共查询到20条相似文献，搜索用时 25 毫秒
1.
As a result of rock dissolution processes, karst aquifers exhibit highly conductive features such as caves and conduits. Within these structures, groundwater flow can become turbulent and therefore be described by nonlinear gradient functions. Some numerical groundwater flow models explicitly account for pipe hydraulics by coupling the continuum model with a pipe network that represents the conduit system. In contrast, the Conduit Flow Process Mode 2 (CFPM2) for MODFLOW-2005 approximates turbulent flow by reducing the hydraulic conductivity within the existing linear head gradient of the MODFLOW continuum model. This approach reduces the practical as well as numerical efforts for simulating turbulence. The original formulation was for large pore aquifers where the onset of turbulence is at low Reynolds numbers (1 to 100) and not for conduits or pipes. In addition, the existing code requires multiple time steps for convergence due to iterative adjustment of the hydraulic conductivity. Modifications to the existing CFPM2 were made by implementing a generalized power function with a user-defined exponent. This allows for matching turbulence in porous media or pipes and eliminates the time steps required for iterative adjustment of hydraulic conductivity. The modified CFPM2 successfully replicated simple benchmark test problems. 相似文献
2.
Evaluation of the MODFLOW-2005 Conduit Flow Process 总被引：1，自引：0，他引：1
The recent development of the Conduit Flow Process (CFP) by the U.S. Geological Survey (USGS) provides hydrogeologic modelers with a new tool that incorporates the non-Darcian, multiporosity components of flow characteristic of karst aquifers. CFP introduces new parameters extending beyond those of traditional Darcian groundwater flow codes. We characterize a karst aquifer to collect data useful for evaluating this new tool at a test site in west-central Florida, where the spatial distribution and cross-sectional area of the conduit network are available. Specifically, we characterize: (1) the potential for Darcian/non-Darcian flow using estimates of specific discharge vs. observed hydraulic gradients, and (2) the temporal variation for the direction and magnitude of fluid exchange between the matrix and conduit network during extreme hydrologic events. We evaluate the performance of CFP Mode 1 using a site-scale dual-porosity model and compare its performance with a comparable laminar equivalent continuum model (ECM) using MODFLOW-2005. Based on our preliminary analyses, hydraulic conductivity coupled with conduit wall conductance improved the match between observed and simulated discharges by 12% to 40% over turbulent flow alone (less than 1%). 相似文献
3.
by Tom Clemo 《Ground water》2010,48(1):68-78
A model coupling fluid hydraulics in a borehole with fluid flow in an aquifer is developed in this paper. Conservation of momentum is used to create a one-dimensional steady-state model of vertical flow in an open borehole combined with radially symmetric flow in an aquifer and with inflow to the well through the wellbore screen. Both laminar and turbulent wellbore conditions are treated. The influence of inflow through the wellbore screen on vertical flow in the wellbore is included, using a relation developed by Siwoń (1987) . The influence of inflow reduces the predicted vertical variation in head up to 15% compared to a calculation of head losses due to fluid acceleration and the conventional Colebrook-White formulation of friction losses in a circular pipe. The wellbore flow model is embedded into the MODFLOW-2000 ground water flow code. The nonlinear conservation of momentum equations are iteratively linearized to calculate the conductance terms for vertical flow in the wellbore. The resulting simulations agree favorably with previously published results when the model is adjusted to meet the assumptions of the previous coupled models. 相似文献
4.
This study demonstrates the utilization of a multi-objective hybrid global/local optimization algorithm for solving managed aquifer recharge (MAR) design problems, in which the decision variables included spatial arrangement of water injection and abstraction wells and time-variant rates of pumping and injection. The objective of the optimization was to maximize the efficiency of the MAR scheme, which includes both quantitative and qualitative aspects. The case study used to demonstrate the capabilities of the proposed approach is based on a published report on designing a real MAR site with defined aquifer properties, chemical groundwater characteristics as well as quality and volumes of injected water. The demonstration problems include steady state and transient scenarios. The steady state scenario demonstrates optimization of spatial arrangement of multiple injection and recovery wells, whereas the transient scenario was developed with the purpose of finding optimal regimes of water injection and recovery at a single location. Both problems were defined as multi-objective problems. The scenarios were simulated by applying coupled numerical groundwater flow and solute transport models: MODFLOW-2005 and MT3D-USGS. The applied optimization method was a combination of global (the non-dominated sorting genetic algorithm [NSGA-2]) and local (the Nelder-Mead downhill simplex search algorithms). The analysis of the resulting Pareto optimal solutions led to the discovery of valuable patterns and dependencies between the decision variables, model properties, and problem objectives. Additionally, the performance of the traditional global and the hybrid optimization schemes were compared. 相似文献
5.
A Parallel PCG Solver for MODFLOW 总被引：2，自引：0，他引：2
In order to simulate large-scale ground water flow problems more efficiently with MODFLOW, the OpenMP programming paradigm was used to parallelize the preconditioned conjugate-gradient (PCG) solver with in this study. Incremental parallelization, the significant advantage supported by OpenMP on a shared-memory computer, made the solver transit to a parallel program smoothly one block of code at a time. The parallel PCG solver, suitable for both MODFLOW-2000 and MODFLOW-2005, is verified using an 8-processor computer. Both the impact of compilers and different model domain sizes were considered in the numerical experiments. Based on the timing results, execution times using the parallel PCG solver are typically about 1.40 to 5.31 times faster than those using the serial one. In addition, the simulation results are the exact same as the original PCG solver, because the majority of serial codes were not changed. It is worth noting that this parallelizing approach reduces cost in terms of software maintenance because only a single source PCG solver code needs to be maintained in the MODFLOW source tree. 相似文献
6.
A previously published regional groundwater‐flow model in north‐central Nebraska was sequentially linked with the recently developed soil‐water‐balance (SWB) model to analyze effects to groundwater‐flow model parameters and calibration results. The linked models provided a more detailed spatial and temporal distribution of simulated recharge based on hydrologic processes, improvement of simulated groundwater‐level changes and base flows at specific sites in agricultural areas, and a physically based assessment of the relative magnitude of recharge for grassland, nonirrigated cropland, and irrigated cropland areas. Root‐mean‐squared (RMS) differences between the simulated and estimated or measured target values for the previously published model and linked models were relatively similar and did not improve for all types of calibration targets. However, without any adjustment to the SWB‐generated recharge, the RMS difference between simulated and estimated base‐flow target values for the groundwater‐flow model was slightly smaller than for the previously published model, possibly indicating that the volume of recharge simulated by the SWB code was closer to actual hydrogeologic conditions than the previously published model provided. Groundwater‐level and base‐flow hydrographs showed that temporal patterns of simulated groundwater levels and base flows were more accurate for the linked models than for the previously published model at several sites, particularly in agricultural areas. 相似文献
7.
Northern rivers experience freeze‐up over the winter, creating asymmetric under‐ice flows. Field and laboratory measurements of under‐ice flows typically exhibit flow asymmetry and its characteristics depend on the presence of roughness elements on the ice cover underside. In this study, flume experiments of flows under a simulated ice cover are presented. Open water conditions and simulated rough ice‐covered flows are discussed. Mean flow and turbulent flow statistics were obtained from an Acoustic Doppler Velocimeter (ADV) above a gravel‐bed surface. A central region of faster flow develops in the middle portion of the flow with the addition of a rough cover. The turbulent flow characteristics are unambiguously different when simulated ice covered conditions are used. Two distinct boundary layers (near the bed and in the vicinity of the ice cover, near the water surface) are clearly identified, each being characterized by high turbulent intensity levels. Detailed profile measurements of Reynolds stresses and turbulent kinetic energy indicate that the turbulence structure is strongly influenced by the presence of an ice cover and its roughness characteristics. In general, for y/d > 0·4 (where y is height above bed and d is local flow depth), the addition of cover and its roughening tends to generate higher turbulent kinetic energy values in comparison to open water flows and Reynolds stresses become increasingly negative due to increased turbulence levels in the vicinity of the rough ice cover. The high negative Reynolds stresses not only indicate high turbulence levels created by the rough ice cover but also coherent flow structures where quadrants one and three dominate. Copyright © 2012 John Wiley & Sons, Ltd. 相似文献
8.
The ground water flow model MODFLOW inherently implements a nongeneralized integrated finite-difference (IFD) numerical scheme. The IFD numerical scheme allows for construction of finite-difference model grids with curvilinear (piecewise linear) rows. The resulting grid comprises model cells in the shape of trapezoids and is distorted in comparison to a traditional MODFLOW finite-difference grid. A version of MODFLOW-88 (herein referred to as MODFLOW IFD) with the code adapted to make the one-dimensional DELR and DELC arrays two dimensional, so that equivalent conductance between distorted grid cells can be calculated, is described. MODFLOW IFD is used to inspect the sensitivity of the numerical head and velocity solutions to the level of distortion in trapezoidal grid cells within a converging radial flow domain. A test problem designed for the analysis implements a grid oriented such that flow is parallel to columns with converging widths. The sensitivity analysis demonstrates MODFLOW IFD's capacity to numerically derive a head solution and resulting intercell volumetric flow when the internal calculation of equivalent conductance accounts for the distortion of the grid cells. The sensitivity of the velocity solution to grid cell distortion indicates criteria for distorted grid design. In the radial flow test problem described, the numerical head solution is not sensitive to grid cell distortion. The accuracy of the velocity solution is sensitive to cell distortion with error <1% if the angle between the nonparallel sides of trapezoidal cells is <12.5 degrees. The error of the velocity solution is related to the degree to which the spatial discretization of a curve is approximated with piecewise linear segments. Curvilinear finite-difference grid construction adds versatility to spatial discretization of the flow domain. MODFLOW-88's inherent IFD numerical scheme and the test problem results imply that more recent versions of MODFLOW 2000, with minor modifications, have the potential to make use of a curvilinear grid. 相似文献
9.
Hiroyuki Yamada Futoshi Nakamura Yasuharu Watanabe Marie Murakami Takeshi Nogami 《水文研究》2005,19(13):2507-2524
Permeability of a streambed is an important factor regulating nutrient and oxygen availability for aquatic biota. In order to investigate the relationship, an accurate permeability should be measured. However, it is difficult to measure permeability in a coarse gravel bed using a conventional permeability test. Moreover, turbulent flow may occur in coarse bed material, and then deviations from Darcy's law do occur. Thus, permeability calculated following Darcy's law may be overestimated under turbulent flow conditions and should be corrected. The packer test can be used in highly permeable gravel beds. We developed a field method applicable to a gravel bed using the packer test and derived an equation adopting a law of turbulent flow to study the problems under any type of flow condition. The accuracy of the equation was examined using a laboratory flume with a gravel bed. The results suggested that permeability calculated from Hvorslev's equation is overestimated for turbulent flow. In contrast, our equation, developed here, could evaluate permeability accurately under any type of flow condition. Copyright © 2005 John Wiley & Sons, Ltd. 相似文献
10.
A large-eddy simulation study has been undertaken to investigate the turbulent structure of open-channel flow in an asymmetric compound channel. The dynamic sub-grid scale model has been employed in the model, with the partial cell treatment being implemented using a Cartesian grid structure to deal with the floodplain. The numerical model was used to predict the: primary velocity and secondary currents, boundary shear stress, turbulence intensities, turbulent kinetic energy, and Reynolds stresses. These parameters were compared with experimental measurements published in the literature, with relatively close agreement being obtained between both sets of results. Furthermore, instantaneous flow fields and large-scale vortical structures were predicted and are presented herein. These vortical structures were found to be responsible for the significant lateral exchange of mass and momentum in compound channels. 相似文献
11.
12.
Marcelo J. S. De Lemos 《Acta Geophysica》2008,56(3):562-583
The ability to realistically model flows through heterogeneous domains, which contain both solid and fluid phases, can benefit
the analysis and simulation of complex real-world systems. Environmental impact studies, as well as engineering equipment
design, can both take advantage of reliable modelling of turbulent flow in permeable media. Turbulence models proposed for
such flows depend on the order of application of volume-and time-average operators. Two methodologies, following the two orders
of integration, lead to distinct governing equations for the statistical quantities. This paper reviews recently published
methodologies to mathematically characterize turbulent transport in permeable media.
A new concept, called double-decomposition, is here discussed and instantaneous local transport equations are reviewed for
clear flow before the time and volume averaging procedures are applied to them. Equations for turbulent transport follow,
including their detailed derivation and a proposed model for suitable numerical simulations. The case of a moving porous bed
is also discussed and transport equations for the mean and turbulent flow fields are presented. 相似文献
13.
by Tom Clemo Warren Barrash Edward C. Reboulet Timothy C. Johnson Carsten Leven 《Ground water》2009,47(4):515-525
This paper describes a combined field, laboratory, and numerical study of electromagnetic borehole flowmeter measurements acquired without the use of a packer or skirt to block bypass flow around the flowmeter. The most significant finding is that inflow through the wellbore screen changes the ratio of flow through the flowmeter to wellbore flow. Experiments reveal up to a factor of two differences in this ratio for conditions with and without inflow through the wellbore screen. Standard practice is to assume the ratio is constant. A numerical model has been developed to simulate the effect of inflow on the flowmeter. The model is formulated using momentum conservation within the borehole and around the flowmeter. The model is embedded in the MODFLOW-2000 ground water flow code. 相似文献
14.
Direct numerical simulation (DNS) is applied to investigate properties of katabatic and anabatic flows along thermally perturbed
(in terms of surface buoyancy flux) sloping surfaces in the absence of rotation. Numerical experiments are conducted for homogeneous
surface forcings over infinite planar slopes. The simulated flows are the turbulent analogs of the Prandtl (1942) one-dimensional
laminar slope flow. The simulated flows achieve quasi-steady periodic regimes at large times, with turbulent fluctuations
being modified by persistent low-frequency oscillatory motions with frequency equal to the product of the ambient buoyancy
frequency and the sine of the slope angle. These oscillatory wave-type motions result from interactions between turbulence
and ambient stable stratification despite the temporal constancy of the surface buoyant forcing. The structure of the mean-flow
fields and turbulence statistics in simulated slope flows is analyzed. An integral dynamic similarity constraint for steady
slope/wall flows forced by surface buoyancy flux is derived and quantitatively verified against the DNS data. 相似文献
15.
A. Elkhalfi 《Annales Geophysicae》1999,17(4):533-546
The hydrostatic model SALSA is used to simulate a particular event observed during the Greenland Ice Margin EXperiment “GIMEX” (on July 12th, 1991). The time evolution of the large-scale flow was incorporated in the model through time dependent boundary conditions which were updated using the closest upwind sounding. A turbulent scheme for the stable boundary layer and an appropriate parametrization of the surface fluxes implemented in the same model, are used for this study. The simulation results are discussed and compared to the available observations. The computed turbulent fluxes are correctly estimated. The model predicts a mixing zone of about 1500 m high which is in good agreement with tundra site observations. Over the ice cap, the katabatic layer is correctly simulated by the model. Its height of 80–300 m is well estimated. The comparison between the simulation and observations taken at ice cap sites is reasonably valid. The ablation computed along the ice cap corresponds well to the values reconstructed of observations at sites 4 and 9. Finally, a sensibility study to a specified westward geostrophic wind (2 ms−1) shows that the consideration of this latter improves the simulated tundra wind evolution. 相似文献
16.
Robust classification for the joint velocity‐intermittency structure of turbulent flow over fixed and mobile bedforms 
下载免费PDF全文
下载免费PDF全文 Christopher J. Keylock Arvind Singh Jeremy G. Venditti Efi Foufoula‐Georgiou 《地球表面变化过程与地形》2014,39(13):1717-1728
Two datasets of turbulence velocities collected over different bedform types under contrasting experimental conditions show similarity in terms of velocity‐intermittency characteristics and suggest a universality to the velocity‐intermittency structure for flow over bedforms. One dataset was obtained by sampling flow over static bedforms in different locations, and the other was based on a static position but mobile bedforms. A flow classification based on the velocity‐intermittency behaviour is shown to reveal some differences from that based on an analysis of Reynolds stresses, boundary layer correlation and turbulent kinetic energy. This may be attributed to the intermittency variable, which captures the local effect of individual turbulent flow structures. Locations in the wake region or the outer layer of the flow are both shown to have a velocity‐intermittency behaviour that departs from that for idealized wakes or outer layer flow because of the superposition of localized flow structures generated by bedforms. The combined effect of this yields a velocity‐intermittency structure unique to bedform flow. The use of a time series of a single velocity component highlights the potential power of our approach for field, numerical and laboratory studies. The further validation of the velocity‐intermittency method for non‐idealized flows undertaken here suggests that this technique can be used for flow classification purposes in geomorphology, hydraulics, meteorology and environmental fluid mechanics. © 2014 The Authors. Earth Surface Processes and Landforms published by John Wiley & Sons Ltd. 相似文献
17.
18.
Although flow turbulence in rivers is of critical importance to earth scientists, ecologists and engineers, its relations with larger flow scales are not well understood, thus leaving a fundamental gap in our knowledge. From an analysis of a long time series of the streamwise and vertical flow velocity fluctuations measured in a gravel‐bed river, we show that the signature of the fundamental turbulent flow structures (e.g. ejections and sweeps) is embedded within increasingly larger flow scales in a self‐similar manner. The imbrication of turbulent structures into large flow pulsations of flow acceleration and deceleration covers more than two‐orders of magnitude from a few seconds to nearly 10 minutes. This property is explained by the clustering of turbulent events creating an emergent pattern at larger scales. The size of the larger flow pulsations scales with the spacing of the pools and riffles in the river. This implies a mutual adjustment between turbulence generation mechanisms and long pulsations of flow acceleration and deceleration controlled by the bed morphology. These results bridge a gap in our understanding of flows in rivers and offer a new perspective on the interactions between the turbulent flow with larger scales of flow motion that are critical for sediment transport, habitat selection and fish behaviour. Copyright © 2011 John Wiley & Sons, Ltd. 相似文献
19.
Detailed echo‐sounder and acoustic Doppler velocimeter measurements are used to assess the temporal and spatial structure of turbulent flow over a mobile dune in a wide, low‐gradient, alluvial reach of the Green River. Based on the geometric position of the sensor over the bedforms, measurements were taken in the wake, in transitional flow at the bedform crest, and in the internal boundary layer. Spatial distributions of Reynolds shear stress, turbulent kinetic energy, turbulence intensity, and correlation coefficient are qualitatively consistent with those over fixed, two‐dimensional bedforms in laboratory flows. Spectral and cospectral analysis demonstrates that energy levels in the lee of the crest (i.e. wake) are two to four times greater than over the crest itself, with minima over the stoss slope (within the developing internal boundary layer). The frequency structure in the wake is sharply defined with single, dominant peaks. Peak and total spectral and cross‐spectral energies vary over the bedform in a manner consistent with wave‐like perturbations that ‘break’ or ‘roll up’ into vortices that amalgamate, grow in size, and eventually diffuse as they are advected downstream. Fluid oscillations in the lee of the dune demonstrate Strouhal similarity between laboratory and field environments, and correspondence between the peak frequencies of these oscillations and the periodicity of surface boils was observed in the field. Copyright © 2005 John Wiley & Sons, Ltd. 相似文献
20.
NUMERICAL SIMULATION OF HEAD-CUT WITH A TWO-LAYERED BED 总被引：1，自引：0，他引：1
Yafei JIA Tadanofi KITAMURA S. S. Y. WANG 《国际泥沙研究》2005,20(3):185-193
1INTRODUCTION The rate of gully erosion is dominated by the upstream migration of existing nick-points called headcut.Due to the shape of the headcut,the flow from the upstream channel impinges into the pool of the scour hole and forms a complex three-dimensional flow structure.The turbulent flow deepens the scour hole,transports the eroded material downstream,undercuts the headcut wall and creates gravitational slumping of the gully head material.In reality,the occurrence of a head cut i… 相似文献