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。

Fluid flow tests are conducted on an artificial model constituted by two crossed fractures with an intersecting angle of 90°. The nonlinear fluid flow patterns within the fracture intersection are observed by using the visualization technique with a charge-coupled device (CCD) camera. Two discrete fracture network (DFN) models are established with and without considering the fracture surface roughness, respectively. The nonlinear flow behaviors of fluid in these two DFN models are characterized by directly solving the Navier-Stokes equations under two kinds of boundary conditions. Experimental results show that the obvious nonlinear flow behaviors in the segment connected to outlet_3 are observed, and measurements also exhibit a nonlinear correlation between the flow rate Q and pressure P. Numerical results demonstrate that, when the hydraulic gradient J is to some extent high (i.e., J > 0.1), the flow rate Q through the DFN is nonlinearly related to the pressure P between two opposite boundaries. However, when J is relatively low (i.e., J < 10-4), Q is linearly proportional to P. Based on these two DFN models in the current study, the critical condition of applying the local cubic law to calculate fluid flow in every fracture in the DFNs is J ≤10-4. The fracture surface roughness can significantly influence the permeability of the DFNs, however, it has negligible influences on the relative flow rate errors.