基于LBM-DEM耦合计算模型的含水层内悬浮颗粒运动特性

Flow Characteristics of Suspended Particles in Aquifer Using Coupled LBM-DEM Numerical Model

基于格子Boltzmann法(LBM)与离散单元法(DEM)的基本理论,通过对格子单松弛模型演化方程进行修正,建立LBM-DEM耦合计算模型。引入浸没移动边界方法(IMB)处理复杂的流-固边界问题,利用编程软件MATLAB开发LBM-IMB-DEM耦合求解程序,对悬浮颗粒在含水介质中的运动过程进行模拟计算。通过开展层析柱强制渗流试验,模拟DKT算例,验证了计算模型与求解方法的正确性;从孔隙尺度分析地下水动力、颗粒形貌和尺寸效应对含水层渗透性能的影响。结果表明:当进口流速为5×10^(-5) m/s,向含水介质注入球形硅微粉悬浮液时,含水层各区域的渗流速度均呈现下降、回升、稳定的3个连续阶段;随着渗流速度的降低,颗粒滞留率增加了47.4%,所选3个断面平均流速恢复率降低。将悬浮颗粒由球形硅微粉替换为非球形硅微粉后,所选3个断面的平均流速最大降幅分别提高了15.0%、11.4%、2.6%,恢复率依次降低了37.5%、69.6%、71.9%,含水介质的颗粒滞留率增加了47.9%。基于颗粒受力平衡分析可知,球形硅微粉转动惯量较低,易于滚动并脱离含水介质表面;同等效体积的非球形硅微粉比表面积更大,而且多絮凝成团,迁移过程中出现沉积或被孔喉捕获的机率提高,并且沉积后难以发生再释放过程。随着渗流速度的降低与含水介质计算区域的增加,悬浮颗粒形貌的变化对于含水层水动力场演化过程的影响程度增强。

A coupled LBM-DEM numerical model was developed by modifying the Lattice Bhatnagar Gross Krook evolution equation based on the theory of lattice Boltzmann method(LBM)and discrete element method(DEM).The immersed moving boundary method(IMB)was intro-duced for solving complex fluid-particle interactions using a fast linear approximation of partially intersected volume between a particle and a lat-tice cell.MATLAB was used to develop an LBM-IMB-DEM coupling solving program to simulate the movement of suspended particles in the aquifer medium.The accuracy of the model and the solution method was verified by using a chromatographic column forced seepage experiment and simulation of the drafting,kissing and tumbling(DKT)phenomenon.The effects of groundwater dynamics,particle morphology and size ef-fect on aquifer permeability were analyzed on the pore scale.The results showed that when the suspension of spherical silica powder was injected with an inlet velocity of 5×10^(-5) m/s,the seepage velocity in the aquifer presented three successive stages of declining,rising and stabilizing.While the seepage velocity decreased,particle retention rate increased by 47.4%and recovery rates for the average velocity of each section declined.After the suspended particle was changed from spherical silica powder to non-spherical silica powder,the maximum drop of the average velocity of each section increased by 15%,11.4%,and 2.6%,respectively,whereas the recovery rate decreased by 37.5%,69.6%,and 71.9%,and the particle retention rate increased by 47.9%.According to the particle force balance analysis,spherical silica powders can be easily detached from the grain surface of the aquifer medium due to their low rotary inertia.Since the specific surface area of the non-spherical silica powder with the equivalent volume is larger and the flocculating lumping is often observed in it,the probability of the attachment in matrix surface or being captured by the pore throat are enhanced during transport,and it is difficult for non-spherical silica powders to re-release after deposition.While the seepage velocity decreases and the calculation area of the aqueous medium increases,the influence degree of suspended particle morphology on the hydrodynamic evolution of the aquifer is strengthened.