Dual-Mode Isolator Shock-Train Unsteadiness Due to Downstream Perturbations

The shocktrain in a Mach 1.6 dual-mode isolator is computed using Large Eddy Simulations and analyzed to identify the key physics driving unsteady streamwise motions. The shocktrain consists of sequential shock-wave/turbulent boundary layer interactions (STBLIs), beginning with an initial separation and reattachment, with progressive boundary layer thickening by subsequent shocks. The primary objective is to identify waves capable of perturbing the leading STBLI and driving unsteadiness. Three analysis methods of increasing complexity are used to isolate and measure characteristic wave velocities. First, space-time plots of near-wall fluctuations reveal propagating signals, with velocities extracted from visual streaks. Comparisons are made between raw pressure fluctuations and the acoustic variable from Doak’s Momentum Potential Theory. Second, these variables are analyzed in frequency-wavenumber space to further resolve the propagating waves, supported by a wave angle analysis upstream of the first STBLI. Finally, a new method is introduced that solves a convective wave equation on the LES mean flow to propagate a prescribed impulse along the shocktrain, providing localized disturbance propagation speeds. This method shows that the propagating signal wave angle is an important variable for dynamics in the turbulent zone downstream of the first STBLI.