Comparative analysis on flow field effect of general artificial reefs in China

Abstract: Artificial reef (AR) is widely used to improve marine ecological environment. Some fisheries developed countries, such as America, Japan, Korea, Australia, New Zealand and some EU (European Union) states, have acquired a lot of achievements on the exploitation and protection of fishery resources by use of ARs. Flow field effect, bait effect and avoidance effect will be caused when the ARs are placed on the sea floor. Flow field effect is deemed to the main affecting mechanism of ARs, and the upwelling and back vortex are important indices to measure the flow field effect of ARs. So far, the study on the flow field effect of ARs has been limited to one or several reefs in China. However, no one has yet scientifically investigated the flow field effect of the Chinese general ARs. This study focused on the 18 types of general ARs, which were divided into 6 kinds, to investigate different flow field effects among them. And several representative reefs were selected for further study, which was the study on the effects of various flow velocity on the characteristics of upwelling and back vortex. Based on theory of computational fluid dynamics, numerical models of turbulent flow were built. Three-dimensional Navier-Stokes equations were solved by the finite volume method. The software CFX (computational fluid X) was used to study the performances of the flow field around reefs when flow passed through them. The 2 evaluation methods, the reef height ratio and the concrete volume ratio, were used to analyze the upwelling area, the back vortex area and the height of upwelling on the central plane. The reef height ratio is the ratio of the maximum upwelling height, upwelling area or back vortex area to reef height. And the concrete volume ratio is the ratio of the maximum upwelling height, upwelling area or back vortex area to concrete volume of reef. The paper was aimed to discuss the advantages and disadvantages of the 2 methods, and to analyze the different flow field effects among 18 types of ARs. Besides, the better reefs would be selected for diverse requirements. The results suggested the concrete volume ratio was a better criterion when studying the upwelling area and the back vortex area, and the height ratio was more appropriate when studying the height of upwelling. Regardless of the upwelling or the back vortex, triangle reefs had the largest relative area and complex reefs were the second, while frame reefs were the least. Then one reef was selected from each type to conduct the research to find out whether there were the effects of different flow velocity (0.2, 0.4, 0.6, 0.8 and 1.0 m/s) on upwelling area, back vortex area, the maximum upwelling velocity and maximum height of upwelling. The results demonstrated that the height of upwelling, the upwelling area, and the back vortex area fluctuated in an extremely narrow range under the 5 flow velocities. The maximum flow velocity of upwelling was linear with the flow velocity, and the slope showed the difference among different reefs. Moreover, reefs for different demands had been selected. AR11, AR10 and AR13 had a larger back vortex area with low cost. AR13, AR11 and AR18 should be chosen for the need of larger upwelling area. AR11 had not only the largest relative area of back vortex but also the largest relative influence area (sum of the upwelling area and the back vortex area). In addition, AR11 had the highest height of upwelling at the same reef height (followed by AR16 and AR02). The top 3 types of reefs with the maximum flow velocity were AR16, AR10 and AR13. The study provides the theoretical basis for the selection of AR, which can cater to various demands. Moreover, it also has certain reference value for the design of AR.