Assessing different methods of generating a three-dimensional numerical model mesh for a complex stream bed topography

Three-dimensional numerical models of flow over complex bed geometry are becoming widely used in river and coastal engineering. Boundary-fitted coordinate grids are typically used to deal with this problem in natural channels. Recently, a regular structured grid method based on numerical porosity has been developed for high-resolution gravel-bed models. A simpler alternative approach is to use a Cartesian mesh with an interpolated 3D solid object representing the river bed. The objective of this study is to assess the impact of these three methods of mesh generation on the simulated flow field above complex bed topography around stream deflectors in a laboratory setting. Results show marked differences between the three types of simulations when running mesh sensitivity analysis. Because the bed porosity approach uses the digital elevation model (DEM) information in each cell to represent bed topography, it requires a finer mesh resolution in order to reach an accurate solution. Although there was good qualitative agreement between the simulated flow fields and Acoustic Doppler Velocity measurements, the quantitative comparison showed relatively poor agreement for all mesh design types. However, the three mesh types were in good agreement when compared to each other for velocity and pressure variables (average correlation coefficient, r, of 0.95), with the 3D object bed and bed porosity showing the best agreement (r =0.97). Simulated turbulence variables (KE and EP), however, showed more scatter (r =0.85) and slopes markedly different from unity.