| Granular materials such as gravel and sand are commonly used in practical geotechnical engineering.In marine engineering,island reef filling and platform construction above offshore sandbed also often involve granular materials such as calcareous sand.Granular materials often cause soil damage and deformation problems due to seepage,such as soil seepage damage between supporting structure and soil heave of the suction anchor penetration in sand.The coupling between particles and fluid is extremely complex,and the fluid-particles coupled numerical simulation method based on discrete element method(CFD-DEM)can effectively reflect the interaction between particles and fluid,and study the problem of granular material seepage from a meso-level perspective.However,the current drag force model used to characterize the effect of fluid on particles still cannot take particle gradation into consideration,which makes CFD-DEM method have defects in simulating the problem of grading particle seepage.In this paper,the internal principles of CFD-DEM numerical method are deconstructed from the three aspects of particle-to-particle interaction,fluid movement and fluid-particle interaction.The framework and steps of coupling calculation are analyzed.The CFD-DEM numerical calculation is realized through the application of open source software,which lays the foundation for the subsequent establishment and embedding of a new drag force model considering the gradation through the secondary development.In order to establish a drag force model considering grading,seepage tests were carried out on the particle accumulations of different shape,particle size,porosity and gradation,and the influence of these parameters on the permeability characteristics of the particle accumulation was explored.The test results proved the porosity The rate has the greatest impact.At the same time,the drag force equation containing 5 parameters(equivalent particle diameter,gradation,Reynolds number,porosity,shape coefficient)was obtained by symbolic regression algorithm based on genetic programming,which integrated the drag force coefficient results obtained from the test.The equation is used to predict the seepage test results,and the correlation coefficient is as high as 0.89,which has a certain generalization.The drag force model considering gradation was embedded into the CFD-DEM numerical method for granular accumulations,and it was applied to the simulation of seepage failure of the soil supported by the structure and suction anchor penetration with negative pressure in sand.By simulating the whole process of seepage failure of sands supported by structures,the development mechanism of seepage failure in three stages is proposed.At the same time,from the perspective of the properties of sandy soil,its influence on seepage damage and soil resistance to damage is analyzed.This paper also analyzes the supporting effect of the structure in the seepage damage,and provides a certain theoretical basis for the actual engineering.In contrast to the indoor model test of suction anchor under 1g situation,a CFD-DEM fluid-particle coupled numerical simulation study was carried out on the suction anchor penetration under negative pressure in the sand.Through soil observation at the particle scale and flow field monitoring,the changes in the sand layer and seepage field during the penetration of the suction anchor are analyzed,and the negative pressure loss outside the anchor and the change in the permeability of the soil inside the anchor are obtained,which explained the mechanism of suction anchor penetrating in sand.The above simulation shows that the drag force model considering the gradation has applicability and effectiveness in the CFD-DEM fluid-structure coupling numerical method,and also shows that the numerical analysis method has great reference for engineering problems involving granular material seepage in the field of geotechnical engineering. |
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