Efficient non-hydrostatic modelling of flow and bed shear stress in a pier scour hole

摘要

Predicting 3-D flow in a pier scour hole and the associated bed shear stress τ_b is important for the safe and economical design of bridge piers. This paper combines layered, hydrostatic hydrodynamic computations with non-hydrostatic pressure corrections, exploring a new modelling approach for efficient and reliable predictions of 3-D flow velocity. The law of the wall method is used for estimating τ_b. Its suitability for incorporation into layered models for bedload transport and pier scour simulations is also discussed. The predicted flow shows realistic features: strong downward flow adjacent to the upstream nose of a circular pier, vortex motions in the vertical and horizontal direction, and meandering flow wakes. The velocity results compare well with available experimental data. In the approach region, τ_b is uniform. It attains a local maximum immediately before flow enters the scour hole and then drops non-linearly in the scour-hole region toward the pier. In the wake region,τ_b has very low values. The τ_b predictions are consistent with the experimental data. In multi-layer models, when applying the law of wall method, one should use near-bed velocities as opposed to bottom-layer velocities to obtain more reliable τ_b estimates and avoid noisy results, which can cause a numerical instability problem in bedload transport simulations.