Effect of cavity-induced perturbation interactions on the transitional flow after the trailing edge

TL;DR

This study analyzes how cavity-induced perturbations and their interactions influence flow transition mechanisms in subsonic cavity flows, revealing dominant amplification processes and instability structures through resolvent analysis.

Contribution

It introduces a combined resolvent analysis approach to identify key flow perturbations and their interactions that lead to flow transition downstream of a cavity.

Findings

Cavity-generated perturbations resemble Tollmien-Schlichting waves.

A low-frequency centrifugal instability dominates inside the cavity.

Stationary streaks are amplified via a lift-up mechanism downstream.

Abstract

We investigate the modal and non-modal linear amplification mechanisms in the flow over a subsonic open cavity and their subsequent interactions to identify optimal flow perturbations that propagate downstream the cavity and trigger flow transitions. Using both the stationary and time-varying base flows from a Direct Numerical Simulation of a cavity flow at Mach 0.6, we employ classical and harmonic resolvent analyses to explain the role of the cavity-generated perturbations in destabilizing the flow downstream. Our analysis of perturbation amplification about the mean flow identifies structures that resemble Tollmien-Schlichting (T-S) waves at the Rossiter frequency in the attached boundary layer region after the cavity. A low-frequency centrifugal instability dominates inside the cavity. The mean flow also amplifies stationary streaks via a lift-up mechanism that extends throughout the boundary layer region downstream of the cavity. The harmonic resolvent analysis (HRA) reveals the amplification of additional perturbations by the unsteady Rossiter base flow. By restricting the input and output at the same frequency in the HRA, we find the amplification of the stationary perturbation to be the most dominant 3D instability mechanism. This amplification is driven by the interaction of the 3D streaks with the unsteady Rossiter base flow, which generates internal forcing in the form of oblique T-S waves, thereby further amplifying the streaks. The interaction between the centrifugal perturbation and the unsteady flow also generates streamwise elongated structures in the boundary layer region after the cavity. Together, the centrifugal-Rossiter and streak-Rossiter interactions synergistically amplify perturbations downstream of the cavity.