Comparison of Turbulent Flows over Clusters of Varying Density

Abstract

Gravel bed rivers commonly form an armored surface layer within which cluster bedforms develop. Clusters can form in isolation or develop cluster groups of increasing density. Flume experiments are presented in which a clustered, armored surface was formed on a gravel bed. Flows were measured over an area without a cluster and areas with an isolated cluster, a coupling of two clusters, and a group of three clusters. Turbulent flow parameters, Reynolds stresses, and turbulent kinetic energy were calculated from acoustic Doppler velocimeter measured flows and visualizations of the flow patterns around clusters. Flow separated over the cluster crest, creating an increase in the turbulent flow properties and a high-turbulence intensity region when flow reattached downstream. Turbulent kinetic energy increased around the coupled clusters, but around the grouped clusters, the flow field was distinguished by a high degree of uniformity. The net effect of increasing density of cluster grouping was an increase in the magnitude and variability of the turbulent flow field around coupled clusters, followed by an overall dampening of turbulent flows around grouped clusters in which the flow patterns generated interfered with one another, and the cluster forms sheltered the bed surface.