Prediction of bypass transition from wall measurements

Prediction of bypass transition from wall measurements in a zero-pressure-gradient boundary-layer flow over a flat plate with a leading edge is investigated using adjoint-variational optimisation techniques and Hessian analyses. The reference wall data are obtained from separate direct numerical simulations subjected to inflows with increasing levels of complexity. The prediction is achieved by reconstructing an inflow profile that generates wall data matching the reference. Initially, the algorithm is validated through steady inflows constructed from a single vortical mode. Positive streaks are reconstructed accurately, while negative streak predictions worsen with increasing turbulence intensity due to their reduced wall impact. Hessian analysis reveals that, under low turbulence intensity, the most observable inflow structures deviating from the optimal profile align with classical streak-generating modes, whereas at higher turbulence levels, the dominant observable directions shift toward spanwise uniform perturbations in high-shear regions, highlighting a reduced estimation robustness. Secondly, an inflow constructed from multiple vortical modes effectively demonstrates the prediction of streak meandering motions, with the optimised inflow profile differing significantly from the reference one. Finally, a more elaborate inflow in the form of free-stream turbulence is used to trigger bypass transition, and the results indicate that primary flow structures can be reconstructed, while the accuracy of the details improves when more downstream measurement data are included. These findings highlight the potential for accurate reconstruction of boundary-layer transition from wall measurement data.