Mechanism Of Local Scour Reduction Around Cylindrical Piers Using Permeable Collars

With the rapid development of the national economy and the strategy of " Traffic Power",China has made a series of brilliant achievements in the transportation industry.As an important part of transportation,bridges play an indispensable role in the development of the country and society.Unlike land structures,bridge foundations are mostly located in turbulent rivers,suffering from the long-term effects of water currents,waves and traffic loads,which are very likely to produce local scour around the bridge piers.As a result,the bearing capacity and stability of bridge piers are gradually reduced,which seriously affects the safety of traffic.Therefore,it is particularly important to carry out research on local scour and its protection technology for bridge piers.Adopt scientific and effective protective measures,not only can save maintenance costs,but also can provide an important guarantee for the safe and stable operation of the bridge.In this paper,a new form of a local scour reduction measure,the permeable collar,was proposed for the problem of local scour of cylindrical bridge piers.A combination of indoor model tests and numerical simulations was used to study the scour reduction characteristics and the scour reduction mechanism of the permeable collar under clear water conditions.The main study contents and conclusions are as follows:(1)Through reading domestic and foreign literature and theoretical analysis,the influence factors of solid collar were summarized and the influence of different influence factors on the impact scour reduction effect of solid collar was compared and analyzed.Firstly,the main influence factors were identified as follows: collar size,collar installation height and collar shape.Later,it was summarized that the better scour reduction effect of the collar design: the size of the collar is 3 times to 3.5 times the diameter of the bridge pier is appropriate;the height of the collar installation to close to the riverbed surface is appropriate;the rectangular collar scour reduction effect is slightly better than the circular collar.Finally,for the permeable collar,porosity,collar size,collar installation height and collar thickness were selected as the main study factors.(2)For the above main study factors,the local scour characteristics around the bridge pier when using permeable collar protection were studied through indoor model tests,and the effects of different factors on the scour reduction effect of permeable collar were analyzed in terms of scour pit morphology,maximum scour depth and average scour depth.The results show that the porosity of the collar has a great influence on the local scour reduction.The scour reduction efficiency increases first and then decreases with the increase in collar porosity.When the collar porosity is 50%,the reduction effect is the best,and can reach up to 78.1%.The influence of the installation height and size of the permeable collar on its scour reduction efficiency is generally consistent with that of the solid collar.The scour reduction efficiency of the permeable collar decreases with the increase of the installation height,and the best reduction effect is achieved when the permeable collar is placed close to the riverbed.When the diameter ratio of permeable collar to bridge pier reaches 3.5,the increase of scour reduction efficiency starts to slow down,and the efficiency is up to 89.6% at this time.The thickness of the permeable collar has a greater influence on its scour reduction effect,and its scour reduction efficiency is negatively correlated with the thickness.When the ratio of the thickness of the permeable collar to the diameter of the bridge pier reaches 0.45,the scour reduction efficiency is sharply reduced to 22.4%,and the collar basically loses its effect.(3)Based on the classical physical test data,the accuracy of the simulation results of the numerical simulation software Flow-3D was verified in terms of flow morphology,flow velocity and pressure distribution around the bridge pier.The results show that the numerical model can better reflect the flow morphology,flow velocity and pressure distribution around the bridge pier,and the overall change pattern is consistent with the measured values.(4)Based on the accuracy verification of the numerical simulation software,the numerical simulation of some test conditions of the indoor model was conducted to reveal the scour reduction mechanism of the permeable collar in terms of the flow morphology and flow velocity distribution near the bed surface of the bridge pier.The results show that the permeable collar divides the flow on the waterward side in front of the pier into flow above the collar,flow below the collar and flow in the pore space inside the collar.Due to the special structure of the permeable collar,it can effectively dissipate energy and inhibit the formation of horseshoe vortex at the foot of the pier to weaken the local scour.