Response of velocity and turbulence in submerged wall jets to abrupt changes from smooth to rough beds and its application to scour downstream of an apron

Abstract

This paper addresses how the turbulent flow field in submerged wall jets responds to an abrupt change from smooth to rough beds. Experiments were conducted for submerged wall jets having different submergence factors and jet Froude numbers. The bed configurations investigated consisted of different combinations of the lengths of smooth beds and the roughness of rough beds. The vertical profiles of time-averaged velocity components, turbulence intensity components and Reynolds stress were detected by an acoustic Doppler velocimeter at different streamwise distances; and the horizontal distributions of bed shear stress were estimated from the Reynolds stress profiles. The flow field displays the decay of jet velocity due to abrupt changes from smooth to rough beds. The boundary layer grows more quickly with increase in roughness of rough beds. The change in bed roughness induces an increased depression of the free surface over the smooth bed. The Reynolds and bed shear stresses are also computed by solving the Navier Stokes equations. The response of the turbulent flow characteristics of submerged wall jets to abrupt changes from smooth to rough beds is analysed from the point of view of similarity, growth of the length scale, and decay of the velocity and turbulence characteristics scales. The significant observation is that the flow in the fully developed zone is plausibly self-preserving on both smooth and rough beds. Also, the use of a common length scale makes it possible to collapse all the flow data onto a single band; and there is a gradual variation of flow at the junction of the smooth and rough beds. The equilibrium scour profiles downstream of a smooth apron due to submerged wall jets are computed from the threshold condition of the sediment particles on the scoured bed. Use of the modified bed shear stress for the downstream variation of scoured bed permits the computation of the equilibrium scour profiles. The time-variation of maximum scour depth is computed from the bed shear stress with a modification for the time dependence. The agreement between the results obtained from the model and the experimental data is satisfactory.