一种CFD-DEM流固耦合方法在渗流导致城市地面沉降问题中的应用

Application of CFD-DEM coupling method in seepage-induced ground subsidence

城市地面沉降的主要原因之一是地下水渗流,有限元方法难以揭示地面沉降的细观机制,而能够揭示细观机制的离散元方法(discrete element method,DEM)却难以建立具有复杂形状的流场模型.基于DEM和计算流体动力学(computational fluid dynamics,CFD)耦合原理,借助DEM程序PFC3D与CFD程序Fluent,实现了基于颗粒细观的流固耦合计算.通过建立单颗粒沉降、基坑临空面渗漏、河道边坡渗流3个算例验证了CFD-DEM方法的正确性与可行性.模拟结果表明,这种CFD-DEM方法可从细观角度展现渗流引起地面沉降的整个动态过程,也可实现对各种复杂形状流场的模拟求解计算.在渗流作用下土体稳定性发生变化,土颗粒运动造成土体流失,最终形成地面沉降.此外,水力梯度越大,地表沉降值越大,从土体细观角度展现了渗流作用下地面沉降的整个动态过程,可对相应城市地质病害防治进行机理研究.

One of the main causes of urban ground subsidence is groundwater seepage.Finite element method is not an ideal method to reveal the microscopic mechanism of ground settlement.With discrete element method,the microscopic mechanism may be revealed,but it doesn’t help with the building of a flow filed model with complex shape.Based on discrete element method(DEM)and computational fluid dynamics(CFD)coupling principle,a fluid-solid coupling method is proposed.This method uses PFC to simulate the solid phase and Fluent to simulate the liquid phase.It achieves fluid-solid coupling calculation based on particle microscopic view.Then this method is applied to the study of ground settlement caused by seepage.The effectiveness and feasibility of this method are verified by three examples.It is found that the PFC-Fluent method can show the whole dynamic process of ground settlement caused by seepage from a microscopic perspective,and can also realize the simulation and calculation of various complex shape conditions.The stability of soil was changed under the influence of seepage,and the soil migration lead to ground subsidence.Land subsidence increases with the rise of hydraulic gradient.This study reveals the whole process of ground subsidence under the action of hydraulic gradient,which provides a basis for the prevention and control of urban geological hazards.