Author(s): Xinqian Leng, Pierre Lubin, Hubert Chanson
Linked Author(s): Xinqian Leng
Keywords: Tidal bores, CFD modelling, Large Eddy Simulation LES, physical model validation, turbulence
Abstract: A tidal bore may form during spring tide conditions when the tidal range exceeds 4 to 6 m in a natural estuary with a funnel shaped river mouth and shallow initial water level. The propagation of tidal bores is a highly unsteady turbulent process associated with intensive sediment scouring and mixing. To date, few physical and numerical studies documented the unsteady turbulent process of tidal bore propagation. Recent numerical CFD models lacked careful experimental validations. The present study aims to provide new results on CFD numerical modelling of tidal bore propagation with a wide range of Froude numbers (1. 2 to 2. 1) and systematic experimental validations. The model solved the incompressible Navier-Stokes equations in its two-phase flow forms using Large Eddy Simulation (LES). Both 2D and 3D simulations were conducted; the inlet turbulence of the 3D models was simulated by a Synthetic Eddy Method (SEM). The physical experiments were based upon an ensemble-average technique, with measurements of water depth and velocity repeated 25 times for each flow condition. The 2D CFD simulations showed good agreement in terms of free-surface elevations with experimental results, for the range of tested Froude numbers. Mesh grid refinement only improved the accuracy for some but not all flow conditions. The 2D velocity data showed qualitative and quantitative agreement, but only at a selective range of vertical elevation beneath the free-surface, where the inlet velocity were correctly reproduced. The 3D simulation highlighted a numerical boundary layer, the thickness of which agreed with the experimental results. The time-averaged velocity and velocity RMS of the numerical model data showed a closer agreement with the physical model outside the boundary layer. The development of the numerical boundary layer was clearly observed in the CFD model results
Year: 2017