| 初始裂隙对岩溶水紊流形成的影响 |
| |
| 引用本文： | 焦友军,黄奇波,于青春.初始裂隙对岩溶水紊流形成的影响[J].中国岩溶,2022,41(4):501-510. |
| |
| 作者姓名： | 焦友军 黄奇波 于青春 |
| |
| 作者单位： | 1.中国地质大学(北京)水资源与环境学院, 北京 100083 |
| |
| 基金项目： | 国家自然科学基金项目(41877196, U1612441, 41272387)；中国地质调查项目（DD20221758） |
| |
| 摘 要： | 岩溶地区地下发育着大量的溶洞和地下河管道，地下水流状态既有层流也有紊流，而紊流是溶洞管道形成的重要条件。紊流的形成受到岩石初始裂隙的影响，初始裂隙的张开度、分布、走向、迹长、密度等因素都影响着裂隙发育过程中水流状态的变化。通过对不同统计特征的初始裂隙网络进行水流和溶蚀的数值模拟发现，以张开度标准差反映的裂隙网络非均匀性越强，模拟紊流出现的时间就越早；主要裂隙的存在使裂隙网络的非均性增强，主要裂隙与水力梯度总方向的角度越小，紊流出现的时间就越早；当裂隙平均迹长过小时会导致裂隙连通性较差，影响裂隙水流和溶蚀作用；裂隙密度，尤其是主要裂隙密度，对岩溶发育的影响较大。相对于次要裂隙，如果主要裂隙密度偏小，紊流形成时间会大大增加，甚至很难形成紊流。当初始裂隙张开度小于0.001 cm，增大水力梯度仍没有紊流发生，岩溶几乎不发育。
|
| 关 键 词： | 初始裂隙 岩溶管道 紊流 溶蚀 数值模拟 |
| 收稿时间： | 2022-02-10 |
Influence of initial fractures on the occurrence of karst turbulent flow |
| |
| Institution: | 1.School of Water Resources and Environment, China University of Geosciences (Beijing) , Beijing 100083, China2.Institute of Karst Geology, CAGS/ Key Laboratory of Karst Dynamics,MNR&GZAR, Guilin, Guangxi 541004, China |
| |
| Abstract: | In karst areas of China, there are many karst caves and underground river conduits in the underground aquifer. The development of large cave and conduit are controlled by many factors and it is not every karst aquifer would develop into conduit system, for example, the southern karst area with ample rainfall has many underground rivers, but in the northern arid and semi-arid karst area, underground rivers are relatively rare. The flow states in conduits may be laminar or turbulent. Turbulent flow is an important condition of forming large-scale caves and conduits. In turbulent state, the water flow begin to have the ability of mechanical transportation, which would carry the solid granule and cause the impact and erosion into surrounding rock. The powerful mechanical erosion of turbulence is very important for karst aquifer to develop into large scale conduits and caves. Furthermore, the dissolution rate of carbonate rock in turbulent flow is at least one order of magnitude faster than laminar flow condition. Therefore, the mechanical erosion and chemical dissolution in turbulent flow make the development of conduit and cave more possible. However, the occurrence of turbulent flow is rigidly affected by the hydraulic condition and the initial medium of the aquifer, such as the initial rock fracture, including the aperture, direction, length and density of initial fractures. So we designed different statistic features of fractures and different hydraulic gradients to study the flow state and dissolution widening rates of fractures by numerical simulation. The cubic law and Lomize equation were used to model the laminar and turbulent flow state in fracture. The Newton-Raphson iteration is high-performance to solve the nonlinear flow equation system of laminar and turbulent flow. Then the dissolution rate equation and the Ca2+ concentration equation system was employed to model the widening of the fractures. The simulation results and discussions were all under given outer environment with PCO2 0.8% and hydraulic gradient 0.02. The modeling results of fracture aperture showed that when the standard deviation of aperture was 0.0005 cm and the mean of aperture is 0.006, there was no turbulence in the modeling period of 5,000 thousand years. But when the standard deviation was increased to 0.001 cm, the turbulent flow emerged on 189 thousand year and the time was greatly shorten. In the 8 modeling aperture statistic situations, as the mean and the standard deviation of aperture increased and the heterogeneity of fractures was more intensive, the turbulent flow began to appear and the time of turbulence became earlier. The existence of primary fractures led to much heterogeneous aquifer and earlier turbulent time. The results of fracture direction modeling scenes showed that when the angle between the direction of primary fractures and the direction of main hydraulic gradient was smaller, the turbulence time would be shorter. If the mean of fracture length is too little, the connectivity of fracture would become poor and the karst dissolution would be heavily restricted. The fracture density, especially the primary fracture density, had much influence on the karst development. Compared with the secondary fractures, if the density of primary fractures was too smaller, the turbulence time would largely increase, and even no turbulence in the whole simulation. We also discussed the influence of hydraulic gradient on the turbulent time. The hydraulic gradient varied from 0.001 to 1 and the mean of aperture was from 0.001 to 0.005. The results showed that for each aperture, it had the corresponding smallest hydraulic gradient to the occurrence of turbulence and the greater gradient the turbulent time would become earlier. Below the smallest hydraulic gradient the turbulence would never occur. If the mean of aperture is less than 0.001cm, no matter how the hydraulic gradient is increased, the flow state in fractures remained laminar and no turbulence occurred, in which karst is nearly not developed in the aquifer. In conclusion, the occurrence time of turbulent flow reflects the possibility of forming large karst conduits and caves in present aquifer. The shorter the time is, the greater the possibility will be. |
| |
| Keywords: | |
|
| 点击此处可从《中国岩溶》浏览原始摘要信息 |
| 点击此处可从《中国岩溶》下载免费的PDF全文 |
|
