Water Migration Model Research Articles

Migration behavior of water molecules and chloride ions at the nanopore scale based on molecular properties under temperature effect

The durability of cement-based materials is closely related to the transmission characteristics of water and chloride ions in the pores of calcium silicate hydrate (C-S-H) gel, especially in the climate conditions with significant temperature difference. The local structure and transport process of water molecules and chloride ions in gel pores are affected by interfacial effects. The unsaturated transport model of sodium chloride solution in C-S-H gel pores is established on the molecular scale, under different temperature and pore size conditions. Divide the regions in the C-S-H gel pore model into internal adsorption zone, external adsorption zone, and free zone. Taking into account the intrinsic mechanisms and molecular properties of water molecules and ions during transport in the C-S-H gel pores, the local structures and dynamic properties of water molecules and ions in the gel pores are quantitatively described. The electrical double layer and the double-layer water film overlap in the 1.5 nm gel pore, the water molecule and the chloride ion are closely arranged. Thus the diffusion coefficient is far lower than others, and is rarely affected by temperature. Considering the changes in the molecule properties of water and ion in different zones in molecular simulation results, a theoretical model of nanopore water and ion migration was established, revealing the migration behavior of water and ions. The results indicate that the steady-state diffusion fluxes of water and chloride ions are positively correlated with the effects of temperature and pore size, and the increasing amplitude of their diffusion fluxes is characterised by a similar phenomenon with that. This research provides a theoretical basis for the mechanism and durability of cement-based materials corroded by water and chloride ions in marine environments.

A Computational Model of Water Migration in a Closed System of Unsaturated Expansive Clay

Expansive clay behaves like mechanical performances of multi-fissures, and deterioration with the changing moisture. To ensure the safe operation of foundation, the effects of intensive water migration on earth constructions founded on semiarid area, have therefore been received much more attention together with the global warming and climate changes. Herein, analytical models considering respectively liquid water and water vapor migration were presented to address changes in moisture content of unsaturated expansive clay under various water content gradients. Moreover, measured data obtained from migration experiments performed under various water content gradients in a closed system, were utilized to check the validity of analytical equations. Results indicate that the migration of both water vapor and mixed water shows nonlinear relationships with time and location. For the expansive clay with low moisture content, major components of migration water was water vapor, which accounted for 83.61–88.62%; while for the clay with higher water content, the liquid water was dominant migration of water due to more free water around the soil particles. The proposed model is helpful to investigate moisture migration law in the unsaturated expansive clay.

Dynamic Simulation of Soil Salt Transport in Arid Irrigation Areas under the HYDRUS-2D-Based Rotation Irrigation Mode

This research aims to explore the dynamic transport of water and salt in soil under different irrigation modes and disclose the desalting effect of different irrigation factors in inland arid irrigation areas. Jingdian Irrigation District in Gansu Province, which is located in the arid region of Northwest China, is selected as a typical experimental area. Soil water and salt dynamic migration model is constructed based on HYDRUS-2D software, and field irrigation experiments are designed according to actual irrigation conditions in crop growth period. The model is used to simulate the salt transport process between saturated soil and unsaturated soil as well as the effects of different irrigation quotas, irrigation rounds and quota allocations on soil desalting. Through simulation, the vertical movement pattern of water and salt in the soil of the test area is analyzed, and the optimal design of the soil desalting irrigation plan is carried out. The results demonstrate that: after two-round irrigation, the average soil salinity of the tillage layer at each test points decreases significantly under evapotranspiration; the infiltration of irrigation water has an obvious impact on soil desalting; the deviation between the simulated and the measured value is less than 0.5%, which proves the model fitting result reliable; with fixed irrigation cycle and times, the soil desalting efficiency is higher when the irrigation quota falls in the range of 4000 ~ 6000 m3/hm2; when the first round of irrigation water only saturates the moisture content and gradually increases the irrigation quota in the subsequent rounds, it is more conducive to the dissolution and discharge of soil salinity. In summary, the results of this study can provide technical support for the protection of water and soil resources, as well as the improvement of saline-alkali land in inland arid irrigation areas.

Mathematical and Experimental Investigation of Water Migration in Plant Xylem

AbstractPlant can take water from soil up to several metres high. However, the mechanism of how water rises against gravity is still controversially discussed despite a few mechanisms have been proposed. Also, there still lacks of a critical transportation model because of the diversity and complex xylem structure of plants.This paper mainly focuses on the water transport process within xylem and a mathematical model is presented. With a simplified micro channel from xylem structure and the calculation using the model of water migration in xylem, this paper identified the relationship between various forces and water migration velocity. The velocity of water migration within the plant stem is considered as detail as possible using all major forces involved, and a full mathmetical model is proposed to calculate and predict the velocity of water migration in plants.Using details of a specific plant, the velocity of water migration in the plant can be calculated, and then compared to the experimental result from Magnetic Resonance Imaging (MRI). The two results match perfectly to each other, indicating the accuracy of the mathematical model, thus the mathematical model should have brighter future in further applications.

A model of migration potential for moisture migration during soil freezing

Frost heave is attributed to water migration to the freezing front and the ice lens develops. Therefore, evaluation of frost heave requires the determination of the water migration in the freezing process. In order to predict the water migration in freezing soil, a water migration model which introduced the concept of migration potential (MP) was presented. This model presents a new approach for describing the water migration in freezing soils, in which the determination of the permeability coefficient in the frozen fringe and unfrozen zone was avoided. To verify the correctness of the proposed model, a series of unidirectional freezing experiments were conducted. Results show that small amount of water is intake to the soil due to the fast freezing rate, and that once the freezing rate becomes slow, the total amount of water intake is approximately increased with elapsed time. The water intake flux always changing with the elapsed time. As a result, the migration potential is a function of elapsed time. Finally, it is demonstrated that the predicted water intake flux is similar to the measured ones.

The hydrothermal alteration of cooling lava domes

Hydrothermal alteration is a recognized cause of volcanic instability and edifice collapse, including that of lava domes or dome complexes. Alteration by percolating fluids transforms primary minerals in dome lavas to weaker secondary products such as clay minerals; moreover, secondary mineral precipitation can affect the porosity and permeability of dome lithologies. The location and intensity of alteration in a dome depend heavily on fluid pathways and availability in conjunction with heat supply. Here we investigate postemplacement lava dome weakening by hydrothermal alteration using a finite element numerical model of water migration in simplified dome geometries. This is combined with the rock alteration index (RAI) to predict zones of alteration and secondary mineral precipitation. Our results show that alteration potential is highest at the interface between the hot core of a lava dome and its clastic talus carapace. The longest lived alteration potential fields occur in domes with persistent heat sources and permeabilities that allow sufficient infiltration of water for alteration processes, but not so much that domes cool quickly. This leads us to conclude that alteration-induced collapses are most likely to be shallow seated and originate in the talus or talus/core interface in domes which have a sustained supply of magmatic heat. Mineral precipitation at these zones of permeability contrast could create barriers to fluid flow, potentially causing gas pressurization which might promote deeper seated and larger volume collapses. This study contributes to our knowledge of how hydrothermal alteration can affect lava domes and provides constraints on potential sites for alteration-related collapses, which can be used to target hazard monitoring.

Moisture migration tests on unsaturated expansive clays in Hefei, China

Expansive clay in semiarid area behaves features of typical unsaturated soil, which is sensitive to climate and moisture, and appears multi-fissures, deterioration of mechanical performances with the change of moisture. Consequently, to ensure the rational design of foundation treatment, and safe operation, research on moisture migration of expansive clays is of great significance. Herein, test models of mixed water migration and vaporous water migration were presented to find out the mechanism. Series of tests, in which the moisture content in left soil column was the same as 24%, and in right soil column was 0%, 6%, 12% and 18% respectively, were conducted under constant temperature. It is concluded that moisture content changed with the increase of migration time in both migration models. And the migration behaves nonlinearly when the initial water content increases up to 6% for mixed water migration model and 12% for vaporous water migration model. But the migration characteristics of mixed water and vaporous water were different. In the initial 30days the change of moisture water was larger in mixed migration model, however within 60 to 90days, it was larger in vaporous water migration model. The research results have been applied to guide relevant engineering practice.

Non-isothermal, three-phase simulations of near-surface flows in a model permafrost system under seasonal variability and climate change

Permafrost responses to a changing climate can affect hydrological and biogeochemical cycling, ecosystems and climate feedbacks. We have simulated a model permafrost system in the temperature range associated with discontinuous permafrost focusing on interactions between permafrost and hydrology using a non-isothermal, three-phase model of water migration coupled to heat transport in partially frozen porous media. We explore the subsurface hydraulic property controls on the formation and dynamics of permafrost, and how this impacts seasonal variability of subsurface runoff to surface waters. For all subsurface conditions considered, the main common hydrological signal of permafrost degradation in a warming trend is decreasing seasonal variability of water flow. This is due to deeper and longer flow pathways with increasing lag times from infiltration or thawing through subsurface flow to surface water discharge. These results show how physically based numerical modelling can be used to quantitatively and qualitatively improve the understanding of how permafrost thawing relates to, and may be detected in, hydrological data. This is advantageous since hydrological data is considerably easier to obtain, may be available in longer time series, and generally reflects larger-scale conditions than direct permafrost observations.

Three-phase numerical model of water migration in partially frozen geological media: model formulation, validation, and applications

Water in the subsurface of the Earth’s cold regions—and possibly the subsurface of Mars—resides in the liquid, vapor, and ice phases. However, relatively few simulations addressing full three-phase, nonisothermal water dynamics in below-freezing porous media have been undertaken. This paper presents a nonisothermal, three-phase approach to modeling water migration in partially frozen porous media. Conservation equations for water (as ice, liquid, and vapor) and a single gas species (in the gas phase and dissolved in water) are coupled to a heat transport equation and solved by a finite-volume method with fully implicit time stepping. Particular attention is given to the method of spatial differencing when the pore space is partially filled with ice. The numerical model is able to reproduce freezing-induced water redistribution observed in laboratory experiments. Simulations of Earth permafrost dynamics and of the formation and evolution of a planetary-scale cryosphere on Mars demonstrate the new capabilities.

A study on concrete degradation during molten core/concrete interactions

A series of MCCI tests was performed in COTELS project at NUPEC to examine concrete degradation characteristics during MCCI with and without water addition onto the debris. Molten stainless steel or a mixture composed of UO 2, ZrO 2, Zr and stainless steel was slumped into a two-dimensional concrete trap, where volumetric decay heat generation was simulated by an induction heating technique. The results of dry MCCI tests implied that concrete ablation was dominated by melting of aggregates when the debris was crusted and cement was thermally weaker than aggregates. Without presence of stable crust, unmolten aggregates were possible to relocate upward due to the density difference from the debris. Concrete responses under a wet condition showed a tendency that water migrated into thermally degraded concrete. A preliminary water migration model was incorporated into COCO code for transient heat conduction. The prediction by COCO code agreed with the tendency of concrete thermal responses observed in the dry and wet MCCI tests.

Reversible changes in fluorescence of bacterial endospores found in aerosols due to hydration/drying

In this study we show that the room temperature multiwavelength autofluorescence signature of wet Bacillus endospores is considerably different from that of the dry endospores. The fluorescence is measured with excitation wavelengths of 300–500 nm with 50 nm steps. Small differences are observed between the dry and the re-dried spores. The fluorescence spectra support the model of reversible water migration between inner spore compartments and the environment. Characteristic information is extracted from spectral fluorescence excitation–emission contours for these spores by applying the dimensional reduction of data techniques known as principal component analysis. Aerosol detection and identification techniques relying on fluorescence must take into account the hydration state of the spores and recognize the ability of the spores to rapidly change their state of hydration.

Spatial Distribution and Temporal Change of Water Volume and Quality of Throughfall and Stemflow

Water volume and quality of throughfall and stemflow were examined for their spatial distribution and temporal change by observations under a single conifer tree on rainy days and by simulations using “tree crown water migration model”. Observation results showed that throughfall decreased on an average as the distance from stem decreased but some spots had more throughfall than rainfall, that reaching rate increased as the total of throughfall and stemflow increased, that acid rainfall was neutralized by tree crown, that stemflow had more solute than rainfall but stemflow was both less or more acid than rainfall, that throughfall was enriched with Cl-, N03-N, S04-S and NH4-N than rainfall, that the concentration of throughfall was higher near the stem than near the tree crown edge, and that the concentration of throughfall became low as time passed. Spatial distribution of throughfall was examined using the tree crown water migration model which represents the interception and migration within tree crown as those in and between boxes covering the profile of tree crown. Simulations using this model showed that both interception and migration to the direction of the tree crown edge accounted for the spatial distribution of throughfall.