Large-eddy simulation of underexpanded round jets impinging on a flat plate 4 to 9 radii downstream from the nozzle

Abstract

Supersonic round jets have been computed by compressible Large Eddy Simulation(LES) using low-dispersion and low-dissipation schemes. The jets are underexpanded, and are characterized by a Nozzle Pressure Ratio of NPR = Pr/Pamb = 4.03, where Pr is the stagnation pressure and Pamb is the ambient pressure. They have a fully expanded Mach number of Mj = 1.56, an exit Mach number of Me = 1, and a Reynolds number of Rej = ujD/mu = 5 × 104, where uj and D are the jet fully expanded exit velocity and the nozzle diameter, respectively. A free jet is first considered. Four jets impinging on a flat plate normally are then examined. The distance L between the nozzle lip and the flat plate varies from L = 4.16r0 up to L = 9.32r0 where r0 = D/2, for the impinging jets. The effects of the plate on the aerodynamic and acoustic properties of the jets are thus studied. For the free jet, snapshots of density, pressure and vorticity are presented. Mean velocity fields are displayed, they are in good agreement with experimental data. The near pressure field of the jet is investigated using Fourier decomposition. A screech tone component is found, at a frequency comparing well with experimental data and theoretical models. For the four impinging jets, similarly, flow snapshots and mean flow fields are shown. The results obtained are similar to the corresponding measurements. The convection velocity of large-scale structures in the jet shear layers is then evaluated and an expression giving the average convection velocity between the nozzle lips and the flat plate is proposed. The near pressure fields are then explored, and the main properties of the aeroacoustic feedback mechanism occurring between the nozzle lip and the flat plate are presented. The results are consistent with theoretical models and experimental data.