Large-Scale Horizontal Coherent Structures in a Deep Open Channel Flow: an Experimental Study

Abstract

This work is a study of the size, dynamics and morphological consequences of large-scale horizontal coherent structures (HCS's) in a deep open-channel flow. The study has the following three objectives: 1- to assert the existence of HCS's in a deep open-channel flow and establish their characteristic times and lengths; 2- to penetrate the dynamics of the life cycle of HCS's in such a flow; and 3- to gain insight into why the bed forms known as alternate bars disappear in such flows, even though meandering can still occur. The study is intended as an extension of previous work carried out at Queen's University and focusing on a shallow, alternate bar inducing flow. For the present purposes, six series of flow velocity measurements were carried out in a 21 m long, 1 m wide straight channel, conveying a 0.14 m deep flow. The bed surface consisted of a silica sand having an average grain size of 2 mm; its surface was flat. The bed slope was such that the bed shear stress was substantially below the threshold for the initiation of motion. The measurements were carried out with the aid of 16 MHz 2D Son Tek Micro-ADV's. The presence of HCS's affecting (or occupying) the entire body of fluid was unambiguously revealed by the present measurements. The average horizontal burst length and period of the measured flow were found to be equal to 5.5 m. The results of a quadrant analysis of the joint behaviour of the longitudinal and transversal components of fluctuating velocity, and the analysis of the plan distribution of planar Reynolds shear stress show that these structures originate at the upper layers of the flow. This study further demonstrates that some form of upstream discontinuity is able to regularize the plan configuration of the HCS's, so as induce the internal meandering of the flow. The HCS's were found to be of a rather diffuse nature towards the lower layers of flow, which likely explains why they are not able to produce alternate bars.