چكيده به لاتين
Excessive scour close to vertical breakwaters may ultimately lead to instability of these
structures. Scour is caused to dig out the bed in the direct vicinity of breakwaters due to local accelerations and decelerations of the near-bed water velocities and the associated turbulence leading to an increase of the local sand transport capacity. The key mechanism of scour is the steady streaming caused by the action of partially or fully standing waves in front of vertical breakwaters.
The unconstructive effect of scour has provoked many researchers to evaluate the scour
pattern via both experimental and numerical methods. However, the experimental study is
rather difficult because it can be very time and space consuming, costly, and acquire the
robust data is quite involved. On the other hand, the development of computer hardware and numerical solution methods has persuaded the numerical models as other good alternative to study the scour process in the vicinity of breakwaters.
In this study, a Lagrangian two-phase flow model was developed to simulate the scour
process in front of a vertical breakwater under action of different wave conditions. Both fluid and sediment phase were simulated by implementing two different particle-based approach.
The fluid phase was simulated using two-dimensional Navier–Stokes equations based on
Smoothed Particle Hydrodynamics (SPH) formulation in conjunction with the Sub Particle
Scale (SPS) turbulence closure model. The sediment phase was simulated by using Newton
second law via the Movable Bed Simulator (MBS) Model. The effects of interparticle and
particle-wall collisions were considered with activating a spring-dashpot system. The results are verified by experimental data. Comparison between the results of two-phase flow model and experimental measurement confirms that the numerical model successfully predicts the bed configuration and maximum scour depth. The numerical model demonstrated an extra recirculating mode of sediment transport similar to the steady streaming re-circulating cells in the fluid phase, which directly affected the scour hole. The results show that by increasing the steady streaming velocity, the deposition rate and the length of scour hole were increasing.
Also in this study, the previous model was developed to simulate the scour in front of
breakwaters with different slopes. For coupling the fluid and the sediment phases, we used
the steady streaming velocity instead of the wave orbital velocity through a one-way coupling procedure. This technique not only efficiently reduced the simulation time but also practically matched the numerical model to the experimental model. The 2D scour at the trunk section of breakwaters was estimated for the case of rough materials. Based on experimental data, a vertical-wall breakwater and two breakwater models with slopes of 1:1.2 and 1:1.75 were engaged for the numerical study of the 2D scour. Comparison between the numerical results and experimental measurement confirmed that the numerical model successfully predicted the bed configuration and maximum scour depth. It was found that the maximum scour depth of the sloped breakwater was smaller than that of the vertical breakwater case. In fact, the scour depth decreased with lowering the slope of breakwaters Moreover, two equality conditions of scour/deposition process was proposed and taken to account through a new criteria as the time changing of the mass center of sediment particles.
Keywords: 2D scour, vertical and sloped breakwaters, numerical two-phase flow model,
numerical particle method.
