Experimental Investigations of Unsteady Flow Over Rough Bed Channels with and Without Emergent Rigid Vegetation

Abstract

The Ph.D. thesis deals with the experimental investigation on flow structures over rough bed channels with and without emergent rigid vegetation. Experiments over dense grass bed are conducted at National Institute of Technology Rourkela (NITR) to investigate the lateral velocity profiles, bed shear stress, Reynolds shear stress and turbulence characteristics under unsteady flow conditions as compared to the steady flow conditions for a given flow depth. The flow structures of unsteady open-channel flow over a rough bed with and without emergent rigid vegetation are also investigated in an 18 m long and 3 m wide laboratory flume at INRAE Lyon-Villeurbanne, France. Steady flows are also studied and served as reference flows. For both steady and unsteady flows, four geometries are tested: (1) uniform bed roughness (uniform dense synthetic grass modelling meadow); (2) a uniform staggered distribution of emergent wooden circular cylinders (model of rigid vegetation) set on bed roughness; (3) a uniform unsteady (longitudinal roughness transition from woodland to dense grass); and (4) a non-uniform unsteady (longitudinal roughness transition from dense grass to woodland). For lower unsteady flow cases (longer period flow hydrograph), lateral distribution of depth-averaged velocity and bed shear stress is plotted at three different cross-sections and compared with the results from steady flow conditions. Variations of Reynolds stress along the vertical and lateral direction are analyzed. Turbulence characteristics, i.e., turbulent kinetic energy (TKE), mean kinetic energy (MKE) and dissipation rate has also been analyzed. For higher unsteady flow cases (short period flow hydrograph), transient flow depths are simultaneously measured at six longitudinal positions using ultra-sonic sensors. Transient velocities are measured at one longitudinal position over the water column using a side looking ADV probe to estimate depth-averaged velocity. In order to compute ensemble averages of the flow parameters, 109 runs of the same hydrographs are injected repeatedly at the flume entrance. Two consecutive runs are separated by a base flow. The ensemble averages of the measured discharge, flow depths and velocity are found to be converged when using 45, 50, and 72 runs respectively. After the data convergence, the discharge, velocity, Reynolds shear stress etc. have been finalized for the present unsteady flow analysis. The present study therefore focuses on these. A particular attention is paid to the convergence of flow parameters based on ensemble averages: (1) to confirm the mean velocity profiles for unsteady accelerated, and decelerated flows, and uniform flows over a rough bed; (2) to study the vertical distribution of mean flow, turbulent statistics and Reynolds shear stress for unsteady accelerated, decelerated, and uniform flows over a rough bed with emergent rigid vegetation, (3) to study the vertical distribution of mean flow, turbulent statistics and Reynolds shear stress for unsteady accelerated, decelerated, and uniform flows over a rough bed (downstream of roughness transition), and (4) to study the vertical distribution of mean flow, turbulent statistics and Reynolds shear stress for unsteady accelerated, decelerated, and non-uniform flows over a rough bed (upstream of roughness transition). The variations of vertical profiles of mean streamwise velocity, turbulence statistics and Reynolds shear stress for the unsteady flow cases along the vertical direction are analysed and also compared to the steady flow conditions. The hysteresis loops in the depth-averaged velocity/flow depth relationships for unsteady flow conditions have been illustrated for the four geometries highlighting the weak effect compared to the effects of un-stationarity.