Abstract: Transitional hypersonic boundary layers due to passive and active trips on a flat plate are studied by using direct numerical simulations (DNS). In the case of the passive trips (diamond-shaped and cylindrical), three dynamically prominent flow structures are consistently observed in both their isolated and distributed configurations. These flow structures are the upstream vortex system, the shock system, and the shear layers and counter-rotating streamwise vortices from the wake of the trips.
Analysis of the power spectral density (PSD) reveals the dominant source of instability due to the diamond-shaped trips as a coupled system of the shear layers and the counter-rotating streamwise vortices irrespective of spanwise roughness-spacing. However, the dominant source of instability due to an array of cylindrical trips is observed to be the upstream vortex system. Therefore, the shape of a roughness element plays an essential role in the instability mechanism. Furthermore, dynamic mode decomposition (DMD) of three-dimensional (3-D) snapshots of pressure fluctuations unveil globally dominant modes consistent with the PSD analysis in all the roughness configurations.
In the case of active trips, a two-dimensional (2-D) sonic jet from a straight slot is injected into Mach-10 3-D laminar boundary layers. The dynamically dominant flow structures observed in the vicinity of the jet correspond to upstream and downstream separation bubbles, where the number and the size of these bubbles vary with the injector pressure. A higher injector pressure leads to the formation of larger bubbles that cause the flow to become more unstable, resulting in a sequence of three successive bifurcations:
- Steady 2-D bubble formation
- Transition from 2-D steady to 3-D quasi-steady bubble
- Transition from 3-D quasi-steady to 3-D unsteady bubble
This finding indicates that specific injector pressures are required to control the onset of transition in the laminar boundary layers. Streamwise streaks with a dominant spanwise wavelength are observed in both 3-D quasi-steady and 3-D unsteady flows. DMD of spanwise velocity reveals that the streamwise streaks originate from the upstream bubbles. In particular, the streaks arise from the coupled oscillations of a primary upstream bubble and the upstream bubble, which causes the flow to bifurcate from 2-D steady to 3-D quasi-steady. These two bubbles emanate two flow structures that lie on top of each other and have opposite spanwise velocities. These flow structures then travel to the top of the downstream bubbles to form a streamwise streak. The spanwise wavelength of the dominant DMD mode agrees with that of the streaks observed in the DNS.
The simulation data in all cases agree well with their corresponding experiment.