ReSPOD analysis of Freestream Acoustic Fluctuations Induced by High-Speed Turbulent Boundary Layers*

Resolvent-extended spectral proper orthogonal decomposition (ReSPOD) analysis is performed to identify the acoustic sources inside a high-speed turbulent boundary layer that give rise to the acoustic disturbances in the free stream. In the ReSPOD framework, the source takes the form of nonlinear fluctuation terms that act as a forcing on the linearized Navier–Stokes equations, and we identify from direct numerical simulation data such a forcing that produces the leading spectral proper orthogonal decomposition modes in the acoustic field for a Mach 5.86 turbulent boundary layer. The study isolates the “acoustically meaningful structures” hidden in the turbulent boundary layer for developing a source model for boundary-layer noise. Such a source model is important for predicting boundary layer transition in conventional (i.e., noisy) high-speed wind tunnels and for extrapolating that data to in-flight transition.