Experimental investigation on local shear stress and turbulence intensities over a rough non-uniform bed with and without sediment using 2D particle image velocimetry

Abstract

The current work focuses on locally resolving velocities, turbulence, and shear stresses over a rough bed with locally non-uniform character. A nonporous subsurface layer and fixed interfacial sublayer of gravel and sand were water-worked to a nature-like bed form and additionally sealed in a hydraulic flume. Two-dimensional Particle Image Velocimetry (2D-PIV) was applied in the vertical plane of the experimental flume axis. Runs with clear water and weak sediment transport were done under slightly supercritical flow to ensure sediment transport conditions without formation of considerable sediment deposits or dunes. The study design included analyzing the double-averaged flow parameters of the entire measurement domain and investigating the flow development at 14 consecutive vertical subsections. Local geometrical variabilities as well the presence of sediment were mainly reflected in the vertical velocity component. Whereas the vertical velocity decreased over the entire depth in presence of sediment transport, the streamwise velocity profile was reduced only within the interfacial sublayer. In the region with decelerating flow conditions, however, the streamwise velocity profile systematically increased along the entire depth extent. The increase in the main velocity (reduction of flow resistance) correlated with a decrease of the turbulent shear and main normal stresses. Therefore, effects of rough bed smoothening and drag force reduction were experimentally documented within the interfacial sublayer due to mobile sediment. Moreover, the current study leads to the conclusion that in non-uniform flows the maximum Reynolds stress values are a better predictor for the bed shear stress than the linearly extrapolated Reynolds stress profile. This is an important finding because, in natural flows, uniform conditions are rare.