Mode Conversion in Acoustically Modulated Confined Jets

Abstract

This paper is concerned with the response of confined low-Reynolds-number jets to external modulations of large amplitudes. The jet formed inside a duct by an orifice plate is submitted to flow perturbations impinging on the constriction from the upstream side. Vortices are generated in the downstream region, giving rise to conversion from acoustic oscillation to convective modes. Experiments and simulations are carried out to characterize this important mode conversion process. It is shown that the convective perturbation phase is a linear function of the distance from the orifice plate and of the operating frequency. Data can be collapsed into a single curve with some scatter providing a scaling rule for the phase. Numerical simulations are carried out and a method is developed to decompose the local velocity perturbations into acoustic and convective components. This is used to determine the amplitudes of upstream and downstream acoustic velocities and of the convective perturbations on the downstream side of the orifice. It is shown that the acoustic velocity approximated by a bulk oscillation in the orifice vicinity is reduced at the constriction and that this is simultaneously compensated by the generation of convective perturbations. The conversion efficiency of this process is characterized. Results obtained can be used to design flow control devices; they provide insight into the generation of vortices in orifice plate systems submitted to large pressure oscillations and could be useful to the analysis of vortex instabilities in segmented solid rocket engines.