Cfd-fem 2d modelling of a local water flow. Some numerical results

Abstract: Pasculli A.: CFD-FEM 2D Modelling of a local water flow. Some numerical results (Chieti, Italy) (IT ISSN 0394-3356, 2008). The paper describes a first step towards the elaboration of flexible numerical research tools to study fluvial erosion, sediment transport, Fluid-Structures Interaction (FSI), through the application of the Computational Fluid Dynamics (CFD) and the Finite Element Method (FEM) techniques. A preliminary but necessary item to be explored is the analyses of the robustness of both mathematical and numerical features of the selected equations and the implemented algorithms. The unsteady Navier-Stokes equations, aimed to model a local 2D water flow, are introduced. To include turbulence, the URANS (Unsteady Reynolds Averaged Navier Stokes) Two Equations κ - ε model, with the Wall-Functions proposed by S. FAN et alii (1993), have been employed. At this step, important phenomena, such as free-surface influence, bed and banks morphologies changes, coupling between particles and fluid flow, have not considered. Then the adopted numerical approach is discussed. Spatial discretization is carried out by the linear Finite Element Method (FEM). A structured meshing with h like adaptability algorithm was developed. Thus to avoid velocities and pressure instabilities related, respectively, to convective mode predominance respect to diffusive mode and incompressibility constraint (divergence free flow), the Characteristic-based split (CBS) algorithm and the method of Artificial Compressibility (AC) have been applied. The Dual Time Stepping method has been adopted with Internal External Local Time Stepping. The three steps of the CBS based on AC or the CBS-AC method (NITHIARASU & LIU 2006) are then applied and discussed. As first step, only some steady state parametric numerical experiments have been performed, considering a semi-implicit, approach. Poiseuille flow test has been carried out also to clarify and to investigate in depth CBS performance. Both laminar and turbulent regimes have been considered. The numerical results of the selected turbulence modelling are compared with some simple analytical expressions, valid for Poiseuille flow. Useful and important suggestions regarding the numerical tuning of some intrinsic parameters, specific to the selected algorithms, are acquired through the discussion of the general performance of the implemented algorithms. Key words: CBS stabilization, RANS turbulence, numerical modelling.