The Modelling of a 2D Local Turbulent Water Flow by the Application of a CFD and FEM Approaches. Some Parametric Numerical Experiments

The developing of a numerical modelling regarding a local 2D water flow, important for example to study erosion and sediment transport phenomena, is described. The Computational Fluid Dynamics (CFD) is applied. Thus the Navier-Stokes equations are solved. Turbulence is considered through the URANS (Unsteady Reynolds Averaged Navier Stokes) approach. The two equations K-epsilon and K-omega models have been employed. Important issues, such as a complete simulations of the fluid-structures interaction, the effects of the free-surface, bed and banks morphology changes, have not yet considered. The turbulence phenomena near solid boundaries is explored by the means of Wall-Functions. The first stage of the research activity is aimed to check the robustness and the main characteristics of the algorithms. Thus the selected numerical approach is discussed. Spatial discretization is carried out using the Finite Element Method (FEM). A structured meshing with h like adaptability is developed. Then to avoid velocities and pressure instabilities, the Characteristic-based split algorithm (CBS) and the Method of Artificial Compressibility (AC) have been applied and the related three steps (Nithiarasu & Liu 2006) are then applied and discussed. Some steady state parametric numerical experiments have been performed, considering a semi-implicit, approach. Poiseuille flow test has been carried out to clarify and investigate in which way CBS method works. The numerical outcomes of the selected turbulence models are compared with simplified approaches results like Prandtl's Mixing Length Theory, Deissler's sublayer and buffer zones and so on. The discussion of the overall results points out the sensitivity of the employed algorithms not only to the mesh size and distribution and to the number of iterations, but also to some intrinsic “experimental numerical dials” (safe coefficients, explicit vs implicit ratio), specific of the selected approach.