Effects of different salinity on water delamination under laboratory conditions

Salinity stratification has caused the uneven distribution of nutrients in the water body for aquaculture. A barrier layer can be normally formed to hinder the exchange of quality and energy, leading to the deterioration of water quality. Therefore, stratification can pose a great threat to the growth of organisms in the water body. In this study, two kinds of water bodies were dyed with different salinity in a glass tank in the laboratory. Four stages of water body delamination were observed: preparation, start-up, mixing, and homogenization, when depending only on buoyancy. Among them, the pressure difference was driven to form the upwelling convection of the "upwelling" type. The upwelling was then floated up to the layered interface for the mixing and thick thermocline. Eventually, the layered water body triggered the instability and destruction. The experimental system consisted of a glass tank, a backwater collecting tank, a water injection, and a backwater part. Specifically, the glass tank was the main body of the system. The backwater collecting tank was composed of sample bottles, whose upper edge of the horizontal position was aligned with the experimental tank. The water injection part also included the small submersible pump, diverter, plastic hose, and acrylic pipe. The water in the return collection tank was firstly delivered by the small submersible pump, and then carried to the predetermined water layer of the test tank via the diverter and plastic hose. In the backwater part, the inlet of acrylic connectors was set on the surface of the experimental water tank, while the outlet was on the bottom of the backwater tank. The water level of the experimental water tank was kept stable by the siphoning during operation. The salinity change was recorded on the surface, subsurface, middle, sub bottom, and bottom layer. Five groups of sensors were used to online monitor the process, when the water pipe was delivered the fresh water of different flows to the bottom layer of the tank. The results show that there was a significant impact of the water delivery flow on the delamination of the water body in the tank. Furthermore, there was the strongest mixing effect of 100 L/h flow, and the shortest time of complete mixing. By contrast, there was the weakest mixing effect of 25 L/h flow, and the longest time of complete mixing. A large amount of experimental data was summarized to obtain the fitting formulas. The function curve was then achieved in the impact of water delivery on the start-up time of the sub bottom, bottom, sub surface, and surface layer. A specific relationship was obtained between the water delivery volume and the time required for the full mixing under the given working conditions in the laboratory. The layered destruction of the water body was summarized to determine the influence of the water delivery flow on the salinity, starting, and mixing time of the water layer. The finding can also provide a strong reference for aquaculture production.