The density near-field of a non-uniformly heated supersonic jet

This work presents analyses of high-speed schlieren images that depict the spatio-temporal structure of near-field sound in uniformly and non-uniformly heated supersonic round jets. The non-uniformly heated jet has a concentrated region of locally lower total temperature flow around the centerline of an ideally expanded jet. Compared to the uniform jet, the non-uniform jet is shown to reduce jet noise by up to 2 +/- 0.5 dB in the peak narrowband sound pressure level at polar angles upstream of the peak directivity. Space-time correlations are performed on frequency-filtered time series of fluctuating schlieren image intensities, an analog for the fluctuating near-field density gradients. The effect of path integration is evaluated using synthetic schlieren of the dominant azimuthal jet modes, which are simulated using the azimuthal basis function of the Fourier transform. Hydrodynamic structures are identified at low frequencies and are shown to be modified by the thermal non-uniformity at axial locations in the near- and far-nozzle regions. The mid-frequency range is dominated by convecting Mach waves that are decorrelated in the thermally non-uniform jet in the near- and far-nozzle regions. Correlations of the high frequency content capture the emission of an acoustic beam. Results indicate the perturbations induced by the thermal non-uniformity can persist far into the developing flow field and reduce the length scale of coherent structures in regions far from the nozzle exhaust. This suggests centerline base flow changes can be optimized to reduce the acoustic efficiency of unsteady flow structures present near strong noise-producing areas such as the potential core collapse region. [GRAPHICS] .

Automated summary

This study presents analyses of high-speed schlieren images to illustrate the spatio-temporal structure of near-field sound in uniformly and non-uniformly heated supersonic round jets. The non-uniformly heated jet is found to reduce jet noise compared to the uniform jet, demonstrating a decrease of up to 2 +/- 0.5 dB in the peak narrowband sound pressure level. Space-time correlations, path integration effects, and hydrodynamic structures are evaluated, showing modifications by thermal non-uniformity in different regions of the jet. The perturbations induced by the thermal non-uniformity are shown to persist far into the developing flow field, suggesting potential optimization of centerline base flow changes to reduce acoustic efficiency near noise-producing areas.