A locally adaptive time-stepping algorithm for\ud petroleum reservoir simulations

An algorithm for locally adapting the step-size for large scale finite volume simulations of multi-phase flow in petroleum reservoirs is suggested which allows for an “all-in-one” implicit calculation of behaviour over a very large time scale. Some numerical results for simple two-phase flow in one space dimension illustrate the promise of the algorithm, which has also been applied to very simple 3D cases. A description of the algorithm is presented here along with early results. Further development of the technique is hoped to facilitate useful scaling properties

Relative dispersion in the subsurface North Atlantic

Pair statistics are calculated for subsurface floats in the North Atlantic. The relative diffusivity (the derivative of the mean square particle separation) is approximately constant at large scales in both eastern and western basins, though the implied scale of the energy-containing eddies is greater in the west. But the behavior at times soon after pair deployment is quite different in the two basins; in the west the diffusivity grows approximately as distance to the 4/3 power, consistent with an inverse turbulent cascade of energy (or possibly of mixing superimposed on a mean shear), but in the east the diffusivity grows more slowly, as for instance in simple stochastic systems. Exponential stretching, expected in an enstrophy cascade, is not resolved in any region; however, this may reflect only that the present pair separations are too large initially

The Spectrum and Variability of Circular Polarization in Sagittarius A* from 1.4 to 15 GHz

We report here multi-epoch, multi-frequency observations of the circular polarization in Sagittarius A*, the compact radio source in the Galactic Center. Data taken from the VLA archive indicate that the fractional circular polarization at 4.8 GHz was -0.31% with an rms scatter of 0.13% from 1981 to 1998, in spite of a factor of 2 change in the total intensity. The sign remained negative over the entire time range, indicating a stable magnetic field polarity. In the Summer of 1999 we obtained 13 epochs of VLA A-array observations at 1.4, 4.8, 8.4 and 15 GHz. In May, September and October of 1999 we obtained 11 epochs of Australia Telescope Compact Array observations at 4.8 and 8.5 GHz. In all three of the data sets, we find no evidence for linear polarization greater than 0.1% in spite of strong circular polarization detections. Both VLA and ATCA data sets support three conclusions regarding the fractional circular polarization: the average spectrum is inverted with a spectral index ~0.5 +/- 0.2; the degree of variability is roughly constant on timescales of days to years; and, the degree of variability increases with frequency. We also observed that the largest increase in fractional circular polarization was coincident with the brightest flare in total intensity. Significant variability in the total intensity and fractional circular polarization on a timescale of 1 hour was observed during this flare, indicating an upper limit to the size of 70 AU at 15 GHz. The fractional circular polarization at 15 GHz reached -1.1% and the spectral index is strongly inverted during this flare. We conclude that the spectrum has two components that match the high and low frequency total intensity components. (abridged)Comment: Accepted for publication in ApJ, 40 pages, 18 figure

Sunyaev - Zel'dovich fluctuations from spatial correlations between clusters of galaxies

We present angular power spectra of the cosmic microwave background radiation anisotropy due to fluctuations of the Sunyaev-Zel'dovich (SZ) effect through clusters of galaxies. A contribution from the correlation among clusters is especially focused on, which has been neglected in the previous analyses. Employing the evolving linear bias factor based on the Press-Schechter formalism, we find that the clustering contribution amounts to 20-30% of the Poissonian one at degree angular scales. If we exclude clusters in the local universe, it even exceeds the Poissonian noise, and makes dominant contribution to the angular power spectrum. As a concrete example, we demonstrate the subtraction of the ROSAT X-ray flux-limited cluster samples. It indicates that we should include the clustering effect in the analysis of the SZ fluctuations. We further find that the degree scale spectra essentially depend upon the normalization of the density fluctuations, i.e., \sigma_8, and the gas mass fraction of the cluster, rather than the density parameter of the universe and details of cluster evolution models. Our results show that the SZ fluctuations at the degree scale will provide a possible measure of \sigma_8, while the arc-minute spectra a probe of the cluster evolution. In addition, the clustering spectrum will give us valuable information on the bias at high redshift, if we can detect it by removing X-ray luminous clusters.Comment: 11 pages, 4 figures, submitted to Astrophysical Journa

Dynamic model of gene regulation for the lac operon

Gene regulatory network is a collection of DNA which interact with each other and with other matter in the cell. The lac operon is an example of a relatively simple genetic network and is one of the best-studied structures in the Escherichia coli bacteria. In this work we consider a deterministic model of the lac operon with a noise term, representing the stochastic nature of the regulation. The model is written in terms of a system of simultaneous first order differential equations with delays. We investigate an analytical and numerical solution and analyse the range of values for the parameters corresponding to a stable solution

Different mechanics of snap-trapping in the two closely related carnivorous plants Dionaea muscipula and Aldrovanda vesiculosa

The carnivorous aquatic Waterwheel Plant (Aldrovanda vesiculosa L.) and the closely related terrestrial Venus Flytrap (Dionaea muscipula SOL. EX J. ELLIS) both feature elaborate snap-traps, which shut after reception of an external mechanical stimulus by prey animals. Traditionally, Aldrovanda is considered as a miniature, aquatic Dionaea, an assumption which was already established by Charles Darwin. However, videos of snapping traps from both species suggest completely different closure mechanisms. Indeed, the well-described snapping mechanism in Dionaea comprises abrupt curvature inversion of the two trap lobes, while the closing movement in Aldrovanda involves deformation of the trap midrib but not of the lobes, which do not change curvature. In this paper, we present the first detailed mechanical models for these plants, which are based on the theory of thin solid membranes and explain this difference by showing that the fast snapping of Aldrovanda is due to kinematic amplification of the bending deformation of the midrib, while that of Dionaea unambiguously relies on the buckling instability that affects the two lobes.Comment: accepted in Physical Review

The Evolution of Diffuse Radio Sources in Galaxy Clusters

We investigate the evolution and number distribution of radio halos in galaxy clusters. Without re-acceleration or regeneration, the relativistic electrons responsible for the diffuse radio emission will lose their energy via inverse-Compton and synchrotron losses in a rather short time, and radio halos will have lifetimes $\sim$ 0.1 Gyr. Radio halos could last for $\sim$ Gyr if a significant level of re-acceleration is involved. The lifetimes of radio halos would be comparable with the cosmological time if the radio-emitting electrons are mainly the secondary electrons generated by pion decay following proton-proton collisions between cosmic-ray protons and the thermal intra-cluster medium within the galaxy clusters. Adopting both observational and theoretical constraints for the formation of radio halos, we calculate the formation rates and the comoving number density of radio halos in the hierarchical clustering scheme. Comparing with observations, we find that the lifetimes of radio halos are $\sim$ Gyr. Our results indicate that a significant level of re-acceleration is necessary for the observed radio halos and the secondary electrons may not be a dominant origin for radio halos.Comment: 22 pages, 6 figures, ApJ, in press (v2:Corrected typos.

Catalog of Galaxy Morphology in Four Rich Clusters: Luminosity Evolution of Disk Galaxies at 0.33<z<0.83

Hubble Space Telescope (HST) imaging of four rich, X-ray luminous, galaxy clusters (0.33<z<0.83) is used to produce quantitative morphological measurements for galaxies in their fields. Catalogs of these measurements are presented for 1642 galaxies brighter than F814W(AB)=23.0 . Galaxy luminosity profiles are fitted with three models: exponential disk, de Vaucouleurs bulge, and a disk-plus-bulge hybrid model. The best fit is selected and produces a quantitative assessment of the morphology of each galaxy: the principal parameters derived being B/T, the ratio of bulge to total luminosity, the scale lengths and half-light radii, axial ratios, position angles and surface brightnesses of each component. Cluster membership is determined using a statistical correction for field galaxy contamination, and a mass normalization factor (mass within boundaries of the observed fields) is derived for each cluster. In the present paper, this catalog of measurements is used to investigate the luminosity evolution of disk galaxies in the rich-cluster environment. Examination of the relations between disk scale-length and central surface brightness suggests, under the assumption that these clusters represent a family who share a common evolutionary history and are simply observed at different ages, that there is a dramatic change in the properties of the small disks (h < 2 kpc). This change is best characterized as a change in surface brightness by about 1.5 magnitude between z=0.3 and z=0.8 with brighter disks at higher redshifts.Comment: 53 pages, including 13 figures and 7 tables. Accepted for publication in the Astrophysical Journal Supplement Serie

An automated archival VLA transients survey

In this paper we present the results of a survey for radio transients using data obtained from the Very Large Array archive. We have reduced, using a pipeline procedure, 5037 observations of the most common pointings - i.e. the calibrator fields. These fields typically contain a relatively bright point source and are used to calibrate `target' observations: they are therefore rarely imaged themselves. The observations used span a time range ~ 1984 - 2008 and consist of eight different pointings, three different frequencies (8.4, 4.8 and 1.4 GHz) and have a total observing time of 435 hours. We have searched for transient and variable radio sources within these observations using components from the prototype LOFAR transient detection system. In this paper we present the methodology for reducing large volumes of Very Large Array data; and we also present a brief overview of the prototype LOFAR transient detection algorithms. No radio transients were detected in this survey, therefore we place an upper limit on the snapshot rate of GHz frequency transients > 8.0 mJy to rho less than or equal to 0.032 deg^-2 that have typical timescales 4.3 to 45.3 days. We compare and contrast our upper limit with the snapshot rates - derived from either detections or non-detections of transient and variable radio sources - reported in the literature. When compared with the current Log N - Log S distribution formed from previous surveys, we show that our upper limit is consistent with the observed population. Current and future radio transient surveys will hopefully further constrain these statistics, and potentially discover dominant transient source populations. In this paper we also briefly explore the current transient commissioning observations with LOFAR, and the impact they will make on the field.Comment: Accepted for publication in MNRA