5,732 research outputs found
Design study and evaluation of a hypergolic engine for a space power system, phase 2 Final report, 1 Jul. 1964 - 10 Feb. 1966
Design feasibility of hypergolic engine for space power syste
Vertical variation of mixing within porous sediment beds below turbulent flows
River ecosystems are influenced by contaminants in the water column, in the pore water and adsorbed to sediment particles. When exchange across the sediment-water interface (hyporheic exchange) is included in modelling, the mixing coefficient is often assumed to be constant with depth below the interface. Novel fibre-optic fluorometers have been developed and combined with a modified EROSIMESS system to quantify the vertical variation in mixing coefficient with depth below the sediment-water interface. The study considered a range of particle diameters and bed shear velocities, with the permeability PĂ©clet number, image between 1,000 and 77,000 and the shear Reynolds number, image between 5 and 600. Different parameterisation of both an interface exchange coefficient and a spatially variable in-sediment mixing coefficient are explored. The variation of in-sediment mixing is described by an exponential function applicable over the full range of parameter combinations tested. The empirical relationship enables estimates of the depth to which concentrations of pollutants will penetrate into the bed sediment, allowing the region where exchange will occur faster than molecular diffusion to be determined
OH (1720 MHz) Masers: A Multiwavelength Study of the Interaction between the W51C Supernova Remnant and the W51B Star Forming Region
We present a comprehensive view of the W51B HII region complex and the W51C
supernova remnant (SNR) using new radio observations from the VLA, VLBA,
MERLIN, JCMT, and CSO along with archival data from Spitzer, ROSAT, ASCA, and
Chandra. Our VLA data include the first 400 cm (74 MHz) continuum image of W51
at high resolution (88 arcsec). The 400 cm image shows non-thermal emission
surrounding the G49.2-0.3 HII region, and a compact source of non-thermal
emission (W51B_NT) coincident with the previously-identified OH (1720 MHz)
maser spots, non-thermal 21 and 90 cm emission, and a hard X-ray source.
W51B_NT falls within the region of high likelihood for the position of TeV
gamma-ray emission. Using the VLBA three OH (1720 MHz) maser spots are detected
in the vicinity of W51B_NT with sizes of 60 to 300 AU and Zeeman effect
magnetic field strengths of 1.5 to 2.2 mG. The multiwavelength data demonstrate
that the northern end of the W51B HII region complex has been partly enveloped
by the advancing W51C SNR and this interaction explains the presence of W51B_NT
and the OH masers. This interaction also appears in the thermal molecular gas
which partially encircles W51B_NT and exhibits narrow pre-shock (DeltaV 5 km/s)
and broad post-shock (DeltaV 20 km/s) velocity components. RADEX radiative
transfer modeling of these two components yield physical conditions consistent
with the passage of a non-dissociative C-type shock. Confirmation of the
W51B/W51C interaction provides additional evidence in favor of this region
being one of the best candidates for hadronic particle acceleration known thus
far.Comment: Accepted to Ap
Finite Temperature Path Integral Method for Fermions and Bosons: a Grand Canonical Approach
The calculation of the density matrix for fermions and bosons in the Grand
Canonical Ensemble allows an efficient way for the inclusion of fermionic and
bosonic statistics at all temperatures. It is shown that in a Path Integral
Formulation fermionic density matrix can be expressed via an integration over a
novel representation of the universal temperature dependent functional. While
several representations for the universal functional have already been
developed, they are usually presented in a form inconvenient for computer
calculations. In this work we discuss a new representation for the universal
functional in terms of Hankel functions which is advantageous for computational
applications. Temperature scaling for the universal functional and its
derivatives are also introduced thus allowing an efficient rescaling rather
then recalculation of the functional at different temperatures. A simple
illustration of the method of calculation of density profiles in Grand
Canonical ensemble is presented using a system of noninteracting electrons in a
finite confining potential.Comment: 13 pages 3 figure
Avatars of Eurocentrism in the critique of the liberal peace
Recent scholarly critiques of the so-called liberal peace raise important political and ethical challenges to practices of postwar intervention in the global South. However, their conceptual and analytic approaches have tended to reproduce rather than challenge the intellectual Eurocentrism underpinning the liberal peace. Eurocentric features of the critiques include the methodological bypassing of target subjects in research, the analytic bypassing of subjects through frameworks of governmentality, the assumed ontological split between the âliberalâ and the âlocalâ, and a nostalgia for the liberal subject and the liberal social contract as alternative bases for politics. These collectively produce a âparadox of liberalismâ that sees the liberal peace as oppressive but also the only true source of emancipation. However, the article suggests that a repoliticization of colonial difference offers an alternative âdecolonizingâ approach to critical analysis through repositioning the analytic gaze. Three alternative research strategies for critical analysis are briefly developed
Berry's conjecture and information theory
It is shown that, by applying a principle of information theory, one obtains
Berry's conjecture regarding the high-lying quantal energy eigenstates of
classically chaotic systems.Comment: 8 pages, no figure
Segue Between Favorable and Unfavorable Solvation
Solvation of small and large clusters are studied by simulation, considering
a range of solvent-solute attractive energy strengths. Over a wide range of
conditions, both for solvation in the Lennard-Jones liquid and in the SPC model
of water, it is shown that the mean solvent density varies linearly with
changes in solvent-solute adhesion or attractive energy strength. This behavior
is understood from the perspective of Weeks' theory of solvation [Ann. Rev.
Phys. Chem. 2002, 53, 533] and supports theories based upon that perspective.Comment: 8 pages, 7 figure
High Resolution 8 mm and 1 cm Polarization of IRAS 4A from the VLA Nascent Disk and Multiplicity (VANDAM) Survey
Magnetic fields can regulate disk formation, accretion and jet launching.
Until recently, it has been difficult to obtain high resolution observations of
the magnetic fields of the youngest protostars in the critical region near the
protostar. The VANDAM survey is observing all known protostars in the Perseus
Molecular Cloud. Here we present the polarization data of IRAS 4A. We find that
with ~ 0.2'' (50 AU) resolution at {\lambda} = 8.1 and 10.3 mm, the inferred
magnetic field is consistent with a circular morphology, in marked contrast
with the hourglass morphology seen on larger scales. This morphology is
consistent with frozen-in field lines that were dragged in by rotating material
entering the infall region. The field morphology is reminiscent of rotating
circumstellar material near the protostar. This is the first polarization
detection of a protostar at these wavelengths. We conclude from our
observations that the dust emission is optically thin with {\beta} ~ 1.3,
suggesting that mm/cm-sized grains have grown and survived in the short
lifetime of the protostar.Comment: Accepted to ApJL. 13 pages, 4 figure
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