Frequency-Wavenumber Spectrum of Acoustic Radiation from High-Speed Turbulent Boundary Layers*

Spatio-temporal structure of the acoustic radiation field emanating from high-speed turbulent boundary layers is analyzed by using a database of direct numerical simulations (DNS). Specifically, DNS are used to examine the frequency-wavenumber spectrum of both surface and free stream pressure fluctuations generated by the nozzle wall boundary layer within a Mach 6 Ludwieg Tube facility. Effects of acoustic propagation within the confined environment of the nozzle are investigated by comparing the results with those for a single, flat wall in an unconfined setting at similar values of freestream Mach number and frictional Reynolds number. The study provides insights into the scaling of pressure disturbance spectrum and the variation of acoustic wave orientation with respect to the boundary-layer parameters and the flow configuration. Such information is important for developing physics-based models for boundary layer transition in conventional (i.e., noisy) hypersonic wind tunnels and for extrapolating that data to in-flight transition.