Two-flow Model Research Articles

Meshless method with ridge basis functions for time fractional two-flow domain model

In this paper, a meshless method with ridge basis functions for solving the time fractional two-flow domain model problem is proposed. The method uses the L1 approximation formula based on piecewise linear interpolation to discretize the Caputo time fractional derivative $$ (0 < \alpha < 1) $$ , and by means of the ridge basis function to construct the approximation function, and uses the collocation method to discretize the governing equation. The existence and uniqueness of the numerical solution are analyzed. The error between the proposed method and the finite difference method is compared by numerical examples; then the affecting factors of the calculation accuracy are discussed. The results show that the proposed method is feasible and simple.

A stratified jet model for AGN emission in the two-flow paradigm

Context.High-energy emission of extragalactic objects is known to take place in relativistic jets, but the nature, the location, and the emission processes of the emitting particles are still unknown. One of the models proposed to explain the formation of relativistic ejections and their associated non-thermal emission is the two-flow model, where the jets are supposed to be composed of two different flows, a mildly relativistic baryonic jet surrounding a fast, relativistically moving electron positron plasma. Here we present the simulation of the emission of such a structure taking into account the main sources of photons that are present in active galactic nuclei (AGNs).Aims.We try to reproduce the broadband spectra of radio-loud AGNs with a detailed model of emission taking into account synchrotron and inverse-Compton emission by a relativistically moving beam of electron positron, heated by a surrounding turbulent baryonic jet.Methods.We compute the density and energy distribution of a relativistic pair plasma all along a jet, taking into account the synchrotron and inverse-Compton process on the various photon sources present in the core of the AGN, as well as the pair creation and annihilation processes. We use semi-analytical approximations to quickly compute the inverse-Compton process on a thermal photon distribution with any anisotropic angular distribution. The anisotropy of the photon field is also responsible for the bulk acceleration of the pair plasma through the “Compton rocket” effect, thus imposing the plasma velocity along the jet. As an example, the simulated emerging spectrum is compared to the broadband emission of 3C 273.Results.In the case of 3C 273, we obtain an excellent fit of the average broadband energy distribution by assuming physical parameters compatible with known estimates. The asymptotic bulk Lorentz factor is lower than what is observed by superluminal motion, but the discrepancy could be solved by assuming different acceleration profiles along the jet.

Close, but not close enough? Audience’s reactions to domesticated distant suffering in international news coverage

Journalists domesticate news about distant events to bring such events closer to the audience and thus make them more relevant and appealing; however, knowledge about the actual audience’s reactions toward domesticated news is lacking. Central to this study is understanding how an audience makes use of domestication strategies in viewing and reacting to mediated distant suffering. Earlier text-based research has found several ways of domesticating distant suffering that can invite an audience to care. Building further on this media-centered study, 10 focus groups reveal a two-flow model of domestication, consisting of first-level domestication on the production side by journalists and second-level domestication, in which audience members themselves use strategies of domestication to make sense of distant suffering.

Variable hyperemia of biological tissue as a noise source in the input optical signal of a medical laser Doppler flowmeter

As applied to the problems of medical laser Doppler flowmetry and based on a modified Kubelka–Munk two-flow model, analytic expressions have been obtained for the radiation power backscattered by biological tissue, taking into account the variable hyperemia of its microvasculature. An estimate is made of the power contribution of the Doppler component of the flow to the overall backscattered-radiation signal recorded by the device, which appears when light is scattered at moving erythrocytes. It is shown that the power contribution of the Doppler component to the overall backscattered radiation flux is no greater than 5% on average. The variable hyperemia that results from various physiological processes causes the radiation flux recorded by the Doppler flowmeter to be amplitude modulated. The power of the amplitude-modulated component can be of the same order of magnitude as, and in certain cases even greater than, the power of the useful Doppler signal, creating noise in the input signal of the device.

Two-flow model for piping erosion based on liquid-solid coupling

Previous studies have indicated that piping erosion greatly threatens the safe operation of various hydraulic structures. However, few mathematical models are available to perfectly describe the erosion process due to the complexity of piping. The focus of the present work is to propose a new fluid-solid coupling model to eliminate the shortcomings of existing work. A “pseudo-liquid” assumption is suggested to simulate the particle movement in the erosion process. Then, based on the mass and momentum conservations of the moving particles and flowing water, a new two-flow model is established by using the continuity equations and motion equations. In the model, the erosion rate of soil is determined with a particle erosion law derived from tests results of STERPI. And ERGUN’s empirical equation is used to determine the interaction forces between the liquid and the solid. A numerical approach is proposed to solve the model with the finite volume method and SIMPLE algorithm. The new model is validated with the tests results of STERPI. And the soil erosion principles in piping are also explored.

Simulation of chloride transport in an aggregated soil using three conceptual models

Breakthrough curves (BTCs) of chloride displaced through columns of loessial soil aggregates of different sizes were measured under saturated steady flow conditions. The data were simulated using three conceptual models. Model I (CDE) assumed that all soil water was mobile and physical equilibrium existed in the system. Model II (two-region model) partitioned the soil water into mobile and immobile regions, and convective diffusive solute transport was limited to the mobile water region. Model III (two-flow region model) also divided the soil water into two regions based on their flow velocities, but both of the regions had a non-zero flow rate. Transfer of the chloride solute between the two soil water regions was assumed to occur at a rate proportional to the difference in solute concentration. The two unknown parameters in model I, three in model II, and four in model III were estimated by fitting the experimental data. The three models could well describe all the BTCs measured for columns packed with all the aggregate sizes at the low pore water velocity (0.68 cm/h); however, the values of the fitted parameters varied greatly. The Peclet numbers derived from both the two-region (model II) and two-flow region (model III) models behaved similarly and increased with increases in aggregate size. But the Peclet numbers derived from the convection dispersion equation (model I) were about two orders of magnitude greater than those derived from the other two models. The mobile water fraction obtained for the two-flow region model decreased with increases of aggregate size. The mass transfer coefficient decreased with an increase in pore water velocity due to the shorter residence time of the chloride solute in the soil columns.

Numerical Simulation of Absorbing CO2 with Ionic Liquids

AbstractAlthough separating CO2 from flue gas with ionic liquids has been regarded as a new and effective method, the mass transfer properties of CO2 absorption in these solvents have not been researched. In this paper, a coupled computational fluid dynamic (CFD) model and population balance model (PBM) was applied to study the mass transfer properties for capturing CO2 with ionic liquids solvents. The numerical simulation was performed using the Fluent code. Considering the unique properties of ionic liquids, the Eulerian‐Eulerian two‐flow model with a new drag coefficient correlation was employed for the gas‐liquid fluid dynamic simulation. The gas holdup, interfacial area, and bubble size distribution in the bubble column reactor were predicted. The mass transfer coefficients were estimated with Higbie's penetration model. Furthermore, the velocity field and pressure field in the reactor were also predicted in this paper.

Why is the Ratio of Reflectivity Effective for Chlorophyll Estimation in the Lake Water?

The reasons why it is effective to estimate the chlorophyll-a concentration with the ratio of spectral radiance reflectance at the red light region and near infrared regions were shown in theory using a two-flow model. It was found that all of the backscattering coefficients can consequently be ignored by using the ratio of spectral radiance reflectance, which is the ratio of the upward radiance to the downward irradiance, at the red light and near infrared regions. In other words, the ratio can be expressed by using only absorption coefficients, which are more stable for measurement than backscattering coefficients. In addition, the band selection is crucial for producing the band ratio when the chlorophyll-a concentration is estimated without the effects of backscattering. I conclude that the two wavelengths selected must be close, but one must be within the absorption range of chlorophyll-a, and the other must be outside of the absorption range of chlorophyll-a, in order to accurately estimate the chlorophyll-a concentration.

Accreting corona model of the X-ray variability in soft state X-ray binaries and active galactic nuclei

We develop a two-flow model of accretion onto a black hole which incorporates the effect of the local magneto-rotational instability. The flow consists of an accretion disk and an accreting corona, and the local dynamo affects the disk/corona mass exchange. The model is aimed to explain the power spectrum density of the sources in their soft, disk-dominated states. The local perturbations of the magnetic field in the disk are described as in King et al. (2004) and Mayer and Pringle (2005), but the time-dependent local magnetic field is assumed to affect the local supply of the material to the corona. Since the viscous timescale in the corona is much shorter than in the disk, the local perturbations are not smeared in the corona. Simple analytical estimates and full time-dependent computations of the disk-corona system are performed. The accreting corona model can reproduce the broad power spectra of Soft State X-ray binaries and AGN. The model, however, predicts that (i) sources undergoing radiation pressure instability (GRS 1915+105) should have systematically steeper power spectra than other sources, (ii) AGN should have systematically steeper power spectra than GBH, even if their disks are described using viscosity proportional to the gas pressure. More precise measurements of power spectra of Soft State sources are clearly needed.

Accreting corona model of the X-ray variability in soft state GBH and AGN

AbstractWe develop a two-flow model of accretion onto a black hole which incorporates the effect of the local magneto-rotational instability. The flow consists of an accretion disk and an accreting corona, and the local dynamo affects the disk/corona mass exchange. The model is aimed to explain the power spectra of the sources in their soft, disk-dominated states. The local perturbations of the magnetic field in the disk are described as in King et al. (2004) and Mayer &amp; Pringle (2006), but the time-dependent local magnetic field is assumed to affect the local supply of the material to the corona. The accreting corona model can reproduce the broad power spectra of Soft State X-ray binaries and AGN. The model, however, predicts that (i) sources undergoing radiation pressure instability (GRS 1915+105) should have systematically steeper power spectra than other sources, (ii) AGN should have systematically steeper power spectra than GBH. More measurements of power spectra of Soft State sources are clearly needed.

Comprehensive assessment of the two-flow model’s shape factors in aquatic environments

The applicability of the two-flow model to problems is dependent on assumptions that are used to reduce the model to two equations with two unknowns. Such a reduction requires proper parameterization of the shape factors. An extensive data set that reveals the relationships of shape factors to scattering-to-absorption ratios (0.1 < or = b/a < or = 30) and to the mean cosine of scattering (0.66 < or = micro(s) < or = 0.95) as obtained with a Monte Carlo model is presented. The sensitivity of the computed irradiance reflectance to the shape factors is also addressed. Results show increasing sensitivity of irradiance reflectance to the shape factors with increasing scattering-to-absorption ratio and with increasing anisotropy of the scattering phase function. For waters with highly anisotropic scattering phase functions, high scattering-to-absorption ratios, or both, the proper parameterization of the shape factors must be considered. A lookup table of the shape factors' depth-dependent values for various optical conditions is also presented.

State transitions in the 2001/2002 outburst of XTE J1650–500

We present a study of the X-ray transient and black hole candidate XTE J1650-500 during its 2001/2002 outburst. The source made two state transitions between the hard and soft states, at luminosity levels that differed by a factor of ~5-10. The first transition, between hard and soft, lasted for ~30 days and showed two parts; one part in which the spectral properties evolve smoothly away from the hard state and another that we identify as the 'steep power law state'. The two parts showed different behavior of the Fe K emission line and QPO frequencies. The second transition, from soft to hard, lasted only \~15 days and showed no evidence of the presence of the 'steep power law state'. Comparing observations from the early rise and the decay of the outburst, we conclude that the source can be in the hard state in a range of more 10^4 in luminosity. We briefly discuss the state transitions in the framework of a two-flow model.

Low-Liquid Loading Multiphase Flow in Natural Gas Pipelines

Pressure and temperature variations of natural gas flows in a pipeline may cause partial gas condensation. Fluid phase behavior and prevailing conditions often make liquid appearance inevitable, which subjects the pipe flow to a higher pressure loss. This study focuses on the hydrodynamic behavior of the common scenarios that may occur in natural gas pipelines. For this purpose, a two-fluid model is used. The expected flow patterns as well as their transitions are modeled with emphasis on the low-liquid loading character of such systems. In addition, the work re-examines previous implementations of two-flow model for gas-condensate flow.

Time-dependent modeling of high energy emission of TeV blazars

We present a self-consistent and time-dependent model in order to explain the variability behaviors of extreme or TeV-blazars. This model is based on the ‘Two-Flow model’ involving a relativistic pair plasma beam surrounded by a classical MHD structure. Particular symbiosis between the ultra-relativistic pairs and the turbulence generated by the MHD jet may lead to a preys–predators comportment. We also consider that the pair energy distribution is not a power-law but a ‘pile-up’ one.

A biooptical model of irradiance distribution and photosynthesis in seagrass canopies

Although extremely vulnerable to coastal eutrophication, seagrasses represent important structuring elements and sources of primary production in shallow waters. They also generate an optical signature that can be tracked remotely. Accurate knowledge of light absorption and scattering by submerged plant canopies permits the calculation of important plant‐ and ecosystem‐level properties, including rates of photosynthesis, vegetation abundance, and distribution. The objectives of this study were to develop a realistic, yet simply parameterized two‐flow model of plane irradiance distribution through a seagrass canopy submerged in an optically active water column, to evaluate its performance against in situ measurements, and to explore the impacts of variations in canopy architecture on irradiance distribution and photosynthesis within the canopy. Allometric functions derived from leaf length‐frequency data enabled simple parameterization of canopy architecture. Model predictions of downwelling spectral irradiance distributions in seagrass canopies growing in both oligotrophic and eutrophic waters were within 15% of field measurements. Thus, the model provides a robust tool for investigating photosynthetic performance of seagrass canopies as functions of water quality, depth distribution, canopy architecture, and leaf orientation. Model predictions of upwelling irradiance were less reliable, particularly in the upper half of the canopies. The model was more sensitive to leaf orientation than leaf optical properties, seabed reflectance, or the average cosine of downwelling irradiance. Better knowledge of leaf orientation appears to be a fruitful avenue for improving our understanding of the interaction between seagrasses and the submarine light environment.

Bathymetric and bottom effects on CZCS chlorophyll-like pigment estimation: Data from the Kerkennah Shelf (Tunisia)

The Coastal Zone Color Scanner (CZCS)-retrieved chlorophyll-like pigments for the Mediterranean Sea show zones of high concentrations (10mgm-3) such as the shallow Kerkennah Shelf, off the southern Tunisian coast. This occurrence pointing to probable erroneous estimates, exaggerated by the effect of the bottom signal is discussed in the general framework of the applicability of water colour techniques to coastal areas, as a source of environmental data suitable for inclusion in a multi-purpose Geographic Information System (GIS). The state of knowledge on the Kerkennah Shelf test site, organized into a regional geo-referenced data base (including CZCS and TM imagery), is presented. A two-flow model has been applied to in situ reflectance measurements, to derive some information on the interplay between water colour and bottom typology. The results have been used to evaluate nature, intensity and extent of the sedimentary and vegetated sea bottom effect on the chlorophyll-like pigment concentrations derived from CZCS in the Kerkennah area. Finally, the applicability of satellite-derived ocean colour data to coastal zone mapping is discussed in some detail.

The QUELL experiment as a validation tool for the numerical code Gandalf

One goal of the QUELL experiment is the validation of numerical codes for the simulation of thermo-hydraulic transients in Cable-in-Conduit Conductors with Central Channel (C 5). The validation of Gandalf has focused on 17 quench propagation runs. In general, the good global agreement of the simulations with the experiments—the standard deviation is in the range between 5% and 25%—as well as the good computer performance qualifies Gandalf as a reliable engineering tool. One limitation of the simplified two-flow model implemented in Gandalf is that it tends to overestimate the quench propagation, and hence the helium pressure increase, in the initial phase of the quench evolution, and to underestimate them in the final phase. In standard quenches this limitation can be overcome by full two-flow modeling (Mithrandir code). Two further problem areas have been identified: the high sensitivity (in common with full two-flow modeling) to conductor parameters, if the coil is operated at low temperature margin, and the lack of accuracy when using simplified hydraulic boundary conditions.

TRANSPORT OF A NONREACTIVE SOLUTE IN SOILS

We investigated the validity of a two-flow domain approach for describing physical nonequilibrium behavior of tritium transport in soils. Tritium breakthrough curves (BTCs) were generated from soil columns using miscible displacement methods. Three soils were used: Mahan, Cecil, and Dothan. The predictive capability of the two-flow domain approach in describing two types of tritium BTCs, one with a pulse duration of only few minutes (about 0.03 pore volumes) and the other with a large pulse of approximately one pore volume, was investigated. Large tritium pulse BTCs were used to calibrate the two-flow domain models. Small tritium pulse BTCs were used for model validation where all model parameters were independently obtained. The use of the two-flow domain model improved descriptions of all BTCs over the one-domain model. Moreover, observed double or bimodal peaks associated with small pulse BTCs were well described using the two-flow domain model. Model sensitivity analyses provided theoretical bases for interpretation of tritium bimodal results and differences of BTCs from large and small input pulses. Distinct double peaks were apparent when a small input pulse (impulse) was applied. In addition, high water flux and small mass transfer coefficient between the two flow domains enhanced the development of double BTC peaks

Energetic particle beams in quasars and active galactic nuclei

New developments of a two-flow model for extragalactic jets are presented. We study the possibility of ascribing Very Long Baseline Interferometry (VLBI) jet features and superluminal motion to beams of relativistic particles streaming through an ambient plasma such as a wind from the accretion disc. It is shown that stability of the beams relative to the excitation of Langmuir and Alfvénic waves can be ensured in the case of electron–positron beams with bulk Lorentz factor γ smaller than 43 (the square root of proton to electron mass ratio) as long as the magnetic field parallel to the beams and jets is larger than a critical value |${B}_\text{c}=3.2\times{10}^{-3}\,n_\text{p}^{1/2},\,\text{where}\,{n}_\text{p}$| is the density of the ambient plasma. Conversely, electron–positron beams with higher bulk Lorentz factor, and electron–proton beams are rapidly destroyed. Therefore we propose that electron–positron beams with |$\gamma \leq 43$| are responsible for VLBI features. Two main models for VLBI structures are then expected according to the relative values of parallel magnetic field and ambient plasma density close to the central engine, corresponding to two different regimes for the beams, namely propagation when |$B\gt{B}_\text{c}$| and dissolution when |$B\lt{B}_\text{c}.$|

Relativistic electron-positron beam formation in the framework of the two-flow model for active galactic nuclei

A model is proposed for relativistic beam formation in active galactic nuclei, in which electrons and positrons are generated by pair cascade on the UV photons of the accretion disk. In situ turbulent acceleration by Alfven waves in a pair plasma balances the Compton losses and prevents annihilation. The origin of the turbulence is in the subrelativistic MHD accretion ejection flow, which guarantees an extended energy reservoir up to distance level, easily achieved, the pair creation becomes catastrophic, and so the dramatic increase of the relativistic pressure expels the pair plasma along the widening flux tubes.