Pavlenko A.M.   Katasonov M.   Kozlov V.V.   Довгаль А.В.  

Generation of localized disturbances by the vibrations of surface behind the ledge in laminar flow

Reporter: Pavlenko A.M.

In recent years, localized disturbances of laminar flow, denoted by the term of “longitudinal structures”, attracted interest of researchers, who are engaged in the study of laminar-turbulent transition. Observed in the experiment longitudinal structures are quasi-stationary deformation of the shear layer, oriented along the flow and limited in the transverse direction to him. The recently discovered effect of instability associated with the formation of longitudinal structures, is generation of Tollmien-Schlichting packages on fronts of structures. Origin and evolution of the packages have been studied in detail in previous experimental investigations with the participation of the authors of this article by a modeling of longitudinal structures of the boundary layer in various ways: generation of structures by blowing-suction of air through the slots in the surface; generation of structures by localized vibrations on the surface; generation of structures of vorticity disturbances of the incoming flow in the gradient and non-gradient flows.
In continuation of previous experiments, this study of localized flow disturbances with the speed shift in the conditions of boundary layer separation was conducted. Experiments were carried out in subsonic low-turbulence T-324 wind tunnel based at the Khristianovich Institute of Theoretical and Applied Mechanics of the Siberian Division of the Russian Academy of Sciences (Novosibirsk). Free stream velocity was U= 3.5 m/s and turbulence level less than Tu ≤0.04%. A flat plate with a streamlined nose part in the form of two conjugated semi-ellipses was used as the experimental model. Sizes of plate are 1500x1000x10mm (length x width x thickness). Pad, rear section of which forms a rectangular ledge (height = 3.0 mm) of the surface, was placed on the working side of plate at a distance of 144 mm from the leading edge. Sizes of pad are 99x300mm (length x width). The low-frequency perturbation of the laminar flow was generated by the controlled oscillations of elastic a lavsan membrane. Membrane had square shape with a side of 17 mm, located on the plate surface near the ledge in the central section of the model. Results of investigations were obtained by method of hot-wire anemometry. The origin and development of hydrodynamic disturbances generated by low-frequency vibration of the local section of the streamlined surface flow in separated flow behind the rectangular ledge on a flat plate were investigated experimentally. Oscillations of the wall leads to the generation of two disturbances types in the separation zone, typical for transitional shear flows: packages of instability waves and low-frequency longitudinal deformations of velocity field. The process of the laminar-turbulent transition behind the ledge of the surface is determined by the growth of wave packets in relatively weak changes of longitudinal structures in the direction of flow and their damping with increasing distance from the ledge. A comparison of the obtained data with similar results for the flow on the straight wing shows that the separation of the boundary layer, as well as an adverse pressure gradient, stimulates the development of fluctuations packets and their transformation into turbulent spots.
This work was supported by the project of the President of the Russian Federation for Leading Scientific Schools NSH-НШ-8788.2016.1 and Russian Foundation for Basic Research No15-08-01945.

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