Transient Dispersion Research Articles

Transient solution for viscoacoustic wave propagation in a double porosity medium, and its limitations

It is well known that Biot theory (Biot, 1956;1962) of porous-media acoustics ignores all wave-induced flow at mesoscopic scales, i.e. scales greater than the grain size but less that the wavelength. Biot?s theory can not explain high level attenuation observed in natural porous media such as fluid-filled sands or sandstone over the seismic frequency range ( 10- 200 Hz). This attenuation is successfully described by the mesoscopic heterogeneity models (e.g., Gurevich and Lopatnikov, 1995; Gelinksy and Shapiro, 1997; Johnson, 2001). By applying the volume averaging theory to the local Biot poroelastic law, Pride an Berryman (2003a,b) developed the double-porosity, dual permeability (DPDP) model It gives a theoretical framework, including the field equations governing the linear acoustics of composites with two isotropic porous constituents (phase 1 and phase 2), to model acoustic wave propagation through heterogeneous porous structures. Under the assumption that phase 2 is entirely embedded in phase 1, the double-porosity theory is reduced to the effective Biot theory with the complex frequency-dependent elastic moduli in which the internal mesoscopic flow is incorporated. This theory provides good agreement with measurements of attenuation over the seismic and ultrasonic frequency bands (Pride et al., 2004). In this paper, the analytical transient solution and dispersion characteristics for the double-porosity model are obtained over the whole frequency range for a homogeneous medium. A homogeneous poro-viscoacoustic model is constructed and analytically solved to approximate the double porosity model. The comparison between the results of the two models shows the likely validity and limitations of numerical solutions using a poro-viscoacoustic model to represent a double porosity medium in the heterogeneous case. Our calculations show that the dissipation by local mesoscopic flow of the double porosity model is very hard to fit over the entire frequency range by a single Zener element. We choose the relaxation function which just approximates the dispersion behaviour of the double porosity model around the source centre frequency. It is shown that if the frequency is much lower than the peak attenuation frequency of the double porosity model, then wave propagation can be well described by the poro-viscoacoustic model with a single Zener element. For most water-filled sandstones having a double porosity structure, this holds true across the seismic frequency range.

Comparing Three Models for Treatment of Stagnant Zones in Riverine Transport

The modified advection dispersion equation (MADE) model is compared to the transient storage model (TSM) and fractional advection dispersion equation (FRADE) model for modeling riverine transport. The comparisons are illustrated using field examples. The parameter F of the FRADE model is found to be inconsistent to its physical meaning and is unrelated to dead zone fraction. Also, the estimated parameters of TSM are found to be inconsistent to their physical meaning. On the contrary, the parameters of MADE are consistent and related to the dead zone fraction. It is observed that the MADE model is easy to use and yields reliable and consistent results.

The effect of the spontaneously generated coherence on the dynamical behaviors of the dispersion and the absorption

This paper investigates the effects of the spontaneously generated coherence (SGC) and incoherent pumping field on the dynamical behaviors of the dispersion and the absorption. Using a three-level Λ-type atomic medium, the influence of the relative phase between the applied fields on the transient dispersion and absorption is discussed. It is shown that the phase dependence of the dispersion and the absorption not only depend on the existence of SGC, but they depend on the existence of the incoherent pumping field as well. We show that the medium can be used as an optical switch in which the propagation of the laser pulse can be controlled by another laser field.

Reducing Risk of Airborne Transmitted Infection in Hospitals by Use of Hospital Curtains

An in-patient suffering from an airborne infectious disease should be properly isolated in a negative-pressure isolation room to prevent hospital-acquired infection. However, before the infectious status is identified by clinical diagnosis, the patient may be assigned to stay with others in a hospital ward with multiple beds. Under these circumstances, there may be a risk of infection of the neighboring patients. Therefore, reasonable infection control measures should be implemented in multi-bed hospital wards, intensive care units, among others. The use of hospital curtains as physical barriers against transmission of disease is potentially a simple but effective method to reduce the risk of infection. In this investigation, the effectiveness of hospital curtains in prevention against airborne transmission of diseases in hospital rooms was evaluated by numerical modeling analysis. A computational fluid dynamics (CFD)-statistical hybrid modeling method, which was validated by tracer gas measurements, was employed to determine the transient dispersion of airborne contaminants. It was found that among all the case studies, the use of curtains between two beds could reduce the peak concentration for each individual neighboring patient in a bioaerosol dispersion process. A curtain fully extended to the length of a bed yielded significantly higher protection than a partially extended curtain and the reduction in peak concentration could be as much as 65%.

Development and applications of CPR—A 3D consequence analysis software for educational use

In this research, a simple package called CPR 2.0 software was developed according to the algorithm suggested by AIChE/CCPS and implemented using Compaq Visual Fortran (CVF 6.6) programming language and METFOR 3.0 Fortran 90/95 Library. This multipurpose software includes nine sets of programs that can be used in combination or separately to calculate physical, effect, and vulnerability consequences; their results can also be easily processed into different tables and figures. A set of three-dimensional (3D) gas release contours with different concentration isosurfaces and transient dispersion states floating above a local map can be easily observed from different angles in order to facilitate emergency response planning and educational training. The software has been benchmarked with CHEMS-PLUS software via a methane release case and the result from the two packages is compared and analyzed.

Comparison of the Eulerian and Lagrangian methods for predicting particle transport in enclosed spaces

The computational fluid dynamics (CFD) methods have been widely used in modeling particle transport and distribution in enclosed spaces. Generally, the particle models can be classified as either Eulerian or Lagrangian methods while each has its own pros and cons. This investigation is to compare the two modeling methods with an emphasis on their performance of predicting particle concentration distributions in ventilated spaces. Both the Eulerian and Lagrangian models under examination were performed based on the same airflow field calculated by solving the RANS equations with the k – ε turbulence model. The numerical results obtained with the two methods were compared with the experimental data. The comparison shows that both of the methods can well predict the steady-state particle concentration distribution, while the Lagrangian method was computationally more demanding. The two models were further compared in predicting the transient dispersion of the particles from a coughing passenger in a section of airliner cabin. In the unsteady state condition, the Lagrangian method performed better than the Eulerian method.

Effect of ethanol on cell growth of budding yeast: genes that are important for cell growth in the presence of ethanol.

The budding yeast Saccharomyces cerevisiae has been used in the fermentation of various kinds of alcoholic beverages. But the effect of ethanol on the cell growth of this yeast is poorly understood. This study shows that the addition of ethanol causes a cell-cycle delay associated with a transient dispersion of F-actin cytoskeleton, resulting in an increase in cell size. We found that the tyrosine kinase Swe1, the negative regulator of Cdc28-Clb kinase, is related to the regulation of cell growth in the presence of ethanol. Indeed, the increase in cell size due to ethanol was partially abolished in the SWE1-deleted cells, and the amount of Swe1 protein increased transiently in the presence of ethanol. These results indicated that Swe1 is involved in cell size control in the presence of ethanol, and that a signal produced by ethanol causes a transient up-regulation of Swe1. Further we investigated comprehensively the ethanol-sensitive strains in the complete set of 4847 non-essential gene deletions and identified at least 256 genes that are important for cell growth in the presence of ethanol.

Chemical-specific representation of air--soil exchange and soil penetration in regional multimedia models.

In multimedia mass-balance models, the soil compartment is an important sink as well as a conduit for transfers to vegetation and shallow groundwater. Here a novel approach for constructing soil transport algorithms for multimedia fate models is developed and evaluated. The resulting algorithms account for diffusion in gas and liquid components; advection in gas, liquid, or solid phases; and multiple transformation processes. They also provide an explicit quantification of the characteristic soil penetration depth. We construct a compartment model using three and four soil layers to replicate with high reliability the flux and mass distribution obtained from the exact analytical solution describing the transient dispersion, advection, and transformation of chemicals in soil layers with different properties but a fixed boundary condition at the air-soil surface. The soil compartment algorithms can be dynamically linked to other compartments (air, vegetation, groundwater, surface water) in multimedia fate models. We demonstrate and evaluate the performance of the algorithms in a model with applications to benzene, benzo[a]pyrene, MTBE, TCDD, and tritium.

Transient dispersion in porous media: A comparison between exact and approximate solutions in a case study

Three methods are proposed for studying solute dispersion over short periods of time in the case study of a one-dimensional flow: the non-local method, the spatial moments method, and the volume averaging method distinguishing two zones using both rigorous closure and classical closure. The non-local method (exact) uses the Green function. Its application in calculating the generalized dispersion tensor and solving the macroscopic problem, is a cumbersome task. The methods of moments and volume averaging distinguishing two zones using rigorous closure can be used to find a correct description in terms of the spatial moments of the solute concentration distribution up to the second order. Nevertheless, the rigorous closure cannot, in general, be applied to periodic media. Volume averaging with classical closure gives coefficients which are different from those obtained by the method of moments. Surprisingly, the macroscopic solution is very similar to the exact one, in some of the tested cases.

Interpretation of short-circuit discharge current in film capacitors based on an equivalent circuit

Misfire phenomena in a discharge circuit composed of a capacitor and thyristor were investigated for operating frequencies to 198 Hz. Misfire was found to occur in polyethylene terephthalate (PET) capacitor circuits at low temperatures, whereas polypropylene (PP) capacitor circuits did not misfire under any conditions. Transient currents in the range 1 mA to 40 A were measured using a digital oscilloscope. A PET capacitor was shown to discharge initially with a given time constant, but subsequently discharge at a lower rate, extending the discharge period. It is this delayed gradual decay that causes misfire, and it is associated with residual charges trapped in the bulk of the PET film. The correlation between the transient discharge current and dielectric dispersion in PET and PP capacitors was investigated over a wide frequency range with the aid of the Debye model for dipolar polarization. It was found that the time dependence of the discharge current is related closely to the broad distribution of dipole relaxation time, with a discrepancy between the discharge current and the dielectric response attributed to charge trapping.

Approximate solutions for two dimensional problems in water pollution – component suppression schemes

In this work, a new hybrid method for solving problems in water pollution is proposed. The method furnishes approximate solutions for the two-dimensional transport equation in analytical form, in cases when the coupling between the hydrological and transport models can be neglected. The high speed processing of the scheme allows to simulate the transient dispersion in real time using low performance microcomputers.

Transient current spectroscopy and frequency dispersion studies of low temperature GaAs and Al0.3Ga0.7As metal-insulator-semiconductor diodes

Low temperature (LT)-grown GaAs and Al0.3Ga0.7As metal-insulator-n+-GaAs (MIN) diodes have been fabricated and their electrical properties analyzed. Studies were carried out to evaluate the interfacial quality of the LT layer and the underlying n+-GaAs layer using transient current spectroscopy (TCS) and capacitance-frequency (C-f) characterization. TCS studies on LT-GaAs revealed a high concentration of a continuum of states anda dominant electron trap with an activation energy of 0.52eV. In LT-Al0.3Ga0.7As, a shallow trap at 0.36eV and two deep level traps at 0.85eV and 1.12eV were observed. Frequency dispersion was observed to be less for LT-GaAs samples with an AlAs barrier layer than without an AlAs barrier layer. However, LT-Al0.3Ga0.7As MIN diodes displayed a smaller frequency dispersion than LT-GaAs MIN diodes. Upon further investigation into MISFET devices, it was found that LT-Al0.3Ga0.7As MISFET devices had better transconductance frequency dispersion characteristics than LT-GaAs MISFET devices did.

Transient and stationary axial dispersion in vortex array flows—II. Decoupling of inter- and intra-vortex transport phenomena

In flows consisting of arrays of vortex units, the transient dispersion regime is of a very complex nature: the effective dispersion coefficient varies strongly with time, comprises both inter- and intra-vortex phenomena and depends heavily upon the injection condition. However, in an infinite array of identical vortices, it is found that when the cross-section of a vortex is injected homogeneously, the effective dispersion coefficient remains strictly constant and is independent of all intra-vortex transport phenomena. In other words, regardless of the presence of strong intra-vortex concentration gradients, the second-order moment of the tracer distribution varies with the time as if all the vortices are perfectly mixed. A mathematical proof for this phenomenon is given. A measurement method based upon this feature allows the separate and direct determination of the inter-vortex exchange coefficient in laminar Couette-Taylor flow, without the need to quantify the intra-vortex transport phenomena, or without the need to wait until all intra-vortex concentration gradients have disappeared. The obtained experimental correlation validates recently performed theoretical calculations and shows that the mass transfer occurs by means of a continuous, strictly ordered surface renewal mechanism. For the transient dispersion effects which are obtained when nonhomogeneous injections are applied, an approximate expression for the variation of the effective dispersion coefficient with the time is established. Within the (very good) accuracy of this expression, the effect of the inter- and intra-vortex transport upon the tracer dispersion rates could also be decoupled.

Transient and stationary axial dispersion in vortex array flows—I. Axial scan measurements and modeling of transient dispersion effects

Whereas conventional RTD experiments yield incomplete and potentially misleading information, the axial scan method is shown to be a powerful technique to analyze the transient dispersion effects in vortex array flows (VAFs). Applying different initial tracer distributions to a given vortex, and making axial scans of the spreading tracer distribution, allowed, for the first time, to quantify and classify the complete set of strongly different transient dispersion modes. As a model system, the laminar Couette-Taylor flow has been selected. By working under high viscosity conditions, the time scale of the different acting phenomena has been enlarged to such an extent that even the fastest dispersion events could be extensively studied and quantified. It is shown that in laminar VAFs effective axial dispersion coefficients can be obtained which vary over orders of magnitude, just by applying different initial tracer distributions to a given vortex. A first principles two-dimensional model (valid when the mixing along the streamlines occurs fast) with which all observed transient dispersion effects can be accurately represented is proposed. The insights obtained in the present study are especially useful for the development of VAF reactors for the treatment of strongly viscous fluids.

MODELLING OF RELEASE OF GAS FROM HIGH-PRESSURE PIPELINES

SUMMARY The problem investigated is the break of a high-pressure pipeline carrying natural single-phase gas which may condensate (retrograde) when the pressure drops. Single-phase non-ideal gas is assumed using a generalized equation of state. Taking advantage of the choked massflow condition, the break is split into a pipe flow problem and a dispersion flow problem, both solved using a finite difference control volume scheme. The transient flow field from the pipeline break location is expanded analytically, using an approximation of the governing equations, until ambient pressure is reached and matched to the corresponding gas dispersion flow field using as subgrid model a jet box with a time-varying equivalent nozzle area as an internal boundary of the dispersion domain. The turbulence models used for the pipe and dispersion flow fields are an empirical model of Reichard and the k-* model for buoyant flow respectively. The pipe flow simulations indicate that the flow from the pipeline might include dispersed condensate which will affect quantitatively the mass flow rate from the pipeline and qualitatively the gas dispersion if the condensate rains out. The transient dispersion simulation shows that an entrainment flow field develops and mixes supersaturated gas with ambient warmer air to an unsaturated mixture. Because of the inertia of the ambient air, it takes time to develop the entrainment flow field. As a consequence of this and the decay of the mass Aow with time, the lower flammability limit of the gasair mixture reaches its most remote downstream position relatively early in the simulation (about 15 s) and withdraws closer to the break location.

On the transient non-Fickian dispersion theory

The Fickian dispersion equation is the basic relationship used to describe the nonconvective mass flux of a solute in a porous medium. This equation prescribes a linear relationship between the dispersive mass flux and the concentration gradient. An important characteristic of the Fickian relationship is that it is independent of the history of dispersion (e.g. the time rate of change of the dispersion flux). Also, the dispersivities are supposed to be medium constants and invariant with temporal and spatial scales of observation. It is believed that in general these restrictions do not hold. A number of authors have proposed various alternative relationships. For example, differential equations have been employed that prescribe a relationship between the dispersion flux and its time and space derivatives. Also, stochastic theories result in integro-differential equations in which dispersion tensor grow asymptotically with time or distance. In this work, three different approaches, which lead to three different non-Fickian equations with a transient character, are discussed and their primary features and differences are highlighted. It is shown that an effective dispersion tensor defined in the framework of the transient non-Fickian theory, grows asymptotically with time and distance; a result which also follows from stochastic theories. Next, principles of continuum mechanics are employed to provide a solid theoretical basis for the non-Fickian transient dispersion theory. The equation of motion of a solute in a porous medium is used to provide a rigorous derivation of various dispersion relationships valid under different conditions. Under various simplifying assumptions, the generalized theory is found to agree with the conventional Fickian theory as well as several other non-Fickian relationships found in the literature. Moreover, it is shown that for nonconservative solutes, the traditional dispersion tensor is affected by the rate of mass exchange of the solute.

Transient photoconductivity in a-Si1−x C x :H thin films

Abstract Excess electron and hole drift mobilities in a-Si1−x Cx:H thin films have been investigated using the ‘time-of-flight’ transient photoconductivity technique. A detailed analysis of the temperature and field dependence of these parameters has been carried out, permitting the computation of the energy distributions of conduction and valence band tail localized states as a function of carbon concentration in the films. It is apparent from these data that increased carbon incorporation is associated with an increase in the total number of conduction band tail states, and with a broadening of their energy distribution. The valence band tail exhibits an increased density of localized tail states, but the broadening is less pronounced in this case. The computed energy distributions of the trapping centres in both the conduction and valence band tails are supported by the experimental behaviour of the respective carrier pre-transit currents. The temperature dependences of the transient dispersion paramet...

Circulation model for absorption and dispersion in cocurrent bubble columns

AbstractLiquid circulation and a finite entrance zone are accounted for in modeling gas absorption into cocurrent flows of pure oxygen and CMC solutions. The new models lead to a Taylor‐like correction factor to account for circulation in both absorption and transient dispersion experiments. Steady‐state experiments for viscosities of 1, 12, 25 and 50 mPa·s were conducted by measuring dissolved gas concentration profiles along the column axis. Liquid mixing experiments were performed using the transient acid‐base neutralization method, leading to measured values of axial dispersion coefficients which depended on viscosity to the power 0.75. Measurements suggest that circulation was minuscule owing to uniform gas injection by Flexisparger in the present column.

Analysis of dispersion structure of fermentation media by dynamic gas disengagement method

In this communication, a digital dynamic gas disengagement sensor which is capable of measuring the transient dispersion height continuously in real time, is used to evaluate the dispersion structures of some synthetic media to demonstrate the usefulness of the technique

Turbulence effects on dust explosions in the 20-liter spherical vessel

The method of collecting a large amount of data and taking ensemble average over all measurement periods using computer is employed to determine the turbulence parameters during the transient dispersion process in the 20 liter spherical vessel. The system studied is typical for laboratory pneumatic dispersion systems in explosion test vessels. Measurements show that a decaying turbulence feature is generated by the dispersion system and characterized by a decay period of 150–300 ms with the integral scale of about 1 cm and initial intensity of about 3 m/s, which has no analogue with the expected natural conditions of accidental explosions in industry. Two kinds of dust-air mixtures (aluminum and cornstarch) and methane-air mixtures, three different sizes of aluminum dust particle (5, 15, 30 μm) and one average size of corntarch dust (15 μm) are tested in this sphere. The maximum burning velocity is used instead of Kst factor to characterize the highest combustion rate occurred during the explosion process in the closed vessel for both dust and gas-air mixtures. The present experimental results demonstrate that the maximum burning velocities determined as a linear function of the initial rms turbulence velocity produced by the pneumatic dispersion system in the aluminum- and cornstarch-air mixtures. The experimental results also demonstrate that the reduction of dust particle size will increase significantly the turbulent buring velocity in dust-air mixtures, but the kind of dust seems to have no significant influence on explosion data, if their particle sizes are the same. These suggest that the dispersion induced turbulence may play more important role in formation of dust-air mixtures than in direct effect dust combustion process, and that the influence of the above turbulence on the combustion characteristics mainly enters through the mechanisms of creating uniform suspension of dust clouds.