831 research outputs found
Investigations of Protostellar Outflow Launching and Gas Entrainment: Hydrodynamic Simulations and Molecular Emission
We investigate protostellar outflow evolution, gas entrainment, and star
formation efficiency using radiation-hydrodynamic simulations of isolated,
turbulent low-mass cores. We adopt an X-wind launching model, in which the
outflow rate is coupled to the instantaneous protostellar accretion rate and
evolution. We vary the outflow collimation angle from =0.01-0.1 and
find that even well collimated outflows effectively sweep up and entrain
significant core mass. The Stage 0 lifetime ranges from 0.14-0.19 Myr, which is
similar to the observed Class 0 lifetime. The star formation efficiency of the
cores spans 0.41-0.51. In all cases, the outflows drive strong turbulence in
the surrounding material. Although the initial core turbulence is purely
solenoidal by construction, the simulations converge to approximate
equipartition between solenoidal and compressive motions due to a combination
of outflow driving and collapse. When compared to a simulation of a cluster of
protostars, which is not gravitationally centrally condensed, we find that the
outflows drive motions that are mainly solenoidal. The final turbulent velocity
dispersion is about twice the initial value of the cores, indicating that an
individual outflow is easily able to replenish turbulent motions on sub-parsec
scales. We post-process the simulations to produce synthetic molecular line
emission maps of CO, CO, and CO and evaluate how well
these tracers reproduce the underlying mass and velocity structure.Comment: Accepted to ApJ, 17 pages, 15 figure
Coupled Elastodynamics of Piston Compression Ring Subject to Sweep Excitation
The piston compression ring's primary function is to seal the combustion chamber, thus mitigating gas leakage to the crankcase and avoiding loss of pressure loading. As a result, the ring is meant to conform closely to the cylinder surface which promotes increased friction. The compression ring is subjected to combustion pressure loading, ring tension, varying inertial force and friction. It is a slender ring of low mass, thus undergoes complex elastodynamic behaviour, when subjected to a multitude of forces. These motions occur in the ring's radial in-plane and axial out-of-plane dynamics, which comprise flutter, ring axial jump, compression-extension, ring twist and rotational drag. An implication of these motions can be loss of sealing, gas blow-by, loss of power and lubricant degradation/oil loss, to name but a few. Consequently, understanding and accurately predicting ring dynamic behaviour under transient conditions is an important step in any subsequent modelling for evaluation of cylinder system efficiency. There have been a plethora of investigations for ring dynamics, often decoupling the ring behaviour in its in-plane and out-of-plane motions. This approach disregards any transfer of dynamic energy from one degree of freedom to another which is only applicable to rectangular ring cross-sections. Alternatively, there are computationally intensive approaches such as finite element analysis which are not conducive for inclusion in any subsequent system level engine modelling where ring response alters in an instantaneous manner. This would require embedded finite element analysis within a transient analysis. This paper presents a finite difference numerical analysis for coupled in-plane and out-of-plane motions of compression rings with practical cross-sectional geometries, which are mostly not rectangular. The formulated method can be integrated into a system level transient cyclic analysis of ring-bore contact. The presented approach takes into account the energy transfer between different degrees of freedom. The predictions are validated against precise non-contact measurements of ring elastodynamic behaviour under amplitude-frequency sweeps. This approach has not hitherto been reported in literature and constitutes the main contribution of the paper
The Green Bank Ammonia Survey: Unveiling the Dynamics of the Barnard 59 star-forming Clump
Understanding the early stages of star formation is a research field of
ongoing development, both theoretically and observationally. In this context,
molecular data have been continuously providing observational constraints on
the gas dynamics at different excitation conditions and depths in the sources.
We have investigated the Barnard 59 core, the only active site of star
formation in the Pipe Nebula, to achieve a comprehensive view of the kinematic
properties of the source. These information were derived by simultaneously
fitting ammonia inversion transition lines (1,1) and (2,2). Our analysis
unveils the imprint of protostellar feedback, such as increasing line widths,
temperature and turbulent motions in our molecular data. Combined with
complementary observations of dust thermal emission, we estimate that the core
is gravitationally bound following a virial analysis. If the core is not
contracting, another source of internal pressure, most likely the magnetic
field, is supporting it against gravitational collapse and limits its star
formation efficiency.Comment: 18 pages, 18 figure
An Investigation into the Oil Transport and Starvation of Piston-Ring Pack
In order to accurately predict the lubricant film thickness and generated friction in any tribological contact, it is important to determine appropriate boundary conditions, taking into account the oil availability and extent of starvation. This paper presents a two-dimensional hydrodynamic model of a piston ring pack for prediction of lubricant film thickness, friction and total power loss. The model takes into account starvation caused by reverse flow at the conjunctional inlet wedge, and applied to a ring pack, comprising a compression and scraper ring. Inlet boundaries are calculated for an engine cycle of a four-cylinder, four-stroke gasoline engine operating at 1500 r/min with conditions pertaining to the New European Drive Cycle. The analysis shows the two main sources of starvation: first, due to a physical lack of inlet meniscus and second, due to reverse flow at the inlet wedge significantly affecting the prevailing conditions from the generally assumed idealised boundary conditions. Such an approach has not hitherto been reported in literature
RTL551 Treatment of EAE Reduces CD226 and T-bet+ CD4 T Cells in Periphery and Prevents Infiltration of T-bet+ IL-17, IFN-Îł Producing T Cells into CNS
Recombinant T cell receptor ligands (RTLs) that target encephalitogenic T-cells can reverse clinical and histological signs of EAE, and are currently in clinical trials for treatment of multiple sclerosis. To evaluate possible regulatory mechanisms, we tested effects of RTL therapy on expression of pathogenic and effector T-cell maturation markers, CD226, T-bet and CD44, by CD4+ Th1 cells early after treatment of MOG-35-55 peptide-induced EAE in C57BL/6 mice. We showed that 1–5 daily injections of RTL551 (two-domain I-Ab covalently linked to MOG-35-55 peptide), but not the control RTL550 (“empty” two-domain I-Ab without a bound peptide) or Vehicle, reduced clinical signs of EAE, prevented trafficking of cells outside the spleen, significantly reduced the frequency of CD226 and T-bet expressing CD4+ T-cells in blood and inhibited expansion of CD44 expressing CD4+ T-cells in blood and spleen. Concomitantly, RTL551 selectively reduced CNS inflammatory lesions, absolute numbers of CNS infiltrating T-bet expressing CD4+ T-cells and IL-17 and IFN-γ secretion by CNS derived MOG-35-55 reactive cells cultured ex vivo. These novel results demonstrate that a major effect of RTL therapy is to attenuate Th1 specific changes in CD4+ T-cells during EAE and prevent expansion of effector T-cells that mediate clinical signs and CNS inflammation in EAE
Probing the evolution of molecular cloud structure: From quiescence to birth
Aims: We derive the probability density functions (PDFs) of column density
for a complete sample of prominent molecular cloud complexes closer than 200
pc. Methods: We derive near-infrared dust extinction maps for 23 molecular
cloud complexes, using the "nicest" colour excess mapping technique and data
from the 2MASS archive. The extinction maps are then used to examine the column
density PDFs in the clouds. Results: The column density PDFs of most molecular
clouds are well-fitted by log-normal functions at low column densities (0.5 mag
< A_v < 3-5 mag). However, at higher column densities prominent, power-law-like
wings are common. In particular, we identify a trend among the PDFs: active
star-forming clouds always have prominent non-log-normal wings. In contrast,
clouds without active star formation resemble log-normals over the whole
observed column density range, or show only low excess of higher column
densities. This trend is also reflected in the cumulative PDFs, showing that
the fraction of high column density material is significantly larger in
star-forming clouds. These observations are in agreement with an evolutionary
trend where turbulent motions are the main cloud-shaping mechanism for
quiescent clouds, but the density enhancements induced by them quickly become
dominated by gravity (and other mechanisms) which is strongly reflected by the
shape of the column density PDFs. The dominant role of the turbulence is
restricted to the very early stages of molecular cloud evolution, comparable to
the onset of active star formation in the clouds.Comment: 7 pages, 11 figures, accepted to A&A Letter
Interactions between brown-dwarf binaries and Sun-like stars
Several mechanisms have been proposed for the formation of brown dwarfs, but
there is as yet no consensus as to which -- if any -- are operative in nature.
Any theory of brown dwarf formation must explain the observed statistics of
brown dwarfs. These statistics are limited by selection effects, but they are
becoming increasingly discriminating. In particular, it appears (a) that brown
dwarfs that are secondaries to Sun-like stars tend to be on wide orbits, a\ga
100\,{\rm AU} (the Brown Dwarf Desert), and (b) that these brown dwarfs have a
significantly higher chance of being in a close (a\la 10\,{\rm AU}) binary
system with another brown dwarf than do brown dwarfs in the field. This then
raises the issue of whether these brown dwarfs have formed {\it in situ}, i.e.
by fragmentation of a circumstellar disc; or have formed elsewhere and
subsequently been captured. We present numerical simulations of the purely
gravitational interaction between a close brown-dwarf binary and a Sun-like
star. These simulations demonstrate that such interactions have a negligible
chance () of leading to the close brown-dwarf binary being captured by
the Sun-like star. Making the interactions dissipative by invoking the
hydrodynamic effects of attendant discs might alter this conclusion. However,
in order to explain the above statistics, this dissipation would have to favour
the capture of brown-dwarf binaries over single brown-dwarfs, and we present
arguments why this is unlikely. The simplest inference is that most brown-dwarf
binaries -- and therefore possibly also most single brown dwarfs -- form by
fragmentation of circumstellar discs around Sun-like protostars, with some of
them subsequently being ejected into the field.Comment: 10 pages, 8 figures, Accepted for publication in Astrophysics and
Space Scienc
Effect of cylinder de-activation on the tribological performance of compression ring conjunction
The paper presents transient thermal-mixed-hydrodynamics of piston compression ring-cylinder liner conjunction for a 4-cylinder 4-stroke gasoline engine during a part of the New European Drive Cycle (NEDC). Analyses are carried out with and without cylinder de-activation (CDA) technology in order to investigate its effect upon the generated tribological conditions. In particular, the effect of CDA upon frictional power loss is studied. The predictions show that overall power losses in the piston-ring cylinder system worsen by as much as 10% because of the increased combustion pressures and liner temperatures in the active cylinders of an engine operating under CDA. This finding shows the down-side of this progressively employed technology, which otherwise is effective in terms of combustion efficiency with additional benefits for operation of catalytic converters. The expounded approach has not hitherto been reported in literature
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A multigroup radiation diffusion test problem: Comparison of code results with analytic solution
We consider a 1D, slab-symmetric test problem for the multigroup radiation diffusion and matter energy balance equations. The test simulates diffusion of energy from a hot central region. Opacities vary with the cube of the frequency and radiation emission is given by a Wien spectrum. We compare results from two LLNL codes, Raptor and Lasnex, with tabular data that define the analytic solution
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