Experimental Study On Local Scour About The Down Flow In Front Of A Cylinder Pier

Local scour of piers refers to the process in which the flow of water is blocked by the piers to produce different flow structures that cause the sediment near the piers to start or lose,which is considered to be one of the main factors for the washout of bridges.As for the research on local scour of bridge piers,the existing results usually integrate a variety of flow structures and analyze the coupling effect of water flows,but there are few research results on the characteristics of the flow structure itself.From the study of the characteristics of down flow itself,this paper studied the characteristic of down flow,analyzed the functional mechanism of down flow in the local scour of bridge pier,discussed the coupling relationship between down flow and other flow structure,perfected flow structures in the local scour of bridge pier,and the relationship between flow structures and riverbed sediment,provides reference for the study of the local scour of bridge piers.The paper is divided into three parts,which are experimental study on the characteristics of scour action of down flow,the experimental study on the pressure stress distribution of pier local scour and the numerical simulation of local scour of bridge piers.Among them,the experimental study on the characteristics of scour action of down flow aims to explore the impact of down flow on the riverbed,and discuss the rationality of using the value of compressive stress to characterize the impact.Under the condition of excluding the interference of horseshoe vortex system and taking the hydrodynamic medium as the test condition,the descending flow directly impacts the riverbed,the distribution of the riverbed pressure stress is measured,and the relationship between the riverbed pressure stress and the descending flow parameters,including horizontal distance,vertical height3)and horizontal velocity(1and falling velocity(1,is analyzed.The critical condition for the down flow to impact the riverbed is derived.The results show that the compressive stress of sediment can represent the impact of down flow on riverbed.In the experimental study of local scour stress distribution of bridge pier,the common cylindrical bridge pier is selected as the research object.Under the condition of flow velocity of 0.2m/s and water depth of 0.2m,the local scour depth of bridge pier is measured to obtain the shape of scour pit.The slope of measuring point is introduced to characterize the change rate of scour depth and scour range in a fixed period of time;According to the measured scour pit shape,the pressure sensor is embedded in the fixed measuring point and the pressure stress value is measured to obtain the pressure stress and its variation of the measuring point of the scour pit.The impact of the descending flow on the scour pit shape is characterized by the pressure stress,and the influence of the descending flow on the scour pit shape is analyzed.The results show that the variation trend of the riverbed compressive stress is almost the same as that of the tangential slope,which indicates that there is a close relationship between the stress state of sediment and the shape of scour pit;The change trend of compressive stress is first decreased and then increased,which indicates that in the formation process of horseshoe vortex system,the downward flow will provide energy for the horseshoe vortex system,resulting in the decrease of compressive stress of sediment.After the horseshoe vortex system is fully formed,the downward flow can bring more energy into the riverbed and increase the compressive stress of sediment.Therefore,the formation stage of horseshoe vortex system is the stage of strong coupling between down flow and horseshoe vortex system.The numerical simulation of local scour of bridge piers is carried out by using Flow-3D software to simulate the changing process of local scour of bridge piers.The accuracy of the model is verified by comparing with the experimental data.The variation of shear stress at the corresponding measuring points in front of the pier is calculated,the relationship between the stress state of the sediment in front of the pier and the shape of the scour pit is discussed,and the influence of the flow structure on the shape of the scour pit is analyzed.The results show that compared with the horseshoe vortex system,the influence of the down flow on the scour depth is more obvious.