Computations on driftwood jamming around obstacles with a 3D-3D model

We developed a new three-dimensional computational model for simulating driftwood behaviour, including collisions and stacking around bridge piers. In this model, both river flow and driftwood motion were modelled three dimensionally (3D-3D model). The model of water flow was URANS type, and the driftwood was expressed as connected small spheres. The collision between driftwood was evaluated by the discrete element method (DEM). Computations were carried out under the same conditions as the previous experiment. In the experiments, the stacking of driftwood around obstacles follows 2D or 3D patterns, depending on hydraulic conditions. We proposed the “modified” driftwood Richardson number (DRI) as a dimensionless number governing the three-dimensionality of driftwood jamming. We also defined “capture ratio” (number of captured driftwood pieces/number of supplied pieces) and “sunk ration” (number of sunk pieces/number of stacked pieces) to quantify 3D motions. Computational results showed that the capture ratio increases, and the sunk ratio decreases with the increase of DRI. The tendency was in qualitative agreement with the experimental result. However, the present computation overestimated the three-dimensionality of driftwood jamming.