| The karst watersheds in southwestern China are widely distributed,and the heterogeneity of their aqueous media is often characterized by the coupling of pipeline-fractured aquifer systems.Although pipes and caves only account for a small part of the entire karst geological system,the groundwater flow here mainly exists in the form of nonlinear laminar and turbulent flows,and Darcy's law cannot be applied to this type of flow,which makes the traditional numerical model of groundwater flow does not apply to karst aquifers.Under this background,the simulation of the movement law of groundwater flow in complex karst aqueous media has received extensive attention from scholars at home and abroad.At present,it is an effective method and hotspot to study the hydrodynamic process in karst areas by using some simplifications and assumptions to establish suitable and effective physical models and mathematical models and conducting indoor experiments.In order to achieve the purpose of this research,a sandbox device was designed to simulate the groundwater flow and solute transport process between the pipeline and the porous media.The sandbox device described in this time includes 6 water tanks,1 sandbox main unit,29 pressure sensors,3 metering rehydration pumps,3 liquid level sensors,6 temperature sensors,and 1 XMT-J800 eight channel controller and 1 paperless recorder.Among them,the main unit of the sandbox has two parallel pipes: the upper pipe has an inner diameter of 1 cm and a height of 31 cm;the inner diameter of the lower pipe is 1.5 cm and the height is 1 cm.In this study,55 cm thick quartz sand was uniformly filled in the sandbox main unit,and the left and right boundaries of the substrate and the two pipes were set at the fixed water head boundary.By changing the boundary condition values,a total of 12 water flow tests and 12 solute tests were designed.After the test is completed,it can be seen from the analysis of multiple sets of measured water pressure data and solute transport images that there is an interaction between the substrate and the pipeline and between the two pipelines.The water head value determines the flow direction of the groundwater.Changing the boundary conditions of the substrate or the upper and lower pipes will change the direction of water exchange between the pipeline and the matrix and the head value of groundwater in the sandbox,which will have an extremely important impact on the solute transport process.Based on the above test device and test plan,the research scope of the model established this time is 120 cm × 2 cm × 46 cm.The permeability coefficient of the aquifer is set at 0.02 cm/s,and the effective porosity is set at 30.5% while the dispersion is set at 0.1 cm.The model is defined as a stress period with a duration of 20 min and a total of 10 time steps.The aquifer is divided into 12 layers,1 row,20 columns,a total of 240 cells,of which the aquifer is divided into 1 layer and the confined aquifer is divided into 11 layers.The upper and lower tubes are located on the 4th and 12 th layers of the aquifer,respectively.The two pipes are divided into 38 sections and 40 nodes.Numerical simulations were carried out by the MODFLOW-CFP program and the MT3 DMS program.The results obtained were slightly different from the measured results,but basically met the actual situation. |
PDF Full Text Request