Vortices in the Boundary Layers of Decelerating Water and Air Flows

A review of experimental and numerical studies on the boundary layer of water and air flows with decreasing velocity along the flow direction is presented. It has been experimentally established that an adverse pressure gradient in water and air flows leads to periodic deceleration of the boundary layer in plane-parallel flow. During deceleration, the vertical velocity profile deforms, creating a local minimum located between two inflection points above the viscous layer. It is shown that within the thin layer containing the upper inflection point, the plane-parallel flow is unstable, as confirmed by experiments—within this layer, the formation of a cylindrical vortex is observed at a critical value of the adverse pressure gradient at the end of the deceleration cycle. The parameters of the vortex chain have been obtained, and the deformation of vortices in a flow with velocity shear, as well as the impact of vortices on the underlying surface, has been investigated. Physical mechanisms have been identified for the formation of ridges on an erodible surface under water flow, the generation of wind waves on an initially flat water surface, and the formation of a wall of water and sediment in flows during dam breaks.