The role of entropic-instabilities in laminar-turbulent transition on a blunted flat plate

TL;DR

This study investigates how entropic-instabilities influence the laminar-turbulent transition on a blunted flat plate at Mach 4, revealing mechanisms of wave interactions, streak formation, and turbulence onset through numerical analysis.

Contribution

It provides a detailed numerical analysis of entropic-instabilities' role in transition, highlighting the nonlinear wave interactions and streak dynamics leading to turbulence.

Findings

Oblique first-modes and entropy-layer disturbances amplify during transition.

Entropy-layer wave interactions induce streamwise streaks and subharmonic oscillations.

Spectral broadening signals flow approaching turbulence.

Abstract

The effects of entropic-instabilities on the laminar-turbulent transition dynamics of a blunted flat plate at Mach~$4$ are numerically investigated through linear and nonlinear approaches. Linear wavepacket analysis reveals amplifying oblique first-modes as well as planar and oblique entropy-layer disturbances. The receptivity of entropic-instabilities is found to be largest for actuation seeded in the entropy-layer; the corresponding linear evolution is characterized by an intensification in the wall-normal plane, coupled with streamwise tilting. The transition process that arises from entropy-layer wave interactions via the oblique breakdown mechanism is examined in detail. Oblique entropy-layer disturbances interact non-linearly with each other and induce streamwise streaks in the boundary-layer. These undergo further destabilization downstream, through a combination of lift-up and Orr-like mechanisms, leading to turbulence onset. Sinuous subharmonic oscillations induced by the entropic-disturbances are the dominant streak instabilities. Slanted hook-shaped structures are observed in the temperature perturbations, and are attributed to an Orr-like mechanism in the entropy-layer. Further, temperature perturbations amplify on the crests of the low-speed streaks in the entropy-layer and generate disturbances through triadic interactions. These disturbances aid in the transfer of perturbation energy into the boundary-layer, eventually leading to spanwise homogenization and near-wall streak generation. Towards the end of the breakdown region, spectral broadening accompanied by the appearance of an inertial sub-range in the boundary-layer indicates the approach of the flow towards turbulence.