Distributed Roughness-Induced Transition on a Blunt Body at Mach 6: a Numerical Investigation

TL;DR

This study uses direct numerical simulation to investigate how surface roughness influences laminar-turbulent transition on a blunt body at Mach 6, revealing the dominant instability modes and a novel feedback mechanism involving acoustics.

Contribution

First DNS of Mach 6 blunt cylinder with distributed roughness, uncovering the transition mechanisms and the influence of roughness configuration on instability modes.

Findings

Aligned roughness induces sinuous streaks; staggered and random roughness induce T-S waves.

Roughness configuration affects the destabilization of waves and the transition location.

Acoustic feedback from turbulence can excite T-S waves without external forcing.

Abstract

Surface roughness significantly impacts transition to turbulence, especially over high-speed, blunt geometries where surface ablation is necessary to mitigate heat loads during atmospheric entry. Inspired by sand-grain roughness experiments performed by Hollis (2017), we perform the first direct numerical simulation (DNS) of a blunt cylinder in Mach 6 cross-flow with roughness elements distributed along the entire surface. Such simulations aimed to uncover the precise means by which laminar-turbulent transition occurs given the limited measurements attainable from experiments and non-existent high-fidelity simulations. Element heights were held fixed at approximately 35% boundary layer thickness, while the relative phasing between streamwise rows was varied. All configurations exhibited convective instabilities driving the transition process, with the mode type being set by the roughness configuration. A fundamental sinuous streak mode dominated the aligned roughness element case, whereas both the staggered and randomly phased cases saw 2D T-S waves dominating. These instability waves, when grown to sufficient amplitude, triggered the steady streaks seeded by the underlying roughness pattern to begin forming hairpin vortices and breakdown occurred soon thereafter. The roughness arrangement was found to dramatically influence the degree to which the waves were destabilised, as well as the strength of the underlying steady streaks, thereby combining to dictate the position along the surface where LTT occurred. Finally, exogeneous forcing was not required for the T-S dominated cases as the acoustics generated by the turbulence in the subsonic flow excited T-S waves on the other side - a feedback mechanism hitherto unknown.