102,419 research outputs found
The Asymmetric Rotor. IX. The Heavy Water Bands at 2787 cm^–1 and 5373 cm^–1
The combination band (110) of the two stretching fundamentals of D2O is reported and analyzed to yield nu0=5373.2 cm^–1 and the excited state moments of inertia 1.910, 3.931, and 5.929×10^–40 g cm^2. The same method of analysis applied to the unsymmetrical fundamental band (100) envelope gives nu0=2787.5 cm^–1 and the excited state moments 1.881, 3.876, and 5.843×10^–40 g cm^2
Measuring the Effects of Artificial Viscosity in SPH Simulations of Rotating Fluid Flows
A commonly cited drawback of SPH is the introduction of spurious shear
viscosity by the artificial viscosity term in situations involving rotation.
Existing approaches for quantifying its effect include approximate analytic
formulae and disc-averaged be- haviour in specific ring-spreading simulations,
based on the kinematic effects produced by the artificial viscosity. These
methods have disadvantages, in that they typically are applicable to a very
small range of physical scenarios, have a large number of simplifying
assumptions, and often are tied to specific SPH formulations which do not
include corrective (e.g., Balsara) or time-dependent viscosity terms. In this
study we have developed a simple, generally applicable and practical technique
for evaluating the local effect of artificial viscosity directly from the
creation of specific entropy for each SPH particle. This local approach is
simple and quick to implement, and it al- lows a detailed characterization of
viscous effects as a function of position. Several advantages of this method
are discussed, including its ease in evaluation, its greater accuracy and its
broad applicability. In order to compare this new method with ex- isting ones,
simple disc flow examples are used. Even in these basic cases, the very roughly
approximate nature of the previous methods is shown. Our local method pro-
vides a detailed description of the effects of the artificial viscosity
throughout the disc, even for extended examples which implement Balsara
corrections. As a further use of this approach, explicit dependencies of the
effective viscosity in terms of SPH and flow parameters are estimated from the
example cases. In an appendix, a method for the initial placement of SPH
particles is discussed which is very effective in reducing numerical
fluctuations.Comment: 15 pages, 9 figures, resubmitted to MNRA
Star Formation in Transient Molecular Clouds
We present the results of a numerical simulation in which star formation
proceeds from an initially unbound molecular cloud core. The turbulent motions,
which dominate the dynamics, dissipate in shocks leaving a quiescent region
which becomes gravitationally bound and collapses to form a small multiple
system. Meanwhile, the bulk of the cloud escapes due to its initial supersonic
velocities. In this simulation, the process naturally results in a star
formation efficiency of 50%. The mass involved in star formation depends on the
gas fraction that dissipates sufficient kinetic energy in shocks. Thus, clouds
with larger turbulent motions will result in lower star formation efficiencies.
This implies that globally unbound, and therefore transient giant molecular
clouds (GMCs), can account for the low efficiency of star formation observed in
our Galaxy without recourse to magnetic fields or feedback processes.
Observations of the dynamic stability in molecular regions suggest that GMCs
may not be self-gravitating, supporting the ideas presented in this letter.Comment: 5 pages, 3 figures, accepted for MNRAS as a lette
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