NUMERICAL AND EXPERIMENTAL INVESTIGATION OF SUSPENDED SEDIMENT TRANSPORT IN LAB-SCALE TURBULENT OPEN CHANNEL FLOW

Single-phase Euler-Euler based wall-resolving LES with the dynamic Smagorinsky model is used to investigate suspended sediment transport in a turbulent open channel flow. Aspect ratio of the open channel is high. Bottom bed is smooth. For the clear water flow, streamwise and vertical turbulence intensities in experiments of Muste et al. [2005] are much bigger than those in the wall-resolving LES and DNS by Hoyas and Jimenez [2008]. Bulk velocity in the sediment-laden flow is lesser than that in the clear water flow. Friction velocity is same for both flows. While Muste et al. [2005] recorded that in the inner region, the streamwise velocity of the sediment-laden flow was higher than that of the clear water flow, wall-resolving LES shows no alteration. In the outer region, the depth-resolved streamwise velocity of the sediment-laden flow is lower than that of the clear water flow. Introduction of suspended sand particles into the turbulent open channel flow of the clear water results in the decrease of the drag force while shear stress on the channel bed is constant. To get reduction of the bulk velocity, the fast Eulerian method in the two-way coupling should be employed. Single-phase Euler-Euler based unresolved wall-function LES with the Smagorinsky model under the equilibrium stress assumption is implemented to understand interactions between turbulence and suspended sand particles in a turbulent open channel flow. Channel bed is rough. Aspect ratio of the open channel is low. To treat erosion from the bed, the reference concentration method together with the Shields diagram is used. Results are compared against experiments of Cellino [1998]. Suspended particles engender reduction of the friction velocity, bulk velocity and roughness in contrast to the clear water flow. Streamwise velocity is decreased in the outer region while it is expedited in the super-saturated region near the channel bed. It is due to high inter-particle collisions between sediment particles which are not bounded by viscosity. In upper levels, remarkable weakening of the vertical turbulence intensity is seen. When the buoyancy term is deactivated, the sediment concentration gets high and unsatisfactory turbulence statistics are obtained. Wall shear stress on the sidewalls of narrow open channels must be considered. Effects of lateral and bottom macro-rough boundaries on the propagation of a suspended sediment wave in a turbulent open channel flow are shown experimentally. Least decay of the normalized concentration is for the reference case in absence of trapping zone. Least sedimentation among lateral configurations is for case L4 with the highest roughness aspect ratio, cavity density and medium flow discharge. Length of the inlet reach is lowest and it boosts mixing. Highest deposition of the Polyurethane particles is for case L5 with the lowest roughness aspect ratio and cavity density. Discharge is also low. Turbulence is most attenuated for case L2 with the medium flow discharge due to adequate lengths of the inlet and outlet reach. Deposition of the Polyurethane particles and turbulence in the lateral macro-rough flows depend on the cavity aspect ratio, flow discharge, roughness aspect ratio, cavity density and location of the lateral cavities. In the bottom macro-rough flows, effects of spacing between bottom macro-rough elements on the deposition and turbulence characteristics are seen. Turbulence characteristics of upstream C1 and downstream C2 signals are identical for reference, B1 and B1.5 cases. When spacing is 2, highest trapping and deposition take place. Most weakening of the turbulence is seen for spacing 2. When spacing is augmented from 2 to 5, the turbulence is enhanced and trapping becomes lesser. Bottom macro-rough elements change significantly flow pattern and turbulence characteristics in a specific spacing. As spacing gets larger, effects of bottom macro-roughness elements on each other reduces more.