Hydrodynamic performance investigation on an Upper and Lower Water Exchange Device

Water exchange is an important way to guarantee its good quality between surface and bottom seawater, and it plays a significant role in water environment and ecology as well as fish farming systems. The conceptual experiment (Yin et al., 2018) was extended to present an Upper and Lower Water Exchange Device for strengthening the vertical water exchange performance, and a 3-D CFD mathematical model combining with Reynolds-Averaged Navier-Stokes equations, standard k-ε equations and VOF technique was developed to investigate its hydrodynamic performance and the vertical water exchange capacity for regular waves. The extrapolated relative error and the grid convergence index were used to estimate the discretization error. A series of experiments were conducted to measure the wave surface and water velocity histories, the experimental data were used to validate the mathematical results, and they agreed well with each other. It was found that the inside water motion is determined mainly by the wave overtopping and water entrainment. Generally, the net flow rate decreases at first and then increases with increasing wave steepness. In addition, the ratio of crest freeboard height to incident wave height plays a significant role in water exchange performance. A small ratio leads to an upward movement of the inside water, whereas a high value contributes to a downward net flow to some extent. Finally, a simple formula on the net flow rate was deduced with the dimensional analysis method and the least squares method, and it was validated well with the related data.