Experimental investigation of 3D flow over cluster microforms

Cluster microforms are a type of small scale bedform found in the surface layer of some gravel bed rivers. These bedforms are comprised of discrete، organized groupings of particles that sit above the average elevation of the surrounding bed. As part of the structural organization of the bed، clusters are believed to impact the local dynamics of the fluvial system through the feedback process involving the flow field، entrainable sediment، and stable bed morphology. To investigate the effects of the cluster on average and turbulent characteristics a laboratory model has been used. An acoustic Doppler velocimeter (ADV) has been used to measure average velocities and fluctuations. Results of this study show that compared with nonclusterd bed، clusters have significant effects on flow characteristics. Clusters cause to change the location of maximum RMS (root mean square) and Reynolds shear stress and move it upward. Clusters create a shear layer downstream of it. The results of analysis show that one can distinguish five different areas over a cluster: flow acceleration up the stoss-side of the cluster; recirculation behind the cluster in the wake region; vortex shedding from the pebble crest and shear layer; flow reattachment downstream of the cluster; upwelling of flow downstream of the point of reattachment; and recovery of flow.