Effect Of Density Inhomogeneity Research Articles

Возникновение концентрационной конвекции в длинной прямоугольной области пористой среды

We solve a problem of the convective flow stability in an elongated rectangular region of a porous medium. The transport of a mixture is described using the MIM (mobile-immobile media) approach. This approach consists in dividing the concentration of the solute into mobile and immobile compo-nents. The effect of density inhomogeneity on the flow of a solution in a porous medium is taken in-to account by the filtration equation within the Darcy-Boussinesq approximation. The problem is solved numerically with the use of the finite difference method. The fields of pressure and concen-tration are obtained. The influence of the parameters of the problem on the disturbance profiles is analyzed. A modified problem of stability is solved. The modification is necessary for finding the oscillatory convective mode. To obtain the solution, the spectrum of Lyapunov exponents is calcu-lated with Gram-Schmidt orthogonalization. Neutral curves are found that make it possible to detect the threshold for the onset of convection. The ranges of parameters in which oscillatory perturba-tions are realized are obtained. The effect of sorption parameters on the occurrence of oscillations is analyzed.

Effects of density inhomogeneity in green body on the structure and properties of ferroelectric ceramics

Effects of density inhomogeneity in green body on the structure and properties of ferroelectric ceramics

The local luminosity function of star-forming galaxies derived from the Planck Early Release Compact Source Catalogue

The Planck Early Release Compact Source Catalogue (ERCSC) has offered the first opportunity to accurately determine the luminosity function of dusty galaxies in the very local Universe (i.e. distances ≲100 Mpc), at several (sub-)millimetre wavelengths, using blindly selected samples of low-redshift sources, unaffected by cosmological evolution. This project, however, requires careful consideration of a variety of issues including the choice of the appropriate flux density measurement, the separation of dusty galaxies from radio sources and from Galactic sources, the correction for the CO emission, the effect of density inhomogeneities and more. We present estimates of the local luminosity functions at 857 GHz (350 μm), 545 GHz (550 μm) and 353 GHz (850 μm) extending across the characteristic luminosity L⋆, and a preliminary estimate over a limited luminosity range at 217 GHz (1382 μm). At 850 μm and for luminosities L ≳ L⋆ our results agree with previous estimates, derived from the Submillimeter Common-User Bolometer Array (SCUBA) Local Universe Galaxy Survey, but are higher than the latter at L ≲ L⋆. We also find good agreement with estimates at 350 and 500 μm based on preliminary Herschel survey data.

Stellar envelope inflation near the Eddington limit

(shortened) It has been proposed that the envelopes of luminous stars may be subject to substantial radius inflation. The inflation effect has been discussed in relation to the radius problem of WR stars, but has yet failed to explain the large observed radii of Galactic WR stars. We wish to obtain a physical perspective of the inflation effect, and study the consequences for the radii of WR stars, and LBVs. For WR stars the observed radii are up to an order of magnitude larger than predicted by theory, whilst S Doradus-type LBVs are subject to humongous radius variations, which remain as yet ill-explained. We use a dual approach to investigate the envelope inflation, based on numerical models for stars near the Eddington limit, and a new analytic formalism to describe the effect. An additional new aspect is that we take the effect of density inhomogeneities (clumping) within the outer stellar envelopes into account. Due to the effect of clumping we are able to bring the observed WR radii in agreement with theory. Based on our new formalism, we find that the radial inflation is a function of a dimensionless parameter W, which largely depends on the topology of the Fe-opacity peak, i.e., on material properties. For W>1, we discover an instability limit, for which the stellar envelope becomes gravitationally unbound, i.e. there no longer exists a static solution. Within this framework we are also able to explain the S Doradus-type instabilities for LBVs like AG Car, with a possible triggering due to changes in stellar rotation. The stellar effective temperatures in the upper HR diagram are potentially strongly affected by the inflation effect. This may have particularly strong effects on the evolved massive LBV and WR stars just prior to their final collapse, as the progenitors of SNe Ibc, SNe II, and long GRBs.

Quantum scattering time and its implications on scattering sources in graphene

Supplementary Information Content: 1. Sample preparation; 2. Background subtraction of Shubnikov-de Haas (SdH) oscillations; 3. The effect of density inhomogeneity on the quantum scattering time tau_q; 4. Determine the concentration of charged impurity n_imp at a distance z; 5. Scattering from charges in the bulk of the SiO_2 substrate.

Interpretation of geoid anomalies in the contact zone between the East European Craton and the Palaeozoic Platform-I. Estimation of effects of density inhomogeneities in the crust on geoid undulations

SUMMARY In our paper, we compile a detailed 3-D velocity model of the crust for the territory of Poland and investigate the effect of a 3-D density distribution in the crust on geoid undulations. The major tectonic feature in the study area is the Trans European Suture Zone (TESZ) separating the Precambrian East European Craton from younger tectonic units of Palaeozoic Europe. High-resolution seismic models of the crust and upper mantle from recent controlled-source experiments were interpolated into a 3-D P-wave velocity model for the whole Poland. The model was converted to a density model assuming different equations connecting seismic velocities to densities for crustal rocks, sediments and the uppermost mantle. The crustal density model was used to calculate synthetic geoid undulations for the territory of Poland and to estimate influence of separate layers (topography, sediments, crust and uppermost mantle) on the geoid. To calculate the gravity effect of a large-scale 3-D density model, we developed a code based on a point mass algorithm as well as prisms algorithm that uses some easy to implement optimizations. The synthetic geoid undulations were compared to an existing gravimetric quasi-geoid solution. The modelling showed that the difference in crustal structure between the East European Platform and Palaeozoic Platform explains significant part of the observed geoid undulations in the study area. The remained residual could be related to density inhomogeneities in the upper mantle.

Effect of density inhomogeneity on YORP: The case of Itokawa

The effect of density inhomogeneity on the YORP effect for a given shape model is investigated. A density inhomogeneity will cause an offset between the center of figure and the center of mass and a re-orientation of the principal axes away from those associated with the shape alone. Both of these effects can alter the predicted YORP rate of change in angular velocity and obliquity. We apply these corrections to the Itokawa shape model and find that its YORP angular velocity rate is sensitive to offsets between its center of mass and center of figure, with a shift on the order of 15 m being able to change the sign of the YORP effect for that asteroid. Given the non-detection of YORP for Itokawa as of 2008, this can shed light on the density distribution within that body. The theory supports a shift of the asteroid center of mass towards Itokawa's neck region, where there is an accumulation of finer gravels, or towards the asteroid's “Head” region. Detection of the YORP effect for Itokawa should provide some strong constraints on its density distribution. This theory could also be applied to asteroids visited by future spacecraft to constrain density inhomogeneities.

Diode-laser-based near-resonantly enhanced flow visualization in shock tunnels

Two new near-resonantly enhanced flow visualization techniques suitable for hypersonic low-density flows in shock or arc tunnels have been developed using seeded lithium (Li) metal as the refractivity-enhancing species. Two semiconductor lasers, single-longitudinal-mode and multimode, are compared with respect to their suitability as light sources for the technique. Transient wake-flow structures around a cylinder and a model of a planetary entry vehicle are visualized to demonstrate the capabilities of this comparatively inexpensive and simple visualization system. The images show flow features which are undetectable with conventional schlieren, shadowgraph, or interferometry techniques. Furthermore, the effect of density inhomogeneities along the line-of-sight outside the region of interest can be reduced by enhancing the refractivity only in selected parts of the flowfield.

Erratum: Growth of a supersymmetric bubble: Inhomogeneity effects [Phys. Rev. D72, 055005 (2005)

In a dense star, the Pauli exclusion principle functions as an enormous energy storage mechanism. Supersymmetry could provide a way to recapture this energy. If there is a transition to an exactly supersymmetric (susy) phase, the trapped energy can be released with consequences similar to gamma ray burst observations. Previous zeroth order calculations have been based on the behavior in a prototypical white dwarf of solar mass and earth radius (such as Sirius B) and have neglected density inhomogeneity. In this article we show that the effects of density inhomogeneity and of variations in masses and radii are substantial enough to encourage further exploration of the susy star model. In addition, the effects discussed here have possible applications to the growth of bubbles in other phase transition models in dense matter.

Growth of a supersymmetric bubble: Inhomogeneity effects

In a dense star, the Pauli exclusion principle functions as an enormous energy storage mechanism. Supersymmetry could provide a way to recapture this energy. If there is a transition to an exactly supersymmetric (SUSY) phase, the trapped energy can be released with consequences similar to gamma ray burst observations. Previous zeroth order calculations have been based on the behavior in a prototypical white dwarf of solar mass and earth radius (such as Sirius B) and have neglected density inhomogeneity. In this article we show that the effects of density inhomogeneity and of variations in masses and radii are substantial enough to encourage further exploration of the SUSY star model. In addition, the effects discussed here have possible applications to the growth of bubbles in other phase transition models in dense matter.

Modification of Projected Velocity Power Spectra by Density Inhomogeneities in Compressible Supersonic Turbulence

The scaling of velocity fluctuation ?v as a function of spatial scale L in molecular clouds can be measured from size-line width relations, principal component analysis, or line centroid variation. Differing values of the power-law index of the scaling relation ?v21/2 L in three dimensions are given by these different methods: the first two give ?3D 0.5, while line centroid analysis gives ?3D 0. This discrepancy has previously not been fully appreciated, as the variation of projected velocity line centroid fluctuations (?v1/2 L) is indeed described, in two dimensions, by ?2D ? 0.5. However, if projection smoothing is accounted for, this implies, in three dimensions, that ?3D ? 0. We suggest that a resolution of this discrepancy can be achieved by accounting for the effect of density inhomogeneity on the observed ?2D obtained from velocity line centroid analysis. Numerical simulations of compressible turbulence are used to show that the effect of density inhomogeneity statistically, but not identically, reverses the effect of projection smoothing in the case of driven turbulence, so that velocity line centroid analysis does indeed predict that ?2D ? ?3D ? 0.5. For decaying turbulence, the effect of density inhomogeneity on the velocity line centroids diminishes with time so that at late times, ?2D ? ?3D + 0.5 as a result of projection smoothing alone. Deprojecting the observed line centroid statistics thus requires some knowledge of the state of the flow. This information can be inferred from the spectral slope of the column density power spectrum and a measure of the standard deviation in column density relative to the mean. Using our numerical results, we can restore consistency between line centroid analysis, principal component analysis, and size-line width relations, and we derive ?3D ? 0.5, corresponding to shock-dominated (Burgers) turbulence, which also describes the simulations of driven turbulence at scales on which numerical dissipation is negligible. We find that this consistency requires that molecular clouds are continually driven on large scales or are only recently formed.

Turbulence-driven tearing modes in inhomogeneous magnetized plasma

The effects of density inhomogeneity in the nonlinear growth of tearing modes in the presence of background density, electrostatic and magnetic fluctuations are investigated. The nonlinear interaction is assumed to occur in the inner layer of the tearing mode, where the fields and their variations are greatest. Our results demonstrate that turbulence-generated dissipative effects replace the collisional resistivity as the driving force of the tearing mode. The inhomogeneity introduces a real frequency to the nonlinear tearing mode, as well as new unstable tearing-mode–related drift waves.

Comparative study of density-functional theories of the exchange-correlation hole and energy in silicon

We present a detailed study of the exchange-correlation hole and exchange-correlation energy per particle in the Si crystal as calculated by the variational Monte Carlo method and predicted by various density-functional models. Nonlocal density-averaging methods prove to be successful in correcting severe errors in the local-density approximation (LDA) at low densities where the density changes dramatically over the correlation length of the LDA hole, but fail to provide systematic improvements at higher densities where the effects of density inhomogeneity are more subtle. Exchange and correlation considered separately show a sensitivity to the nonlocal semiconductor-crystal environment, particularly within the Si bond, which is not predicted by the nonlocal approaches based on density averaging. The exchange hole is well described by a bonding-orbital picture, while the correlation hole has a significant component due to the polarization of the nearby bonds, which partially screens out the anisotropy in the exchange hole.

Amorphous thin film growth: effects of density inhomogeneities.

A nonlinear stochastic growth equation for the spatiotemporal evolution of the surface morphology of amorphous thin films in the presence of potential density variations is derived from the relevant physical symmetries and compared to recent experimental results. Numerical simulations of the growth equation exhibit a saturation of the surface morphology for large film thickness originating from the inclusion of the density inhomogeneities. Furthermore, we argue why moundlike surface structures observed on vapor deposited amorphous films are not the result of the Grinfeld instability.

Simulation of the Collision of Magnetic Flux Tubes in the Quiet Solar Photosphere

Recent high-resolution photospheric magnetograms made with the SOHO/Michelson Doppler Imager instrument and the Swedish Vacuum Solar Telescope on La Palma show that concentrations of magnetic flux in the quiet photospheric network of the solar photosphere are highly dynamic objects with small-scale substructure. These observations reveal many details in the dynamics of flux emergence, fragmentation, and cancellation. In order to understand such phenomena we investigate the dynamics of two colliding magnetic flux tubes in weakly ionized plasmas with high plasma beta (β 1), using the three-dimensional neutral-MHD equations. First we investigate the collision of two parallel flux tubes for the two cases of partial and complete magnetic reconnection. We find that, when one flux tube with weak current and small radius collides with another flux tube with strong current and large radius, the weak-current flux tube splits into two small flux tubes because of magnetic reconnection. We also find that the collision of magnetic flux tubes with weak current leads to the emission of strong fast magnetosonic waves, resulting in shock formation, while the collision site of two strong-current loops shows no strong wave emission. Next we investigate the collision of two noncollinear flux tubes with X-type configuration, taking into account the effect of density inhomogeneity along the flux tubes due to gravity. We find strong upward plasma flows along the flux tubes and also shock wave emission from the X-type collision region. Finally we discuss the application of these simulation results to coronal heating.

VLA Observations of Hydrogen and Carbon Recombination Lines toward W3A at 1.4 GHz

We present high-sensitivity, high-resolution VLA observations of the W3 complex of H II regions in the 168α recombination lines of hydrogen, carbon, and sulphur. The H 168α line from W3A consists of two components: a broad line (width ~27 km s-1) and a narrow line (width ~7 km s-1). The narrow hydrogen and carbon line emissions over W3A, although overlapping, are not entirely coextensive. The carbon line is possibly correlated with the molecular gas near W3A. Stimulated emission is the main mechanism for the narrow hydrogen line emission. The width of the H0 line gives an upper limit of ~1000 K for the electron temperature of the partially ionized gas. The electron density ranges from 10 to 80 cm-3 in the narrow hydrogen line region and from 10 to 60 cm-3 in the carbon-line region. We determined the electron temperature of the classical H II region W3A from the continuum brightness to be ~9000 K. The rms ne of this H II region is ~2200 cm-3, and the true ne, determined from a pressure-broadened profile of the H 171η (8.6 GHz) line, is ~2 × 104 cm-3. Using these two values of electron densities, we determine a lower limit to the filling factor (0.01). Such a low value can be interpreted as an effect of density inhomogeneities in the medium.

Evolution of planetesimals. I - Dynamics: Relaxation in a thin disk. II - Numerical simulations

The study examines the effects of density inhomogeneity and differential rotation as well as inelastic collisions on the dynamical evolution of planetesimals. Consideration is given to a three-step analysis: the dynamical evolution of the planetesimals, collisions and mass accumulation, and interaction with gas. It is shown that the velocity dispersion of a cold system of planetesimals increases rapidly due to elastic gravitational scattering. When the dispersion in the epicycle amplitude becomes comparable to the planetesimals' Roche radius, energy is transferred from the systematic Keplerian shear to the dispersive motion. With a numerical N-body scheme, gravitational scattering and physical collisions among a system of planetesimals is simulated. It is shown that dynamical equilibrium is attained with a velocity dispersion comparable to the surface escape velocity of those planetesimals which contribute most of the system mass.

The effect of density inhomogeneity on standing Alfven wave structure

At geomagnetic latitudes well within the plasmasphere (~25°–40°), ionospheric O + ions have a significant effect on hydromagnetic wave frequencies, waveforms and wave mode coupling. We apply a physically realistic plasmasphere model to obtain the eigenfunctions of the toroidal mode standing waves in a dipole field. Ion mass density inhomogeneities along the field lines (e.g. ionospheric O +, hemispheric asymmetry, or equatorial enhancements) result in a confinement of the wave fields within any significant density increase. Near latitude 30° the fundamental toroidal mode electric field has its largest values well away from the equatorial plane. The effects of the ionospheric F-region need to be considered when mapping pulsation fields and evaluating hydromagnetic wave coupling at low latitudes. The irregular spacing of the toroidal mode harmonic frequencies near latitude 30° is a direct consequence of the Alfvén velocity inhomogeneity over a substantial portion of the field line length.

The generation of sound by the convection of inhomogeneities through space‐varying flows, by analogy with the scattering of acoustic and evanescent waves

It is pointed out that the scattering theory of Rayleigh for sound waves interacting with inhomogeneities of a stationary medium can also be applied to evanescent waves. By considering the fluid to be moving and the wave system stationary and caused by a fixed wavy wall, the problem of the sound generated by inhomogeneities of a fluid carried through a supersonic or subsonic space‐varying flow field past the wall can then be addressed. As in that scattering theory, the inhomogeneities must be small in extent compared to the scale of the pressure gradient of the basic flow and also of the resultant sound: This is the low‐frequency approximation (which excludes application of the theory to shockwaves). It is found that inhomogeneities in density and compressibility of the gas (i.e., of the gas constant) both result in sound power that varies as the sixth power of the speed in all the cases of supersonic and subsonic flow, and when the interaction takes place far from, or close to, the wall or body, except that, at supersonic speed, interactions close to the body result in the sound power due to density fluctuations varying with only the fourth power of the Mach number and that due to the gas constant (compressibility) at subsonic speeds varying as the eighth power. In all cases, the sound power depends acutely on the shortness of the length characterizing the pressure gradients in the flow; specifically, it varies inversely as the sixth power of the characteristic length. It is pointed out that, in most engineering applications, the effects of density inhomogeneities are likely to overwhelm those due to variations in the compressibility of the gas, and interactions in the subsonic case must be close to the body to be significant, due to the very rapid decay of the pressure field away from the body.