265,915 research outputs found
Trends in Unauthorized Immigration: Undocumented Inflow Now Trails Legal Inflow
Analyzes trends in the populations and annual inflows of undocumented and legal immigrants in the United States between 2000 and 2008. Presents findings on the composition of the foreign-born population in terms of arrival year and country of birth
Nonstationary flow for the Navier-Stokes equations in a cylindrical pipe
In cylindrical domain, we consider the nonstationary flow with prescribed
inflow and outflow, modelled with Navier-Stokes equations under the slip
boundary conditions. Using smallness of some derivatives of inflow function,
external force and initial velocity of the flow, but with no smallness
restrictions on the inflow, initial velocity neither force, we prove existence
of solutions in $W^{2,1}_2.
Flow rate of transport network controls uniform metabolite supply to tissue
Life and functioning of higher organisms depends on the continuous supply of
metabolites to tissues and organs. What are the requirements on the transport
network pervading a tissue to provide a uniform supply of nutrients, minerals,
or hormones? To theoretically answer this question, we present an analytical
scaling argument and numerical simulations on how flow dynamics and network
architecture control active spread and uniform supply of metabolites by
studying the example of xylem vessels in plants. We identify the fluid inflow
rate as the key factor for uniform supply. While at low inflow rates
metabolites are already exhausted close to flow inlets, too high inflow flushes
metabolites through the network and deprives tissue close to inlets of supply.
In between these two regimes, there exists an optimal inflow rate that yields a
uniform supply of metabolites. We determine this optimal inflow analytically in
quantitative agreement with numerical results. Optimizing network architecture
by reducing the supply variance over all network tubes, we identify patterns of
tube dilation or contraction that compensate sub-optimal supply for the case of
too low or too high inflow rate.Comment: 11 pages, 4 figures, 8 pages supplemen
Instability driven by boundary inflow across shear: a way to circumvent Rayleigh's stability criterion in accretion disks?
We investigate the 2D instability recently discussed by Gallet et al. (2010)
and Ilin \& Morgulis (2013) which arises when a radial crossflow is imposed on
a centrifugally-stable swirling flow. By finding a simpler rectilinear example
of the instability - a sheared half plane, the minimal ingredients for the
instability are identified and the destabilizing/stabilizing effect of
inflow/outflow boundaries clarified. The instability - christened `boundary
inflow instability' here - is of critical layer type where this layer is either
at the inflow wall and the growth rate is (as found by Ilin \&
Morgulis 2013), or in the interior of the flow and the growth rate is where measures the (small) inflow-to-tangential-flow
ratio. The instability is robust to changes in the rotation profile even to
those which are very Rayleigh-stable and the addition of further physics such
as viscosity, 3-dimensionality and compressibility but is sensitive to the
boundary condition imposed on the tangential velocity field at the inflow
boundary. Providing the vorticity is not fixed at the inflow boundary, the
instability seems generic and operates by the inflow advecting vorticity
present at the boundary across the interior shear. Both the primary bifurcation
to 2D states and secondary bifurcations to 3D states are found to be
supercritical. Assuming an accretion flow driven by molecular viscosity only so
, the instability is not immediately relevant for accretion
disks since the critical threshold is and the inflow boundary
conditions are more likely to be stress-free than non-slip. However, the
analysis presented here does highlight the potential for mass entering a disk
to disrupt the orbiting flow if this mass flux possesses vorticity.Comment: 44 pages, 14 figure
Black Hole growth and AGN obscuration by instability-driven inflows in high-redshift disk galaxies fed by cold streams
Disk galaxies at high redshift have been predicted to maintain high gas
surface densities due to continuous feeding by intense cold streams leading to
violent gravitational instability, transient features and giant clumps.
Gravitational torques between the perturbations drive angular momentum out and
mass in, and the inflow provides the energy for keeping strong turbulence. We
use analytic estimates of the inflow for a self-regulated unstable disk at a
Toomre stability parameter Q~1, and isolated galaxy simulations capable of
resolving the nuclear inflow down to the central parsec. We predict an average
inflow rate ~10 Msun/yr through the disk of a 10^11 Msun galaxy, with
conditions representative of z~2 stream-fed disks. The inflow rate scales with
disk mass and (1+z)^{3/2}. It includes clump migration and inflow of the
smoother component, valid even if clumps disrupt. This inflow grows the bulge,
while only a fraction ~ 10^-3 of it needs to accrete onto a central black hole
(BH), in order to obey the observed BH-bulge relation. A galaxy of 10^11 Msun
at z~2 is expected to host a BH of ~10^8 Msun, accreting on average with
moderate sub-Eddington luminosity L_X ~ 10^42-43 erg/s, accompanied by brighter
episodes when dense clumps coalesce. We note that in rare massive galaxies at
z~6, the same process may feed 10^9 Msun BH at the Eddington rate. High central
gas column densities can severely obscure AGN in high-redshift disks, possibly
hindering their detection in deep X-ray surveys.Comment: ApJL in pres
Constraint on the inflow/outflow rates in star-forming galaxies at z~1.4 from molecular gas observations
We constrain the rate of gas inflow into and outflow from a main-sequence
star-forming galaxy at z~1.4 by fitting a simple analytic model for the
chemical evolution in a galaxy to the observational data of the stellar mass,
metallicity, and molecular gas mass fraction. The molecular gas mass is derived
from CO observations with a metallicity-dependent CO-to-H2 conversion factor,
and the gas metallicity is derived from the H{\alpha} and [NII]{\lambda} 6584
emission line ratio. Using a stacking analysis of CO integrated intensity maps
and the emission lines of H{\alpha} and [NII], the relation between stellar
mass, metallicity, and gas mass fraction is derived. We constrain the inflow
and outflow rates with least-chi-square fitting of a simple analytic chemical
evolution model to the observational data. The best-fit inflow and outflow
rates are ~1.7 and ~0.4 in units of star-formation rate, respectively. The
inflow rate is roughly comparable to the sum of the star-formation rate and
outflow rate, which supports the equilibrium model for galaxy evolution; i.e.,
all inflow gas is consumed by star formation and outflow.Comment: 5 pages, 2 figures, Accepted for publication in the Ap
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