Large Eddy Simulation Of Local Scour At Sand-bed

With the rapid development of water conservancy project,a large number of underwater structures like pipelines and piers are arranged in rivers,oceans,lakes and other water bodies.When local sour occurs near the structure due to the strong change of flow field,it will have a great impact on the stability of the structure,even endanger the safety of the structure.The flow field near the structure has strong three-dimensional turbulent structures,and its calculation has a high requirement for the simulation method.Compared with other methods,large eddy simulation has the advantages of accurate simulation of turbulent structure and saving computing resources.In this paper,based on large eddy simulation,a three-dimensional numerical model of flow,sediment and bed deformation is established,and two classical scour problem,local scour around pipelines and piers,is studied.The main work and results are as follows:Applying the Smagorinsky subgrid turbulence model,building the bed deformation model based on the Exner equation,and using the immersed boundary method to deal with the complex boundary and moving boundary problem,a three-dimensional flow and sediment numerical model suitable for structured grid is developed,which only considers bed-load transport.In the bed deformation model,a new method dealing with the sand slide is put forward,which makes the numerical result closer to the real situation.In order to verify the reliability of the model,the bed shear stress on a wavy rigid bed is compared,and it shows good agreement.The model is applied to the numerical simulation of local scour around pipelines with the Reynolds number of 1500.The calculated maximum scour depth is in good agreement with the result of experiments.Also,it is determined that the Meyer-Peter bed-load transport formula can give a more accurate result of the maximum scour depth in three-dimensional flow and sediment simulation.The model can accurately describe the flow field around the pipeline,and there is obvious wake vortex at the downstream of the pipeline as well as vortex streets.On this basis,through the comprehensive analysis of the bed shape,Shields number and flow field at different times in the process of the local scour development,the local scour process around pipeline is divided into three stages – early stage,middle stage and balanced stage.The characteristics of each stage are also summarized.The model is applied to the numerical simulation of local scour around piers with the Reynolds number of 1500.The calculated maximum scour depth is verified by the classical empirical formula.And the simulated bed shape is also in fine agreement with the previous experimental results.Finally,the formation and development mechanism of local sour around piers are explained by the comprehensive analysis of the flow field,bed-load transport rate and bed morphology.The correct simulation of the horseshoe vortex and the wake vortex is the key to get reasonable sour process and shape of the sour pit.