude of the impingement tone excitation by varying the nozzle-to-obstacle distance and the obstacle immersion. Proper sh aping of the obstacles made it possible to reduce the thrust penalty s ignificantly.