Outer-Bank Shear Stress in River Bends

Abstract

In the dimensional averaged numerical models, that are used for practicle purposes, there need to be accounted for three-dimensional processes. Amongst others a bank shear stress parameterization need to be incorperated. This master thesis focusses on the outer-bank shear stress in order to obtain a parameterization for the outer-bank shear stress for naturally curved flows. The outer-bank shear stress is obtained from three-dimensional numerical computations. The numerical modeling is based on the Large Eddy Simulator, available at the Fluid Mechanics Department of the Technical University of Delft. Despite the considerable progress achieved, the software is still under development and a thorough review of its code is therefore necessary. The aim of this master thesis is to assess the effect of the outer-bank roughness, the outer-bank angle and the transverse bed slope on the outer-bank shear stress and to test and improve the performance of the boundary method of the computational algorithm. The magnitude of the outer-bank shear stress and the outer-bank cell increase for increasing roughness of the outer-bank, leading to a less uniform distribution of the outer-bank shear stress. The magnitude of the outer-bank shear stress decreases for increasing inclination of the outer-bank. For a more inclined outer-bank, the magnitude of the outer-bank shear stress is more dependent on the outer-bank roughness. No significant dependency of the distribution of the outer-bank shear stress on the outer-bank inclination can be found from the results. Inclusion of the point bar related transverse bed slope does not lead to a significant change of the magnitude and distribution of the outer-bank shear stress. Clearly, the helical motion outscores the effect of topographic steering. The boundary method adds the frictional effects of solid boundaries to the computational algorithm. The implemented boundary method, the Immersed Boundary Method, allows for the implementation of complex boundaries on a structured grid. Although in some cases the boundary method has a good performance, it is not very robust and prone to errors. It is recognized that the near wall velocity profile using the Immersed Boundary Method does not often coincide with the actual momentum `loss' at the wall. This problem, not recognized in earlier studies, can be attributed to inaccuracy in the description of the turbulent viscosity. The latter is understood and partly solved. The accuracy can be further improved by coarsening the grid or reconsidering the implemented turbulence closure model, the Smagorinsky Model. Different improved Smagorinsky Models are available.