3D numerical simulation of dam break waves over erodible beds: the role of turbulence and mesh size.

Kamal El kadi Abderrezzak, Riadh Ata, Pablo Tassi, Jean-Michel Hervouet, Dongchen Wang, Qinghui Zhang

Monday 29 june 2015

17:33 - 17:36h at Mississippi (level 1)

Themes: (T) Sediment management and morphodynamics, (ST) Sediment transport mechanisms and modelling, Poster pitches

Parallel session: Poster pitch: 3A. Sediment - Erosion

Three-Dimensional (3D) numerical simulations of dam-break flows over erodible beds are performed with the open source Telemac-Mascaret modeling system (www.opentelemac.org). The hydrodynamic model, Telemac-3D, internally coupled with the sediment transport module Sisyphe, is used. Telemac-3D is based on the solution of the Reynolds Averaged Navier-Stokes (RANS) equations with a non-hydrostatic pressure distribution. Bed-load rate is calculated using the Meyer-Peter and Müller (1948) formula with a critical dimensionless shear stress of 0.047. Bed elevation is updated using the sediment continuity equation. The roughness Manning coefficient is estimated on the basis of the bed material diameter. Resolution of the governing equations relies on a finite element method on an unstructured mesh of tetrahedral elements. The computational domain was discretized with prismatic elements, obtained by first dividing the 2D domain with non-overlapping linear triangles and then by extruding each triangle along the vertical direction into linear prismatic columns that exactly fitted the bottom and the free-surface. Then, each column was partitioned into non-overlapping layers, requiring that two adjacent layers comprised the same number of prisms. Emphasis is focused on investigating numerically the effect of turbulence models and mesh size. In this work, the turbulence stresses are modeled with the aid of the Boussinesq relations and the performance of four isotropic turbulence closure models is evaluated: the constant viscosity model, the standard k–_ model, the Smagorinsky-Lilly subgrid scale model and the mixing length model. Results are compared with published experimental data on dam-break flows (Fig. 1). Comparisons between numerical results and experimental data are based on water level evolution measurements at selected gauging stations and bed topography changes. Satisfactory results are obtained using the standard k-_ model (Figs. 2 and 3). The mesh refinement (by increasing the number of layers in the vertical direction and/or the number of nodes in the 2D unstructured triangular horizontal mesh) allows only a slight improvement of the computed final bed geometry.