Flow and Acoustic Features of a Mach 0.9 Jet Using High Frequency Excitation

摘要

This study experimentally investigates the effect of high frequency forcing on the now field and acoustic characteristics of a Mach 0.9 (Re_D ≈ 6 × 10~5) jet. Eight Resonance Enhanced Microactuators (REM) (f ≈ 25kHz, St_D ≈ 2.2) are used to excite the shear layer at frequencies that are approximately an order of magnitude higher than the jet preferred frequency. The primary goal of control is to suppress the evolution of the acoustically dominant large scale structures in the shear layer. Flow field measurements are performed using planar and stereoscopic PIV to assess the effect of the localized high-frequency forcing on the mean and turbulent characteristics of the jet. The resulting impact on the noise signature is estimated via far-field acoustic measurements. Noise reduction was observed at low to moderate frequencies as a result of control for all observation angles. Cross stream PIV measurements for control cases revealed that the emergence of strong streamwise vortex pairs significantly modifies the mean flow field, resulting in a wavy or undulated shear layer. These vortex pair grow as they convect downstream, increasing the local entrainment and significantly thickening the initial shear layer. Moreover, it was also observed that high frequency forcing results in increased turbulence levels for the initial region of the jet followed by a rapid dissipation, resulting in overall reduced fluctuation values downstream. Reduction in the mean shear and unsteady effects associated with the forcing of fine scale structures at high frequencies are hypothesized to be the main mechanisms that lead to noise reduction.