Numerical Simulations Of The Flow Field Around Artificial Reefs With Truncated-cone Shape

For more than decade, marine ranching, characterized by the construction of artificial reefs and artificial propagation releasing, has become an important measure to restore and protect the fishery resources in the offshore area of our country and remarkable achievements have been made. Artificial reefs, built in the proper sea area, change the flow pattern of local waters and thus improve the biological and fish habitat environment, which promotes the restoration of the fishery ecological environment and achieves the proliferation of fishery resources. Artificial reefs improve the ecosystem through their effects on the flow field in the local area. The flow field should be modified by artificial reefs in such a way that it enriches the food organisms to lure and gather fishes,The flow field around artificial reefs is mainly affected by the reef shape and their deployment configuration, as well as the local topography and current. It is of key significance, for the selection of reef shape and optimization of their arrangement, to make a good understand of the hydrodynamic characteristics of reefs.In the present work, based on the theory of computational fluid mechanics, the flow field around artificial reefs with truncated-cone shape is numerically simulated by means of the software FLUENT. The flow phenomena are generated by the reefs, such as the upwelling, back vortexes, flow separation and vortex development, are analyzed and discussed. The design plan of optimization and corresponding structural parameters of truncated-cone shaped reefs are suggested.Firstly, the maximum upwelling speed, normalized by the velocity of the free incoming stream, increases with the increase of the incident slope until reaching a peak value and then it decreases as the slope further increases. The normalized average upwelling speed varies from 0.14 to 0.18 without significant fluctuations. The maximum height of upwelling flow first decreases and then grows with the increase of the incident slope. The flow pattern of upwelling is optimally affected when the incident slope of reefs takes values between 200% and 400%.Secondly, a series of numerical simulation is made for various reef heights, incident slopes and reef openings to investigate the effects of these factors on the flow field. As a result, a group of parameters for the optimal design of truncated-cone shaped reefs are proposed, i.e. the reef height is 2.4m while the diameter of the upper and lower plane is 1.5m and 3.0m respectively with a slope 320%. The optimal width of openings on the reef surface is equal to 0.1 m.Thirdly, the flow field around two truncated-cone shaped reefs, aligned longitudinally in the direction of the free stream, is simulated. Optimal space between reefs is 1-1.5 times of bottom diameter. The maximum upwelling height, the value of which is generally greater than that of single-bodied reef, first increases and then decreases with the increase of space between reefs. The maximum upwelling speed is smaller compared with that of single-bodied. The flow inside the reef and in the area of back vortex presents a complicated pattern. The size of back vortexes is not apparently affected by the speed of the incoming flow. With the generation of various vortexes, the water layers in the wake of back vortexes are intensively mixed up to form a chaotic flow pattern as required.Finally, tank experiments are performed to verify the numerical results. Comparison between numerical and experimental results shows that the relative error is within twenty percent for the more than eighty percent of measuring points. It may be concluded that the numerical simulation is a method fairly reliable.