Effect of turbulence on collisional growth of cloud droplets

We investigate the effect of turbulence on the collisional growth of um-sized droplets through high- resolution numerical simulations with well resolved Kolmogorov scales, assuming a collision and coalescence efficiency of unity. The droplet dynamics and collisions are approximated using a superparticle approach. In the absence of gravity, we show that the time evolution of the shape of the droplet-size distribution due to turbulence-induced collisions depends strongly on the turbulent energy-dissipation rate, but only weakly on the Reynolds number. This can be explained through the energy dissipation rate dependence of the mean collision rate described by the Saffman-Turner collision model. Consistent with the Saffman-Turner collision model and its extensions, the collision rate increases as the square root of the energy dissipation rate even when coalescence is invoked. The size distribution exhibits power law behavior with a slope of -3.7 between a maximum at approximately 10 um up to about 40 um. When gravity is invoked, turbulence is found to dominate the time evolution of an initially monodisperse droplet distribution at early times. At later times, however, gravity takes over and dominates the collisional growth. We find that the formation of large droplets is very sensitive to the turbulent energy dissipation rate. This is due to the fact that turbulence enhances the collisional growth between similar sized droplets at the early stage of raindrop formation. The mean collision rate grows exponentially, which is consistent with the theoretical prediction of the continuous collisional growth even when turbulence-generated collisions are invoked. This consistency only reflects the mean effect of turbulence on collisional growth

X-ray Variability Characteristics of the Seyfert 1 Galaxy NGC 3783

We have characterized the energy-dependent X-ray variability properties of the Seyfert~1 galaxy NGC 3783 using archival XMM-Newton and Rossi X-ray Timing Explorer data. The high-frequency fluctuation power spectral density function (PSD) slope is consistent with flattening towards higher energies. Light curve cross correlation functions yield no significant lags, but peak coefficients generally decrease as energy separation of the bands increases on both short and long timescales. We have measured the coherence between various X-ray bands over the temporal frequency range of 6e-8 to 1e-4 Hz; this range includes the temporal frequency of the low-frequency power spectral density function (PSD) break tentatively detected by Markowitz et al. and includes the lowest temporal frequency over which coherence has been measured in any AGN to date. Coherence is generally near unity at these temporal frequencies, though it decreases slightly as energy separation of the bands increases. Temporal frequency-dependent phase lags are detected on short time scales; phase lags are consistent with increasing as energy separation increases or as temporal frequency decreases. All of these results are similar to those obtained previously for several Seyfert galaxies and stellar-mass black hole systems. Qualitatively, these results are consistent with the variability models of Kotov et al. and Lyubarskii, wherein the X-ray variability is due to inwardly propagating variations in the local mass accretion rate.Comment: Accepted for publication in The Astrophysical Journal, 2005, vol. 635, p. 180; version 2 has minor grammatical changes; 23 pages; uses emulateapj

Do group dynamics affect colour morph clines during a range shift?

Funded by Strategic Research Area Biodiversity and Ecosystem Services in a Changing Climate (BECC) Lund and Gothenburg Universities Wenner-Gren Foundation EU FP7 Swedish Research Council Royal Swedish Academy of Sciences (KVA) Stiftelsen Anna-Greta and Holger Crafoords Fund Crafoord FoundationPeer reviewedPostprin

Truthful Facility Assignment with Resource Augmentation: An Exact Analysis of Serial Dictatorship

We study the truthful facility assignment problem, where a set of agents with private most-preferred points on a metric space are assigned to facilities that lie on the metric space, under capacity constraints on the facilities. The goal is to produce such an assignment that minimizes the social cost, i.e., the total distance between the most-preferred points of the agents and their corresponding facilities in the assignment, under the constraint of truthfulness, which ensures that agents do not misreport their most-preferred points. We propose a resource augmentation framework, where a truthful mechanism is evaluated by its worst-case performance on an instance with enhanced facility capacities against the optimal mechanism on the same instance with the original capacities. We study a very well-known mechanism, Serial Dictatorship, and provide an exact analysis of its performance. Although Serial Dictatorship is a purely combinatorial mechanism, our analysis uses linear programming; a linear program expresses its greedy nature as well as the structure of the input, and finds the input instance that enforces the mechanism have its worst-case performance. Bounding the objective of the linear program using duality arguments allows us to compute tight bounds on the approximation ratio. Among other results, we prove that Serial Dictatorship has approximation ratio $g/(g-2)$ when the capacities are multiplied by any integer $g \geq 3$. Our results suggest that even a limited augmentation of the resources can have wondrous effects on the performance of the mechanism and in particular, the approximation ratio goes to 1 as the augmentation factor becomes large. We complement our results with bounds on the approximation ratio of Random Serial Dictatorship, the randomized version of Serial Dictatorship, when there is no resource augmentation

Plasma Ejection from Magnetic Flares and the X-ray Spectrum of Cygnus X-1

The hard X-rays in Cyg X-1 and similar black hole sources are possibly produced in an active corona atop an accretion disk. We suggest that the observed weakness of X-ray reflection from the disk is due to bulk motion of the emitting hot plasma away from the reflector. A mildly relativistic motion causes aberration reducing X-ray emission towards the disk. This in turn reduces the reprocessed radiation from the disk and leads to a hard spectrum of the X-ray source. The resulting spectral index is Gamma=1.9B^{1/2} where B=gamma(1+beta) is the aberration factor for a bulk velocity beta=v/c. The observed Gamma=1.6 and the amount of reflection, R=0.3, in Cyg X-1 in the hard state can both be explained assuming a bulk velocity beta=0.3. We discuss one possible scenario: the compact magnetic flares are dominated by e+- pairs which are ejected away from the reflector by the pressure of the reflected radiation. We also discuss physical constraints on the disk-corona model and argue that the magnetic flares are related to magneto-rotational instabilities in the accretion disk.Comment: The final version, accepted for publication in ApJ Letter

The Decline of the Source Population of Gamma-Ray Bursts and Their Luminosity Function

The source population of gamma-ray bursts (GRBs) declines towards the present epoch being consistent with the measured decline of the star formation rate. We show this using the brightness distribution of 3255 long BATSE GRBs found in an off-line scan of the BATSE continuous 1.024 s count rate records. The significance of this conclusion is enhanced by the detection of three GRBs with known redshifts brighter than 10^{52} erg/s during the last two years. This is an argument in favor of the generally believed idea that GRBs are strongly correlated with the star production, at least on cosmological time scales, and favors the association of long GRBs with collapses of supermassive stars. However, we still cannot rule out neutron star mergers if the typical delay time for binary system evolution is relatively short. If we assume a steep decline of the GRB population at z>1.5, then their luminosity function can be clearly outlined. The luminosity function is close to a power law, dN/dL ~ L^{-1.4}, for low luminosities over at least 1.7 orders of magnitude. Then the luminosity function breaks to a steeper slope or to an exponential decline around L = 3*10^{51} erg/s in the 50-300 keV range assuming isotropic emission.Comment: revised version, to appear in ApJ (2002, V.569, N.1), 12 pages, 2 tables, 10 figure

Lineshape of the thermopower of quantum dots

Quantum dots are an important model system for thermoelectric phenomena, and may be used to enhance the thermal-to-electric energy conversion efficiency in functional materials. It is therefore important to obtain a detailed understanding of a quantum-dot's thermopower as a function of the Fermi energy. However, so far it has proven difficult to take effects of co-tunnelling into account in the interpretation of experimental data. Here we show that a single-electron tunnelling model, using knowledge of the dot's electrical conductance which in fact includes all-order co-tunneling effects, predicts the thermopower of quantum dots as a function of the relevant energy scales, in very good agreement with experiment.Comment: 10 pages, 5 figure

Electron-Positron Pairs in Hot Accretion Flows and Thin Disk Coronae

We investigate equilibrium accretion flows dominated by $e^+ e^-$ pairs. We consider one- and two-temperature accretion disk coronae above a thin disk, as well as hot optically thin two-temperature accretion flows without an underlying thin disk; we model the latter in the framework of advection-dominated accretion flows (ADAFs). In all three cases we include equipartition magnetic fields. We confirm the previous result that the equilibrium density of pairs in two-temperature ADAFs is negligible; and show that the inclusion of magnetic fields and the corresponding synchrotron cooling reduces the pair density even further. Similarly, we find that pairs are unimportant in two-temperature coronae. Even when the corona has significantly enhanced heating by direct transfer of viscous dissipation in the thin disk to the corona, the inefficient Coulomb coupling between protons and electrons acts as a bottleneck and prevents the high compactness required for pair-dominated solutions. Only in the case of a one-temperature corona model do we find pair-dominated thermal equilibria. These pair-dominated solutions occur over a limited range of optical depth and temperature.Comment: 38 pages, including 10 figures, LaTeX; to appear in Ap

Oxygen-deficient perovskite-related (Nd0.4Sr0.6)2Ni0.8M0.2O4-δ as oxygen electrode materials for SOFC/SOEC

Perovskite-related Ln2NiO4+δ (Ln = La, Pr, Nd) nickelates with layered Ruddlesden-Popper combine redox stability with noticeable oxygen stoichiometry changes, yielding enhanced mixed transport and electrocatalytic properties. These unique features are promising for applications as oxygen electrodes with good electrochemical performance in reversible SOFC/SOEC (solid oxide fuel/electrolysis cell) systems. To date, most efforts were focused on oxygen-hyperstoichiometric Ln2NiO4+δ-based phases, whereas nickelates with oxygen-deficient lattice remain poorly explored. Recent studies demonstrated that the highest electrical conductivity in (Ln2-xSrx)2NiO4±δ series at elevated temperatures is observed for the compositions containing ~ 60 at.% of strontium in A sublattice [1,2]. The present work was focused on the characterization of (Nd0.4Sr0.6)2Ni0.8M0.2O4-δ (M = Ni, Co, Fe) nickelates for the possible use as materials for reversible oxygen electrodes. The ceramic materials were prepared by Pechini method with repeated annealings at 650-1200°C and sintered at 1250-1300°C for 5 h under oxygen atmosphere. Variable-temperature XRD studies confirmed that all studied compositions retain tetragonal K2NiF4-type structure in the temperature range 25-900°C. The results of thermogravimetric analysis showed that the prepared nickelates has oxygen-deficient lattice under oxidizing conditions at temperatures above 700°C. Partial substitution of nickel by cobalt or iron results in a decrease of p-type electronic conductivity and the concentration of oxygen vacancies in the lattice (Fig.1), but also suppresses dimensional changes associated with microcracking effects (due to anisotropic thermal expansion of tetragonal lattice). Electrochemical performance of porous (Nd0.4Sr0.6)2Ni0.8M0.2O4-δ electrodes in contact with Ce0.9Gd0.1O2-δ solid electrolyte was evaluated at 600- 800°C employing electrochemical impedance spectroscopy and steady-state polarization (anodic and cathodic) measurements.publishe

Oxygen-deficient Nd0.8Sr1.2Ni0.8M0.2O4-δ (M = Ni, Co, Fe) nickelates as oxygen electrode materials for SOFC/SOEC

Ruddlesden-Popper Nd0.8Sr1.2Ni0.8M0.2O4±δ (M = Ni, Co, Fe) nickelates have been characterized as prospective oxygen electrode materials for solid electrolyte cells. XRD studies showed that these oxides retain tetragonal K2NiF4-type structure in air until at least 900°C. Average thermal expansion coefficients of Nd0.8Sr1.2Ni0.8M0.2O4±δ calculated from the structural data are in the range 14.5-15.8 ppm/K. TGA studies revealed that these nickelates are oxygen-deficient in air at temperature above 700°C but tends to oxygen stoichiometry or minor excess on cooling. Incorporation of cobalt or iron into nickel sublattice of Nd0.8Sr1.2NiO4-δ reduces oxygen deficiency and electrical conductivity. Electrochemical impedance spectroscopy studies of symmetrical cells showed that porous Nd0.8Sr1.2Ni0.8M0.2O4-δ electrodes applied onto Ce0.9Gd0.1O2-δ electrolyte exhibit quite similar performance, with lowest values of polarization resistance (0.8 Ohm×cm2 at 800°C) observed for M = Ni. The polarization resistance can be further decreased (down to 0.04 Ohm×cm2 at 800°C for M = Ni) by surface modification with PrOx.publishe