Fluid-dynamic feed-back in shear layer oscillation below a submerged sluice gate

Abstract

The phenomenon of fluid-dynamic feed-back, which leads to organised periodic vortex shedding and oscillation of the free shear layer below a submerged sluice gate, is investigated. The feedback mechanism occurs through the impingement of the vortices onto a second parallel sluice gate installed at some distance downstream. The pressure fluctuations at the point of impingement are fed back upstream along the shear layer to the sensitive region of flow separation near the lip of the sluice gate. This amplifies the instability and enhances the vortex formation at the source of instability. The process of vortex impingement, upstream propagation of the disturbances, and the incitement of the velocity field at the vortex source, eventually lead to organised periodic oscillation of the shear layer and the accompanying vortex formation process. Over the present tests ranges of 30 <Lc /θO < 170, and 2,000 < Re θ < 10,000, fluid-dynamic feedback was observed to occur within a Strouhal number range of 0.005 < S θ < 0.025, where Re θ and S θ are the Reynolds and Strouhal numbers defined in terms of the initial momentum layer θo, and L c the impingement length. The present results agree well with those of previous studies, as well as that predicted from linear stability analysis.