Knowledge of the distribution and variation of water turbidity directly represent important information related to the marine ecology and multiple biogeochemical processes, including sediment transport and resuspension and heat transfer in the upper water layer. In this study, a neural network (NN) approach was applied to derive the water turbidity using the geostationary ocean color imager (GOCI) data in turbid estuaries of the Yellow River and the Yangtze River. The results showed a good agreement between the GOCI-derived turbidity and in situ measured data with a determination coefficient (R2) of 0.84, root mean squared error (RMSE) of 58.8 nephelometric turbidity unit (NTU), mean absolute error of 25.1 NTU, and mean relative error of 34.4%, showing a better performance than existing empirical algorithms. The hourly spatial distributions of water turbidity in April 2018 suggested that high turbidity regions were distributed in the Yellow River estuary, Yangtze River estuary, Hangzhou Bay, and coastal waters of Zhejiang Province. Furthermore, the relationship between water turbidity and tide were estimated. A defined turbid zone was defined to evaluate the diurnal variations of turbidity, which has subtle changes at different times. Our results showed an inverse relationship between turbidity and tide over six selected stations, i.e., when the value of turbidity is high, then the corresponding tidal height is usually low, and vice versa. The combined effects of tidal height and tidal currents could explain the phenomena, and other factors such as winds also contribute to the turbidity distributions.
Cavitation is a complex flow phenomenon that hinders the efficient, safe, and stable operation of hydraulic machinery. To investigate the effect of cavitation on energy performance and flow characteristics of hydraulic machinery, cavitating flow in a slanted axial-flow pump based on entropy production theory and vortex dynamics is studied. The results show that the impeller chamber is a primary region of cavitation and energy loss generation under different cavitation conditions, including the incipient, growing, and wedge-shaped cavitation stages. In the incipient cavitation stage, as degree of cavitation strengthens, the flow at the impeller blade is smooth with little cavitation, and the variation in entropy production is constant, resulting in a stable energy performance. As it evolves into the wedge-shaped stage, the cavitation grows from the tip region near the impeller blade to the hub. At this time, the entropy production increases in the impeller chamber, resulting in a drop in energy performance. Meanwhile, flow separation appears at the impeller blade, and a secondary tip leakage vortex is promoted. The region with high vorticity basically matches the region with the high local entropy production rate. According to the relative vorticity transport equation, compressibility of cavitation strongly affects the relative vorticity in the impeller chamber, indicating that cavitation indirectly increases entropy production and energy loss by affecting the vorticity distribution, resulting in the drop in energy performance.
The siphon outlet is widely used in pumping stations due to its reliable and convenient cut-off performance. Long siphoning time or high hydraulic loss caused by the inappropriate design of the siphon outlet can significantly affect the safety of stations. The air compressibility volume-of-fluid (VOF) method is conducted to simulate the two-phase flow in the siphoning formation process at the design points selected by the optimal Latin hypercube design (OLHD), the results of which show good agreement with the experimental data. In this work, the siphoning time and hydraulic loss coefficient are selected as the objective functions, and a multi-objective shape optimization strategy is proposed for the siphon outlet in conjunction with the response surface method (RSM). This optimization strategy can not only reconcile conflicting objective functions but also obtain the effect and interaction of design variables. Sensitivity analysis on the constructed response surface models indicates that among three design variables, the aspect ratio has the greatest effect on the objective functions, the descending angle has the second greatest effect, and the ascending angle has almost no effect. Compared with the original design, the hydraulic loss coefficient and siphoning time of the optimized design are reduced by 2.95% and 26.76%, respectively. A higher vorticity magnitude and more uniform outflow are created in the optimized design, which results in the improvement of hydraulic performance.
As a low-cost scheme for small-scale hydropower generation, PATs are used at different hydrosites around the world. Nevertheless, considerable studies on PAT performance have mainly focused on the centrifugal type, despite the fact that axial-flow type has a comparatively large flow capacity, thus disposing of higher power density. Therefore, this article seeks to investigate the flow dynamics of an axial-flow PAT and associated energy loss characteristics, under both pump and turbine modes. It adopts the numerical simulation method, and uses entropy production theory to propose an energy loss intensity model in the cylindrical coordinate system, which quantitatively gives the spatial variation pattern for energy losses in pump and turbine modes. In addition, the correlation between energy loss and flow instability is deeply analyzed, where the energy characteristics in pump and turbine modes are quantitatively evaluated. It is shown that the energy loss within the impeller and the guide vane flow fields, for both operating modes, is mainly composed of the turbulent entropy production. The proportion of direct entropy production and wall entropy production is found to be relatively small. The velocity gradient, flow vorticity, turbulence intensity, and energy losses within the flow passages of the axial-flow PAT have been closely related. However, owing to the difference in PAT operating modes, there is a significant difference in the location of energy losses. The unstable flow phenomena such as the impact at blade inlet, flow deviation at blade outlet, flow separation, back-flow, and vortex are the main reasons for entropy production.
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