Department of Earth Sciences, University of Aarhus, Høegh-Guldbergs Gade 2, 8000 Aarhus C, Denmark.
Ground Water. 2012 Jan-Feb;50(1):118-32. doi: 10.1111/j.1745-6584.2011.00826.x. Epub 2011 May 27.
This work studies costs and benefits of utilizing local-grid refinement (LGR) as implemented in MODFLOW-LGR to simulate groundwater flow in a buried tunnel valley interacting with a regional aquifer. Two alternative LGR methods were used: the shared-node (SN) method and the ghost-node (GN) method. To conserve flows the SN method requires correction of sources and sinks in cells at the refined/coarse-grid interface. We found that the optimal correction method is case dependent and difficult to identify in practice. However, the results showed little difference and suggest that identifying the optimal method was of minor importance in our case. The GN method does not require corrections at the models' interface, and it uses a simpler head interpolation scheme than the SN method. The simpler scheme is faster but less accurate so that more iterations may be necessary. However, the GN method solved our flow problem more efficiently than the SN method. The MODFLOW-LGR results were compared with the results obtained using a globally coarse (GC) grid. The LGR simulations required one to two orders of magnitude longer run times than the GC model. However, the improvements of the numerical resolution around the buried valley substantially increased the accuracy of simulated heads and flows compared with the GC simulation. Accuracy further increased locally around the valley flanks when improving the geological resolution using the refined grid. Finally, comparing MODFLOW-LGR simulation with a globally refined (GR) grid showed that the refinement proportion of the model should not exceed 10% to 15% in order to secure method efficiency.
这项工作研究了利用 MODFLOW-LGR 中实现的局部网格细化（LGR）来模拟与区域含水层相互作用的埋地隧道谷地下水流的成本和效益。使用了两种替代的 LGR 方法：共享节点（SN）方法和幽灵节点（GN）方法。为了保持流量守恒，SN 方法需要在细化/粗网格界面处的单元格中修正源和汇。我们发现，最优的修正方法取决于具体情况，在实践中难以确定。然而，结果表明差异很小，这表明在我们的案例中，确定最优方法并不重要。GN 方法不需要在模型接口处进行修正，并且它使用比 SN 方法更简单的水头插值方案。该方案速度更快，但精度较低，因此可能需要更多的迭代。然而，GN 方法比 SN 方法更有效地解决了我们的流量问题。将 MODFLOW-LGR 的结果与使用全局粗化（GC）网格获得的结果进行了比较。LGR 模拟所需的运行时间比 GC 模型长一个到两个数量级。然而，与 GC 模拟相比，在埋地山谷周围提高数值分辨率显著提高了模拟水头和流量的准确性。当使用细化网格提高地质分辨率时，山谷侧翼周围的准确性进一步提高。最后，将 MODFLOW-LGR 模拟与全局细化（GR）网格进行比较表明，为了确保方法的效率，模型的细化比例不应超过 10%到 15%。
