Effect Of Density Fluctuations Research Articles

A field study of the coupled effects of aquifer stratification, fluid density, and groundwater fluctuations on dispersivity assessments

A number of experimental studies have tackled the issue of solute transport parameter assessments either in the laboratory or in the field. But yet, the behavior of a plume in the field under density driven forces, is not well known due to possible development of instabilities. Some field tracer tests on the fate of plumes denser than native groundwater such as those encountered under waste disposal facilities, have pointed out the processes of sinking and splitting at the early stage of migration. The process of dispersion was widely investigated, but the range of dispersivity values obtained from either experimental tests, or numerical and theoretical calculations is still very large, even for the same type of aquifers. These discrepancies were considered to be essentially caused by soil heterogeneities and scale effects. In the meantime, studies on the influence of sinking and fingering have remained more scarce. The objective of the work is to analyze how transport parameters such as dispersivities can be affected by unstable conditions, which lead to plume sinking and fingering. A series of tracer tests were carried out to study under natural conditions, the transport of a dense chloride solution injected in a shallow two-layered aquifer. Two types of experiments were performed: in the first type, source injection was such that the plume could travel downward from one layer to the other of higher pore velocity, and in the second one, the migration took place only in the faster layer. The results suggest some new insights in the processes occurring at the early stages of a dense plume migration moving in a stratified aquifer under groundwater fluctuations, which can be summarized through the following points: (i) Above a stability criterion threshold, a fingering process and a multi modal plume transport take place, but local dispersivities can be cautiously derived, using breakthrough curves matching. (ii) When water table is subject to some cycling or rising, the plume can be significantly distorted in the transverse direction, leading to unusual values of the ratio between longitudinal and transverse dispersivities. (iii) Under stable conditions, for example in the case of straightforward injection in the faster aquifer layer, longitudinal dispersivity is greater than the transverse component as usually encountered, and the obtained transport parameters are closed to macro dispersivity values, which reach their asymptotic limit at very short distances. (iv) The classical scale effect about the varying dispersivity at short distances could be a process mainly due to the distance required for a plume stabilization.

Small angle x-ray scattering of a supercritical electrolyte solution: The effect of density fluctuations on the hydration of ions

Synchrotron small angle x-ray scattering measurements on water and zinc bromide ZnBr2 aqueous solutions were carried out from ambient to supercritical conditions. For both systems several isobars (between 285 and 600 bars) were followed beyond the critical isochore. The data were analyzed through an Ornstein-Zernike formalism in terms of correlation length and null angle structure factor. The results for pure water are in agreement with previously published values. Solutions of different electrolyte concentrations were studied. In each case, the values of the correlation length and null angle structure factor are larger than those of pure water. This effect is more pronounced for higher concentrations and/or for pressure closer to the critical point of pure water. This is in agreement with the shift of the critical point determined in the literature for NaCl solutions. Comparing these results to previous x-ray absorption measurements carried out on identical samples we propose the following two step sequence for ionic hydration up to supercritical conditions: (1) from ambient to about 300 degrees C, an increase of ion pairing and formation of multi-ionic complexes which can be correlated to the decrease of the dielectric constant; (2) an enhancement of the local solvation shell of ions due to the onset of the thermal density fluctuations at high temperature, leading to a screening effect between ions and inhibiting the ion pairing processes.

Quasi-particle properties of trapped Fermi gases

We develop a consistent formalism in order to explore the effects of density and spin fluctuations on the quasi-particle properties and, as a consequence, on the pairing critical temperature of a trapped Fermi gas on the attractive side of a Feshbach resonance. We first analyse the quasi-particle properties of a gas due to interactions far from resonance (effective mass and lifetime, quasi-particle strength and effective interaction) for the two cases of a spherically symmetric harmonic trap and of a spherically symmetric infinite potential well. We then explore the effect of each of these quantities on Tc and point out the important role played by the discrete level structure.

The effect of density fluctuations in supercritical fluids: new science and technology for polymer thin films

Supercritical carbon dioxide (scCO 2) is being used increasingly as a green solvent in polymer processing. However, the major disadvantage thus far is that only a limited class of polymers, such as fluorinated or silicone-based polymers, can be dissolved in scCO 2. Here we show that large density fluctuations in scCO 2 can significantly enhance the solubility of scCO 2 with polymer thin films even when the bulk polymers have very poor miscibility with scCO 2. In addition, by utilizing quick solvent evaporation of CO 2, we could preserve the swollen structures, resulting in low-density polymer thin films where nanometer-scale porosity was significantly introduced. The use of the low-density polymer thin films could allow us to develop a new gas membrane that could selectively permit a large flow of small molecule gases, such as O 2 and CO 2, while completely blocking out larger gases or particulates.

Design of an X-mode swept frequency modulation reflectometer for the measurement of KSTAR plasma density profiles (invited)

An X-mode swept frequency modulation (FM) reflectometry system has been designed to measure the electron density profiles of the “initial” KSTAR plasma. Fast swept HTO oscillators are employed to avoid density fluctuation effects, and frequency quadruplers are utilized to expand the HTO frequency range of 8–18 GHz to completely cover the X-mode cutoff frequency range of 33–66 GHz. The system can also be utilized to measure the edge profiles of the “Day One” KSTAR plasma by either switching from X- to O-mode reflectometry or by employing higher frequency millimeter-wave components and retaining the X-mode reflectometry configuration. To facilitate engineering design and optimization, a 3D drawing tool is utilized to effectively deal with any technical problems that may happen under actual KSTAR conditions. Details of the KSTAR FM reflectometry system are described together with important design issues.

Doppler spectral line shapes in low frequency turbulent plasmas

AbstractIn this paper we investigate the influence of low frequency, i.e. drift wave like turbulence on the spectral line shapes in magnetized plasmas. The measured spectrum, which is obtained through both spatial and time averaging processes, is shown to contain information on turbulence. Using a statistical description of the turbulent fluctuations, we investigate the effects of density, fluid velocity and temperature fluctuations on the Doppler profile of a spectral line. (© 2004 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Dimensional and temperature crossover in trapped Bose gases

We investigate the long-range phase coherence of homogeneous and trapped Bose gases as a function of the geometry of the trap, the temperature, and the mean-field interactions in the weakly interacting limit. We explicitly take into account the (quasi)condensate depletion due to quantum and thermal fluctuations, i.e., we include the effects of both phase and density fluctuations. In particular, we determine the phase diagram of the gas by calculating the off-diagonal one-particle density matrix and discuss the various crossovers that occur in this phase diagram and the feasibility of their experimental observation in trapped Bose gases.

Possible mechanism for cold denaturation of proteins at high pressure.

We study cold denaturation of proteins at high pressures. Using multicanonical Monte Carlo simulations of a model protein in a water bath, we investigate the effect of water density fluctuations on protein stability. We find that above the pressure where water freezes to the dense ice phase (approximately 2 kbars) the mechanism for cold denaturation with decreasing temperature is the loss of local low-density water structure. We find our results in agreement with data of bovine pancreatic ribonuclease A.

Do the KamLAND and solar neutrino data rule out solar density fluctuations?

We elucidate effects of solar density fluctuations on neutrino propagation through the Sun. Using data from the recent solar neutrino and KamLAND experiments we provide stringent limits on solar density fluctuations. It is shown that the neutrino data constrains solar density fluctuations to be less than beta = 0.05 at the 70 % confidence level, where beta is the fractional fluctuation around the value given by the Standard Solar Model. We find that the best fit to the combined solar neutrino and KamLAND data is given by beta = 0.

Radiative transfer in clumpy environments: absorption and scattering by dust

We study the effects of stochastic density fluctuations on the transport of far-ultraviolet radiation in interstellar clouds. The physical processes considered are absorption and anisotropic scattering by dust grains. We consider clouds that are, on average, homogeneous and have a continuous stochastic density distribution. The spatial variation of the density n is given in a statistical sense only. It is assumed to correspond to a continuous Markov process along each line of sight. Caused by the stochastic nature of the density, the intensity becomes a stochastic variable, too. As a consequence, the ordinary radiative transfer equation has to be replaced by a generalized transfer equation, of Fokker–Planck type. Results are presented for a plane-parallel slab and a spherical geometry. In both cases, we find a substantial higher radiation field inside the cloud than in the case of a homogeneous density distribution. Also the amount of reflected radiation is affected significantly by an inhomogeneous, stochastic density distribution. Furthermore, we compare our findings with the results drawn from the two-phase model of Boissé, in which the spatial variation of the extinction coefficient is also modelled by a Markov process. Finally, we address the question of whether the results obtained from the generalized radiative transfer equation can be approximated by calculations for a homogeneous medium consisting of grains with effective optical properties.

The Role of Elasticity in the Anomalous Swelling of Polymer Thin Films in Density Fluctuating Supercritical Fluids

In situ neutron reflectivity was used to investigate the effects of density fluctuations on the solubility of supercritical carbon dioxide (scCO2) in polymer thin films. Deuterated polystyrene, deuterated polybutadiene, and the corresponding random copolymer, deuterated styrene-random- butadiene copolymer, as well as deuterated poly(methyl methacrylate) were investigated. Data were obtained as a function of pressure under two isothermal conditions (T ) 36 and 50 °C). All the polymer films used showed anomalous swelling and CO2 sorption on the density fluctuation ridge in the P-T phase diagram of CO2. We found that the magnitude of the swelling was a function of the elasticity of the films rather than the bulk solubility of CO2. The enhanced miscibility of the rubber/CO2 systems, which are very poor in bulk, was found to be almost identical to that of the silicon rubber/CO2 mixture, which is one of the highly miscible polymeric materials under moderate CO2 conditions.

On the critical behavior of phase changes of a forward-scattered light in an isotropic chiral fluid

The effect of density fluctuations on the changes of azimuth and ellipticity are analytically obtained in an isotropic chiral fluid, when the incident light is completely linearly polarized above (or below) the horizontal at 45°. The results are discussed in two extreme cases of the critical region.

Small-angle scattering and spatial resolution of fluctuation reflectometry: comparison of 2D analytical theory with numerical calculations

Three methods to study the effect of density fluctuations on the reflected signal, based on a 2D full-wave analysis and Born approximation numerical and analytical calculations, are developed. The explicit analytical expression for the signal produced by an island-like perturbation running in poloidal direction is obtained and shown to be in nice agreement with results of numerical techniques for small level density perturbations. It is shown, both numerically and analytically, that due to small-angle scattering effects, fluctuations far from the cut-off layer may produce a significant and, in some realistic cases, dominant response. The conditions under which the fluctuation reflectometry signal is generated only in the cut-off vicinity are given.

Effects of random matter density fluctuations on the neutrino oscillation transition probabilities in the Earth

In this paper, we investigate the effects of random fluctuations of the Earth matter density for long baselines on the neutrino oscillation transition probabilities. We especially identify relevant parameters characterizing the matter density noise and calculate their effects by averaging over statistical ensembles of a large number of matter density profiles. For energies and baselines appropriate to neutrino factories, absolute errors on the relevant appearance probabilities are at the level of $|\Delta P_{\alpha \beta}| \sim 10^{-4}$ (with perhaps $|\Delta P_{\mu e}|/P_{\mu e} \sim 1%$ for neutrinos), whereby a modest improvement in understanding of the geophysical data should render such effects unimportant.

Probability density function model equation for particle charging in a homogeneous dusty plasma.

In this paper, we use the direct interaction approximation (DIA) to obtain an approximate integrodifferential equation for the probability density function (PDF) of charge (q) on dust particles in homogeneous dusty plasma. The DIA is used to solve the closure problem which appears in the PDF equation due to the interactions between the phase space density of plasma particles and the phase space density of dust particles. The equation simplifies to a differential form under the condition when the fluctuations in phase space density for plasma particles change very rapidly in time and is correlated for very short times. The result is a Fokker-Planck type equation with extra terms having third and fourth order differentials in q, which account for the discrete nature of distribution of plasma particles and the interaction between fluctuations. Approximate macroscopic equations for the time evolution of the average charge and the higher order moments of the fluctuations in charge on the dust particles are obtained from the differential PDF equation. These equations are computed, in the case of a Maxwellian plasma, to show the effect of density fluctuations of plasma particles on the statistics of dust charge.

On the Role of Density Fluctuations in the Equation of State of a Simple Fluid

We address the wellknown problems intorduced into the theory of fluids by density fluctuations in the form of van der Waals loops and nonclassical critical phenomena. A clean separation of long and short range density fluctuations is achieved by use of cell-constrained models which display well-defined van der Waals loops and classical behaviour around the critical point. For a pure Lennard-Jones fluid with occupancy restricted to 1 or 8 particles per cell, the phase diagram is determined by Monte Carlo simulation. By considering the deviations from the normal simulations without cell constraint, the effects of longer range density fluctuations are exposed. The system size dependence of the van der Waals loops present in all simulations of fluids is analyzed in terms of the GvdW free energy density functional theory, which is formuiated on the basis of the cell concept. The loops are found to gradually disappear either with greater cel occupancy or increasing total particle number in the simulation box.

Matter profile effect in a neutrino factory

We point out that the matter profile effect---the effect of matter density fluctuation on the baseline---is very important to estimate the parameters in a neutrino factory with a very long baseline. To make it clear, we propose the method of Fourier series expansion of the matter profile. By using this method, we can take account of both the matter profile effect and its ambiguity. For very long baseline experiment, such as $L=7332\mathrm{km},$ in the analysis of the oscillation phenomena we need to introduce a new parameter ${a}_{1}$---the Fourier coefficient of the matter profile---as a theoretical parameter to deal with the matter profile effects.

Turbulent Transport Coefficients with Allowance for Density Fluctuations

A model of turbulent transfer processes modified to take into account the effect of density fluctuations is proposed. The model is used for finding expressions for the turbulent transfer coefficients (viscosity, thermal conductivity, and diffusion of admixtures) in flows of the boundary-layer type in the presence of small density gradients. The turbulent Prandtl number distribution in the far wake behind a heated cylinder is compared with the results of measurements.

Monte Carlo simulations of polymer brushes.

Three-dimensional Monte Carlo simulations of flexible and semiflexible polymer brushes at various grafting densities are carried out to study their equilibrium structure and attendant properties by using the bond fluctuation model. Our simulation results of long flexible polymer brushes are, in general, consistent with predictions of the self-consistent field theory. However, a depletion layer near the substrate is only observed at small grafting densities but not at medium densities. We have also measured the brush height and end-to-end distance of various polymer brushes, and their dependence on grafting density, chain length, and chain stiffness are obtained. The distribution of bending angles at various temperatures are calculated for four different forms of bending energy and our simulation results agree with theoretical predictions very well. Moreover, we study the isotropic-to-nematic transition of polymer brushes, which is found to be a continuous phase transition from our simulation results. Finally, we discuss the effects of local grafting density fluctuation on the monomer and end density distributions.

Electron renormalization of sound interaction with two-level systems in superconducting metallic glasses

The crossing of the temperature dependences of sound velocity in the normal and the superconducting state of metallic glasses indicates renormalization of the intensity of sound interaction with two-level systems (TLS's) caused by their coupling with electrons. In this paper we examine different theoretical approaches to a quantitative description of the renormalization, using the results of a low-temperature ultrasonic investigation of ${\mathrm{Zr}}_{41.2}{\mathrm{Ti}}_{13.8}{\mathrm{Cu}}_{12.5}{\mathrm{Ni}}_{10}{\mathrm{Be}}_{22.5}$ amorphous alloy. It is shown that the adiabatic renormalization of the coherent tunneling amplitude can explain only part of the whole effect observed in the experiment. There exists another mechanism of the renormalization affecting only nearly symmetric TLS's, which may be associated with the effect of electron density fluctuations on the interwell potential.