Investigation of the feedback mechanism in ideally expanded round impinging jets using large-eddy simulation

Abstract

Large-eddy simulations (LES) have been performed in order to study the tone generation mechanism in four supersonic ideally expanded round impinging jets. The jets have a Mach number of 1.5, and a Reynolds number of 6 × 10^4. They impinge normally on a flat plate located at a distance from the nozzle exit varying from 6r0 up to 12r0 where r0 is the jet nozzle radius. The aerodynamic properties of the jets are first investigated. In particular, the convection velocity of the turbulent structures in the jet shear layers is computed. In the spectra of pressure fluctuations in the vicinity of the nozzle exit, intense tones emerge. Their associated Strouhal numbers are in agreement with measurements available for round impinging jets with similar exit conditions. The tone frequencies also correspond well to the frequencies predicted by the classical model of the aeroacoustic feedback establishing between the nozzle lips and the flat plate. A study of the feedback mechanism is then proposed by applying Fourier decomposition to the near pressure fields. The feedback mechanism is found to lead to the formation of hydrodynamic-acoustic standing waves. Moreover, for each tone frequency, the corresponding axisymmetric or helical oscillation mode of the jet is examined. Finally, an analysis is conducted using a vortex sheet model of the jet in order to determine the allowable frequency ranges of the upstream-propagating neutral acoustic wave modes. The tone frequencies obtained in the LES fall within these ranges, depending on their axisymmetric or helical nature.