Response and receptivity of a hypersonic boundary layer to free- stream disturbances

Abstract eng:
This paper analyses the response and receptivity of the hypersonic boundary layer over a wedge to free—stream acoustic, vortical and entropy disturbances. In the presence of a shock, the boundary layer supports viscous Tollmien—Schlichting (T—S) instability modes, whose eigenfunctions are oscillatory in the far field. Any of three elementary types of disturbances interacts with the shock to generate a slow acoustic wave, which is reflected between the shock and the wall. Through this induced acoustic wave, vortical and entropy disturbances drive significant fluctuations in the boundary layer, which is impossible without a shock. A quasi—resonance occurs for high frequencies leading to a strong response to a continuum of disturbances. In the vicinity of the lower—branch neutral curve the slow acoustic wave of suitable frequency and wavenumbers is in exact resonance with a neutral eigen mode. As a result, the latter can be generated directly by each of three types of free—stream disturbances without involving any surface roughness element.

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