Numerical Simulation of the Effect of Inflow Velocity on the Flow Field Characteristics of Circular Circulating Aquaculture Ponds

Abstract

To meet the growing demand for animal food among the population, China has proposed the strategy of building a "Blue Granary, " relying on marine space, marine biological resources, and the application of modern marine high-tech. Industrialized recirculating aquaculture (also known as land-based factory aquaculture, factory aquaculture, or industrial fish farming) has the advantages of high production efficiency and small land occupation. It is a high-density, high-yield, high investment, and cost-effective aquaculture method. The recirculating aquaculture system is in line with the "Blue Granary" strategy, effectively reducing water pollution while ensuring food security. Therefore, in recent years, recirculating aquaculture in China has developed rapidly. In this context to achieve intelligent regulation of the inflow velocity of industrial recirculating aquaculture, this article firstly summarizes the three main factors that affect the inflow velocity from previous research: The flow field velocity of the aquaculture pool, the discharge velocity of the bait (residual bait), and the velocity of temperature regulation. With the improvement of computer software and hardware, computational fluid dynamics (CFD) is gradually being applied in various fields. CFD provides cheap tools for simulation, design, and optimization, as well as tools for analyzing three-dimensional complex flows. In complex cases, measurements are often difficult, even impossible, and CFD can easily provide detailed information on all flow fields. Compared with conventional experiments, CFD has the advantages of no restrictions on parameters, lower cost, and no interference in the flow field. This method can be used in aquaculture to solve the problems of temperature control effects, solid particle emission efficiency, and flow zone division that cannot be directly measured in actual production under different flow rates. At present, there are few studies reporting CFD numerical simulation of multiphase flow in aquaculture. Existing research focuses on the impact of the inlet and outlet structure and the shape of aquaculture ponds on the flow field, and the effect of sewage collection. There are few reports on the use of numerical simulation methods to design the regulation scheme of inlet flow velocity. Because of the advantages of CFD, we chose it for simulation. Secondly, the specific model used in the numerical simulation was determined through experimental verification, research discussion, and other methods. Based on this, grid independence verification was conducted, indicating that the simulation results under the number of grids in the text are not affected by the number of grids, and the simulation results are reliable. Thereafter, based on the above research, and taking Scophthalmus maximus as an example, the effects of different inlet flow rates on the flow field, sewage discharge, and water temperature regulation in aquaculture ponds were simulated. The results showed that the inlet flow rate significantly affects these three factors. Therefore, the inlet flow rate can be adjusted according to production needs. Based on the simulation results, a set of inlet flow velocity control schemes were proposed: For the feeding stage, low flow velocity can be used to reduce feed costs (inlet flow velocity = 0.2 m/s); after eating, the feed can be quickly discharged at a high flow rate for a short period of time (inlet flow velocity = 1.2 m/s for 20 s); at abnormal water temperature, different flow rates and times can be used to adjust the water temperature in the breeding pool (water inlet temperature = 15 ℃; an inlet flow velocity of 1.2 m/s can be used to inject water for 230 s, reducing the water temperature in the breeding pool from 22 ℃ to 18 ℃). Numerical simulation experiments can be designed based on this method for different aquaculture environments and organisms to determine the inflow velocity control scheme. Unlike traditional methods, the numerical simulation method proposed in this study for regulating the inflow velocity of recirculating aquaculture systems can be used to determine the inflow velocity control scheme at a lower cost. The method can directly measure the discharge of particulate matter, the overall water temperature, and the flow field in aquaculture ponds. This method is safer than the method of indirectly measuring water quality to determine the inflow velocity control scheme. In terms of regulation time, traditional methods are only accurate to the hour in regulating the inlet flow rate, whereas this method is accurate to the second, which makes it more reliable to determine the regulation of the inlet flow rate. Therefore, from the perspectives of cost, safety, and accuracy, it is better to use numerical simulation methods to regulate the inflow velocity of recirculating aquaculture systems. In actual production, the method described in this study can be used to determine the inflow velocity control scheme, which can be combined with the control system to achieve automatic control, reduce breeding costs, and increase breeding success.

Keywords

• discrete phase model
• particle distribution
• flow field characteristics
• water temperature regulation
• round aquaculture pond