Dependence of wall-pressure measurements on precursor perturbations in hypersonic boundary layers

To maximize the utility of measurements in hypersonic flows, we interpret sensor data with the aid of the equations that govern their dependence on precursor events. Specifically, the adjoint Navier-Stokes equations are used to evaluate the sensitivity of a sensor to upstream perturbations. Particular attention is placed on the domain of dependence (DOD) of wall-pressure measurements, and we report results at Mach 4.5 for boundary layers over flat plates and slender cones in the transitional regime. In backward time, the DOD retreats upstream from the sensor location, taking the form of a wavepacket that splits into two parts: one that propagates into the free stream and the second that remains within the boundary layer. The latter part amplifies as it advects upstream, with most of its energy being concentrated near the boundary-layer edge. The sensitivity to different disturbances, e.g. upstream temperature versus velocity fluctuations, is contrasted as well as the DoD for sensors within the pre-transitional and transitional boundary layers. The introduced framework paves the way for a richer interpretation of sensor data and improved design of measurements in high-speed flows.