Effects of surface roughness on boundary layer transition in a hypersonic flow

Abstract

Boundary layer transition is a critical factor in the design of hypersonic vehicles. It is strongly affected by surface roughness, which is often modelled as either discrete areas or individual isolated elements. This thesis aims to begin to bridge the gap between these two modelling approaches, through a study of the flow around pairs of closely spaced isolated elements using experimental and numerical tools. Experiments investigating the boundary layer transition downstream of diamond- planform roughness elements were performed using thin-film heat transfer gauges in the Imperial College Hypersonic Gun Tunnel, and the wake structure was in- vestigated using oil flow in the Imperial College Supersonic Wind Tunnel. The models were carefully designed to have matching conditions, and the experiments were complemented by numerical simulations exploring the velocity fields around the elements. Isolated diamond-planform roughness elements produced transitional boundary layers containing turbulent spots. The convection and growth of the spots was characterised, and found to vary depending on the size and development of the spots themselves. The oil flow and numerical results revealed each element generated two pairs of counter-rotating vortices, which formed high and low speed streaks that were responsible for transition. The inter-element spacing of a pair of roughness element affected the state of its downstream boundary layer: transition is more advanced when the elements are touching or widely spaced, whilst small spacing suppresses transition. This was attributed to changes in the size and position of the high and low speed streaks in the boundary layer, caused by movement and a change in size of the vortices induced by the change in element spacing. Two-dimensional configurations of pairs of roughness elements have been designed using the experimental observations to manipulate the flow features in order to suppress or advance transition as desired.