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Stability and transition of the flow behind isolated roughness elements in hypersonic boundary layers

Van den Eynde, Jeroen (2015) Stability and transition of the flow behind isolated roughness elements in hypersonic boundary layers University of Southampton, Engineering and the Environment, Doctoral Thesis , 198pp.

Record type: Thesis (Doctoral)


In this work the effect of isolated surface roughness on the behaviour of a hypersonic boundary layer is investigated, with a particular focus on the effect of the three-dimensional roughness shape on the instability of the roughness wake and the subsequent transition process. The analysis is performed computationally using direct numerical simulations, which solve the compressible Navier-Stokes equations, and a new code, developed in the scope of the current work, to analyse the linear stability of these equations. The full three-stage roughness-induced transition process has been investigated: firstly, the receptivity process and generation of boundary layer instabilities from freestream disturbances; secondly, the generation of a roughness wake and its initial linear instability; and finally the non-linear breakdown to turbulence of the roughness wake. In particular the effect of the three-dimensional roughness shape on these processes has been studied, looking at the roughness height, frontal profile, planform shape and upward/downward ramps. Also the effect of freestream disturbance amplitude andwall cooling has been investigated. It has been found that the roughness height and frontal profile have a large influence on the stability characteristics of the resulting wake and the subsequent transition. The roughness planform shape has a marginal effect, although cylindrical and diamond-shaped elements yield more unstable wakes than a square roughness element. Bi-local stability analysis can be used in most cases to predict the most unstable wake mode, but it under-predicts the instability growth rates due to non-parallel effects. The roughness shape has been observed to affect the transition onset location. The criteria commonly used to predict roughness-induced transition, do not take into account the three-dimensional shape, and an alternative transition prediction, based on the amplitude of the roughness-induced streamwise streak, is considered.

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Published date: July 2015
Organisations: University of Southampton, Aerodynamics & Flight Mechanics Group


Local EPrints ID: 386204
PURE UUID: 0809a987-6da1-40cb-aa18-bcc9bbf5354a

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Date deposited: 10 Feb 2016 14:57
Last modified: 17 Jul 2017 19:51

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Author: Jeroen Van den Eynde
Thesis advisor: Neil Sandham

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