Browsing by Author "Fen, Chiu-Shia"

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A general analytical model for pumping tests in radial finite two-zone confined aquifers with Robin-type outer boundary
(2016-09)
Lin, Ye-Chen
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Yang, Shaw-Yang
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Fen, Chiu-Shia
;
Yeh, Hund-Der
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環境工程研究所
This study develops a general analytical model for describing transient drawdown distribution induced by pumping at a finite-radius well in a radial two-zone confined aquifer of finite areal extent with Robin-type condition at both inner and outer boundaries. This model is also applicable to heat conduction problems for a composite hollow cylinder on the basis of the analogy between heat flow and groundwater flow. The time-domain solution of the model is derived by the methods of Laplace transform, Bromwich integral, and residue theorem. This new solution can reduce to the solution for constant-head test (CHT) or constant-rate test (CRT) problem by specifying appropriate coefficients at the Robin inner boundary condition. The solution describing the flow rate across the wellbore due to CHT is further developed by applying Darcy\'s law to the new solution. In addition, steady-state solutions for both CHT and CRT are also developed based on the approximation for Bessel functions with very small argument values. Many existing solutions for transient flow in homogeneous or two-zone finite aquifers with Dirichlet or no-flow condition at the outer boundary are shown to be special cases of the present solution. Furthermore, the sensitivity analysis is also performed to investigate the behaviors of the wellbore flow due to CHT and the aquifer drawdown induced by CRT in response to the change in each of aquifer parameters. (C) 2016 Elsevier B.V. All rights reserved.
Analysis of Transient Flow to an Extended Fully Penetrating Well at Constant-Head Pumping
(2020-01-01)
Jhuang, Wei-Hao
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Fen, Chiu-Shia
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Shen, Yu-Te
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Yeh, Hund-Der
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環境工程研究所
Vertical wells with radial extension at the well bottom can improve the rate of water production. No study has yet investigated the effects of the transient state and anisotropy in directional hydraulic conductivities on the wellbore flux rate for this type of well. This study derives a semianalytical transient drawdown solution for constant-head pumping at a fully penetrating well radially extended at the bottom of a confined, anisotropic aquifer by applying Laplace transform and separation of variables as well as conducting a Fourier analysis. The results of this new solution indicate that transient and steady-state wellbore flux rates can be increased by a factor of two for greater radial extension of the well. Compared with an isotropic aquifer (a ratio of vertical and horizontal hydraulic conductivities equal to one), an anisotropic aquifer with the ratio less than one may produce a higher transient wellbore flux rate and lower steady-state wellbore flux rate. Moreover, the time required to achieve the steady-state wellbore flux rate can be substantially affected by anisotropy of the aquifer.
Effect of Well Radius on Drawdown Solutions Obtained with Laplace Transform and Green's Function
(2012-01-01)
Fen, Chiu-Shia
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Yeh, Hund-Der
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環境工程研究所
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Institute of Environmental Engineering
We have developed a drawdown solution for a partially penetrating well under constant flux pumping in a confined aquifer with finite thickness. The predictions of our solution diverge from the predictions of Hantush's solution (1961), particularly for problems with low ratios of well screen length to aquifer thickness. Furthermore, the predicted drawdown from Hantush's solution (1961) differs from that of Yang et al.'s solution Water Resour Res 42:W0552, (2006) only near the well and at small time values as indicated in Yang et al. Water Resour Res 42:W0552, (2006). Our solution is based on Green's function with a columnar source (sink) that represents pumping from a finite-radius well. Hantush's solution (1961) and Yang et al.'s solution Water Resour Res 42:W0552, (2006), however, were derived from Laplace transform techniques for pumping in a well with an infinitesimal and a finite radius, respectively.
Modeling volatilization of residual VOCs in unsaturated zones: A moving boundary problem
(2012-06-15)
Abbas, Talib R.
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Yu, Jung-Hau
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Fen, Chiu-Shia
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Yeh, Hund-Der
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Yeh, Li-Ming
It is of practical interest to investigate the natural evaporation of volatile organic compounds (VOCs) after the removal of a leaking tank situated on the top of the soil. This study aims to develop a mathematical model to predict mole fraction distributions and migration of evaporation front for two VOCs emanating from residual non-aqueous phase liquid (NAPL) due to the leak from the tank in a homogeneous soil. Considering the location of the front and the regions above and below the front, a numerical model for the diffusive transport of VOCs in unsaturated soils was developed using the finite difference method with a moving grid approach. The model was further simplified to the case of single VOC and solved analytically by Boltzmann's transformation with a moving boundary. Analytical expressions for the depth and moving speed of the front for a single VOC were then obtained for practical use. Finally, the developed model was used to predict the concentration distributions of VOCs below the land surface and examine the factors affecting the location and moving speed of the evaporation front. (C) 2012 Elsevier B.V. All rights reserved.
Solution for Soil Vapor Extraction from a Pressure-Controlled Well
(2017-06-01)
Kuo, Ching-Lin
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Fen, Chiu-Shia
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Chen, Kai-Hsin
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Yeh, Hund-Der
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環境工程研究所
This paper presents a new solution for a soil venting problem, in which a vertical, partially penetrating well is operated at a fixed pressure under covered, uncovered, or leaking ground surface conditions. The method of domain decomposition is applied to solve the problem of a mixed-type boundary condition imposed along a well screen, which in previous solutions was usually handled by discretization of the screen. The new solution is also capable of predicting the flow field beneath the well bottom, which is commonly neglected by other solutions. The authors found that the present solution may predict different pressure distributions from those given by a solution employing a constant flux rate at a well screen. The radius of influence for soil vapor extraction (SVE) operations is strongly affected by the radial permeability, the vertical permeability, and the ratio between the two. Vacuum pressure values were predicted with the proposed solution and compared against measurements at different observation wells and depths under venting tests at a former SVE-operated site. The measured vacuum pressure and gas flow velocity values around the wells were found to be substantially lower than the predicted values. Thus, the wellbore soils of the site were suspected to be positive skin zones. (C) 2017 American Society of Civil Engineers.