756 research outputs found
A census of Meddies tracked by floats
Recent subsurface float measurements in 27 Mediterranean Water eddies (Meddies) in the Atlantic are grouped together to reveal new information about the pathways of these energetic eddies and how they are often modified and possibly destroyed by collisions with seamounts. Twenty Meddles were tracked in the Iberian Basin west of Portugal, seven in the Canary Basin. During February 1994 14 Meddles were simultaneously observed, 11 of them in the Iberian Basin. Most (69%) of the newly formed Meddles in the Iberian Basin translated southwestward into the vicinity of the Horseshoe Seamounts and probably collided with them. Some Meddles (31%) passed around the northern side of the seamounts and translated southwestward at a typical velocity of 2.0 cm/s into the Canary Basin. Some Meddles observed there were estimated to be up to similar to 5 yr old. Four Meddles in the Canary Basin collided with the Great Meteor Seamounts and three Meddles were inferred to have been destroyed by the collision. Overall an estimated 90% of Meddles collided with major seamounts. The mean time from Meddy formation to a collision with a major seamount was estimated to be around 1.7 yr. Combined with the estimated Meddy formation rate of 17 Meddies/yr from previous work, this suggests that around 29 Meddles co-exist in the North Atlantic. Therefore during February 1994 we observed about half of the population of Meddle
Optical fiber fabrication using novel gas-phase deposition technique
We report a highly versatile chemical-in-crucible preform fabrication technique suitable for gas-phase deposition of doped optical fibers. Aluminosilicate and ytterbium-doped phosphosilicate fibers are presented demonstrating the technique and its potential for realizing complex fiber designs that are suitable for the next generation of high-power fiber devices. The results show aluminum-doped fiber with numerical aperture of 0.28 and ytterbium-doped fiber with a measured slope efficiency of 84% with respect to pump launch power
Quasi-separatrix layers and three-dimensional reconnection diagnostics for line-tied tearing modes
In three-dimensional magnetic configurations for a plasma in which no closed
field line or magnetic null exists, no magnetic reconnection can occur, by the
strictest definition of reconnection. A finitely long pinch with line-tied
boundary conditions, in which all the magnetic field lines start at one end of
the system and proceed to the opposite end, is an example of such a system.
Nevertheless, for a long system of this type, the physical behavior in
resistive magnetohydrodynamics (MHD) essentially involves reconnection. This
has been explained in terms comparing the geometric and tearing widths [1, 2].
The concept of a quasi-separatrix layer[3, 4] was developed for such systems.
In this paper we study a model for a line-tied system in which the
corresponding periodic system has an unstable tearing mode. We analyze this
system in terms of two magnetic field line diagnostics, the squashing
factor[3-5] and the electrostatic potential difference used in kinematic
reconnection studies[6, 7]. We discuss the physical and geometric significance
of these two diagnostics and compare them in the context of discerning
tearing-like behavior in line-tied modes. [1] G. L. Delzanno and J. M. Finn.
Physics of Plasmas, 15(3):032904, 2008. [2] Y.-M. Huang and E. G. Zweibel.
Physics of Plasmas, 16(4):042102, 2009. [3] E. R. Priest and P. D\'emoulin. J.
Geophys. Res., 100(A12):23443-23463, 1995. [4] P. D\'emoulin, J. C. Henoux, E.
R. Priest, and C. H. Mandrini. Astron. Astrophys., 308:643-655, Apr. 1996. [5]
V. S. Titov and G. Hornig. Advances in Space Research, 29(7):1087-1092, 2002.
[6] Y. Lau and J. M. Finn. The Astrophysical Journal, 350:672-691, Feb. 1990.
[7] Y. Lau and J. M. Finn. The Astrophysical Journal, 366:577-591, 1991.Comment: 13 pages, 9 figures, Submitted to Commun Nonlinear Sci Numer Simula
Exact Resummations in the Theory of Hydrodynamic Turbulence: I The Ball of Locality and Normal Scaling
This paper is the first in a series of three papers that aim at understanding
the scaling behaviour of hydrodynamic turbulence. We present in this paper a
perturbative theory for the structure functions and the response functions of
the hydrodynamic velocity field in real space and time. Starting from the
Navier-Stokes equations (at high Reynolds number Re) we show that the standard
perturbative expansions that suffer from infra-red divergences can be exactly
resummed using the Belinicher-L'vov transformation. After this exact (partial)
resummation it is proven that the resulting perturbation theory is free of
divergences, both in large and in small spatial separations. The hydrodynamic
response and the correlations have contributions that arise from mediated
interactions which take place at some space- time coordinates. It is shown that
the main contribution arises when these coordinates lie within a shell of a
"ball of locality" that is defined and discussed. We argue that the real
space-time formalism developed here offers a clear and intuitive understanding
of every diagram in the theory, and of every element in the diagrams. One major
consequence of this theory is that none of the familiar perturbative mechanisms
may ruin the classical Kolmogorov (K41) scaling solution for the structure
functions. Accordingly, corrections to the K41 solutions should be sought in
nonperturbative effects. These effects are the subjects of papers II and III in
this series, that will propose a mechanism for anomalous scaling in turbulence,
which in particular allows multiscaling of the structure functions.Comment: PRE in press, 18 pages + 6 figures, REVTeX. The Eps files of figures
will be FTPed by request to [email protected]
Towards an understanding of social networks among organizational self-initiated expatriates: a qualitative case study of a professional services firm
Drawing on a qualitative case study of 51 organizational self-initiated expatriates (OSIEs) in a professional services firm, this article investigates the role of networks during expatriation and, specifically, in the development of learning that is beneficial to both the individual expatriate and the global operations of the firm. First, we investigate the extent to which individual motivations to engage in OSIE impact on the development of networks. Second, we investigate individual’s experiences of network development. Third, we investigate individual perceptions of the benefits of networks for both organizations and individual actors. The paper will report that professionals initiating their own expatriation develop continually expanding and composite networks such that mobility and networks evolve in a seemingly symbiotic relationship. In doing so, it contributes to our understanding of the role of agency in network development and extends our understanding of organizational self-initiated expatriation as a relatively under-researched phenomenon
Criteria for flatness and injectivity
Let be a commutative Noetherian ring. We give criteria for flatness of
-modules in terms of associated primes and torsion-freeness of certain
tensor products. This allows us to develop a criterion for regularity if
has characteristic , or more generally if it has a locally contracting
endomorphism. Dualizing, we give criteria for injectivity of -modules in
terms of coassociated primes and (h-)divisibility of certain \Hom-modules.
Along the way, we develop tools to achieve such a dual result. These include a
careful analysis of the notions of divisibility and h-divisibility (including a
localization result), a theorem on coassociated primes across a \Hom-module
base change, and a local criterion for injectivity.Comment: 19 page
Addressing the [O III]/Hβ offset of dwarf galaxies in the RESOLVE survey
Metal-poor dwarf galaxies in the local universe, such as those found in the RESOLVE galaxy survey, often produce high [O III]/Hβ ratios close to the star-forming demarcation lines of the diagnostic BPT diagram. Modelling the emission from these galaxies at lower metallicities generally underpredicts this line ratio, which is typically attributed to a deficit of photons >35 eV. We show that applying a model that includes empirical abundances scaled with metallicity strongly influences the thermal balance in HII regions and preserves the [O III]/Hβ offset even in the presence of a harder radiation field generated by interacting binaries. Additional heating mechanisms are more successful in addressing the offset. In accordance with the high sSFR typical of dwarf galaxies in the sample, we demonstrate that cosmic ray heating serves as one mechanism capable of aligning spectral synthesis predictions with observations. We also show that incorporating a range of physical conditions in our modelling can create even better agreement between model calculations and observed emission-line ratios. Together these results emphasize that both the hardness of the incident continuum and the variety of physical conditions present in nebular gas clouds must be accurately accounted for prior to drawing conclusions from emission-line diagnostic diagrams
Discrete eddies in the northern North Atlantic as observed by looping RAFOS floats
Author Posting. © The Authors, 2004. This is the author's version of the work. It is posted here by permission of Elsevier B.V. for personal use, not for redistribution. The definitive version was published in Deep Sea Research Part II: Topical Studies in Oceanography 52 (2005): 627-650, doi:10.1016/j.dsr2.2004.12.011.RAFOS float trajectories near the 27.5 density level were analyzed to investigate discrete
eddies in the northern North Atlantic with the objective of determining their geographical
distribution and characteristics. Floats that made two or more consecutive loops in the
same direction (loopers) were considered to have been in an eddy. Overall 15% (24 float
years) of the float data were in loopers. One hundred and eight loopers were identified in
96 different eddies. Roughly half of the eddies were cyclonic (49%) and half were
anticyclonic (51%), although the percentages varied in different regions. A few eddies
were quasi-stationary for long times, one for over a year in the Iceland Basin, and many
others clearly translated, often in the direction of the general circulation as observed by
non-looping floats. Several floats were trapped in eddies in the vicinity of the North
Atlantic Current just upstream (west) of the Charlie Gibbs (52ÂşN) and Faraday (50ÂşN)
Fracture Zones, which seem to be preferred routes for flow crossing the mid-Atlantic
ridge. Five floats looped in four anticyclones which translated southwestward away from
the eastern boundary near the Goban Spur (47ÂşN-50ÂşN). These could have been weak
meddies forming from remnants of warm salty Mediterranean Water advected northward
along the eastern boundary.Funds for
this research were provided by National Science Foundation grants OCE-9531877 to
WHOI and OCE-9906775 to URI. This work was also supported by a grant from the
WHOI Associates
Radiation damage tolerance of a novel metastable refractory high entropy alloy V2.5Cr1.2WMoCo0.04
A novel multicomponent alloy, V2.5Cr1.2WMoCo0.04, produced from elements expected to favour a BCC crystal structure, and to be suitable for high temperature environments, was fabricated by arc melting and found to exhibit a multiphase dendritic microstructure with W-rich dendrites and V–Cr segregated to the inter-dendritic cores. The as-cast alloy displayed an apparent single-phase XRD pattern. Following heat treatment at 1187 °C for 500 h the alloy transformed into three different distinct phases - BCC, orthorhombic, and tetragonal in crystal structure. This attests to the BCC crystal structure observed in the as-cast state being metastable. The radiation damage response was investigated through room temperature 5 MeV Au+ ion irradiation studies. Metastable as-cast V2.5Cr1.2WMoCo0.04 shows good resistance to radiation induced damage up to 40 displacements per atom (dpa). 96 wt% of the as-cast single-phase BCC crystal structure remained intact, as exhibited by grazing incidence X-ray diffraction (GI-XRD) patterns, whilst the remainder of the alloy transformed into an additional BCC crystal structure with a similar lattice parameter. The exceptional phase stability seen here is attributed to a combination of self-healing processes and the BCC structure, rather than a high configurational entropy, as has been suggested for some of these multicomponent “High Entropy Alloy” types. The importance of the stability of metastable high entropy alloy phases for behaviour under irradiation is for the first time highlighted and the findings thus challenge the current understanding of phase stability after irradiation of systems like the HEAs
Anomalous Scaling of Structure Functions and Dynamic Constraints on Turbulence Simulations
The connection between anomalous scaling of structure functions
(intermittency) and numerical methods for turbulence simulations is discussed.
It is argued that the computational work for direct numerical simulations (DNS)
of fully developed turbulence increases as , and not as
expected from Kolmogorov's theory, where is a large-scale Reynolds number.
Various relations for the moments of acceleration and velocity derivatives are
derived. An infinite set of exact constraints on dynamically consistent subgrid
models for Large Eddy Simulations (LES) is derived from the Navier-Stokes
equations, and some problems of principle associated with existing LES models
are highlighted.Comment: 18 page
- …