Previous experimental and numerical studies showed that two-dimensional roughness elements can stabilize disturbances inside a hypersonic boundary layer, and eventually delay the transition onset. The objective of this paper is to evaluate the response of disturbances propagating inside a high-speed boundary layer to various two-dimensional surface deformations of different shapes. We perform an assessment of the impact of various 2D surface non-uniformities, such as backward or forward steps, combinations of backward and forward steps, wavy surfaces, surface dips, and surface humps. Disturbances inside a Mach 5.92 flat-plate boundary layer are excited using periodic wall blowing and suction at an upstream location. The numerical tools consist of a high-accurate numerical algorithm solving for the unsteady, compressible form of the Navier-Stokes equations in curvilinear coordinates. Results show that all types of surface non-uniformities are able to reduce the amplitude of boundary layer disturbances to a certain degree. The amount of disturbance energy reduction is related to the type of pressure gradients that are posed by the deformation (adverse or favorable). A possible cause (among others) of the disturbance energy reduction inside the boundary layer is presumed to be the result of a partial deviation of the kinetic energy to the external flow, along the discontinuity that is generated by the wall deformation. arXiv:1807.02197v1 [physics.flu-dyn] 5 Jul 2018 Khokhlov [26,27], Sakaue et al. [29], Fedorov [30]). In some of these studies, it was found that acoustic waves are very effective in exciting disturbances inside the boundary layer with amplitudes that become much larger than those in the free-stream, but this happens only above some critical Reynolds number as in the incompressible regime. Other studies (Fedorov and Khokhlov [26,27], Sakaue et al. [29], Fedorov [30]) were concerned about the generation of the first and second modes in the vicinity of the leading edge. The effect of all types of waves, i.e. slow and fast acoustic waves, vorticity waves and entropy waves, on supersonic boundary layers was studied and reported in a suite of papers by Balakumar [31,32,33]. The generation and the evolution of three-dimensional disturbances induced by slow and fast acoustic disturbances and isolated roughness elements in a supersonic boundary layer over flat plates and wedges were numerically investigated by solving the full three-dimensional Navier-Stokes equations. It was found that instability waves are generated within one wavelength of the acoustic wave from the leading edge.Previous experimental and numerical studies (Holloway and Sterrett [34], Fujii [35], Fong et al. [15, 16, 17, 18], Duan et al. [19], Park and Park [20], Mortensen and Zhong [36]) showed evidence that twodimensional roughness elements can reduce the amplitude of disturbances inside high-speed boundary layers. Holloway and Sterrett [34], for example, carried out early experiments on flat plate boundary layer disturbed by roughness ele...
