The Impact of Nonequilibrium Flow on the Structure of Turbulence Over River Dunes

Abstract

Most past experimental investigations of flow over river dunes have focused on conditions that match semiempirical flow-depth scaling laws, yet such equilibrium conditions are of limited value because they rarely occur in natural channels. This paper quantifies the structure of mean and turbulent flow over fixed 2-D laboratory dunes across a range of nonequilibrium conditions within the dune flow regime. The flow field was quantified using 2-D particle imaging velocimetry for 12 conditions, including flows that are too deep, too shallow, too fast, or too slow for the size of the fixed dunes. The results demonstrate major departures in the patterns of the mean flow and structure of turbulence when compared to dunes formed under equilibrium flow conditions. The length of flow reattachment scales linearly with the ratio of mean depth-averaged streamwise velocity to shear velocity at the dune crest ((Formula presented.)), which provides a new predictive measure for flow reattachment length. Depth-averaged vertical velocities at the dune crest ((Formula presented.)) show a parabolic relationship with (Formula presented.), peaking at (Formula presented.) ~0.60 m/s, which matches the relationship of dune aspect ratio with transport stage present in mobile bed conditions. The spatial location of the turbulent wake was found to vary with flow depth and velocity, with lower (Formula presented.) and greater flow depths causing the wake to rise toward the free surface. Deeper flows are likely to show less flow convergence over the crests of dunes due to reduced interaction of turbulence with the free surface, resulting in a reduction of transport stage.