Browsing by Author "Chang, C. -M."

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Investigation of solute transport in nonstationary unsaturated flow fields
(2012)
Chang, C. -M.
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Yeh, H. -D.
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環境工程研究所
;
Institute of Environmental Engineering
Most theoretical work on analyzing plume spreading at the field scale in a partially saturated heterogeneous formation assumes weak stationarity of velocity field. While this assumption is not applicable to the case of a bounded flow domain, the nonstationarity in fluctuations of unsaturated velocity fields is induced through the presence of the boundary conditions in the flow domain. In this work, we attempt to quantify the large-time macrodispersion in nonstationary unsaturated velocity fields caused by the presence of a fixed head boundary condition. Application of the perturbation-based nonstationary spectral approach leads to an analytical expression for the macrodispersion describing the field-scale dispersive solute flux in terms of the statistical moments of two formation parameters: the Gardener's parameter (alpha) and the saturated hydraulic conductivity (K-s). The results predicted from the expression indicate that the enhanced unsaturated plume spreading can arise from a larger correlation scale of ln K-s or ln alpha fields. In addition, the alpha-parameter takes the role in reducing the field-scale plume spreading.
Oropharyngeal yeast colonization in HIV-infected outpatients in southern Taiwan: CD4 count, efavirenz therapy and intravenous drug use matter
(2012-05-01)
Wu, C. -J.
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Lee, H. -C.
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Yang, Y. -L.
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Chang, C. -M.
;
Chen, H. -T.
To understand the status of oropharyngeal yeast colonization in human immunodeficiency virus (HIV) -infected outpatients in the era of highly active antiretroviral therapy (HAART), we conducted a prospective, cross-sectional study from October 2009 to January 2010 at a medical centre in southern Taiwan. Fungal cultures of the oropharyngeal swabs were performed on 327 enrolled patients. At enrolment, 258 (79%) patients had been receiving HAART, and 42 (12.8%), 73 (22.3%) and 212 (64.8%) patients had CD4 cell counts =200, 201350, and >350 cells/mm3, respectively. Oral yeast colonization was detected in 193 (59%) patients, among whom 157 (81.3%), 25 (13.0%), and 11 (5.7%) were colonized by a single, two and more than two species, respectively. Multivariate analysis showed that receipt of efavirenz-containing regiments and CD4 cell counts >200 cells/mm3 were associated with lower risks of oral yeast colonization, while intravenous drug users were at a higher risk. Among the 241 isolates recovered, Candida albicans accounted for 69.7%, followed by C.similar to dubliniensis (9.5%), C.similar to glabrata (8.3%), C similar to tropicalis (3.3%), C.similar to intermedia (2.1%), C.similar to parapsilosis (1.7%), and 11 other species (5.4%). Overall, 230 (95.4%), 236 (97.9%) and 240 (99.6%) isolates were susceptible to fluconazole, voriconazole and amphotericin B, respectively. In conclusion, colonization by C.similar to dubliniensis has emerged in recent years. In addition to a CD4 cell count =200 cells/mm3, which is a known risk factor for oropharyngeal yeast colonization in HIV-infected patients that was identified in our previous studies, two risk factors, non-receipt of efavirenz-based combinations and intravenous drug use, were first identified in the present study. Fluconazole remained effective in vitro against the yeasts colonizing the oropharynx in this population.
Spectral approach to seawater intrusion in heterogeneous coastal aquifers
(2010)
Chang, C. -M.
;
Yeh, H. -D.
;
環境工程研究所
;
Institute of Environmental Engineering
This work presents a two-dimensional horizontal plane model of seawater intrusion into a heterogeneous coastal unconfined aquifer under the spatially random recharge condition from a stochastic point of view. The stochastic response of the coastal unconfined aquifer is attributed to uncertainties related to the heterogeneity of aquifer properties and spatial variability of the recharge. Based on the sharp-interface assumption, analytical expressions for the variances of the seawater-freshwater interface elevation and specific discharge in the longitudinal and transverse directions are developed using the perturbation approximation and spectral Fourier-Stieltjes nonstationary representations. We focus on the study of the impact of the heterogeneity of aquifer properties and recharge on these results.
Stochastic analysis of field-scale heat advection in heterogeneous aquifers
(2012)
Chang, C. -M.
;
Yeh, H. -D.
;
環境工程研究所
;
Institute of Environmental Engineering
Owing to the analogy between the solute and heat transport processes, it can be expected that the rate of growth of the spatial second moments of the heat flux in a heterogeneous aquifer over relatively large space scales is greater than that predicted by applying the classical heat transport model. The motivation of stochastic analysis of heat transport at the field scale is therefore to quantify the enhanced growth of the field-scale second moments caused by the spatially varying specific discharge field. Within the framework of stochastic theory, an effective advection-dispersion equation containing effective parameters (namely, the macrodispersion coefficients) is developed to model the mean temperature field. The rate of growth of the field-scale spatial second moments of the mean temperature field in the principal coordinate directions is described by the macrodispersion coefficient. The variance of the temperature field is also developed to characterize the reliability to be anticipated in applying the mean heat transport model. It is found that the heterogeneity of the medium and the correlation length of the log hydraulic conductivity are important in enhancing the field-scale heat advection, while the effective thermal conductivity plays the role in reducing the field-scale heat advection.
Variability of flow discharge in lateral inflow-dominated stream channels
(2015-01-01)
Chang, C. -M.
;
Yeh, H. -D.
;
環境工程研究所
;
Institute of Environmental Engineering
The influence of the temporal changes in lateral inflow rate on the discharge variability in stream channels is explored through the analysis of the diffusion wave equation (i.e. the linearized Saint-Venant equation). To account for variability and uncertainty, the lateral inflow rate is regarded as a temporal random function. On the basis of the spectral representation theory, analytical expressions for the covariance function and evolutionary power spectral density of the random discharge perturbation process are derived to quantify variability in stream flow discharge induced by the temporal changes in lateral inflow rate. The treatment of the discharge variance (square root of the variance) gives us a quantitative estimate of uncertainty in predictions from the deterministic model. It is found that the discharge variability of stream flow is very large in the downstream reach, indicating large uncertainty anticipated from the use of the deterministic model. A larger temporal correlation scale of inflow rate fluctuations, representing more temporal consistency of fluctuations in inflow rate around the mean, introduces a higher variability in stream flow discharge.