A Coarse-Grained DEM-CFD Coupling Method For Engineering-Scale Simulation And Its Application

With the implementation of the"Thirteenth Five-Year Plan" and the start of the"Fourteen Five-Year Plan",the construction of tunnels and underground projects in China has developed rapidly.The total construction ranks among the top in the world.As the construction of the project moves towards the western plateau with more complicated geological conditions and the more severe underwater environment,extreme conditions such as high altitude,high seismic intensity,high ground stress,high water pressure,and high ground temperature have brought severe challenges to safe construction.For example,major geological disasters include tunnel water and mud,landslides,rock bursts,and large deformations.There is an urgent need to further develop methods for active disaster prevention and control.The scientific cognition of the disaster mechanism and evolution process of underground engineering disasters is the theoretical basis for active disaster prevention and control.The unclear catastrophe mechanism and unclear evolution process of major geological disasters lead to lack of theoretical basis for prediction and early warning and lack of technical support for active prevention and control.Compared with other methods,numerical simulation method is proved to be an effective method for underground engineering disaster research due to its short time-consuming,low cost,high precision,and easy control.It is called "one of the three pillars of scientific research".In this study,the discrete element method(DEM)and the efficient computational fluid dynamics method(CFD),which are particularly suitable for the simulation of rock and soil material inhomogeneity,anisotropy,and large deformation failure,are selected as the main research methods of this article.Among them,the DEM method faces the difficulty of large particle calculation and long time-consuming in the simulation of engineering scale problems.The coarse graining theory is chosen as the main theory to solve this key problem.The current coarse-grained DEM theory mainly focuses on the particle system without strong cohesive particles.There is a lack of systematic research on the large-scale simulation of cohesive particles and the simulation of the seepage erosion failure process with strong cohesiveness.Therefore,it is urgent to develop a coarse-grained DEM bonding model and a coarse-grained DEM-CFD-based erosion weakening model to simulate the entire process of engineering-scale fluid-solid coupling disaster evolution.This is of great significance for scientifically understanding the evolution mechanism of major engineering disasters,and it can provide a theoretical basis for monitoring,early warning,and control decision-making.This research mainly studied the DEM-CFD coupling method based on the coarse-graining theory.Through laboratory experiments,theoretical derivation and programming development and other research methods,a coarse-grained DEM bonding model and a coarse-grained erosion weakening model have been established,thereby realizing the engineering-scale simulation of the evolution process of water inrush and mud inrush.The main research results of this study include:(1)Using CT scanning technology,the pore structure,particle size distribution and inter-grain bonding structure of the white sandstone were reconstructed and analyzed.Furthermore,the basic mechanical properties of white sandstone were determined.Based on the API of EDEM,the Hertz-Mindlin with Bonding model was improved,a new particle bonding model was established,and the basic mechanical properties of rock were accurately simulated.In addition,the EDEM numerical simulation software was developed with API developed in C++,which realized the acoustic emission monitoring of rock fracture and true three-axis servo stress loading.(2)A study on the particle size effect of numerical experiments on rock uniaxial compression and Brazilian splitting was carried out.Based on the principle of coarse-graining and the fracture mechanics,the theoretical calculation formula of coarse-graining parameters was deduced for bonded particle model.Combining with C++and EDEM API,a coarse-grained particle bonding model(CG BPM)that can effectively overcome the particle size effect was developed.Furthermore,the effectiveness of the CG BPM was verified by applying the model to a classical engineering-scale excavation problem.(3)The accuracy of the coupling calculation based on the EDEM-Fluent software was verified by the single particle hydrostatic sedimentation test.Combining the traditional rock and soil erosion theory and the DEM-CFD coupling method,a rock and soil erosion weakening model was established,and the rock and soil erosion weakening process was realized.The accuracy of the coarse-grained DEM-CFD coupling theory was verified through programming,and a coarse-grained erosion weakening model was established that can simulate large-scale seepage disasters.(4)Taking the water inrush and mud inrush disaster of Yonglian Tunnel in Jiangxi as the research object,the engineering-scale simulation of the evolution process of water and mud inrush disaster was carried out,revealing the evolution mechanism of water inrush and mud inrush disaster in broken zone of water-rich fault.The results are as follows:(a)The law of erosion and weakening of the internal fillings of the fault was shown as a development model of bottom-up development and point-to-surface connectivity.(b)The velocity field showed the law that the overall disturbance develops from the bottom to the surface,and the local velocity first increases and then becomes stable.(c)The displacement field shows the law that the overall disturbance develops from the bottom to the surface,and the displacement of the filling material gradually increases with the evolution of the catastrophe.The coarse-grained particle bonding model and the coarse-grained erosion weakening model established in this paper based on the coarse-grained theory can effectively reduce the amount of particle calculation in large-scale calculations and improve the calculation efficiency.These models can effectively promote the popularization and application of the DEM-CFD coupling method in the simulation of large deformation disasters and fluid-solid coupling disasters in underground engineering,and have theoretical value and engineering significance.