Domain of dependence of wall-pressure measurements in transitional high-speed boundary layers

Measurements in high-speed flows are extremely difficult to make. To maximize their utility, it is important to understand what influences an instantaneous measurement. Flow events, or perturbations, that are invisible to a sensor may prevent detection of key physics. Conversely, perturbations away from a sensor prior to the measurement may impact its signal at the measurement time. The collection of these latter perturbations defines the domain of dependence (DOD) which can be evaluated efficiently using adjoint methods. For Mach 4.5 transitional flat-plate boundary layers, we consider the DOD of an instantaneous and localized wall-pressure observation, akin to a PCB probe that is common for flight and wind-tunnel experiments. In the time preceding the pressure measurement, the DOD retreats upstream from the probe, and the sensitivity to flow perturbations amplifies. The sensitivity forms a wavepacket structure, concentrates near the boundary-layer edge, and radiates a portion above the boundary layer indicating a receptivity to freestream disturbances. We contrast the measurement sensitivity to different components of the perturbation field, e.g., temperature and velocity perturbations, and the impact of sensor placement in the pre-transitional and turbulent regions.