3,352 research outputs found
Unsteady wake modelling for tidal current turbines
The authors present a numerical model for three-dimensional unsteady wake calculations for tidal turbines. Since wakes are characterised by the shedding of a vortex sheet from the rotor blades, the model is based on the vorticity transport equations. A vortex sheet may be considered a jump contact discontinuity in tangential velocity with, in inviscid hydrodynamic terms, certain kinematic and dynamic conditions across the sheet. The kinematic condition is that the sheet is a stream surface with zero normal fluid velocity; the dynamic condition is that the pressure is equal on either side of the sheet. The dynamic condition is explicitly satisfied at the trailing edge only, via an approximation of the Kutta condition. The shed vorticity is the span-wise derivative of bound circulation, and the trailed vorticity is the time derivative of bound circulation, and is convected downstream from the rotors using a finite-volume solution of vorticity transport equations thus satisfying the kinematic conditions. Owing to an absence in the literature of pressure data for marine turbines, results from the code are presented for the NREL-UAE Phase IV turbine. Axial flow cases show a close match in pressure coefficients at various spanwise stations; however, yawed flow cases demonstrate the shortcomings of a modelling strategy lacking viscosity
Molecular ions in the protostellar shock L1157-B1
We perform a complete census of molecular ions with an abundance larger than
1e-10 in the protostellar shock L1157-B1 by means of an unbiased
high-sensitivity survey obtained with the IRAM-30m and Herschel/HIFI. By means
of an LVG radiative transfer code the gas physical conditions and fractional
abundances of molecular ions are derived. The latter are compared with
estimates of steady-state abundances in the cloud and their evolution in the
shock calculated with the chemical model Astrochem. We detect emission from
HCO+, H13CO+, N2H+, HCS+, and, for the first time in a shock, from HOCO+, and
SO+. The bulk of the emission peaks at blueshifted velocity, ~ 0.5-3 km/s with
respect to systemic, has a width of ~ 4-8 km/s, and is associated with the
outflow cavities (T_kin ~ 20-70 K, n(H2) ~ 1e5 cm-3). Observed HCO+ and N2H+
abundances are in agreement with steady-state abundances in the cloud and with
their evolution in the compressed and heated gas in the shock for cosmic rays
ionization rate Z = 3e-16 s-1. HOCO+, SO+, and HCS+ observed abundances,
instead, are 1-2 orders of magnitude larger than predicted in the cloud; on the
other hand they are strongly enhanced on timescales shorter than the shock age
(~2000 years) if CO2, S or H2S, and OCS are sputtered off the dust grains in
the shock. The performed analysis indicates that HCO+ and N2H+ are a fossil
record of pre-shock gas in the outflow cavity, while HOCO+, SO+, and HCS+ are
effective shock tracers and can be used to infer the amount of CO2 and
sulphur-bearing species released from dust mantles in the shock. The observed
HCS+ (and CS) abundance indicates that OCS should be one of the main sulphur
carrier on grain mantles. However, the OCS abundance required to fit the
observations is 1-2 orders of magnitude larger than observed. Further studies
are required to fully understand the chemistry of sulphur-bearing species.Comment: 12 pages, 5 figures, accepted by A&
A non-equilibrium ortho-to-para ratio of water in the Orion PDR
The ortho-to-para ratio (OPR) of HO is thought to be sensitive to the
temperature of water formation. The OPR of HO is thus useful to study the
formation mechanism of water. We investigate the OPR of water in the Orion PDR
(Photon-dominated region), at the Orion Bar and Orion S positions, using data
from {\it Herschel}/HIFI. We detect the ground-state lines of ortho- and
para-HO in the Orion Bar and Orion S and we estimate the column
densities using LTE and non-LTE methods. Based on our calculations, the
ortho-to-para ratio (OPR) in the Orion Bar is 0.1 0.5, which is
unexpectedly low given the gas temperature of 85 K, and also lower than
the values measured for other interstellar clouds and protoplanetary disks.
Toward Orion S, our OPR estimate is below 2. This low OPR at 2 positions in the
Orion PDR is inconsistent with gas phase formation and with thermal evaporation
from dust grains, but it may be explained by photodesorption
Line formation in solar granulation: I. Fe line shapes, shifts and asymmetries
Realistic ab-initio 3D, radiative-hydrodynamical convection simulations of
the solar granulation have been applied to FeI and FeII line formation. In
contrast to classical analyses based on hydrostatic 1D model atmospheres the
procedure contains no adjustable free parameters but the treatment of the
numerical viscosity in the construction of the 3D, time-dependent,
inhomogeneous model atmosphere and the elemental abundance in the 3D spectral
synthesis. However, the numerical viscosity is introduced purely for numerical
stability purposes and is determined from standard hydrodynamical test cases
with no adjustments allowed to improve the agreement with the observational
constraints from the solar granulation. The non-thermal line broadening is
mainly provided by the Doppler shifts arising from the convective flows in the
solar photosphere and the solar oscillations. The almost perfect agreement
between the predicted temporally and spatially averaged line profiles for weak
Fe lines with the observed profiles and the absence of trends in derived
abundances with line strengths, seem to imply that the micro- and
macroturbulence concepts are obsolete in these 3D analyses. Furthermore, the
theoretical line asymmetries and shifts show a very satisfactory agreement with
observations with an accuracy of typically 50-100 m/s on an absolute velocity
scale. The remaining minor discrepancies point to how the convection
simulations can be refined further.Comment: Accepted for A&
A new method for probing magnetic field strengths from striations in the interstellar medium
Recent studies of the diffuse parts of molecular clouds have revealed the
presence of parallel, ordered low-density filaments termed striations. Flows
along magnetic field lines, Kelvin-Helmholtz instabilities and hydromagnetic
waves are amongst the various formation mechanisms proposed. Through a synergy
of observational, numerical and theoretical analysis, previous studies singled
out the hydromagnetic waves model as the only one that can account for the
observed properties of striations. Based on the predictions of that model, we
develop here a method for measuring the temporal evolution of striations
through a combination of molecular and dust continuum observations. Our method
allows us to not only probe temporal variations in molecular clouds but also
estimate the strength of both the ordered and fluctuating components of the
magnetic field projected on the plane-of-the-sky. We benchmark our new method
against chemical and radiative transfer effects through two-dimensional
magnetohydrodynamic simulations coupled with non-equilibrium chemical modelling
and non-local thermodynamic equilibrium line radiative transfer. We find good
agreement between theoretical predictions, simulations and observations of
striations in the Taurus molecular cloud. We find a value of for the plane-of-sky magnetic field, in agreement with previous
estimates via the Davis-Chandrasekhar-Fermi method, and a ratio of fluctuating
to ordered component of the magnetic field of 10\%.Comment: 12 pages, 14 figures, Accepted for publication in MNRA
Understanding hydrogen recombination line observations with ALMA and EVLA
Hydrogen recombination lines are one of the major diagnostics of H II region
physical properties and kinematics. In the near future, the Expanded Very Large
Array (EVLA) and the Atacama Large Millimeter Array (ALMA) will allow observers
to study recombination lines in the radio and sub-mm regime in unprecedented
detail. In this paper, we study the properties of recombination lines, in
particular at ALMA wavelengths. We find that such lines will lie in almost
every wideband ALMA setup and that the line emission will be equally detectable
in all bands. Furthermore, we present our implementation of hydrogen
recombination lines in the adaptive-mesh radiative transfer code RADMC-3D. We
particularly emphasize the importance of non-LTE (local thermodynamical
equilibrium) modeling since non-LTE effects can drastically affect the line
shapes and produce asymmetric line profiles from radially symmetric H II
regions. We demonstrate how these non-LTE effects can be used as a probe of
systematic motions (infall & outflow) in the gas. We use RADMC-3D to produce
synthetic observations of model H II regions and study the necessary conditions
for observing such asymmetric line profiles with ALMA and EVLA.Comment: MNRAS in pres
Chemical composition of intermediate mass stars members of the M6 (NGC 6405) open cluster
We present here the first abundance analysis of 44 late B, A and F-type
members of the young open cluster M6 (NGC 6405, age about 75 Myrs). Spectra,
covering the 4500 to 5800 \AA{} wavelength range, were obtained using the
FLAMES/GIRAFFE spectrograph attached to the ESO Very Large Telescopes (VLT). We
determined the atmospheric parameters using calibrations of the Geneva
photometry and by adjusting the profiles to synthetic ones. The
abundances of up to 20 chemical elements, were derived for 19 late B, 16 A and
9 F stars by iteratively adjusting synthetic spectra to the observations. We
also derived a mean cluster metallicity of dex
from the iron abundances of the F-type stars. We find that, for most chemical
elements, the normal late B and A-type stars exhibit larger star-to-star
abundance variations than the F-type stars do probably because of the faster
rotation of the B and A stars. The abundances of C, O, Mg, Si and Sc appear to
be anticorrelated to that of Fe, while the opposite holds for the abundances of
Ca, Ti, Cr, Mn, Ni, Y, and Ba about as expected if radiative diffusion is
efficient in the envelopes of these stars. In the course of this analysis, we
discovered five new peculiar stars: one mild-Am, one Am, and one Fm star (HD
318091, CD-32 13109, GSC 07380-01211), one HgMn star (HD 318126), and one
He-weak P-rich (HD 318101) star. We also discovered a new spectroscopic binary,
most likely a SB2. We performed a detailed modelling of HD 318101,the new
He-weak P-rich CP star, using the Montr\'eal stellar evolution code XEVOL which
treats self-consistently all particle transport processes. Although the overall
abundance pattern of this star is properly reproduced, we find that detailed
abundances (in particular the high P excess) resisted modelling attempts even
when a range of turbulence profiles and mass loss rates were considered.Comment: Accepted for publication in Astronomical Journal (Oct.15,2015), 115
pages, 15 figure
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