1,903 research outputs found
Artificial viscosity model to mitigate numerical artefacts at fluid interfaces with surface tension
The numerical onset of parasitic and spurious artefacts in the vicinity of uid interfaces with surface tension is an important and well-recognised problem with respect to the accuracy and numerical stability of interfacial ow simulations. Issues of particular interest are spurious capillary waves, which are spatially underresolved by the computational mesh yet impose very restrictive time-step requirements, as well as parasitic currents, typically the result of a numerically unbalanced curvature evaluation. We present an arti cial viscosity model to mitigate numerical artefacts at surface-tension-dominated interfaces without adversely a ecting the accuracy of the physical solution. The proposed methodology computes an additional interfacial shear stress term, including an interface viscosity, based on the local ow data and uid properties that reduces the impact of numerical artefacts and dissipates underresolved small scale interface movements. Furthermore, the presented methodology can be readily applied to model surface shear viscosity, for instance to simulate the dissipative e ect of surface-active substances adsorbed at the interface. The presented analysis of numerical test cases demonstrates the e cacy of the proposed methodology in diminishing the adverse impact of parasitic and spurious interfacial artefacts on the convergence and stability of the numerical solution algorithm as well as on the overall accuracy of the simulation results
Gravitational Lensing Limits on the Average Redshift of Submillimeter Sources
The submillimeter universe has now been explored with the Submillimeter
Common User Bolometer Array (SCUBA) camera on the James Clerk Maxwell
Telescope, and a claim has been made to the presence of a new population of
optically unidentified starforming galaxies at high redshifts (z \gtrsim 3).
Such a population dramatically alters current views on the star formation
history of the universe as well as galaxy formation and evolution. Recently,
new radio identifications of the Hubble Deep Field submm sources have led to
the suggestion that some of these sources are at low redshifts, however, submm
source redshift distribution is still not well determined. Here, we present an
upper limit to the average redshift by comparing the expected number of
gravitationally lensed submm sources due to foreground cluster potentials to
current observed statistics of such lensed sources. The upper limit depends on
the cosmological parameters, and at the 68% confidence level, < 3.1, 4.8,
5.2, or 8.0 for (Omega,Lambda) values of (0.3,0.7), (0.5,0.5), (0.3,0.0) or
(1.0,0.0) respectively. These upper limits are consistent with redshift
distribution for 850 micron sources implied by starformation history models
based on measured background radiation at far-infrared and submm wavelengths.Comment: Accepted for publication in ApJ Letters (4 pages, including 1 table
Gravitational Lensing as a Probe of Quintessence
A large number of cosmological studies now suggest that roughly two-thirds of
the critical energy density of the Universe exists in a component with negative
pressure. If the equation of state of such an energy component varies with
time, it should in principle be possible to identify such a variation using
cosmological probes over a wide range in redshift. Proper detection of any time
variation, however, requires cosmological probes beyond the currently studied
range in redshift of 0.1 to 1. We extend our analysis to gravitational
lensing statistics at high redshift and suggest that a reliable sample of
lensed sources, out to a redshift of 5, can be used to constrain the
variation of the equation of state, provided that both the redshift
distribution of lensed sources and the selection function involved with the
lensed source discovery process are known. An exciting opportunity to catalog
an adequate sample of lensed sources (quasars) to probe quintessence is now
available with the ongoing Sloan Digital Sky Survey. Writing , we study the expected accuracy to which the equation of state
today and its rate of change can simultaneously be
constrained. Such a determination can rule out some missing-energy candidates,
such as classes of quintessence models or a cosmological constant.Comment: Accepted for publication in ApJ Letters (4 pages, including 4
figures
Perspectives historiques et contemporaines sur l’occulte dans la correspondance Freud-Ferenczi / Historical and Contemporary Perspectives on Occultism in the Freud-Ferenczi Correspondence
International audienc
BOOMERanG Data Suggest a Purely Baryonic Universe
The amplitudes of peaks in the angular power spectrum of anisotropies in the
microwave background radiation depend on the mass content of the universe. The
second peak should be prominent when cold dark matter is dominant, but is
depressed when baryons dominate. Recent microwave background data are
consistent with a purely baryonic universe with Omega(matter) = Omega(baryon) ~
0.03 and Omega(Lambda) ~ 1.Comment: 10 pages AASTeX with 1 color postscript figure. Accepted for
publication in ApJ Letters. And yes, the prediction was in the literature
before the dat
The Ultimate Halo Mass in a LCDM Universe
In the far future of an accelerating LCDM cosmology, the cosmic web of
large-scale structure consists of a set of increasingly isolated halos in
dynamical equilibrium. We examine the approach of collisionless dark matter to
hydrostatic equilibrium using a large N-body simulation evolved to scale factor
a = 100, well beyond the vacuum--matter equality epoch, a_eq ~ 0.75, and 53/h
Gyr into the future for a concordance model universe (Omega_m ~ 0.3,
Omega_Lambda ~ 0.7). The radial phase-space structure of halos -- characterized
at a < a_eq by a pair of zero-velocity surfaces that bracket a dynamically
active accretion region -- simplifies at a > 10 a_eq when these surfaces merge
to create a single zero-velocity surface, clearly defining the halo outer
boundary, rhalo, and its enclosed mass, mhalo. This boundary approaches a fixed
physical size encompassing a mean interior density ~ 5 times the critical
density, similar to the turnaround value in a classical Einstein-deSitter
model. We relate mhalo to other scales currently used to define halo mass
(m200, mvir, m180b) and find that m200 is approximately half of the total
asymptotic cluster mass, while m180b follows the evolution of the inner zero
velocity surface for a < 2 but becomes much larger than the total bound mass
for a > 3. The radial density profile of all bound halo material is well fit by
a truncated Hernquist profile. An NFW profile provides a somewhat better fit
interior to r200 but is much too shallow in the range r200 < r < rhalo.Comment: 5 pages, 3 figures, submitted to MNRAS letter
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