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A wind-tunnel study of the influence of flow suction on laminar boundary-layer separation behind a two-dimensional step on the surface is performed. Hot-wire measurements are carried out at low subsonic flow velocities. It is demonstrated that this method of flow control allows suppressing the formation of large-scale vortices determined by global stability properties of the separation region.Emergence of local regions of laminar-flow separation (so-called separation bubbles) is often accompanied by generation of large-scale vortices, which are quasi-periodically shed from the separation zone in the external flow direction. Dominating in the unsteady flow, the vortices determine its fluctuating and, to a large extent, mean (time-averaged) parameters, which can be optimized by affecting the formation and dynamics of coherent vortex structures.The vortex motion emerging owing to evolution of disturbances of the separated shear layer can be controlled by changing their initial spectrum and local characteristics of flow stability. The initial spectrum of perturbations is modified by periodic excitation of the separation region by external acoustic forcing [1,2] or locally generated oscillations [3,4]. The stability of the separated flow to small-amplitude oscillations can be changed, e.g., by flow suction or by cooling of the body surface [5][6][7].A possibility of controlling quasi-periodic vortex formation is considered in the present paper, the vortices being induced by instability of the separation region to long-wave oscillations ("shedding"-type instability in terminology of [8]) rather than amplification of convective disturbances of the separated layer. The coherent vortex motion determined by the global flow properties at the scale of the entire separation region was examined experimentally in [9, 10]. One method of controlling flow separation in such a regime of instability was considered in [11], where vortex formation was found to depend on weak harmonic excitation of the flow in the frequency range of amplifying small perturbations of the separated boundary layer.A possible method of affecting the shedding of periodic vortices used in the present work is flow suction, which is a well-known flow-control technique, in particular, for modification of the transition to turbulence initiated by flow instability behind the boundary-layer separation point [5].Experimental Technique. The experiments were performed in a T-324 low-turbulent subsonic wind tunnel located at the Institute of Theoretical and Applied Mechanics of the Siberian Division of the Russian Academy of Sciences. The wind tunnel has a closed test section 1 × 1 × 4 m, and the free-stream turbulence is lower than 0.04%. The experimental model schematically shown in Fig. 1 was previously used to study the formation of coherent vortices in the flow behind a rectangular step on a flat-plate surface [9][10][11]. The model consisting of two plates was mounted in the centerplane of the wind-tunnel test section at a zero angle of attack, and the level of ...

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The present paper deals with small amplitude velocity oscillations at laminar boundary layer separation behind a backward-facing step on a flat plate. The focus is the effect of a spatially periodic stationary disturbance of the nominally 2D basic configuration on the linear instability of the separated shear layer. Nonstationary perturbations of the mean flow which is modulated by roughness elements spaced along the step are examined through wind-tunnel tests and stability calculations. Noteworthy is flow destabilization in the presence of streaks behind the line of separation.

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