Abstract
The objective of the present paper is to reveal the intrinsic mechanisms by which two-dimensional surface indentations influence the laminar–turbulent transition of natural route in hypersonic boundary layers. For an oncoming Mach number of 6 and a rectangular indentation, two relevant scattering mechanisms, namely, the local scattering of oncoming Mack modes and the local receptivity to freestream acoustic waves, are studied by direct numerical simulations. A transmission coefficient and a receptivity coefficient are introduced to quantify the two mechanisms, and their dependence on the indentation depths and the oncoming-perturbation frequencies is investigated systematically. Numerical results reveal that both the local scattering and receptivity effects increase with the indentation depth when the latter is shallow, but the opposite is true for deep indentations. Such a phenomenon is found to be related to the topological structure of the base flow inside the indentations, namely, two independent separation bubbles for shallow configurations and one large-scale separation bubble occupying the whole indentation for deep configurations.
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