| In the cold regions north of China’s northern latitude 30°,there are widespread hydrological phenomena of river icing and drifting ice.Due to the economic development and the demand for transportation,a large number of bridges have been established in river courses.When rivers in the cold regions cause river ice by hydraulic action,Ice flow seriously affected the safety of the bridge,at the same time,the bridge pier changed the original boundary conditions of the river flow,affecting the movement of ice,ice piers and ice dams may be formed at the bridge pier section.Field visits to cross-river bridges in cold regions,these bridges often use cutting-edge piers(increasing the vertical tip of the icebreaker at the front end)to reduce the hazards of flow.The shape of the tipped pier is different from that of the traditional cylindrical pier.The flow pattern around the pier has also undergone great changes,and the analysis of the flow field of this type of pier is not thorough enough.At the same time,the mutual hydraulic effects of the tipped piers and drift ice,as well as the ice transfer capacity of the piers,need further study.In this paper,physical model experiments were conducted to observe the water flow patterns around the tip of the pier.The Particle Image Velocimetry technique(PIV)was used to analyze the flow field around the pier.The distribution characteristics of vorticity,turbulence intensity,and Reynolds stress around the pier were calculated.The ice transfer capacity test was conducted by using a high-speed camera(i-SPEED)to perform a single ice single pier test and record the process of ice flow and bridge pier collisions.The mutual hydraulic effects of piers and drift ice were observed,and the influence of the flow field around the pier on the ice movement was analyzed.Then the double pier test is used to analyse the situation of ice passing through the bridge pier under different flow conditions and to determine the influence of the flow velocity on the ice capacity of the pier.In this test,a total of 12 working conditions were set up.The flow of the pier around the pier,the single ice single pier test and the double pier test were 4 groups,and the corresponding flow rates were equal.This study can not only enrich the flow mechanism of bridge piers,providing scientific basis for the design of river bridges in cold regions,which is of great theoretical and practical significance for the prevention of the occurrence of ice storms in bridges.Through the analysis of the test results,the following rules can be drawn.Test flow around the tip piers:(1)Due to the special shape of the tipped pier,a special water flow pattern is formed.The water level is high in front of the pier,and the drowning curve conforms well to the distribution of a quaternary function.The length of muddy water before the pier is less sensitive to the change of column Reynolds number,and the height of muddy water increases with the increase of the Reynolds number of the cylinder.There is a good one-time function relationship between them.Immediately after the ice-breaking body,two symmetrical water-flow depressions form on both sides of the pier,and a backflow zone is generated behind the pier,and a vortex street is generated.(2)The transverse velocity values on both sides of the ice-breaking body are relatively large.When the water flows through the tip-shaped bridge piers,an apparent body flow is formed,and the initial position of the flow is accurately determined by the PIV.The vortex values of the rear side of the icebreaking pier and the pier side of the tip pier are the largest,and the extreme values appear at a distance of 0.3-0.6 times the pier thickness.The turbulence intensity in this area is also greater.(3)After the pier has a small flow velocity,there is a significant low-velocity wake region.The trail increases with the increase of the distance from the pier,shrinks first,and then widens.The upstream boundary layer separation zone is farther from the pier which is the main reason of different change of trailing piers and wakes of cylindrical piers.(4)When Fr is between 0.1 and 0.58 in this experiment,the wake vortex width is within the range of 1.2-2.0 times the pier thickness.With the increase of the flow velocity,the shape of the vortex formed behind the pier is narrower and longer.(5)At the subcritical Reynolds number,the vortex shedding Stroha number of the tip pier is larger than the Strohal number of the sylindrical pier and the square pillar pier.(6)The spatial distribution of Reynolds stress in the wake and the distribution of turbulence intensity are similar.Reynolds stress is smaller when the distance in the flow direction is less than 1 times the pier thickness.As the distance from the bridge pier increases,the Reynolds stress value gradually increases.The maximum value appears in the thickness range of 2-4 piers and 1.5 times the pier thickness.Over-ice capacity test:(1)Single pier test,after the collision of ice blocks and bridge piers,the rotation range is between 45-90 degrees.When the flow rate is small,the flow ice only occurs plane rotation;when the flow rate is large,it produces the strong Venturi effect,the height of the drifting ice on the side far away from the pier is significantly reduced,with a tendency of “submergence”.After the ice flows through the ice-breaking body,it recovers the balance under the action of the supporting force couple;The effect of convection ice in the turbulent area behind the pier is greater than the turbulence area at the pier side(2)The double pier test showed that the existence of bridge piers significantly reduced the ice capacity of the water tank,and the formation of ice plugs was affected by the flow velocity of the water.When the simulated channel ice density is 80%,ice plugs are produced at the cross section of the bridge piers at flow rates of 0.1m/s and 0.18m/s.and the ice capacity is 42.9% and 82.1% respectively.Flow ice at the flow rates of 0.26m/s and 0.34m/s can smoothly pass through the bridge pier section. |
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