Flow Field Upstream of an Orifice under Fixed Bed and Equilibrium Scour Conditions

Flow behavior upstream of a circular orifice with the invert at the bed level (initially) is investigated under fixed bed and equilibrium scour (mobile bed) conditions. The experiments are performed under three different constant heads and sediment sizes. Longitudinal and vertical components of velocity are measured in the vertical and horizontal planes passing through the center of the orifice. The velocity decay upstream of the orifice (along the centerline of the orifice) is slower for the fixed bed case than for the unbounded orifice. Under equilibrium scour condition, the velocity decay transitions from the velocity profile for the unbounded orifice to the fixed bed condition. The longitudinal velocity profiles in the horizontal plane at different locations upstream of the orifice are found to be similar for both fixed and mobile bed cases. The longitudinal velocity profiles in the vertical plane are also found to be similar for each case. The maximum velocity is found to occur below the centerline of the orifice for both cases. The location and decay of maximum velocity with distance upstream of the orifice are quantified. A three-dimensional flow model is used to simulate the flow behavior upstream of the orifice under fixed bed and equilibrium scour conditions. The model accurately predicts the velocity field upstream of the orifice under both fixed bed and mobile bed conditions. (C) 2014 American Society of Civil Engineers.