Study On The Characteristics Of Flow Field Of Artificial Reef In Three-prism And Its Effect On The Hydrodynamics Of Fish Swimming

Upstream of the Yangtze River Chaotianmen to Fuling river channel improvement project starts from Chongqing Chaotianmen(upstream channel mileage 659.0km)and ends at Fuling(upstream channel mileage 536.0km),123.0km long,located in the lower part of the Three Gorges reservoir change backwater area,the river channel improvement project involves the Chongqing section of the Yangtze River four big fish national aquatic germplasm resources protection zone.Artificial reefs are used in the improvement project plan of Luoqi channel in Chaofu section to create the ecological environment of local waters,but the application of artificial reefs in the Yangtze River basin is still in the exploration stage.The effect of flow field after the deployment of artificial reefs and the effect of artificial reefs on the swimming hydrodynamics of fish are not yet clear.Therefore,in this thesis,we take the trigon artificial reef and chub as the research objects,and use the commercial software of computational fluid dynamics to study the flow field distribution characteristics around the artificial reef under different incoming flow conditions,the best combination of artificial reef arrangement and the effect of artificial reef on the swimming hydrodynamics of fish.The findings of the study can provide support for fish habitat creation in the Chaofu section of the channel improvement project,provide a theoretical basis for ecological environmental protection in the upper Yangtze River channel improvement project,and provide a reference for similar projects in the future.The main findings of this thesis are as follows:(1)The headwater and backwater sides of the artificial reef produce different ranges of upwelling areas,and the scale of upwelling is about 13.68m~3.The maximum velocity of the fish reef upflow area in the dry period is 0.80～1.61m/s,and the average velocity range is 0.18～0.34m/s;the maximum velocity of the upflow in the flood period is1.61～2.68m/s,and the average velocity range is 0.34～0.59m/s.The maximum velocity and average velocity are proportional to the incoming velocity,but the ratio of the maximum velocity value and the average velocity both to the incoming velocity is almost does not change with the increase of the incoming velocity.The width of the back eddy area is 1.31 times the width of the reef body,and the height is 1.45 times the height of the reef body.The scale of fish suitable habitat created by the reef is about 4.38m~3 in dry water period,and the reef creation effect is good;about 1.06m~3 in flood period,and the reef creation effect is slightly poor.When the horizontal deployment spacing is 1.0L and the vertical deployment spacing is set to 4.0L,the flow field effect around the artificial reef combination is better,and the longitudinal reef combination has the best effect of upwelling and back eddy flow when it is placed at 90°and the second best when it is placed at 45°.(2)Fish swimming in still water,both sides of the fish body will appear positive and negative pressure zones,respectively,the existence of this part of the pressure difference provides the impetus for fish propulsion,while the surface friction of the fish body provides resistance.As the frequency of tail swing increases,the thrust will increase,and the swimming resistance will increase accordingly.At the flow velocity of the project river section,the fish pressure differential force is positive and increases with the flow velocity as the pressure around the fish body increases,providing resistance.When the flow velocity is 1.5m/s,the longitudinal force on the fish 4Hz swing tail is 10.6N,which is expressed as resistance,that is,the fish is difficult to swim forward or in the cruising state when the flow velocity is greater than 1.5m/s.(3)After the deployment of artificial reefs,the improvement effect is the best when the fish swim in the center of the turbulent area of the reef,but the effect is weak at the boundary of the turbulent area.The average longitudinal force of the fish body is-2.88N under the flow velocity in the dry period,which shows a thrust,and the maximum lateral force is low,and the fish can swim forward independently behind the reef,which can achieve a good effect of flow field remediation;The average longitudinal force of the fish body is-9.6N under the flow velocity in flood period,with the increase of the flow velocity,the thrust of the fish swimming forward is increased,but the maximum lateral force is also increased,the maximum lateral force of the flow velocity is 5.0m/s is 43.44N,at this time,the lateral flat movement of the fish body is intense,and the stability of the fish swimming is reduced,which is not conducive to forward swimming.(4)Hydrodynamic mechanisms of artificial reef habitat creation:(1)Artificial reefs can effectively reduce the velocity of the water around the fish,which in turn reduces the frictional resistance to the surface of the fish;(2)The scattered vortex behind the reef changes the pressure distribution on the surface of the fish,and the differential pressure force formed by the differential pressure on the surface of the fish points in the direction of swimming forward,increasing the thrust;(3)A large number of turbulent scattered eddies exist behind the reef,and the scattered eddies and fish tail eddies merge with each other to form a high-speed jet that enhances the reaction thrust of the fluid on the fish.