Time-series changes in flow in a shallow rectangular tank with inflow and outflow

Although there is no inflowing river into the upper reservoir of a pumped storage power plant, sedimentation can still progress over time. Fine sediments contained in the water pumped up from the lower reservoir gradually accumulate, leading to reduced operational efficiency and making countermeasures necessary. However, the mechanism underlying this accumulation remains unclear. As a first step toward clarifying the sedimentation mechanism, this study aims to elucidate the internal flow in the upper reservoir during pumping and turbine operation. In our experiments, we used a shallow rectangular tank with a single opening that allows inflow and outflow. We investigated time-series changes in the surface flow through PIV (Particle Image Velocimetry) analysis. During the inflow stage, the incoming water was initially deflected to the left; viewed from above, a small counterclockwise vortex and a larger clockwise vortex were formed. Notably, when the inflow rate was relatively high, after flowing straight inward, the incoming water split to the left and right and circulated along the tank walls. As the water depth increased, the surface velocity across the entire tank gradually became more uniform. During the outflow stage, as the water depth decreased, the surface velocity over the entire tank again became nearly uniform. Once the water depth reached the height of the outlet, water began flowing directly toward the outlet. These findings indicate that multiple flow patterns emerge in the upper reservoir, depending on the water depth and inflow rate. It is inferred that such time-dependent flow patterns contribute to sediment deposition.