Decoding wall-pressure sensor data in hypersonic boundary-layer transition on a cone

Hypersonic boundary-layer transition is extremely sensitive to uncertain environmental disturbances, which may mask the underlying transition mechanism. Direct measurements can reduce this uncertainty. However, observations at high speeds are often scarce, e.g., limited to discrete wall-pressure probes. To discover the true flow, we optimize the simulations to reproduce the experimental observations using an ensemble-variational (EnVar) data assimilation approach (Buchta & Zaki, J. Fluid Mech. 2021). We demonstrate our framework on sensor data acquired in the AFRL Mach-6 Ludwieg Tube for boundary-layer transition over a 7-degree cone. Without knowing the freestream condition and using only PCB measurements, we determine the inflow spectra for two types of measurements: (i) the time-history of a migrating wavepacket and (ii) the wall-pressure power spectra recorded by the sensors. The reconstructed wavepacket is compared with quantitative Schlieren that was synchronized with the PCBs. We also evaluate the entire flow field, beyond the original limited wall sensors, and examine the transition mechanisms in detail.