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基于人工交叉裂隙模型,通过室内透水试验,利用电荷耦合元件(CCD)照相机可视化技术,对流体在裂隙交叉点内的非线性流动特性进行研究。建立两种离散裂隙网络(DFN)模型,考虑两种边界条件,改变模型进口和出口之间的压力,直接求解Navier-Stokes(简称N-S)方程,对DFN的非线性渗流特性进行研究。结果表明,室内试验可以观测到与出口3相连的裂隙单元内发生了明显的非线性流动,且通过模型的流量Q和模型两端的压力P具有非线性关系。数值计算结果也表明,在水力梯度J较大时(比如J>0.1),通过DFN的Q和P具有非线性关系,而当J较小时(比如J<10-4),Q与P线性相关;根据文中的算例,建议利用局部立方定律求解DFN内每条裂隙的渗流特性的临界条件为J≤10-4;裂隙表面粗糙会造成通过DFN渗流量的降低,但对相对流量误差的影响可忽略不计。
Based on the artificial cross-fracture model, the nonlinear flow characteristics of the fluid at the intersection of fractures were studied by indoor water permeation test and CCD camera visualization technology. Two kinds of DFN models were established. The Navier-Stokes (N-S) equation was directly solved by considering two boundary conditions and changing the pressure between the inlet and the outlet. The nonlinear seepage characteristics of DFN were studied. The results show that there is a significant non-linear flow in the fracture cell connected to the outlet 3 observed by laboratory tests. The flow Q through the model has a nonlinear relationship with the pressure P at the two ends of the model. The numerical results also show that Q and P have a nonlinear relationship with DFN when the hydraulic gradient J is large (for example, J> 0.1), while Q is linearly related to P when J is small (for example, J <10-4) . According to the example in this paper, it is suggested that the critical condition for the local cubic law to solve the seepage characteristics of each fracture in the DFN is J≤10-4. The rough surface of the fracture will lead to the decrease of the seepage through the DFN, but the influence on the relative flow error Negligible.