Effects of the angle of impact on the aeroacoustic feedback mechanism in supersonic impinging planar jets

Abstract

Three planar impinging supersonic jets of infinite extent are simulated using compressible large eddy simulations in order to study the effects of the angle of impact on the flow and acoustic fields of the jet. At the exit of a nozzle of height h, they are ideally expanded and have an exit velocity uj, yielding a Mach number of 1.28 and a Reynolds number of 5 × 104. They impinge on a flat plate at a distance 5.5 h from the nozzle lips with angles of 60°, 75°, and 90° between the jet direction and the plate. Mean velocity flows and snapshots of density, pressure, and vorticity are first shown. The mean convection velocity of the turbulent structures in the jet shear layers is then determined. The sound pressure levels are computed, and several tones due to the presence of a feedback mechanism are found to establish between the nozzle lips and the flat plate. They agree well with the corresponding measurements and with the classical model of the feedback mechanism. Moreover, when the angle of impact deviates from 90° to 75°, a jump from the third to the fourth mode of the feedback mechanism and a reduction in intensity are noted. By applying a Fourier decomposition to the near pressure fields, hydrodynamic–acoustic standing waves are found for each dominant tone frequency. Moreover, as suggested by amplitude fields and velocity spectra in the jet shear layers, the feedback mechanism seems to establish mainly along the lip that is farther away from the plate when the impact angle is not normal. This jump from the third to the fourth mode is similar to the jump observed experimentally for an angle of impact of 90° when the nozzle-to-plate distance increases from 5.5 h to 5.85 h. Finally, for an angle of impact of 60°, it is seen that none of the modes of the feedback persists in time, but that several modes randomly establish during short periods of time. These rapid switches between different modes lead to several tones that are less energetic on average and centered on St = 0.25.