Numerical simulations of granular dynamics: I. Hard-sphere discrete element method and tests

We present a new particle-based (discrete element) numerical method for the simulation of granular dynamics, with application to motions of particles on small solar system body and planetary surfaces. The method employs the parallel N-body tree code pkdgrav to search for collisions and compute particle trajectories. Collisions are treated as instantaneous point-contact events between rigid spheres. Particle confinement is achieved by combining arbitrary combinations of four provided wall primitives, namely infinite plane, finite disk, infinite cylinder, and finite cylinder, and degenerate cases of these. Various wall movements, including translation, oscillation, and rotation, are supported. We provide full derivations of collision prediction and resolution equations for all geometries and motions. Several tests of the method are described, including a model granular “atmosphere” that achieves correct energy equipartition, and a series of tumbler simulations that show the expected transition from tumbling to centrifuging as a function of rotation rate.     

A numerical model of cohesion in planetary rings

We present a numerical method that incorporates particle sticking in simulations using the N-body code pkdgrav to study motions in a local rotating frame, such as a patch of a planetary ring. Particles stick to form non-deformable but breakable aggregates that obey the (Eulerian) equations of rigid-body motion. Applications include local simulations of planetary ring dynamics and planet formation, which typically feature hundreds of thousands or more colliding bodies. Bonding and breaking thresholds are tunable parameters that can approximately mimic, for example, van der Waals forces or interlocking of surface frost layers. The bonding and breaking model does not incorporate a rigorous treatment of internal fracture; rather the method serves as motivation for first-order investigation of how semi-rigid bonding affects the evolution of particle assemblies in high-density environments.We apply the method to Saturn’s A ring, for which laboratory experiments suggest that interpenetration of thin, frost-coated surface layers may lead to weak cohesive bonding. These experiments show that frost-coated icy bodies can bond at the low impact speeds characteristic of the rings. Our investigation is further motivated by recent simulations that suggest a very low coefficient of restitution is needed to explain the amplitude of the azimuthal brightness asymmetry in Saturn’s A ring, and the hypothesis that fine structure in Saturn’s B ring may in part be caused by large-scale cohesion.This work presents the full implementation of our model in pkdgrav, as well as results from initial tests with a limited set of parameters explored. We find a combination of parameters that yields aggregate size distribution and maximum radius values in agreement with Voyager data for ring particles in Saturn’s outer A ring. We also find that the bonding and breaking parameters define two strength regimes in which fragmentation is dominated either by collisions or other stresses, such as tides. We conclude our study with a discussion of future applications of and refinements to our model.     

The natural thermoluminescence of Antarctic meteorites and their terrestrial ages and orbits: A 2010 update

Abstract– We have examined the relationship between natural thermoluminescence (TL) and ²⁶Al in 120 Antarctic meteorites in order to explore the orbital history and terrestrial ages of these meteorites. Our results confirm the observations of Hasan et al. (1987) which were based on 23 meteorites. For most meteorites there was a positive correlation between natural TL and ²⁶Al, reflecting their similarity in decay rate under Antarctic conditions and thus in terrestrial age. For a small group with low TL and high ²⁶Al a small perihelion was proposed. Within this group, natural TL decreases with terrestrial age as determined by ³⁶Cl measurements, although the rate of TL decay is faster (half‐life approximately 10 ka) and the ages that can be determined are smaller (<200 ka) than for most meteorites. The faster decay rate and lower natural TL levels are a reflection of recent exposure to higher radiation doses and higher temperatures, since this history would populate less stable TL traps with smaller electron densities. We sort the 120 meteorites by perihelion and terrestrial age. The normal perihelion group range up to approximately 1000 ka and the small perihelion group range up to approximately 200 ka. An intermediate perihelion group tends to have short terrestrial ages (20–60 ka). There is acceptable agreement between most (34 out of 43) of our present terrestrial age estimates and those determined by isotopic means, the exceptions reflecting complex irradiation histories, long burial times in the Antarctic, or other issues.     

Mixing efficiency in decaying stably stratified turbulence

Mixing efficiency in stratified flows is a measure of the proportion of turbulent kinetic energy that goes into increasing the potential energy of the fluid by irreversible mixing. In this research direct numerical simulations (DNS) and rapid distortion theory (RDT) calculations of transient turbulent mixing events are carried out in order to study this aspect of mixing. In particular, DNS and RDT of decaying, homogeneous, stably-stratified turbulence are used to determine the mixing efficiency as a function of the initial turbulence Richardson number R_it0=(N_L0/_u0)²$R{i}_{t0}={(N{L}_0/u_0)}^2$, where N   is the buoyancy frequency and _L0$L_0$ and _u0$u_0$ are initial length and velocity scales of the turbulence. The results show that the mixing efficiency increases with increasing R_it0$R{i}_{t0}$ for small R_it0$R{i}_{t0}$, but for larger R_it0$R{i}_{t0}$ the mixing efficiency becomes approximately constant. These results are compared with data from towed grid experiments. There is qualitative agreement between the DNS results and the available experimental data, but significant quantitative discrepancies. The grid turbulence experiments suggest a maximum mixing efficiency (at large R_it0$R{i}_{t0}$) of about 6%, while the DNS and RDT results give about 30%. We consider two possible reasons for this discrepancy: Prandtl number effects and non-matching initial conditions. We conclude that the main source of the disagreement probably is due to inaccuracy in determining the initial turbulence energy input in the case of the grid turbulence experiments.     

The Cali meteorite fall: A new H/L ordinary chondrite

Abstract— The fall of the Cali meteorite took place on 6 July 2007 at 16 h 32 ± 1 min local time (21 h 32 ± 1 min UTC). A daylight fireball was witnessed by hundreds of people in the Cauca Valley in Colombia from which 10 meteorite samples with a total mass of 478 g were recovered near 3°24.3′N, 76°30.6′W. The fireball trajectory and radiant have been reconstructed with moderate accuracy. From the computed radiant and from considering various plausible velocities, we obtained a range of orbital solutions that suggest that the Cali progenitor meteoroid probably originated in the main asteroid belt. Based on petrography, mineral chemistry, magnetic susceptibility, thermoluminescence, and bulk chemistry, the Cali meteorite is classified as an H/L4 ordinary chondrite breccia.     

THE COLONY METEORITE AND VARIATIONS IN CO3 CHONDRITE PROPERTIES

The Colony meteorite is an accretionary breccia containing several millimeter-to centimeter-size chondritic clasts embedded in a chondritic host. Colony is one of the least equilibrated CO3 chondrites; it has an unrecrystallized texture and contains compositionally heterogeneous olivine and low-Ca pyroxene, kamacite with low Ni and Co and high Cr, amoeboid inclusions with low FeO and MnO, a fine-grained silicate matrix with very high FeO, and numerous small chondrules with clear pink glass. However, Colony differs from normal CO chondrites in several respects: Although Al, Sc, V, Cr, Ir, Fe, Au and Ga abundances are consistent with a CO chondrite classification, certain lithophiles (Mg and Mn), siderophiles (Ni and Co) and chalcophiles (Se and Zn) are depleted by factors of 10–40%. The shape of Colony's thermoluminescence (TL) glow curve is similar to that of Allan Hills A77307 (another unequilibrated chondrite with CO3 petrological characteristics) and different from those of normal CO chondrites. [ALHA77307 also resembles Colony in having low Mg, Mn, Ni and Co, compared to normal CO chondrites, but it possesses CO-CV levels of Se and Zn and nearly CV levels of Cd.] Colony is badly weathered; it contains 22.7 wt.% Fe₂O₃ and 5.7 wt.% H₂O. Recalculating the analysis on an H₂O-free basis with all Fe₂O₃, NiO and CoO converted to metal, yields an inferred original metallic Fe, Ni abundance of ～ 19 wt.%. This is similar to that of Kainsaz (an unweathered CO3 fall), but much higher than that of all other CO3 chondrites (< 6.3 wt.%). Although it is possible that Colony and either ALHA77307 or Kainsaz constitute distinct CO3 chemical subgroups, the weathered nature of Colony and ALHA77307 preclude the drawing of firm conclusions. Nevertheless, it is clear that CO3 chondrites vary more in compositional and petrological properties than was previously recognized.     

TERRA BOOKS PALAEOBIOLOGY A SYNTHESIS

1990, £89.50 (hbk), 583 pp. Blackwell Scientific Publications, Oxford ISBN 0–632–02525–5     

Consortium study of the unusual H chondrite regolith breccia,Noblesville

Abstract— The Noblesville meteorite is a genomict, regolith breccia (H6 clasts in H4 matrix). Mössbauer analysis confirms that Noblesville is unusually fresh, not surprising in view of its recovery immediately after its fall. It resembles “normal” H4–6 chondrites in its chemical composition and induced thermoluminescence (TL) levels. Thus, at least in its contents of volatile trace elements, Noblesville differs from other H chondrite, class A regolith breccias. Noblesville's small pre-atmospheric mass and fall near Solar maximum and/or its peculiar orbit (with perihelion <0.8 AU as shown by natural TL intensity) may partly explain its levels of cosmogenic radionuclides. Its cosmic ray exposure age of ～ 44 Ma, is long, is equalled or exceeded by <3% of all H chondrites, and also differs from the 33 ± 3 Ma mean exposure age peak of other H chondrite regolith breccias. One whole-rock aliquot has a high, but not unmatched, ¹²⁹Xe/¹³²Xe of 1.88. While Noblesville is now among the chondritic regolithic breccias richest in solar gases, elemental ratios indicate some loss, especially of He, perhaps b; impacts in the regolith that heated individual grains. While general shock-loading levels in Noblesville did not exceed 4 GPa, individual clasts record shock levels of 5–10 GPa, doubtless acquired prior to lithification of the whole-rock meteoroid.     

Category, narrative, and value in the governance of small-scale fisheries

Since the 1970s, small-scale fisheries have had an important place in fisheries social science and in fisheries management. While there has been substantial discussion of what constitutes the category of small-scale fisheries, its considerable ambiguity is nevertheless often passed over. This paper argues that while the category of scale fisheries can be best understood in terms of scale, the underlying reason for the power of the category lies in the values of social justice and ecological sustainability that it has come to represent in response to dominant high modern narratives of change. Fisheries governance may better be served by prioritising these values rather than by making a fetish out of small-scale fisheries.