Experimental Study On Single Pile Flow And Pile Foundation Erosion Under Unsteady Flow Environment

Seasonal rivers in mountainous areas are widely distributed.The impoundment of the Three Gorges Project has created a new hydrological environment in the upstream reservoir area of the Three Gorges Project.Especially,the river level rises and falls rapidly during the flood period.This leads to a complex hydrological environment with large variation-large flow velocity in the terminal for a long time,which makes the pile foundation suffer from unsteady flow scour damage for a long time.In order to better reveal the flow around the pile and the local scour around the pile under the unsteady flow environment,this paper combined with the National Natural Science Foundation of China(NSFC)project Research on the deterioration mechanism of large-scale overhead pier structure under the coupling of environment and load(51979017),and used the sine function to fit the unsteady flow ephemeral curve as an important basis for the flume test arrangement,and experimental research on the winding characteristics,pressure distribution law,turbulence characteristics and scouring characteristics of the pile column under non-constant flow environment was carried out.The specific works were as follows:(1)The ephemeral curves of the flow velocity in the main flow direction and the tangential direction in the front,left and right areas of the pile basically coincide with each other.And it show the law of synchronous change with the flow change.Due to Carmen vortex street,the main flow direction shows the opposite change law with the tangential direction in the rear area.And there is a phase difference of about 180° between the ephemeral curves of the flow velocity in the two directions.(2)The flow velocity near the bottom of the tank rises gradually with the change of unsteady flow process.The change of unsteady flow has a certain delay,which leads to a more dramatic flow velocity and flow distribution near the bottom of the rising flow section than the falling section.(3)The relative turbulence intensity of flow around the pile is unevenly distributed.The maximum relative turbulence intensity in the mainstream flow direction appears in the area behind the pile.And the relative turbulence intensity around the pile drops to a low when the flood flow is adjacent.The flow velocity in the tangential direction is more strongly influenced by the Carmen vortex motion.The relative turbulence intensity of the tangential flow in the post-pile area is tens of times higher than the mainstream.(4)The pressure variation around the pile is related to the flow rate.The pressure around the pile reaches the maximum at the moment of maximum flow.And the pressure under the pile column at the same moment increases from top to bottom along the water depth.The pressure in front of the pile is the largest.Due to rising water and falling water,the pressure on the side of the pile is the smallest at the same moment.Due to the influence of the motion of Carmen vortex,the pressure distribution on the left side and the right side of the pile is uneven,and the phase difference is about 180°.(5)The maximum value of relative turbulence intensity of pulsation pressure around the pile didn’t appear at the flood stage.It was a certain stage after the flood flow starts to decay.It means that the destructiveness borne by the pile column reaches the maximum at this stage.(6)The change of the local scour pit scale is mainly in the rising flow stage.The percentage of scour in the rising phase increases with the increase of the flow ratio.Compared with other areas around the pile,the scour percentage in the front of the pile is the largest and the scour percentage in the back of the pile is the smallest.(7)The scour rate of the local scour pits was mainly concentrated in the rising flow phase.The maximum scour rate is mainly concentrated in the post-pile area.The scour rate of local scour pits is influenced by the intensity of water turbulence.The phases with larger scour rates all occurred in the phase with higher flow turbulence intensity.