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 9.88 × 106 m−1 . 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.
Bibliographical noteFunding Information:
The authors acknowledge support of the Office of Naval Research through grant numbers N00014-16-1-2452 and N00014-18-1-2521 and the U.S. Air Force Office of Scientific Research (AFOSR) through grant number FA9550-17-1-0250. The views and conclusions contained herein are those of the authors and should not be interpreted as necessarily representing the official policies or endorsements, either expressed or implied, of the Office of Naval Research (ONR), the AFOSR, or the U.S. Government. The authors would like to thank Neal Bitter and Maziar Hemati for insightful discussions, Heather Kostak for providing heat flux data, Greg McKiernan for providing surface pressure data, and Joseph Brock for helpful comments.
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