Variations In Grain Properties Research Articles

Dust as a solar shield

We revisit dust placed near the Earth–Sun L1 Lagrange point as a possible climate-change mitigation measure. Our calculations include variations in grain properties and orbit solutions with lunar and planetary perturbations. To achieve sunlight attenuation of 1.8%, equivalent to about 6 days per year of an obscured Sun, the mass of dust in the scenarios we consider must exceed 1010 kg. The more promising approaches include using high-porosity, fluffy grains to increase the extinction efficiency per unit mass, and launching this material in directed jets from a platform orbiting at L1. A simpler approach is to ballistically eject dust grains from the Moon’s surface on a free trajectory toward L1, providing sun shade for several days or more. Advantages compared to an Earth launch include a ready reservoir of dust on the lunar surface and less kinetic energy required to achieve a sun-shielding orbit.

CFD modelling of physical velocity and anisotropic resistance components in a peaked stored grain with aeration ducting systems

Modelling airflow in large stored grain silos has been dealt with superficial velocities and with uniform airflow resistance across the bulk. However, superficial velocities are always lower than physical velocities in an isotropic medium. Besides, due to varying grain properties, there is a high potential of variation in airflow resistances along the vertical and horizontal directions.Thus, a 3D CFD model was developed considering physical velocity in an anisotropic porous media to predict airflow and static pressure distribution from a V-shaped aeration floor. Varying airflow resistances along the horizontal direction of an element were incorporated into the model to account for the variation in grain properties. Mesh independency tests were conducted for achieving numerical stability during the steady-state simulation. The model was validated using the static pressure taps and airflow data measured in an on-farm grain storage silo.A 0.35 m mesh size was optimized using the static pressure and airflow data in the silo model. Least error for static pressure and airflow predictions were observed when horizontal airflow resistance was 50% of the vertical airflow resistance. Furthermore, the model predicted that 14.3% of the total grain volume exhibited a low airflow rate below the recommended value of 2 L s−1 t−1 including a significant proportion of the top grain volume above the eave height. Due to 85% of the non-perforated section on the silo floor, 12.6% of the lower grain volume up to a maximum height of 4.4 m above the floor exhibited the low airflow regime. As physical experimentations are tedious, the proposed physical velocity based model could be extended for simulating different options for minimizing dead zone proportions for better grain storage management.

In situ X-ray imaging of heterogeneity in dynamic compaction of granular media

Dynamic compaction of granular materials is a complex process, wherein the material response at the continuum scale arises from fracture, comminution, and fragment flow at the grain scale. Granular materials have heterogeneity due to variation in grain properties as well as variation in local packing density and structure. These heterogeneities may affect the dynamic compaction response, producing a structured and time-varying compaction front. The methodology used to characterize the shock behavior in solid materials may be inappropriate for granular materials because of this unsteady behavior and interactions between the granular material and measurement surfaces. To observe the compaction front heterogeneity, in situ x-ray imaging of granular compaction is conducted at a scale between the grain- and continuum scales. To allow for sufficient x-ray transmission, a thin sample geometry is used, for which boundary affects may result in a significantly different load path than would occur in a 1-D compaction configuration. Numerical simulations of the experimental geometry support the analysis of the results so that the feature of the compaction front can be distinguished from artifacts of the experimental configuration. The results suggest that compaction front heterogeneity may have structure with a length scale of tens of particle diameters and demonstrate that finite grain size can inhibit the formation of shear-induced features that would arise in a homogenized representation of the same material.

Cool dust heating and temperature mixing in nearby star-forming galaxies

Physical conditions of the interstellar medium in galaxies are closely linked to the ambient radiation field and the heating of dust grains. In order to characterize dust properties in galaxies over a wide range of physical conditions, we present here the radial surface brightness profiles of the entire sample of 61 galaxies from Key Insights into Nearby Galaxies: Far-Infrared Survey with Herschel (KINGFISH). The main goal of our work is the characterization of the grain emissivities, dust temperatures, and interstellar radiation fields responsible for heating the dust. After fitting the dust and stellar radial profiles with exponential functions, we fit the far-infrared spectral energy distribution (SED) in each annular region with single-temperature modified black bodies using both variable (MBBV) and fixed (MBBF) emissivity indices beta, as well as with physically motivated dust models. Results show that while most SED parameters decrease with radius, the emissivity index beta also decreases with radius in some galaxies, but in others is increasing, or rising in the inner regions and falling in the outer ones. Despite the fixed grain emissivity (average beta~ 2.1) of the physically-motivated models, they are well able to accommodate flat spectral slopes with beta<= 1. We find that flatter slopes (beta<= 1.5) are associated with cooler temperatures, contrary to what would be expected from the usual Tdust-beta degeneracy. This trend is related to variations in Umin since beta and Umin are very closely linked over the entire range in Umin sampled by the KINGFISH galaxies: low Umin is associated with flat beta<=1. Both these results strongly suggest that the low apparent \beta values (flat slopes) in MBBV fits are caused by temperature mixing along the line-of-sight, rather than by intrinsic variations in grain properties. Abstract truncated for arXiv.

Variation in dust properties in a dense filament of the Taurus molecular complex (L1506)

We observed the L1506 filament, which is located in the Taurus molecular complex, with the Herschel PACS and SPIRE instruments. Our aim is to prove the variation in grain properties along the entire length of the filament. In particular, we want to determine above which gas density this variation arises and what changes in the grain optical properties/size distribution are required. We use the 3D radiative transfer code CRT, coupled to the dust emission and extinction code DustEM, to model the emission and extinction of the dense filament. We test a range of optical properties and size distributions for the grains: dust of the diffuse interstellar medium (interstellar PAHs and amorphous carbons and silicates) and both compact and fluffy aggregates. We find that the grain opacity has to increase across the filament to fit simultaneously the near-IR extinction and Herschel emission profiles of L1506. We interpret this change to be a consequence of the coagulation of dust grains to form fluffy aggregates. Grains similar to those in the diffuse medium have to be present in the outer layers of the cloud, whereas aggregates must prevail above gas densities of a few 1000 H/cm3. This corresponds to line-of-sights with visual extinction in the V band of the order of 2 to 3. The dust opacity at 250 microns is raised by a factor of 1.8 to 2.2, while the grain average size is increased by a factor of 5. These exact numbers depend naturally on the dust model chosen to fit the data. Our findings agree with the constraints given by the study of the gas molecular lines. Using a simple approach, we show that the aggregates may have time to form inside the filament within the cloud lifetime. Our model also characterises the density structure of the filament, showing that the filament width is not constant along L1506 but instead varies by a factor of the order of 4.

Variations in Grain Properties of Dry Bean (Phaseolus vulgaris L.)*

Variations in Grain Properties of Dry Bean (Phaseolus vulgaris L.)*

The Effects of Grain-Density Variation on Turbidity Currents and Some Implications for the Deposition of Carbonate Turbidites

Abstract Sediment grains in carbonate turbidity currents vary greatly in size, shape, surface texture, and density depending on the evolution of the source carbonate platform in time and space. Facies character and distribution may vary from traditional siliciclastic turbidite models not only because of the difference in composition but also because variation in grain properties influences flow and depositional behavior. Experimental gravity flows were generated to investigate one control on this variation. The experiments used silicon carbide (ρ = 3214 kg m−3) and glass ballotini (ρ = 2650 kg m−3) with a similar grain-size distribution in varying proportions to assess the influence of particle density on flow and sedimentation processes. Mixed-particle density flows were more efficient than single-particle density flows. Flow propagation was faster in flows with greater initial bulk density determined by grain density and suspended-sediment concentration. With mixed-particle suspensions flow propagation velocity was maintained for longer due to the presence of low-settling-velocity particles. As a result mixed-particle density flows transported more sediment downstream compared to single-particle density flows of similar initial suspended-sediment volume concentration. Because of the wider range of grain types in carbonate turbidity currents they may more effectively transport coarse, sand-grade sediment to more distal sites in a marine basin compared to siliciclastic counterparts of equivalent initial volume. This results in differences in bed and facies geometry and distribution.

Unbiased fitting of B335 dust continuum observations: approach and evidence for variation of grain properties with position

We present the first published unbiased, automated approach to fitting infrared observations of interstellar dust clouds for the purposes of constraining their underlying physical properties. The dust radiative transfer problem is solved self-consistently for a given set of physical inputs. Spherical geometry is chosen to minimize the run time, given the large number (∼ millions) of models necessary. The model output is convolved with appropriate beams to produce simulated observations, which are then compared with observational data. A best fit is achieved using the Levenberg-Marquardt DNLS 1 routine from the SLATEC library, to minimize the chi-squared deviation. The speed of the model allows a large number of initial starting conditions to be considered. While any number of parameters can be fitted, we concentrate on the dust density and grain property distribution. We apply the model to the well-studied source B335. While much of the envelope is well modelled by a power-law exponent ∼1.5-1.9, we find potential evidence for a different inner structure for r 120 arcsec. We also find evidence of a variation in grain properties with radial position. The interior and exterior have larger concentrations of both thick ice mantles and bare grains. The properties in the interior would be consistent with a non-spherical structure seen by Harvey et al. (2003b) and Stutz et al. (2008), and the exterior would be consistent with incomplete refreezing of ice on to mantles after an evaporative event, such as a shock, in the last ∼ 10 5 yr.

350 μm Observations of Local Luminous Infrared Galaxies and the Temperature Dependence of the Emissivity Index

We report 350 μm observations of 18 nearby luminous infrared galaxies (LIRGs) using the Submillimeter High Angular Resolution Camera II (SHARC-II) mounted on the Caltech Submillimeter Observatory (CSO) 10.4 m telescope. Combining our 350 μm flux measurements with the existing far-infrared (FIR) and submillimeter data, we fit a single-temperature model to the spectral energy distributions (SEDs) and find the dust temperatures, emissivity indices, and FIR luminosities having sample medians of Td = 39.4 ± 7.9 K, β = 1.6 ± 0.3, and LFIR = 1011.2±0.6 L☉. An empirical inverse Td-β correlation, best described by Td = (9.86 × 109)1/(4.63 + β), is established for the local LIRG sample, which we argue can be explained by the intrinsic interdependence between the dust temperature and grain emissivity index as physical parameters, as well as variations in grain properties in the interstellar medium (ISM).