3,670 research outputs found
Photoreductive Dissolution of Iron Oxides Trapped in Ice and Its Environmental Implications
The availability of iron has been thought to be a main limiting factor for the productivity of phytoplankton and related with the uptake of atmospheric CO_2 and algal blooms in fresh and sea waters. In this work, the formation of bioavailable iron (Fe(II)_(aq)) from the dissolution of iron oxide particles was investigated in the ice phase under both UV and visible light irradiation. The photoreductive dissolution of iron oxides proceeded slowly in aqueous solution (pH 3.5) but was significantly accelerated in polycrystalline ice, subsequently releasing more bioavailable ferrous iron upon thawing. The enhanced photogeneration of Fe(II)_(aq) in ice was confirmed regardless of the type of iron oxides [hematite, maghemite (γ-Fe_2O_3), goethite (α-FeOOH)] and the kind of electron donors. The ice-enhanced dissolution of iron oxides was also observed under visible light irradiation, although the dissolution rate was much slower compared with the case of UV radiation. The iron oxide particles and organic electron donors (if any) in ice are concentrated and aggregated in the liquid-like grain boundary region (freeze concentration effect) where protons are also highly concentrated (lower pH). The enhanced photodissolution of iron oxides should occur in this confined boundary region. We hypothesized that electron hopping through the interconnected grain boundaries of iron oxide particles facilitates the separation of photoinduced charge pairs. The outdoor experiments carried out under ambient solar radiation of Ny-Ålesund (Svalbard, 78°55′N) also showed that the generation of dissolved Fe(II)_(aq) via photoreductive dissolution is enhanced when iron oxides are trapped in ice. Our results imply that the ice(snow)-covered surfaces and ice-cloud particles containing iron-rich mineral dusts in the polar and cold environments provide a source of bioavailable iron when they thaw
Compressible Boundary Layer Predictions at High Reynolds Number using Hybrid LES/RANS Methods
Simulations of compressible boundary layer flow at three different Reynolds numbers (Re(sub delta) = 5.59x10(exp 4), 1.78x10(exp 5), and 1.58x10(exp 6) are performed using a hybrid large-eddy/Reynolds-averaged Navier-Stokes method. Variations in the recycling/rescaling method, the higher-order extension, the choice of primitive variables, the RANS/LES transition parameters, and the mesh resolution are considered in order to assess the model. The results indicate that the present model can provide good predictions of the mean flow properties and second-moment statistics of the boundary layers considered. Normalized Reynolds stresses in the outer layer are found to be independent of Reynolds number, similar to incompressible turbulent boundary layers
Binary black hole late inspiral: Simulations for gravitational wave observations
Coalescing binary black hole mergers are expected to be the strongest
gravitational wave sources for ground-based interferometers, such as the LIGO,
VIRGO, and GEO600, as well as the space-based interferometer LISA. Until
recently it has been impossible to reliably derive the predictions of General
Relativity for the final merger stage, which takes place in the strong-field
regime. Recent progress in numerical relativity simulations is, however,
revolutionizing our understanding of these systems. We examine here the
specific case of merging equal-mass Schwarzschild black holes in detail,
presenting new simulations in which the black holes start in the late inspiral
stage on orbits with very low eccentricity and evolve for ~1200M through ~7
orbits before merging. We study the accuracy and consistency of our simulations
and the resulting gravitational waveforms, which encompass ~14 cycles before
merger, and highlight the importance of using frequency (rather than time) to
set the physical reference when comparing models. Matching our results to PN
calculations for the earlier parts of the inspiral provides a combined waveform
with less than half a cycle of accumulated phase error through the entire
coalescence. Using this waveform, we calculate signal-to-noise ratios (SNRs)
for iLIGO, adLIGO, and LISA, highlighting the contributions from the
late-inspiral and merger-ringdown parts of the waveform which can now be
simulated numerically. Contour plots of SNR as a function of z and M show that
adLIGO can achieve SNR >~ 10 for some intermediate-mass binary black holes
(IMBBHs) out to z ~ 1, and that LISA can see massive binary black holes (MBBHs)
in the range 3x10^4 100 out to the earliest epochs
of structure formation at z > 15.Comment: 17 pages, 20 figures. Final published versio
Evolving a puncture black hole with fixed mesh refinement
We present an algorithm for treating mesh refinement interfaces in numerical
relativity. We detail the behavior of the solution near such interfaces located
in the strong field regions of dynamical black hole spacetimes, with particular
attention to the convergence properties of the simulations. In our applications
of this technique to the evolution of puncture initial data with vanishing
shift, we demonstrate that it is possible to simultaneously maintain second
order convergence near the puncture and extend the outer boundary beyond 100M,
thereby approaching the asymptotically flat region in which boundary condition
problems are less difficult and wave extraction is meaningful.Comment: 18 pages, 12 figures. Minor changes, final PRD versio
Analysis of ``Gauge Modes'' in Linearized Relativity
By writing the complete set of (ADM) equations for linearized waves,
we are able to demonstrate the properties of the initial data and of the
evolution of a wave problem set by Alcubierre and Schutz. We show that the
gauge modes and constraint error modes arise in a straightforward way in the
analysis, and are of a form which will be controlled in any well specified
convergent computational discretization of the differential equations.Comment: 11pages LaTe
Getting a kick out of numerical relativity
Recent developments in numerical relativity have made it possible to follow
reliably the coalescence of two black holes from near the innermost stable
circular orbit to final ringdown. This opens up a wide variety of exciting
astrophysical applications of these simulations. Chief among these is the net
kick received when two unequal mass or spinning black holes merge. The
magnitude of this kick has bearing on the production and growth of supermassive
black holes during the epoch of structure formation, and on the retention of
black holes in stellar clusters. Here we report the first accurate numerical
calculation of this kick, for two nonspinning black holes in a 1.5:1 mass
ratio, which is expected based on analytic considerations to give a significant
fraction of the maximum possible recoil. We have performed multiple runs with
different initial separations, orbital angular momenta, resolutions, extraction
radii, and gauges. The full range of our kick speeds is 86--116 km s,
and the most reliable runs give kicks between 86 and 97 km s. This is
intermediate between the estimates from two recent post-Newtonian analyses and
suggests that at redshifts , halos with masses will have difficulty retaining coalesced black holes after major
mergers.Comment: Updated. Accepted by ApJ Letter
Intense field stabilization in circular polarization: 3D time-dependent dynamics
We investigate the stabilization of a hydrogen atom in circularly polarized
laser fields. We use a time-dependent, fully three dimensional approach to
study the quantum dynamics of the hydrogen atom subject to high intensity,
short wavelength laser pulses. We find enhanced survival probability as the
field is increased under fixed envelope conditions. We also confirm wavepacket
dynamics seen in prior time-dependent computations restricted to two
dimensions.Comment: 4 pages, 3 figures, submitte
In vitro and in vivo assessments of an optimal polyblend composition of polycaprolactone/gelatin nanofibrous scaffolds for Achilles tendon tissue engineering
In this study, we manufactured various ratios of polycaprolactone (PCL)/gelatin (GE) highly aligned electrospun nanofibrous scaffolds (ENs) to investigate the effects of polymer ratio on tenogenic differentiation activity. For biological assessments, the cell proliferation rate was optimal in the PCL/GE (9:1) group. Interestingly, however, the tenogenic differentiation rate was best for the PCL/GE (7:3) group. From our outcomes, we established that a poly-blending mix of PCL/GE (7:3) is a promising ratio for tenogenic differentiation. Thus, our findings may provide for an effective mesh to promote tenogenic differentiation of ENs in future tendon tissue engineering applications.This work was supported by the Bio & Medical Technology
Development Program of the National Research Foundation (NRF)
& funded by the Korean government (MSIP&MOHW) (No.
2017M3A9E4048170)
Three-dimensional adaptive evolution of gravitational waves in numerical relativity
Adaptive techniques are crucial for successful numerical modeling of
gravitational waves from astrophysical sources such as coalescing compact
binaries, since the radiation typically has wavelengths much larger than the
scale of the sources. We have carried out an important step toward this goal,
the evolution of weak gravitational waves using adaptive mesh refinement in the
Einstein equations. The 2-level adaptive simulation is compared with unigrid
runs at coarse and fine resolution, and is shown to track closely the features
of the fine grid run.Comment: REVTeX, 7 pages, including three figures; submitted to Physical
Review
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