| The formation of local scour on bridge piers will reduce the effective embedment depth of bridge piers,thereby affecting the bearing capacity of bridges.It has always been one of the important factors affecting bridge construction and operation.With the frequent occurrence of bridge piers scour and water damage accidents,it is particularly important to propose reasonable and efficient anti scour countermeasures based on the local scour mechanism of bridge piers.In this paper,the mechanism and influencing factors of local scour on bridge piers were analyzed,the characteristic and applicability of various anti scour countermeasures were summarizes,and the research status of in-situ tests,flume tests,and numerical simulation tests aimed at local scour issues were expounded.Due to the fact that accurate prediction of the maximum scour depth is an important basis for the design of bridge pier foundations,this paper also collated the scour depth prediction formula for single bridge pier and the pier under the protection of devices.Based on a summary of existing research,and in view of the current status and shortcomings of research on local scour protection measures for cylindrical bridge piers,this paper proposed a composite anti scour countermeasure that combined a perforated baffle in front of the pier and a ring-wing plate on the pier.Using a combination of theoretical analysis,flume tests,and CFD numerical simulation,the following research has been conducted on optimizing the combined anti-scour device and mechanism:(1)The classification basis of anti-scour devices with changed flow field and riverbed is proposed,and the characteristics and applicability of each device are compared and analyzed.Based on the mechanism and influencing factors of bridge pier local scour,a new type of combined anti-scour device was proposed.And the advantages and disadvantages of in-situ tests,flume tests,and numerical simulation tests for local scour were investigated to provide theoretical guidance for subsequent research.(2)A series of flume tests for local scour of bridge pier under the protection of combined devices were designed and carried out.In order to determine the optimal parameters of the combined device,a three factor and three level orthogonal tests were carried out using the area ratio S of the baffle opening,the distance L from the perforated baffle to the bridge pier,and the relative height H of the ring-wing plate as the test factors.In order to obtain the primary and secondary relationship between the impact of various experimental factors on the optimization index and the optimal level of each factor.The maximum scour depth in front of and at the pier side were used as the optimization index for range analysis.(3)The CFD software ANSYS FLUENT was used to conduct numerical simulation studies on various conditions of the flume tests.The flow field analysis before the start of scour(streamlines,axial velocity vector,vertical velocity)and equilibrium scour hole morphology analysis were performed on individual piers,verifying the reliability of the numerical model in this paper.Based on numerical simulation orthogonal experiments,the primary and secondary relationships of the impact of various experimental factors on the optimization index and the optimal level of each factor were obtained.(4)A comprehensive analysis of the primary and secondary relationships among the three experimental factors under the flume tests and numerical simulation ultimately determined the optimal parameters for the combined device.The same flume test was conducted on the bridge piers protected by the optimal combination device,and the scour prevention effect and self-protection performance of the optimal combination device were verified using the maximum scour depth reduction rate in front of the pier and the scour depth reduction rate in front of the baffle as evaluation indicators.The same numerical simulation test was conducted,and the streamlines,velocity vectors,and turbulent kinetic energy on different horizontal and vertical planes were compared and analyzed in combination with the single bridge pier and the single ring-wing plate pier,clarifying the scour prevention mechanism of the combination device.(5)The existing formulas for calculating the scouring depth of single cylindrical piers are compared and introduced,and the characteristics of each formula were summarized.And the calculation formulas for local scour of bridge piers under the protection of anti-scour device were listed.Based on the experiments in this paper,a calculation formula for local scour of bridge piers under the protection of combined devices in this paper is proposed,and the mathematical relationship between the area ratio S of the baffle opening,the distance L from the perforated baffle to the bridge pier,the relative height H of the ring-wing plate,and the scour depth was established.This has important guidance and engineering significance for the embedded depth of bridge piers and the design of similar erosion prevention devices. |
