Numerical Simulation of Instabilities in the Boundary-Layer Transition Experiment Flowfield

Abstract

The development of disturbances in the boundary-layer transition (BoLT) flight experiment flowfield is investigated using a recently developed “quiet direct numerical simulation (DNS)” approach. By using a combination of low-dissipation numerics and a novel shock capturing method, numerical noise is significantly reduced, enabling the simulation and analysis of early stages of the transition process that are governed by the linear growth of small-amplitude disturbances. The freestream conditions of the present simulations correspond to experiments conducted in the Purdue Boeing/U.S. Air Force Office of Scientific Research Mach 6 quiet tunnel under quiet flow at a unit Reynolds number of . First, the steady-state flowfield is computed and then compared to infrared images, demonstrating excellent agreement with experimentally measured steady streamwise wall heat flux streaks. Next, an unsteady numerical simulation is performed with continuous stochastic forcing to excite boundary-layer instabilities. Two-dimensional time-series snapshots of the disturbance flowfield are saved at several streamwise locations. Sparsity-promoting dynamic mode decomposition (SPDMD) is used to extract dominant modes from the snapshot sequences. The frequencies of dominant modes are compared to power spectral densities obtained from wall pressure fluctuations measured in the experiments. Based on numerical and experimental data, three distinct instabilities in the BoLT flowfield are identified. The quiet DNS approach used in conjunction with controlled disturbances to excite instabilities, as well as SPDMD to analyze them, represents a new high-fidelity method of investigating stability and transition in complex three-dimensional flowfields.