Study On Local Scour Mechanism Of Marine Pile Foundations Based On CFD-DEM

Offshore wind power pile foundations have been subjected to long-term wave and current loads,and a series of hydrodynamic characteristics such as underwater currents,horseshoe shaped vortices,and wake vortices can occur around the pile foundation.After interacting with soil particles around the pile,it is extremely easy to cause local erosion problems in varying degrees of the soil around the pile foundation,seriously threatening the safety and stability of the offshore wind power foundation throughout its life cycle,Therefore,it is necessary to study the mechanism of local erosion around piles to propose more effective erosion protection measures.Based on computational fluid dynamics-discrete element method(CFD-DEM),this paper studies the mechanism of local scour around piles.The main research contents and conclusions are as follows:1)The hydrodynamic model was validated through a three-dimensional cylindrical flow simulation experiment under different roughness bed conditions.The simulation results showed that the simulation results of the three-dimensional cylindrical flow field were basically consistent with the measured water velocity distribution,indicating that the hydrodynamic model and the selected standard k-ε turbulence model can be used to accurately calculate the three-dimensional flow field around the pile.Based on this,static water free sedimentation simulation tests of sediment particles with different particle sizes were carried out,and the final sedimentation velocity of the particles was close to the theoretical solution of sedimentation,indicating that the Gidaspow drag model selected in this paper can accurately and reliably calculate the force between the particles and the fluid,laying a foundation for the subsequent establishment of local erosion models.2)A local scouring model for a single pile under unidirectional flow is constructed,and the effectiveness of the model is verified through clear water scouring tests.The research shows that when the local scouring equilibrium is reached,the scouring depth at 45 ° around the pile is greater than that at 90 ° around the pile and 0 ° in front of the pile,while during the entire scouring process,the sediment carrying rate at 90 ° around the pile is greater than that at 45 ° around the pile and 0 ° in front of the pile,that is,the local scouring rate does not entirely depend on the sediment carrying rate of the water flow,and the cumulative output and input of sediment jointly determine the changes in the local scouring depth.Before sediment is captured by high-speed turbulence around the pile,the scouring mechanism mainly depends on the entrainment of activated sediment particles;when sediment particles enter the high-speed turbulent zone around the pile,the scouring mechanism is mainly dominated by local hydrodynamic effects.3)A simulation experiment was conducted using the physical properties of seabed sediment particles as research variables to study the impact mechanism of factors such as particle size,density,angle of repose,and cohesion on local erosion.The study showed that the angle of repose of sediment has a small impact on the mechanical mechanism and movement state of particles,and only has a certain impact on the morphology of erosion pits.The increase in particle size and density of sediment particles will lead to a decrease in the drag force on sediment particles,and the increase in sediment cohesion will significantly increase the normal viscous contact force and tangential collision contact force between sediment particles,resulting in a continuous decrease in the scouring depth.The scouring equilibrium time is highly sensitive to the particle size and cohesion of sediment,and the increase in particle size and cohesion significantly reduces the time for local scouring to reach equilibrium.