Study On The Flow Field Characteristics And Hydrodynamic Performance Of Top Tilted Artificial Reefs

In recent years,the habitats of marine organisms in China’s offshore waters have been severely damaged.In order to repair the damaged habitats and conserve fishery resources,the construction of artificial reefs has been carried out on a large scale in coastal areas.As a kind of fishery ecological engineering facilities,artificial reefs will produce flow field effect,shading effect and biological effect after being put into the sea,which will have a prominent role in the conservation of fishery resources and ecological resources restoration.The size and layout of the reefs are closely related to the size and effect of the upwelling and backwash generated by the reefs,so the research on the flow field effect and hydrodynamics of the artificial reefs is crucial.Recent research on the flow field and hydrodynamic characteristics of artificial reefs has been limited to the size of reef openings and changes in the internal structure,while the effects of changes in the reef top on the flow field and hydrodynamics have rarely been reported.The scale and effect of upwelling generated by artificial reefs is not only related to the size of the reef opening,but also to the degree of tilting of the top surface.The design of inclined deflectors on the top surface of artificial reefs increases the volume of the cavity with little difference in the volume of the coagulum,resulting in better water permeability,which is conducive to the realisation of large eddy flows and is more conducive to the realisation of flow field effects while ensuring stability.Therefore,this paper adopts Fluent as the numerical simulation tool,and takes the top-tilted square artificial reef as the research object,and systematically investigates four frontal opening ratios(φ=0.2,0.3,0.4,0.5),four head-on angles(β=0°,15°,30°,45°)and five top-tilted angles(θ=0°,5°,10°,15°,20°).In order to investigate the effect of different spacing on the flow field of top-tilted artificial reefs,the flow field characteristics of double reefs with horizontal spacing(0.5L,1L,1.5L,2L)and vertical spacing(1L,2L,3L,4L)were investigated;the top-tilted square artificial reefs were also tested by the most dynamic flume model.The flow field characteristics and hydrodynamic performance of the square-shaped artificial reef model were experimentally investigated by the most dynamic flume model test method to further verify the reliability of the numerical simulation.The main results are as follows:(1)The variation of the incoming velocity from 0.2m/s to 1.5m/s has essentially no effect on the volume of the flow field effect and is shown to be essentially constant.(2)For the updraft intensity aspect.Control the top surface inclination angle of 15degrees and the incoming flow angle 0 degrees constant,change the frontal opening ratio0.2～0.5,the volume of the upward flow shows Vα(0.2)>Vα(0.3)>Vα(0.4)>Vα(0.5),the relative volume of the upward flow also decreases with the increase of the opening ratio;control the top surface inclination angle of 15 degrees,the opening ratio of 0.3,change the angle of the incoming flow from 0 degrees to The overall trend of the upflow volume is V_β(30)>V_β(45)>V_β(0)>V_β(15);controlling the angle of 0 degrees and the frontal opening ratio of 0.3 remains unchanged,changing the angle of inclination of the top surface from 0 degrees to 20 degrees,the relative volume of upflow generally increases with the increase of the angle of inclination of the top surface,that is,V_θ(20)>V_θ(15)>V_θ(10)>V_θ(5)>V_θ(0).In summary,at an opening ratio of 0.2,the best upward flow effect is achieved at a top surface inclination angle of 20 degrees and a head-on flow angle of 30 degrees.(3)For back eddy strength.Controlling the top surface inclination angle of 15degrees with the incoming flow angle of 0 degrees unchanged,changing the frontal opening ratio of 0.2～0.5,the back vortex volume shows as Vα(0.2)>Vα(0.3)>Vα(0.4)>Vα(0.5).The back eddy flow is V_β(45)>V_β(30)>V_β(15)>V_β(0),and the back eddy flow volume increases significantly from 0 to 15 degrees and from 30 to 45 degrees when the angle of top surface inclination is controlled at 15 degrees and the opening ratio is 0.3.The overall back eddy flow shows Vα(20)>Vα(0)≈Vα(5)>Vα(10)>Vα(15).In summary,the best effect of back eddy flow was achieved at an opening ratio of 0.2,a top surface inclination angle of 20 degrees and a head-on angle of 45 degrees.(4)Quantitative analysis of the flow field effect of the artificial reef was carried out,and the relationship between the permeability coefficient and the design parameters was obtained,i.e.γtt=-0.1803β+0.8352φ-0.2834θ+0.0464;the relative volume of the upwelling flow showed non-significant correlation with the incoming velocity,and showed a highly significant correlation with the velocity ratio taking and the permeability coefficient,and the equivalence between them was obtained,Vu1=44.656e^(-22.67VR),Vu2=0.0827V+5.4591,Vu3=8.065-37.92γtt+92.20γtt~2-69.79γtt~3;the relative volume of the back eddy shows a non-significant correlation with the incoming velocity,and a highly significant correlation with the velocity ratio and the permeability coefficient,and the equivalence between them is obtained relationship,Vb1=432.37VR^2-531.1VR+175.95,Vb2=0.3531V^2-0.1257V+25.328andVb3=35.25-98.94γtt+216.7γtt^2+64.1γtt^3.(6)Under different opening ratio conditions,the drag force on the reef decreases with increasing opening ratio,and the drag coefficient presents an increase with increasing opening ratio;under different headwater angles,the drag force on the reef presents as F_β(45)≈F_β(30)>F_β(15)>F_β(0),and the relationship of the drag coefficient is Cd(0)>Cd(45)≈Cd(30)>Cd(15);under different top inclination angles,the resistance of the reef body is presented as F_θ(20)>F_θ(15)>F_θ(0)>F_θ(10)>F_θ(5),and the relationship of the resistance coefficient is Cd(0)>Cd(5)≈Cd(10)≈Cd(15)≈Cd(20);the experiments meet the requirements of the safety coefficient of anti-slip and anti-overturning under all parameters and working conditions,and no slip and overturning will occur phenomenon.(7)When the angle of head-on flow is 0 degrees,the relationship between the drag coefficient and the opening ratio is obtained as follows:Cd=1.378-4.648γ＿tt+14.46γ＿tt^2(R~2=85.315,P<0.001);when the angle of head-on flow is not 0 degrees,the relationship between the drag coefficient and the through-opening ratio and the projected opening ratio is obtained as follows:Cd=0.1062+0.341γtt+3.737γtt+3.737γty(R~2=0.87,p<0.001).(8)When the reef is laid out horizontally,the upwelling volume and back eddy volume decrease gradually with the increase of laying spacing,and at the laying spacing of 0.5L,the upwelling volume and back eddy volume are larger,and the upwelling effect and back eddy effect are the best.When the reefs are laid out longitudinally,the upwelling volume and back eddy volume gradually increase with increasing spacing,and at a spacing of 4L,the upwelling volume and back eddy volume are larger and the upwelling effect and back eddy effect are optimal.