Oblique transition in hypersonic double-wedge flow

TL;DR

This paper combines resolvent analysis, weakly nonlinear analysis, and DNS to uncover mechanisms behind oblique transition in hypersonic double-wedge flows, highlighting the role of disturbance amplification and shear layer dynamics.

Contribution

It introduces a comprehensive framework integrating linear, nonlinear, and numerical methods to predict oblique transition mechanisms in hypersonic separated flows.

Findings

Oblique waves are significantly amplified by shear stress growth in the separated shear layer.

Quadratic interactions of unsteady waves generate streaks leading to turbulence.

DNS confirms the predictive capability of the weakly nonlinear input-output analysis.

Abstract

We utilize resolvent and weakly nonlinear analyses in combination with direct numerical simulations (DNS) to identify mechanisms for oblique transition in a Mach 5 hypersonic flow over an adiabatic slender double-wedge. Even though the laminar separated flow is globally stable, resolvent analysis demonstrates significant amplification of unsteady external disturbances to the linearized flow equations. These disturbances are introduced upstream of the separation zone and they lead to the appearance of oblique waves further downstream. We demonstrate that large amplification of oblique waves arises from the growth of fluctuation shear stress due to streamline curvature of the laminar base flow in the separated shear layer. This is in contrast to the attached boundary layers, where no such mechanism exists. We also use a weakly nonlinear analysis to show that the resolvent operator associated with linearization around the laminar base flow governs the evolution of steady reattachment streaks that arise from quadratic interactions of unsteady oblique waves. These quadratic interactions generate vortical excitations in the reattaching shear layer which lead to the formation of streaks in the recirculation zone and their subsequent amplification, breakdown, and transition to turbulence downstream. Our analysis of the energy budget shows that deceleration of the base flow near reattachment is primarily responsible for amplification of steady streaks. Finally, we employ DNS to examine latter stages of transition to turbulence and demonstrate the predictive power of a weakly nonlinear input-output framework in uncovering triggering mechanisms for oblique transition in separated high-speed boundary layer flows.