Numerical Simulation On Wave-affected Flow Field And Local Scour Of Artificial Reefs

Artificial reefs have the ability to renovate ecological environment as long as restore the habitat of marine life.So they have been wildly used in the near shore area to protect the marine fishery resources.Normally artificial reefs are put in deeper water,so their flow fields are only affected by the tidal current.In some circumstances,artificial reefs are also put in shallow waters for ecological renovating use,for instance,Tianjin Dashentang artificial project.It is located in the north of Bohai Sea,closed to the shore,and in shallow water.The flow fields of artificial reefs there are heavily affected by the waves.In the same time,site monitor for three consecutive years shows that the sediment deposit around artificial reefs there is getting more serious year by year,causing damage to the function of the reefs.Therefore,it is important to study waves-affected flow field and local scour around artificial reefs.A three-dimensional numerial wave tank has been developed using Fluent for simulate wave affected flow field and it has been tested and verified to proof its correctness.A method of measuring the scale of vortex to value the flow field of the artificial reef was introduced.The simulation was conducted for different wave height,period,and spacing between two reefs.The results show that the vortex scale increased 103% while the wave height changed from 0.5m to 0.9m under the same period.The ratio of vortex length L and reef side length l increased from 2.30 to 4.66.The vortex scale increased 130% while the period changed from 3s to 5s under the same wave height.And L/l increased from 2.30 to 5.32.When the spacing between two reefs was inside of a certain range,the scale of vortex around both reefs increased.When the spacing grew outside the certain range,the scale around both reefs becomes smaller and eventually stabilized to the size it was around single reef.A three dimensional scour model has been developed using Flow-3D for simulate local scour around artificial reefs.It has been validated against test data.Local scour around two different types of artificial reefs used in Tianjin Dashentang artificial reef project,the reef with large openings and the reef with large and small openings,under different stream velocity have been studied.The results show that the reef with large openings has weak downstream in front of it and can hardly move the sediment,lead to a smaller scour depth.On the contrary,the reef with large and small openings has strong downstream in front of it,lead to a bigger scour depth.When the current velocity was 0.3m/s~0.6m/s,the bigger the velocity is,the shorter the time it needed for reaching equilibrium scour depth,which doesn't change with the velocity.But when the current velocity is 0.7m/s,both rate and depth of the scour decreased.