Numerical Simulation On Features Of Flow Field And Transport Of Nutrient Around Hollow Artificial Reefs

Artificial reef was one or more objects of human origin or natural deployed purposefully on the seafloor to influence biological and physical marine resources, and it was the important component of the construction of marine ranching engineering. Similar to other marine structures, artificial reef were affected by the hydrological environment, and then the flow field effect was closely related to the reef inherent water dynamic performance. Current research on artificial reef mainly involved biological and non-biological factors, productivity, spatial and temporal changes in flora and fauna, the three major benefits, location and layout, recreational fisheries and proliferation resources. With the development and application of computational fluid dynamics, CFD was gradually used to explore the artificial reef surrounding unsteady, unstable flow around a blunt body, to found the numerical solution of this problem.In this paper, two different types of hollow artificial reef for the study, based on the theory of CFD and Fluent software, using three-dimensional turbulence simulation techniques to numerical simulation the flow field effect of artificial reef, the flow field distribution of horizontal section and vertical section was acquired, the flow field structure of artificial reef was revealed, the numerical results with classic numerical experimental and model test were analyzed, the results were relatively consistent.First, the water flow pattern around monomer hollow trapezoidal and hollow square reef in unsteady flow effects were investigated based on Fluent numerical simulation. The maximum height of upwelling was about 2.12 to 2.22 times of reef height, the maximum and average velocities of upwelling were about 0.58 to 0.67 and 0.17 to 0.19 times of incoming velocity. The length of wake vortex region was about 5.28 to 9.04 times of reef height, the width of wake vortex region was about 1.40 to 1.75 times of reef width, and the average velocity of wake vortex region was about 0.35 to 0.36 times of incoming velocity.Secondly, the three-dimensional flow field of different types reefs in unsteady flow effect were investigated. The scale and intensity of upwelling around trapezoidal reef was smaller than square reef, the scale of wake vortex region around trapezoidal reef was smaller than square reef, the average velocity of wake vortex region around trapezoidal reef was larger than square reef. From the point of flow field control effect, under the same simulation condition, the square reef than trapezoidal reef could play a better environmental and resources repair function.Thirdly, the three-dimensional flow field of hollow trapezoidal and hollow square reef under different placement pitches in unsteady flow effects were investigated based on LES turbulence model. Placement pitch in a certain range, the scale and intensity of upwelling with increasing of layout spacing increase first and then decrease, when the placement pitch L=5m,the scale and intensity of upwelling have reached the maximum. With increasing of layout spacing,the length and width of wake vortex region showed an increasing trend, when the placement pitch increased to L=7m, the impact area of wake vortex region have reached the maximum, but its vortex intensity was reduced when setting distance L=5m. When the placement pitch L=5m, the reefs could play the best role in regulation of flow field, and square reef than trapezoidal reef could play a better ecological regulation function, further evidence of previous studies.Then, according to the experimental data of 2012-2013 survey in Xiapu sea reefs, the water flow pattern around three different combinations of structural units of reef in unsteady flow effects were investigated based on CFD numerical simulation, and the vertical transport fluxes of various nutrients were estimated. The average vertical fluxes of PO43--P, NO3--N, Si O32--Si were 241.6 mg/(m2?d), 1257.7 mg/(m2?d)、4445.8 mg/(m2?d) in trapezoidal reef area; the average vertical fluxes of PO43--P, NO3--N, Si O32--Si were 438.3 mg/(m2?d)、2212.5 mg/(m2?d)、7288.3 mg/(m2?d) in square reef area; and the average vertical fluxes of PO43--P, NO3--N, Si O32--Si were 392.4 mg/(m2?d)、2275.6 mg/(m2?d)、7148.9 mg/(m2?d) in ship reef area. The transport fluxes of nutrients in square reef area was the largest, with previous estimation results of flow field effect was consistent.