Ambipolarity condition and possibility of multivalued steady-state solutions to transport equations in stellarators

The possibility of steady-state multivalued solutions to transport equations in stellarators is considered. It is shown that the ambipolarity condition is necessary but not sufficient to find the ambipolar electric field, because the functions entering into it (the plasma density and temperature, as well as their spatial derivatives) depend on the ambipolar field. To do this correctly, it is necessary to solve the full set of time-independent transport equations (including diffusion and heat conduction equations). The possible existence of multivalued solutions to this set of equations is analyzed numerically. It is shown that, under certain conditions that depend on the form and magnitude of particle and heat sources, such solutions can exist. Their form is determined by the initial value of the ambipolar field, the source magnitudes, and the boundary conditions. Discontinuous solutions in which the radial profile of the ambipolar field undergoes jumps are found. In this case, however, the particle and energy fluxes remain continuous, because the discontinuities of the electric field are balanced by the discontinuities of the density and temperature gradients.     

Evaluation of Slurry Wall Construction Using Paper Clay for Containment of Contaminated Groundwater

Slurry walls have been used widely as passive vertical barriers to control the horizontal flow of groundwater and contaminants and therefore limit the migration of contaminants through the subsurface. The material used as slurry to reduce contaminant transport is expected to form a filter cake with low permeability along the sides of the trench wall/excavation face. In this article, paper mill sludge (hereafter referred to as paper clay) is investigated as a possible slurry. Previous research by Moo Young et al. (2000) has shown that paper clay has several properties that make its use in vertical barriers very promising. It can be compacted to low permeability values (10^?7?cm/s to 10^?9?cm/s) and has a high organic content that may act as a potential carbon source for microbial growth and sorption sites for heavy metal attenuation. To determine the feasibility of paper clay in a slurry, column testing to determine the dispersion coefficient for transport modeling, slump testing, and filter cake formation tests were performed. It is shown that the coefficient of hydrodynamic dispersion within paper clay is in the order of 10^?7?cm²/ s. It is also shown that paper clay can achieve the required slump of between 5.08 and 15.24?cm (2 and 6 in) similar to that of a workable soil-bentonite backfill. Furthermore, it is shown that the filter cakes that are formed have permeability values similar to those of soil-bentonite filter cakes.     

Osmotic Transport across Cell Membranes in Nondilute Solutions: A New Nondilute Solute Transport Equation

The fundamental physical mechanisms of water and solute transport across cell membranes have long been studied in the field of cell membrane biophysics. Cryobiology is a discipline that requires an understanding of osmotic transport across cell membranes under nondilute solution conditions, yet many of the currently-used transport formalisms make limiting dilute solution assumptions. While dilute solution assumptions are often appropriate under physiological conditions, they are rarely appropriate in cryobiology. The first objective of this article is to review commonly-used transport equations, and the explicit and implicit assumptions made when using the two-parameter and the Kedem-Katchalsky formalisms. The second objective of this article is to describe a set of transport equations that do not make the previous dilute solution or near-equilibrium assumptions. Specifically, a new nondilute solute transport equation is presented. Such nondilute equations are applicable to many fields including cryobiology where dilute solution conditions are not often met. An illustrative example is provided. Utilizing suitable transport equations that fit for two permeability coefficients, fits were as good as with the previous three-parameter model (which includes the reflection coefficient, σ). There is less unexpected concentration dependence with the nondilute transport equations, suggesting that some of the unexpected concentration dependence of permeability is due to the use of inappropriate transport equations.     

Controlled environment soil‐core microcosm unit for investigating fate,migration, and transformation of chemicals in soils

The controlled environment soil‐core microcosm unit (CESMU) methods embody a collection of techniques that began with soil sampling in the field and continued throughout the laboratory investigation of chemical fate, migration, and transformation in site‐specific soils; it was a cost‐effective investigative methodology that could be used to screen chemical materials before initiating high‐cost environmental field studies. Intact soil cores were collected in the field using a hydraulically controlled probe, delivering intact soil‐cores with minimal disturbance directly into high‐density polyethylene pipe (10.3‐cm ID). The inert polyethylene pipe was an effective hydrophobic barrier that remained an integral part of the soil‐core column, obviating subsequent transfers of soil. In the laboratory, each soil column was fitted with a porous ceramic plate and a polyethylene endcap containing fittings for teflon tubing, so that a tension could be applied at the bottom of each soil column (30–35 kPa) to mimic field conditions, thus preventing the undue buildup of water within columns that otherwise would change the chemical, physical, and biological properties of the soil. The intact soil‐cores were housed in the CESMU chamber, a controlled temperature unit with sufficient capacity for maintaining constant temperature within entire soil‐cores. Synthetic rain was added twice a week by peristaltic pump at rates simulating rainfall. Leachates were collected under tension via teflon tubing into flasks in darkness and kept at soil column temperature inside CESMU until harvested for analyses. Soil columns were harvested at intervals for sectioning by depth, extraction, and soil analyses. CESMU methods are applicable to investigations of water movement, soil chemistry, solute transport/transformation, and effects on plants.     

Hydrodynamic and sediment transport modeling with emphasis on shallow-water,vegetated areas (lakes,reservoirs, estuaries and lagoons)

Modeling capabilities for shallow, vegetated, systems are reviewed to assess hydrodynamic, wind and wave, submersed plant friction, and sediment transport aspects. Typically, ecosystems with submersed aquatic vegetation are relatively shallow, physically stable and of moderate hydrodynamic energy. Wind-waves are often important to sediment resuspension. These are open systems that receive flows of material and energy to various degrees around their boundaries. Bed shear-stress, erosion, light extinction and submersed aquatic vegetation influence each other. Therefore, it is difficult to uncouple these components in model systems. Spatial changes in temperature, salinity, dissolved and particulate material depend on hydrodynamics. Water motions range from wind-wave scales on the small end, which might be important to erosion, to sub-tidal or seasonal scales on the large end, which are generally important to flushing. Seagrass modifies waves and, therefore, affects the relationships among the non-dimensional scaling parameters commonly used in wave analysis. Seagrass shelters the bed, often causing aggradation and changes in grain size, while increasing total resistance to flow. Hydrodynamic friction can not be well characterized by a single-parameter equation in seagrass beds, and models need appropriate enhancement when applied to these systems.Presently, modeling is limited by computational power, which is, however, improving. Other limitations include information on seagrass effects expressed in frictional resistance to currents, bed-sheltering, and wave damping in very shallow water under conditions of both normal and high bed roughness. Moreover, quantitative information on atmospheric friction and shear stress in shallow water and seagrass areas are needed. So far, various empirical equations have been used with wind or wave forcing to describe resuspension in shallow water. Although these equations have been reasonably successful in predicting suspended sediment concentrations, they require site-specific data. More detailed laboratory and field measurements are needed to improve the resuspension equations and model formulation pertaining to seagrass beds.     

Characteristic properties of fluctuating particle fluxes near the last closed flux surface in the course of lower hybrid heating and during a transition into the improved confinement mode in the FT-2 tokamak

Results are presented from experimental studies of the time behavior of the transport processes in the edge plasma of the FT-2 tokamak during auxiliary lower hybrid heating when an internal transport barrier and then an external transport barrier form in the plasma. An analysis of the data on turbulent particle transport in the edge plasma shows that the radial electric field generated inside the plasma column during auxiliary heating plays an important role in both the formation of a transport barrier and the suppression of anomalous transport at the plasma periphery in the postheating phase of the discharge. The mechanism for the formation of a negative radial electric field E _r near the last closed flux surface after the end of the lower hybrid heating pulse is considered. Fluctuation spectra of the particle density and poloidal electric field are presented that characterize the process of suppression of microturbulence at the plasma periphery. The experimental data were obtained with the use of movable multielectrode Langmuir probes.     

Diffusion of TCE Through Soil-Bentonite Slurry Walls

Diffusion experiments performed using both a dissolved solution of trichloroethylene (TCE) and a pool of free phase TCE adjacent to a simulated soil-bentonite (SB) wall are described. These tests examine a multi-layer system that includes both contaminated sand and a SB barrier. Results obtained from experiments with dissolved TCE as the primary source are shown to be consistent with those obtained with free-phase TCE as the source of contaminant. Diffusion and sorption coefficients of a soil-bentonite slurry wall are reported to be 3.5 × 10^?10 m ² /s and 0 cm ³ /g, respectively. These diffusion and sorption coefficients were used to evaluate the effectiveness of a hypothetical SB slurry wall located adjacent to a TCE spill.     

Current status and challenges in connecting models of erythrocyte metabolism to experimental reality

Detailed kinetic models of human erythrocyte metabolism have served to summarize the vast literature and to predict outcomes from laboratory and “Nature's” experiments on this simple cell. Mathematical methods for handling the large array of nonlinear ordinary differential equations that describe the time dependence of this system are well developed, but experimental methods that can guide the evolution of the models are in short supply. NMR spectroscopy is one method that is non-selective with respect to analyte detection but is highly specific with respect to their identification and quantification. Thus time courses of metabolism are readily recorded for easily changed experimental conditions. While the data can be simulated, the systems of equations are too complex to allow solutions of the inverse problem, namely parameter-value estimation for the large number of enzyme and membrane-transport reactions operating in situ as opposed to in vitro. Other complications with the modelling include the dependence of cell volume on time, and the rates of membrane transport processes are often dependent on the membrane potential. These matters are discussed in the light of new modelling strategies.     

Influence of Precipitation and Soil on Transport of Fecal Enterococci in Fractured Limestone Aquifers

Limestone aquifers provide the main drinking water resources of southern Italy. The groundwater is often contaminated by fecal bacteria because of the interaction between rocks having high permeability and microbial pollutants introduced into the environment by grazing and/or manure spreading. The microbial contamination of springwater in picnic areas located in high mountains can cause gastrointestinal illness. This study was carried out in order to analyze the interaction between Enterococcus faecalis and the soil of a limestone aquifer and to verify the influence of this interaction on the time dependence of groundwater contamination. E. faecalis was chosen because, in the study area involved, it represents a better indicator than Escherichia coli. The research was carried out through field (springwater monitoring) and laboratory experiments (column tests with intact soil blocks). The transport of bacterial cells through soil samples was analyzed by simulating an infiltration event that was monitored in the study area. Comparison of laboratory results with data acquired in the field showed that discontinuous precipitation caused an intermittent migration of microorganisms through the soil and produced, together with dispersion in the fractured medium (unsaturated and saturated zones), an articulated breakthrough at the spring. The short distances of bacterial transport in the study area produced a significant daily variability of bacterial contamination at the field scale.     

Rate theory models for ion transport through rigid pores. I. Time-dependent analysis in the case of vanishing interactions

In this first of a series of papers concerning the theoretical analysis of rate theory models for ion transport through rigid pores, the case of vanishing interactions is investigated. "Rigidity" means that ions crossing membranes through pores see a fixed structure of the pores, not changing in time. A single pore is considered to be a sequence of (n + 1) activation barriers separated by n energy minima. The explicit analytical treatment is restricted to pores with regular internal barrier structure, including the nonequilibrium situation of an applied electric field. In this case the connection with continuum diffusion models is demonstrated by performing in the limit n leads to infinity (n = number of binding sites within the pores) the transition to continuum. Thus, from diffusion equations describing a discrete number of jumps, the corresponding diffusion-like partial differential equations and boundary conditions are generated. For regular pores, from the time dependent solutions of the discrete equations, the corresponding solutions of the continuum equations are explicitly generated. The time-dependent relaxation behaviour of the discrete model is in good agreement with the continuum model if one assumes more than two binding sites in the pores.     

Wave Glider Monitoring of Sediment Transport and Dredge Plumes in a Shallow Marine Sandbank Environment

As human pressure on the marine environment increases, safeguarding healthy and productive seas increasingly necessitates integrated, time- and cost-effective environmental monitoring. Employment of a Wave Glider proved very useful for the study of sediment transport in a shallow sandbank area in the Belgian part of the North Sea. During 22 days, data on surface and water-column currents and turbidity were recorded along 39 loops around an aggregate-extraction site. Correlation with wave and tidal-amplitude data allowed the quantification of current- and wave-induced advection and resuspension, important background information to assess dredging impacts. Important anomalies in suspended particulate matter concentrations in the water column suggested dredging-induced overflow of sediments in the near field (i.e., dynamic plume), and settling of finer-grained material in the far field (i.e., passive plume). Capturing the latter is a successful outcome to this experiment, since the location of dispersion and settling of a passive plume is highly dependent on the ruling hydro-meteorological conditions and thus difficult to predict. Deposition of the observed sediment plumes may cause habitat changes in the long-term.     

Sensing of Barrier Tissue Disruption with an Organic Electrochemical Transistor

The gastrointestinal tract is an example of barrier tissue that provides a physical barrier against entry of pathogens and toxins, while allowing the passage of necessary ions and molecules. A breach in this barrier can be caused by a reduction in the extracellular calcium concentration. This reduction in calcium concentration causes a conformational change in proteins involved in the sealing of the barrier, leading to an increase of the paracellular flux. To mimic this effect the calcium chelator ethylene glycol-bis(beta-aminoethyl ether)-N,N,N'',N''-tetra acetic acid (EGTA) was used on a monolayer of cells known to be representative of the gastrointestinal tract. Different methods to detect the disruption of the barrier tissue already exist, such as immunofluorescence and permeability assays. However, these methods are time-consuming and costly and not suited to dynamic or high-throughput measurements. Electronic methods for measuring barrier tissue integrity also exist for measurement of the transepithelial resistance (TER), however these are often costly and complex. The development of rapid, cheap, and sensitive methods is urgently needed as the integrity of barrier tissue is a key parameter in drug discovery and pathogen/toxin diagnostics. The organic electrochemical transistor (OECT) integrated with barrier tissue forming cells has been shown as a new device capable of dynamically monitoring barrier tissue integrity. The device is able to measure minute variations in ionic flux with unprecedented temporal resolution and sensitivity, in real time, as an indicator of barrier tissue integrity. This new method is based on a simple device that can be compatible with high throughput screening applications and fabricated at low cost.     

Plasma equilibrium with barriers

The recently proposed concept of the transport barrier formation in a tokamak plasma as a bifurcation of the equilibrium state with a change in the toroidal magnetic field over the entire plasma column, including the plasma edge, is analyzed. The analysis is performed in the cylindrical approximation. It is shown that, in the framework of the discussed concept, all of the equilibrium solutions are continuous functions of the parameters involved, bifurcations are absent, and the result is determined by the model assumptions that are necessary in order to make the task self-contained. Removing even part of these restrictive assumptions can substantially change the result. Under typical conditions, the effect of the plasma rotation on the plasma equilibrium is negligibly small. Besides, from the viewpoint of the formal analysis of the force balance, the rotation does not facilitate but, in contrast, hampers the formation of a positive pressure jump.     

Estuarine intertidal sandflat benthic microalgal responses to in situ and mesocosm nitrogen additions

Benthic microalgal communities are important components of estuarine food webs and make substantial contributions to coastal materials cycling. Nitrogen is generally the limiting factor for marine primary production; however other factors can limit benthic primary producers because of their access to the additional nutrients found in sediment porewater. Field and laboratory experiments were conducted to test the hypothesis that water column nitrogen supply affects estuarine sandflat benthic microalgal community structure and function. Our field and mesocosm experiments assessed changes at both the population and functional group levels. Simulated water column nitrogen additions increased maximum community photosynthesis in most cases (Pb_max from photosynthesis vs. irradiance curves). Additional changes that resulted from nitrogen additions were decreases in porewater phosphate, increases in porewater ammonium, shifts in community composition from N₂ fixing cyanobacteria toward diatoms, and detectable, though not statistically significant increases in biomass (as chlorophyll a). Results from field and laboratory experiments were quite similar, suggesting that laboratory experiments support accurate predictions of the response of intertidal benthic microalgae to changes in water column nutrient conditions.     

Understanding behavioral responses of fish to pheromones in natural freshwater environments

There is an abundance of experimental studies and reviews that describe odorant-mediated behaviors of fish in laboratory microcosms, but research in natural field conditions has received considerably less attention. Fish pheromone studies in laboratory settings can be highly productive and allow for controlled experimental designs; however, laboratory tanks and flumes often cannot replicate all the physical, physiological and social contexts associated with natural environments. Field experiments can be a critical step in affirming and enhancing understanding of laboratory discoveries and often implicate the ecological significance of pheromones employed by fishes. When findings from laboratory experiments have been further tested in field environments, often different and sometimes contradictory conclusions are found. Examples include studies of sea lamprey (Petromyzon marinus) mating pheromones and fish alarm substances. Here, we review field research conducted on fish pheromones and alarm substances, highlighting the following topics: (1) contradictory results obtained in laboratory and field experiments, (2) how environmental context and physiological status influences behavior, (3) challenges and constraints of aquatic field research and (4) innovative techniques and experimental designs that advance understanding of fish chemical ecology through field research.     

(In)validity of the constant field and constant currents assumptions in theories of ion transport.

Constant electric fields and constant ion currents are often considered in theories of ion transport. Therefore, it is important to understand the validity of these helpful concepts. The constant field assumption requires that the charge density of permeant ions and flexible polar groups is virtually voltage independent. We present analytic relations that indicate the conditions under which the constant field approximation applies. Barrier models are frequently fitted to experimental current-voltage curves to describe ion transport. These models are based on three fundamental characteristics: a constant electric field, negligible concerted motions of ions inside the channel (an ion can enter only an empty site), and concentration-independent energy profiles. An analysis of those fundamental assumptions of barrier models shows that those approximations require large barriers because the electrostatic interaction is strong and has a long range. In the constant currents assumption, the current of each permeating ion species is considered to be constant throughout the channel; thus ion pairing is explicitly ignored. In inhomogeneous steady-state systems, the association rate constant determines the strength of ion pairing. Among permeable ions, however, the ion association rate constants are not small, according to modern diffusion-limited reaction rate theories. A mathematical formulation of a constant currents condition indicates that ion pairing very likely has an effect but does not dominate ion transport.     

Dynamic Model for the Adsorption in a Multibed Three‐Phase Fluidization Column Applied to Wastewater Biological Treatment

It is proposed a dynamic model for adsorption of NH₄⁺ ions from ammonia waters on volcanic tuff in a 10‐bed three‐phase (air – ammonia waters – volcanic tuff) fluidization column. The model consists in the nonstationary material balance differential equations. For each layer the ideal well‐mixing conditions are considered. The effluent ammonia ion concentrations, corresponding to each layer, have been measured at several time values in a laboratory‐scale column. The absolute relative mean error between the calculated and measured values of ammonia ion concentrations into liquid phase for all layers and times is 6.65 %, being in the order of magnitude of experimental errors.     

Influence of precipitation and soil on transport of fecal enterococci in fractured limestone aquifers

Limestone aquifers provide the main drinking water resources of southern Italy. The groundwater is often contaminated by fecal bacteria because of the interaction between rocks having high permeability and microbial pollutants introduced into the environment by grazing and/or manure spreading. The microbial contamination of springwater in picnic areas located in high mountains can cause gastrointestinal illness. This study was carried out in order to analyze the interaction between Enterococcus faecalis and the soil of a limestone aquifer and to verify the influence of this interaction on the time dependence of groundwater contamination. E. faecalis was chosen because, in the study area involved, it represents a better indicator than Escherichia coli. The research was carried out through field (springwater monitoring) and laboratory experiments (column tests with intact soil blocks). The transport of bacterial cells through soil samples was analyzed by simulating an infiltration event that was monitored in the study area. Comparison of laboratory results with data acquired in the field showed that discontinuous precipitation caused an intermittent migration of microorganisms through the soil and produced, together with dispersion in the fractured medium (unsaturated and saturated zones), an articulated breakthrough at the spring. The short distances of bacterial transport in the study area produced a significant daily variability of bacterial contamination at the field scale.     

Mixed Convective Peristaltic Flow of Water Based Nanofluids with Joule Heating and Convective Boundary Conditions

Main objective of present study is to analyze the mixed convective peristaltic transport of water based nanofluids using five different nanoparticles i.e. (Al₂O_3, CuO, Cu, Ag and TiO₂). Two thermal conductivity models namely the Maxwell''s and Hamilton-Crosser''s are used in this study. Hall and Joule heating effects are also given consideration. Convection boundary conditions are employed. Furthermore, viscous dissipation and heat generation/absorption are used to model the energy equation. Problem is simplified by employing lubrication approach. System of equations are solved numerically. Influence of pertinent parameters on the velocity and temperature are discussed. Also the heat transfer rate at the wall is observed for considered five nanofluids using the two phase models via graphs.     

Steady solutions to neoclassical transport equations and the absence of bifurcated states

The question of whether two-valued solutions can exist for an ambipolar electric field in stellarators and rippled tokamaks is considered. Steady solutions to transport equations in the limit of infrequent collisions are obtained in the purely neoclassical transport theory (that is, without allowance for possible anomalous losses). It is shown that, given the particle and heat sources, these equations have only one steady continuous solution, i.e., the steady states are nonbifurcating.