Laminar-turbulent transition in a hypersonic compression ramp flow

The hypersonic flow over a compression ramp is investigated by utilizing direct numerical simulation (DNS) and various stability analysis tools. The free-stream Mach number and Reynolds number based on the length of the flat plate are 8.0 and 3.9×105, respectively. Global stability analysis is applied to confirm the weekly unstable nature of the current flow condition. As a result of the low growth rate, this case is believed to be more susceptible to convective instability than intrinsic instability. Subsequently, across a wide range of frequencies and a globally stable wavelength, resolvent analysis is utilized to investigate the response of two-dimensional base flow to external disturbances. It reveals that the optimal response to upstream disturbances located adjacent to the leading edge manifests in the form of streamwise streaks, which result from transient growth in the flat-plate boundary layer. Downstream of reattachment, the Mack second mode and low-frequency streaks as a manifestation of Görtler instability coexist within the boundary layers. Further downstream, the amalgamation of the amplification of Mack's second mode with the sinuous and varicose breakdown of streaks disrupts the boundary layers via the ejection-sweep motion, resulting in the creation of a strong localized vorticity region and contributing to the concentration of vorticity within the boundary layers. This kicks off the vortex roll-up process, which results in the formation of hairpin vortices, and eventually leads to the breakdown process.