Resolving large bed roughness elements with an unstructured hexahedral grid

A three-dimensional numerical model is used to compute the water flow field in a straight flume with a cobble bed. The geometry of the numerical model is a replication of a physical model experiment where the largest cobbles were approximately 10 cm long. Detailed information on the digital terrain model of the bed was available from a laser scan of the bed. In the experiments, flow velocity profiles and turbulence properties were measured with a 3D-Laser Doppler Anemometer at 24 randomly selected locations. For the numerical simulations, the bed roughness is resolved with a very fine unstructured grid with dominantly hexahedral cells, adapted to the larger stones. At the bed, also a few tetrahedral cells are used. The unstructured grid allows for a varying number of grid cells in the vertical direction, depending on the water depth. The grid has 72 million cells, covering a flume area of 2.1 m in the longitudinal direction, 0.71 m in the width and 0.21 m over the depth. The cell sizes are smaller than 2 mm in the horizontal directions and smaller than 1 mm in the vertical direction. Results from the numerical model are compared with the flume measurements on the basis of the measured velocity profiles.