Issues in Numerical Modelling and Assessing the Flow over Weirs and Groynes

Abstract

In this research two problem statements are defined, namely (1) the validity of the assumption that groynes and weirs are equivalent in hydraulic models, and (2) the difficulty of modelling groynes and weirs in the 3D numerical solver package OpenFOAM. Using the incompressible 3D Navier-Stokes equations solver for multiphase phase flows, the so-called interFoam solver un- der OpenFOAM framework, two different models are set up for tackling the two problem statements.

The interFoam evaluation model addressed the issues of computational mesh types which affect the simulation results, and the methods for regulation of water level at the downstream boundary. The two types of the meshes which are used in the research are structured non-orthogonal mesh and unstructured more-orthogonal mesh. The benchmark test is the flow over the weir in the flume. The use of a non-orthogonal mesh could simulate the flow separation which is found downstream of the weir. However the unstructured meshes which are composed of more orthogonal parts and less non-orthogonal parts, could not reproduce the flow separation in 2D simulations correctly for high and small specific discharges. For the 3D simulations, the unstructured meshes apparently simulate the correct flow profile for low specific discharges. In this interFoam evaluation model the control structure or gate is used for maintaining the water level at the downstream boundary. In other words, it uses the physical method of water level boundary condition.

The weir-groyne comparison model is developed with different settings from the former model for the assessment of the differences between weirs and groynes in open-channel flumes. The water level is regulated by a trial-and-error discharge adjustment method making use of inlet and outlet tanks. The mesh type used is the unstructured more-orthogonal mesh in 3D simulations since the severely non-orthogonal meshes introduce numerical artifacts. A variety of weirs and groynes with different slopes are simulated for the comparison of energy losses and streamlines patterns. From the simulation results, the streamlines of the flow over the weir and the groyne are found to be different and the energy head losses also differ between these two structures.

For recommendations, to evaluate the skill of the interFoam solver the 3D simulations for unstructured meshes should be carried out to make sure that they can reproduce the flow sep- aration correctly. The use of a mathematical boundary condition for defining the water level is recommended to avoid the extra computational power for the control structure. To acquire the knowledge of flow around groynes and weirs, the experimental set-ups are recommended, and well-proved non-linear k − ε and k − ω turbulence closure models should be applied in the numerical modellings.