Numerical Simulations And Laboratory Experiments Of Hydrodynamics For Artificial Reefs

Environmental degradation and overfishing have caused several of the world fisheries to decline. At the same time the demand for fishery products is expanding globally. In order to meet the shortfall, marine ranching is being used in the worldwide to sustain continued production from the marine environment. Artificial reefs are manmade structures that are placed on the seabed deliberately to mimic some characteristics of a natural reef. Artificial constructions have been applied to increase fish abundance and diversity, prosper recreational fishery and reduce coastal erosion. The structural design of artificial reef is closely related to the hydrodynamic characteristics itself. The obvious mutual effect between artificial reef and flow along with waves are the main hydrodynamic process for artificial reef on the seabed, which primarily involves two aspects:flow field effect and physical stability. About the studies of the hydrodynamic capability of artificial reef, the traditional fishing gear hydrodynamics and flume tank experiments are used by researchers. Nevertheless, these methods may be affected or limited by experimental method and precision, theoretical profundity, and the accuracy of the research, etc., which can’t meet the requirements of complete understanding for the hydrodynamics of artificial reefs.This paper aimed at hollow and solid artificial reef model, based on computational fluid dynamics (CFD), utilized Fluent CFD code to simulate the hydrodynamic capability of single and binary artificial reefs qualitatively and quantitatively. Three dimensional flow field effects, pressure distribution and resistance characteristics of different shaped and number artificial reefs are obtained. Non-invasive particle image velocimetry (PIV) laboratory measurements are employed to verify the simulation results. There is good agreement between the simulation and the experimental results regarding the flow fields. Consequently, it ensures that the two methods can verify each other and gain more reliable results.Firstly, a three-dimensional turbulent flow numerical model is set up to calculate the unsteady flow field around hollow cube and star-shaped artificial reefs; the model is based on Reynolds-averaged Navier-Stokes (RANS) equations embedded with a renormalization group (RNG) k-c turbulence model. RANS equations are solved using the finite volume method (FVM) on the unstructured tetrahedral mesh. In order to validate the simulated results, non-invasive particle image velocimetry (PIV) measurements are conducted to measure the flow patterns. The comparison shows a good agreement between the numerical and experimental results. On the basis of the numerical model validation, the flow fields of artificial reefs with different altitudes and layouts were analyzed using the numerical model. In addition, the PIV flow fields test results for compound M, spherical type, V-shaped and circular tube artificial reefs are introduced and discussed in detail.Secondly, by similar three-dimensional numerical model above and PIV technology, the unsteady flow fields around two binary hollow cube and star-shaped artificial reef models are obtained. The consistent results between them can more indicate that both numerical computation and PIV data in this paper achieve a precision to some extent. The effects of combining form and spacing on the flow fields’distribution around two kinds of artificial reefs are acquired by numerical simulation technology. The influence of the arrangement (parallel and vertical) and spacing on the upwelling and horizontal flow field of the two hollow cube artificial reefs were discussed using the results of the numerical method. In the meantime, the effects of increasing streamwise spacing on the flow fields between two star-shaped artificial reefs are elaborated and analyzed.Finally, the stability of artificial reefs is the most significant guarantees to achieve their functions. The stability largely depends on surrounding physical environment and the interaction among wave, flow and artificial reef system. Based on the wave and current flume tests, the drag force of single and two artificial reefs with different spacing distances are obtained. In addition, water flow resistance numerical model is set up to calculate the drag force of artificial reefs under current action. There is a good agreement between the simulation and the experimental results regarding the drag forces. On the basis of experimental verification, the relationship of the drag coefficient and Reynolds number (Re) is obtained. In order to determine the risk assessment of the hollow cube and star-shaped artificial reefs in the bottom of the sea, anti-drift coefficient and anti-roll coefficient are introduced to measure their stability, which provide sufficient technical support for the development of artificial reef career.