Numerical simulations of attachment-line boundary layer in hypersonic flow, Part I: roughness-induced subcritical transitions

TL;DR

This paper uses high-fidelity numerical simulations to analyze how surface roughness induces laminar-turbulent transition in the attachment-line boundary layer of hypersonic flows, revealing detailed transition mechanisms.

Contribution

It provides a detailed simulation of the roughness-induced transition process at the attachment line, including the effects of different roughness heights and the instability mechanisms involved.

Findings

Higher roughness elements can independently trigger transition.

Flow acts as a disturbance amplifier for smaller roughness.

Low-frequency absolute instability leads to streak formation.

Abstract

The attachment-line boundary layer is critical in hypersonic flows because of its significant impact on heat transfer and aerodynamic performance. In this study, high-fidelity numerical simulations are conducted to analyze the subcritical roughness-induced laminar-turbulent transition at the leading-edge attachment-line boundary layer of a blunt swept body under hypersonic conditions. This simulation represents a significant advancement by successfully reproducing the complete leading-edge contamination process induced by surface roughness elements in a realistic configuration, thereby providing previously unattainable insights. Two roughness elements of different heights are examined. For the lower-height roughness element, additional unsteady perturbations are required to trigger a transition in the wake, suggesting that the flow field around the roughness element acts as a disturbance amplifier for upstream perturbations. Conversely, a higher roughness element can independently induce the transition. A low-frequency absolute instability is detected behind the roughness, leading to the formation of streaks. The secondary instabilities of these streaks are identified as the direct cause of the final transition.