50 kHz Doppler global velocimetry for the study of large-scale turbulence in supersonic flows

Large-scale turbulent structures can be correlated over many jet diameters, and thus, a large field of view measurement resolved in time and space is desired to link their evolution to aeroacoustic noise production. This work describes the development and demonstration of a novel time-resolved Doppler global velocimetry system capable of spatially resolving a large field of view of the jet. The technique was applied to image a large axial extent, 8 jet diameters, of the flow field in a heated supersonic jet at 50 kHz. From the single-component measurements, mean velocity and turbulence intensity were measured within a root-mean-square error of 0.02\({\overline{U}}/U_j\) and 0.01\(\overline{u}^{\prime}/U_j\), respectively, to PIV validation measurements. The velocity spectrum measured along the nozzle lip line at \(x/D=6\) was validated by comparison to the existing literature for free jets. The spatial distribution of energy within the jet was also observed through the velocity spectra, showing energy concentrated along the nozzle lip line for all frequencies. Space–time correlations reveal evidence of wavepacket structures, characterized by their long axial correlations and constant convection velocity. The measurement technique developed in this work shows strong suitability for studies in aeroacoustics of high-speed flows, in which large-scale long-lived turbulent structures dominate radiated noise.