Browsing by Subject "Transitional boundary layers"

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    Numerical Study of High-Speed Transition due to Passive and Active Trips
    (2019-04) Shrestha, Prakash
    Transitional hypersonic boundary layers due to passive and active trips on a flat plate are studied using direct numerical simulations (DNS). In the case of 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 the 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 trip-spacing. However, the dominant source of instability due to an array of cylindrical trips (Williams et al. 2018) is observed to be the upstream vortex system similar to Subbareddy et al. 2014 who used an isolated cylindrical trip. Therefore, the shape of a roughness element plays an essential role in the instability mechanism. Furthermore, dynamic mode decomposition (DMD) of three-dimensional snapshots of pressure fluctuations unveil globally dominant modes consistent with the PSD analysis in all the trip configurations. Higher peak-amplitude frequencies and amplitudes characterize dominant instabilities in higher freestream Reynolds number flows. When the trip heights are reduced, the source of instability has been observed to be unchanged, while peak-amplitude frequencies, the mean upstream recirculation zone, the mean instability-onset location, and the maximum turbulent kinetic energy is found to be reduced. When the trip spacing is greater than three times the trip width, each trip of the trip array becomes isolated. In the case of active trips, a two-dimensional (2-D) sonic jet from a straight slot is injected into Mach-10 three-dimensional (3-D) laminar boundary layers (Berry et al. 2004). 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: (1) steady 2-D bubble formation, (2) transition from 2-D steady to 3-D quasi-unsteady bubble, and (3) transition from 3-D quasi-unsteady 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 undulation of a primary upstream bubble and the upstream secondary bubble, which causes the flow to bifurcate from 2-D steady to 3-D quasi-unsteady. It is proposed that the source of the unsteadiness observed is generated by high-pressure fluctuations present between the secondary bubble and the jet. The unsteady interaction between the secondary bubble and the jet selects a specific wavelength of the spanwise undulation of the secondary bubble, which then modulates the primary bubble across span with the same wavelength. These two bubbles emanate two flow structures that 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. No effect of real gas has been found in this current study. The source of instability is observed to be independent of the thermal nature of the wall (isothermal or adiabatic). The angle of injection is observed to play a significant role in flow unsteadiness downstream of the jet. The mean Mach-disk height and the mean upstream recirculation length are compared to existing models in order to access their accuracy under the present flow and jet configurations.