Improved Numerical Model And CFD Simulation Of Local Scour Around Pile Group Foundation

Local scour around bridge piers is recognized as one of the major causes of bridge failures crossing rivers.Reasonable estimation of the maximum scour depth is a important design parameter to ensure the safety of bridge pier.With the development of computer hardware and numerical method,numerical simulation based on the computational fluid dynamics(CFD)has gradually became an effective methods to investigate the local scour around bridge pier.In this paper,local scour around various types of bridge piers has been studies based on CFD method.The researches mainly focus on the following aspects and conclusions can be drawn as following:(1)Several typical calculation model of local scour around piers and their application limitsare introduced,in which the comparasion between the Chinese code and US codes on local scour around bridge piers is highlighted,and the disadvantage of current Chinese Codes on predict local scour is pointed out.With this consideration,a new calculation formula for predict of local scour depth around piers is presented through dimensional analysis and multiple regression analysis,with the help of test data from the Turner–Fairbank Highway Research Centerand United States Geological Survey,.The value of correction factors of pier shape,the correction coefficient of water yaw attack,and the interference coefficient ? of cylindrical piers in tandem,are determined based on CFD simulations.Finally,the applicability and rationality of the present formula are verified through experimental data of different bridge pier shapes.(2)Under the assumption of full development turbulent flow,the equilibrium turbulent boundary layer model and the corresponding inlet boundary conditions are derived for the standard k-?turbulent model.Then,the model parameters are determined based on the least square method in terms of flume test data,followed by parametric sensitivity analysis to optimize model parameter of the standard k-?turbulent model.Finally,the applicability of the presented model is validated by CFD simulations with different inlet boundary conditions.(3)Based on Fluent platform,the local scour module is developed which can consider the effect of gravity on slop and sediment collapse,the CFD simulation on local scour around piers are carried out.The predicted flow pattern,scour depth and shape are consistent with results from both the Melville flume test and Beheshti flume test,which validates the applicability and rationality of present method.The influence of some major parameters on local scour depth,such as pier shape,aspect ratio and water yaw angle are also investigated.The results show that among the four pier shapes,the rectangular pier produces the deepest local scour hole,while the sharp pier gets the smallest.Under zero degree of yaw angle,the variation of the aspect ratio has little effect on the scour depth,but has great influence on the scour hole shape and its affected extent.Under the flow yaw angle ranging from 5o to 45o,the scour depth increases with the increase of flow yaw angle.(4)The influence of interference effect on the Scour depth around the tandem and staggered cylindrical piers is studied.There is the interference effect between the upstream and downstream piers.This effect is weakened with the increase of the pier center distance(L/D)of the tandem piers.When the pier center distance is greater than 5 times the pier diameter,interference effect can not be taken into account.As for the staggered piers,the maximum scour depth increases with the angle of attack.When the distance is less than 5 times the pier diameter,if the upstream pier shields the downstream pier,the maximum scour depth forms in the shielded side.If the upstream pier does not shield the downstream pier,there is the narrow tube effect,the maximum scour depth forms in the slit side.Based on the new calculation formula of local scour around piers,the maximum scour depth of double column pier can be reasonably predicted.(5)The flow characteristics and scour mechanism of the pile group foundation of 2(bridge transvere direction)×4(bridge longitudinal direction)are studied.The results show that the maximum scour depth occurs at the central bed area of the two leading piles and the shape of the scour hole is similar to the shape of “?” ".Similar to the arrangement of piers in tandem or staggered columns,the interference between pile groups is still significant.Because the downstream piles are blocked by the upstream piles,the scour depth around the later one is smaller than that of the former one.The more upstream piles ahead of the downstream piles,the smaller the scour depth occurred.In addition,since the flow is also blocked by the pile cap and pier,the flow will separate and reattach on cap and pier's side.At the same time,the flow will deflect in the depth,resulting in obvious flow characteristics such as down-flow jet.With the development of scour holes,the strength of down-flow jet at the front of the cap increases gradually,and the velocity of down-flow jet at equilibrium scour stage is about three times larger than that of initial scour.Since the water can flow through the interval between the pier cap and pile group,significant horseshoe vortex and backflow is not observed in the wake of pile group.However,in the wake of pile cap and pier,the backflow can be observed and the complex vortex system is formed with high turbulent energy,especially near the trailing edge of the pile cap.