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The evolution of disturbances in a hypersonic viscous shock layer on a flat plate excited by slowmode acoustic waves is considered numerically and experimentally. The parameters measured in the experiments performed with a free-stream Mach number M ∞ = 21 and Reynolds number Re L = 1.44 · 10 5 are the transverse profiles of the mean density and Mach number, the spectra of density fluctuations, and growth rates of natural disturbances. Direct numerical simulation of propagation of disturbances is performed by solving the Navier-Stokes equations with a high-order shock-capturing scheme. The numerical and experimental data characterizing the mean flow field, intensity of density fluctuations, and their growth rates are found to be in good agreement. Possible mechanisms of disturbance generation and evolution in the shock layer at hypersonic velocities are discussed.Introduction. In high-velocity high-altitude flight, the entire space between the surface of the flying vehicle and the bow shock wave (SW) even at a large distance from the leading edges is the viscous flow zone where the socalled viscous shock layer is formed. Like the boundary layer, the laminar shock layer is unstable, and perturbations developed in this layer induce a transition to a turbulent flow regime. The evolution of perturbations in the viscous shock layer and in supersonic flows with lower Mach numbers, however, may be caused by different mechanisms. The presence of numerous instability modes plays an important role in the development of instability at hypersonic velocities. Factors that can also affect the character of instability evolution are the interaction of instability waves and the SW [1], substantial deviations from a parallel flow, and velocity slip and temperature jump on the wall. In addition, one should take into account that instability waves can be excited not only by the conventional mechanism of receptivity but also by means of direct amplification of free-stream perturbations in the SW [2]. Finally, of great importance in flight conditions at high stagnation temperatures of the flow are real gas effects capable of changing the stability characteristics to a large extent.Because of the above-listed factors, a sufficiently large amount of experimental measurements, results of the linear analysis of hydrodynamic stability (see [3,4]), and data of direct numerical simulations [5,6], which were accumulated during long-time research of the laminar-turbulent transition of the boundary layer at moderate hypersonic Mach numbers (M ∞ = 5-8), cannot be extrapolated to the case of a viscous shock layer at extremely high Mach numbers (M ∞ = 15-25). Meanwhile, the knowledge of mechanisms controlling the evolution of disturbances in a viscous shock layer is necessary to develop efficient methods of predicting and controlling the laminar-turbulent transition in hypersonic flows. This will allow a significant reduction of the drag force and heat loads and offer engineering background for production of efficient hypersonic flying vehicles.

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The characteristics of density waves in a shock layer on a flat plate have been measured by the method of electron beam fluorescence of nitrogen for the free-stream Mach number M∞=21 and the local Reynolds number Rex=2×104–3×105. The measurements have been performed for natural perturbations. The data on spatial distribution of fluctuation spectra and phase velocities of the waves in the longitudinal and transverse directions have been obtained. The propagation angles and the increments of density waves have been determined. The characteristics of fluctuation coherence have been measured. The experimental equipment, techniques of measurements, and methods for the reconstruction of the mean density and density fluctuations are described in the paper. The limitations imposed upon the measurement method are discussed.