EVOLUTION OF THE PULSED WAVE PACKET IN A SUPERSONIC

There are two approaches of an experimental study of the laminar-turbulent transition mechanisms in boundary layers. In the first place, it is the evolution of natural disturbances in the boundary layer. In this case, their integral characteristics are generally considered and the power spectra are analyzed. Since the nature of origin of the natural disturbances is random, it is impossible to determine the wave characteristics of pulsations and quantitatively compare with the results of the linear theory of hydrodynamic stability. Another approach is the experiment in controlled conditions when disturbances with known initial characteristics are introduced into stream. This gives to determine the wave characteristics of excited disturbances in the boundary layer and directly compare the results with theoretical calculations. Small scales of compressible shear layers and high frequencies of the Tollmien-Schlichting waves overlay strict conditions to sizes both transducers and the source of controlled disturbances. High-frequency glow discharge is an effective way of introducing controlled pulsations into the supersonic flow. Results of experimental studies on linear and weakly nonlinear development of instability waves in the supersonic boundary layer confirm this [1, 2]. At high-frequency discharge the perturbations, limited in space but periodic in time (so-called wave trains), are excited in the boundary layer. The results of the wave trains evolution can be compared with theoretical calculations, but to find of them in nature is problematic. Wave packets, which are localized not only in space but also in time, are well known in nature. Recent progress in the study of the late stages of the transition at low subsonic speeds are related to the technique of pulsed excitation coherent structures in the boundary layer, localized in space and time [3]. Coherent structures were generated by both the "blowing-suction" method and the vibrating surface. In [3] was shown that the transformation of the "structure" in a turbulent spot was related to the origin and development of high-frequency disturbances on the coherent structure, which occurs near the peak of the local velocity gradient u/z. Streaky structures have the local velocity gradient in the longitudinal direction u/x, on which may also appear the secondary high-frequency disturbances. The occurrence of wave packets "forerunners" in a domain of fronts streaky structures and their development in the subsonic boundary layer was studied by experimentally in [4]. Experiments have shown that the "forerunners" are packets of waves Tollmien – Schlichting. The pressure gradient of the external flow directly affects the occurrence and development of the wave packets "forerunners". The method of excitation of wave packets in a hypersonic boundary layer was used in the experiments [5, 6]. Development of wave packets downstream and their breakdown into turbulent spots on the walls of the nozzle of the wind tunnel have been studied in these works. Measurements were carried out in quiet flow at Mach number 6. The development of both naturally occurring and artificially generated glow discharge wave packets was studied. Frequency of artificial wave packets was 200 Hz. Experiments [5] have shown that under quiet flow the boundary layer on the walls of the nozzle in the measurements domain was laminar. The peak frequencies of these natural wave packets were in agreement with second-mode computations. It is known, the second mode in the hypersonic flow is the most unstable. Wave packets had the linear and the nonlinear development, and then they transformed into turbulent spots. Measurements of the spatial