Volume-filling Factor Research Articles

Wave Dispersion in a Two-Phase Gas Mixture : Comments on Random Motions and Void Modes

Abstract The properties of linear waves in a mixture comprising diffuse and dense phases are discussed using two approaches: (i) In one approach, the repulsive pressure produced in the diffuse phase by ballistically moving dense clouds generates motion of the components when gradients of the volume-filling factor arise; (ii) in the other approach, where this effect is omitted, a mixture is dynamically insensitive to perturbations containing only variations in the volume-filling factor of dense clouds. We show that the wave dynamics in these two approaches is quite different: in the framework of the first approach, the dispersion relation contains two different branches — the acoustic and the void (pattern formation) modes, while in the second approach the void mode disappears. We argue that in most situations for astrophysical gas mixtures, the first approach is more applicable: since effects of repulsive pressure from randomly moving clouds do exist, the void mode must be important in determining the dynamics of small perturbations and the formation of structures.

Aggregation Dependent Absorption Reduction of Indocyanine Green

Indocyanine green in water and in aqueous NaCl solutions forms large J-aggregate globules. With rising aggregate size the apparent absorption cross section per molecule decreases because of specific surface reduction of the aggregates. A theoretical description of the absorption behavior is given for dense spheres, loosely packed spheres (globules), straight chains, and coiled chains. The volume fill factor of the globules is determined from the absorption reduction measurements using the degree of aggregation from Mie scattering experiments described elsewhere.

The Volume Filling Factor of the WIM

AbstractA compilation of data on the volume filling factor fv of the warm ionized medium (WIM) as a function of the local electron density ne indicates that approximately fv ∝ n−1e over 4 decades. This result is expected for a fractal ISM.

The Lyα Forest from Gravitational Collapse in the Cold Dark Matter + Λ Model

We use an Eulerian hydrodynamic cosmological simulation to model the Ly? forest in a spatially flat, COBE-normalized, cold dark matter model with) ? = 0.4. We find that the intergalactic, photoionized gas is predicted to collapse into sheetlike and filamentary structures which give rise to absorption lines having characteristics similar to the observed Ly? forest. A typical filament is ~500 h?1 kpc long with thickness ~50 h?1 kpc (in proper units), and baryonic mass ~ 1010 h?1 M. In comparison our cell size is (2.5, 9) h?1 kpc in the two simulations we perform, with true resolution perhaps a factor of 2.5 worse than this. The gas temperature is in the range 104-105 K, and it increases with time as structures with larger velocities collapse gravitationally. We show that the predicted distributions of column densities, b-parameters, and equivalent widths of the Ly? forest clouds agree reasonably with observations, and that their evolution is consistent with the observed evolution, if the ionizing background has an approximately constant intensity between z = 2 and z = 4. A new method of identifying lines as contiguous regions in the spectrum below a fixed flux threshold is suggested to analyze the absorption lines, given that the Ly? spectra arise from a continuous density field of neutral hydrogen rather than discrete clouds. We also predict the distribution of transmitted flux and its correlation along a spectrum and on parallel spectra, and the He ii flux decrement as a function of redshift. We predict a correlation length of ~80 h?1 kpc perpendicular to the line of sight for features in the Ly? forest. In order to reproduce the observed number of lines and average flux transmission, the baryon content of the clouds may need to be significantly higher than in previous models because of the low densities and large volume-filling factors we predict. If the background intensity JH I is at least that predicted from the observed quasars, ?b needs to be as high as ~0.25 h?2. The model also predicts that most of the baryons at z > 2 are in Ly? clouds, and that the rate at which the baryons move to more overdense regions is slow. A large fraction of the baryons which are not observed at present in galaxies might be intergalactic gas in the currently collapsing structures, with T ~ 105?106 K. All our results on the statistical properties of the simulated spectra are predictions that can be directly tested by applying the same methods to observed spectra. We are making the simulated spectra electronically available.

Anti-matter production in shocked ism clouds

The efficiency of the anti-proton (pBAR) production in interstellar clouds is increased by a factor ≳50 when the engendering cosmic-ray protons (CRs) experience diffusive acceleration by shocks. One can represent the pBAR/p observations by a conventional multi-phase model of the interstellar medium in which the volume-filling factor for shocked clouds is ≤1 % of that for all clouds. The pBAR/p ratio will reach a maximum value at an energy which depends upon a typical cloud size; the present observations give this size to be ≳1 pc. if individual cloud sizes do not get very much larger then thepBAR/p value may peak at a low energy, slightly above ∼10 GeV. The porosity factor of Cox and Smith (1974) is limited (Q≲0.2). Standard CR confinement models are examined using a diffusion equation approach. Most of the production ofpBAR's by any old CR population will occur in shocked clouds near the galactic plane. One does not need to assume an old CR population exists in order to explain thepBAR observations.

Self-regulated star formation and the evolution of the interstellar medium

Calculations of the evolution of the interstellar medium (ISM) in a one-zone model are presented. The purpose is to study the influences of different processes on the evolution of the ISM and the star-formation rate by applying a detailed description of the stars and the ISM as well as their interactions. Different processes and time-scales are taken into account: stellar evolutionary time-scales and nucleosynthesis, stellar mass loss and energy release to the ISM by means of both supernovae and stellar winds, and a multi-component ISM with phase transitions by means of condensation, evaporation, and ionization as well as metal-dependent heating and cooling of the different phases. Moreover, we allow for intrinsic heating of the cool star-forming clouds. The results show that in addition to the heating by means of supernovae that represents the global star-formation regulation mechanism young stars act predominantly already locally within the star-forming sites. Open-box models allowing for inflow of all existing gas phases with similar physical states but restricting the outflow to the hot phase only are able to explain successfully the dilution of metallicity and large fluctuations of the star-formation rate, of the volume-filling factors, and of the amounts of different gas phases.

The physical and kinematic structures of molecular clouds. I - Model

A molecular cloud model that simulates the radiative transfer of the line emissions of a molecular gas is presented, together with methods for calculating the radiative transfer and line intensities in a clumpy medium. Observations of the Orion A cloud were used to study the model. To allow for a more complete distribution of molecular matter than hitherto considered, a new parameter, the volume-filling factor, is introduced. The results indicate that in order to observe the hot core of a cloud in the optically thick CO lines in a molecular gas that is not moving in a systematic way, the volume-filling factor needs to be very small at the cloud surface and increasingly large toward the center. 21 references.

Processing in an Internal Mixer as Affected by Carbon Black Properties

Abstract This paper has attempted to review current knowledge on mixing fundamentals from a carbon black standpoint. The mixing parameters discussed include volume fill factor, specific energy of mixing, temperature buildup, and flow characteristics. Together, these parameters define the economics of mixing in terms of mixing capacity, energy expenditure, limitations imposed by dump temperature on second-stage operations, and extruder flow requirements. The four parameters in turn were related to basic carbon black properties by way of mixing profiles. Ten currently available tread blacks which exhibit a wide range of tread wear resistance have been characterized in terms of the processability criteria. A processability rating system based on the most simple equipment requirement (a Banbury followed immediately by a second-stage mill or an extruder) was applied to the ten blacks. Comparison of the processability ratings with the tread wear ratings clearly demonstrates that a valid assessment of carbon black utility must include processing cost in addition to performance capability. As in many rubber properties, there is clearly an inverse relationship between performance quality and processability. High-structure, high-surface-area blacks that are desirable from a performance standpoint require the highest processing cost. The question of uniformity was also discussed, and it was concluded that the present specifications controlling carbon black surface area and structure are adequate in minimizing processing variations. The challenge to processing technology from a carbon black standpoint then involves the reduction in the cost of processing high-structure, high-surface-area blacks. At this stage, a quantitative measure of the processing characteristics of the currently available tread blacks at least allows a rational product selection in terms of optimum performance and processing cost. In the meantime, it is evident that significant advances in carbon black technology are still needed if performance optimization is to be reconciled with the economics of processing.

Comb-type slow-wave structure for travelling-wave masers

A modified comb-type slow-wave structure (s.w.s.) for travelling-wave masers is discussed. The dispersion relation is expressed in terms of the s.w.s. geometry and the admittances of coupled striplines. The slowing factor and the volume-filling factor, important parameters for travelling-wave masers, are discussed.