2019 Volume 75 Issue 2 Pages I_289-I_300
Hydraulic experiments and numerical predictions were conducted for the unsteady process of local scour on the gravel bed, in which the average diameter of gravel particles is about 7 mm, caused by an impinging vertical water jet. The average velocity of the vertical jet was about 1.2 m/s and the flow was stopped at t = 3.18 [s]. In the computations, the fluid-solid interactions are taken into account in a similar way to the so-called an immersed domain method. This method was introduced into our parallel computational method in which governing equations are discretized with a finite volume method in the three-dimensional collocated grid system. The parallel computations were conducted for the local scour with 16,700 gravel particle models with 1,088 processes. The unsteady process of the local scour is categorized into three stages, (A) unsteady-scouring stage, (B) saltation-collapse equilibrium, and (C) stationary state with angle of repose. It was confirmed that the calculated gravel-bed shapes in all three stages are in good agreement with the experimental results. In particular, it was shown that the numbers of rising saltation particles and falling collapsed ones are approximately equivalent and that as a result almost uniform scoured surfaces are maintained during the saltation-collapse equilibrium. In addition, the total traveling lengths of the gravel particles were tracked in a Lagrangian way with the computational results. The distributions of fluid pressure were obtained in the computations and the pore water pressure near the scoured area is estimated higher than the initial hydrostatic pressure.
