Effects of Expansion Ratio and Nozzle Asymmetry on Flowfield of Diamond Jets

Abstract

A joint experimental and computational study is performed to quantify the effects of various expansion ratios on the flowfields and acoustic properties of a jet, issuing from a diamond-shaped nozzle. Three expansion ratios are studied corresponding to perfectly expanded, overexpanded, and underexpanded regimes, which progressively increase the far-field acoustic impact. Nozzle asymmetry leads to distinct mean flow properties across the two principal (major/minor) axes of the jets, with the potential core and shock-expansion cells exhibiting a faster collapse on the major axis plane. Asymmetry is also observed in the computed near-field pressure spectra, which exhibits higher amplitudes in the minor axis plane, consistent with the higher far-field sound levels experimentally measured on this plane for all three jets. Near-field spectra also identify three specific bands, corresponding to broadband shock-associated noise (BBSAN), mixing noise, and that associated with nonradiating subsonic convecting eddies. Further analysis of noise mechanisms through the turbulent kinetic energy (TKE) budget shows enhanced levels of TKE on the minor axis plane, resulting from correlations between fluctuations in streamwise and cross-stream velocities, and gradients across the shear layers. Fourier analysis of near-field coherent structures in pressure identified axisymmetric features in the most energetic modes, followed by the first helical mode, which was enhanced in the presence of shock-expansion cells in the imperfectly expanded jets. The most energetic modes in the overexpanded shock-expansion cells have a strong harmonic nature to it, corresponding to the peak frequencies of its near-field BBSAN component.