Numerical Modeling of Flow and Scour Using Strict Vertex-Based Terrain Conformal Moving Mesh with Masked Elements

Abstract

Local scour can jeopardize the safety of the hydraulic structures and the ecology of a river system, so it is desirable to understand better the relevant processes and characteristics, and thus it is of primary importance for researchers to propose better research methodologies or numerical models. OpenFOAM is attractive for sediment-scour applications because of its high-quality hydrodynamic solvers and turbulence models, and its advantages of being open source and readily modifiable and extendable. However, the standard OpenFOAM package does not have a sediment scour module. It lacks the ability to perform large-amplitude motions needed for sediment-scour problems, and thus its application is normally restricted to small-amplitude cases to prevent divergence due to mesh deterioration. The first part of this study presents the implementation and validation of a new sediment-scour model with a strict vertex-based terrain conformal moving-mesh technique within the framework of OpenFOAM. The proposed simple moving-mesh technique in OpenFOAM is implemented to overcome the shortcomings of the conventional automatic mesh-motion techniques in handling large-amplitude moving geometries. The model is employed to simulate a simple case of prescribed boundary motion, a previous experiment in the literature, and a new laboratory experiment for local scour due to submerged wall jets. The results are compared with both the experimental and other numerical results. The comparisons demonstrate that the present model has the novel advantage of allowing for more severe topographic variations, and can provide more reliable predictions for the key characteristics and evolution of the bed profiles in wall jet scour problems. The second part of this study extends the model presented in the first part to simulate cases in which there is an embedded structure (here, a pipeline). In contrast to previous moving-mesh models, the newly implemented model uses masked elements to resolve the effects of the stationary object on the flow and sediment scour processes. This approach makes it possible to track directly the moving fluid-sediment interface using a very simple mesh setup, which is more practical for engineering computations than the existing models. The third part of this study further applies the proposed masked-element approach to simulate local scour at submerged weirs with downstream slopes. A new laboratory experiment is reported and used to validate the model. The effects of various parameters and configurations on the numerical predictions are studied, including the roughness height estimation, angle of repose, turbulence model, stochastic model, critical Shields number, and the form of the sand-slide model. The fourth part of this study provides some additional results and discussions, including the applications of the wall jet scour model, numerical modeling of flow around a pipeline near a scoured bed, and mesh configurations and improvement.