Flow Structure Oscillations and Tone Production in Underexpanded Impinging Round Jets

Abstract

Flow structure oscillations and tone generation mechanisms in an underexpanded round jet impinging on a flat plate normally have been investigated using compressible large-eddy simulations. At the exit of a pipe nozzle of diameter , the jet is characterized by a nozzle pressure ratio of 4.03, an exit Mach number of 1, a fully expanded Mach number of 1.56, and a Reynolds number of . Four distances between the nozzle and the plate of , , , and are considered. Snapshots of vorticity, density, pressure, and mean velocity flowfields are first presented. The latter results compare well with data of the literature. In three cases, in particular, a Mach disk appears to form just upstream from the plate. The convection velocity of flow structures between the nozzle and the plate, and its dependence on the nozzle-to-plate distance, are then examined. The properties of the jet near pressure fields are subsequently described using Fourier analysis. Tones emerge in the spectra at frequencies consistent with those expected for an aeroacoustic feedback loop between the nozzle and the plate as well as with measurements. Their amplitudes are particularly high in the presence of a near-wall Mach disk. The axisymmetric or helical natures of the jet oscillations at the tone frequencies are determined. The motions of the Mach disk found just upstream from the plate for certain nozzle-to-plate distances are then explored. As noted for the jet oscillations, axially pulsing and helical motions are observed, in agreement with experiments. Finally, the intermittency of the tone intensities is studied. They significantly vary in time, except for the two cases where the near-wall Mach disk has a nearly periodic motion at the dominant tone frequency.