253 research outputs found
Design, commissioning and performance of a device to vary the turbulence in a recirculating flume
Ambient turbulent flow structures are one of the key drivers that will determine the rate of wake recovery downstream of tidal turbines. For second and third generation arrays or farms such a parameter is critical for the determination of inter-device spacing and the optimisation of energy extraction per unit surface area. At present offshore flow characterisation is dominated by seabed or surface-mounted diverging-beam acoustic Doppler profilers that whilst having a good spatial capture cannot characterise turbulent flow structures to the same accuracy as single point converging laboratory-scale velocimeters. So a paradox presently exists: We can measure the (mean) flow characteristics at real tidal energy sites but lack the ability to accuracy ascertain high-frequency flow characteristic at discrete spatial locations. This is possible at laboratory-scale with convergent-beam devices but as we do not know the real site conditions replication at small-scale can only be approximated. To date there has been few laboratory studies where the ambient flow turbulence has been varied. The standard method is to generate turbulence from a static structure such as a grid. Here we have developed an articulated rig that has the ability to oscillate cylindrical members along two axes in the flow upstream of tidal turbine models. Initial results presented in this paper show the effect upon the ambient flow that the turbulence-generating rig can impose and the effects upon wake dissipation for varying levels of turbulent length and time scales. Also the formation and insistence of turbulent structures shed from the device are reported. As expected increasing ambient turbulence intensity serves to dissipate the turbine wake more rapidly and whilst we cannot directly relate these laboratory flow characteristics to full-scale tidal energy sites at present it is hoped that offshore measurement technology and that of laboratory replication can converge so that device performance prediction can be performed at smaller-scale and at a corresponding lower cost to the technology
What is a Professional Cataloger? Perception Differences between Professionals and Paraprofessionals
This paper examines the roles of professional and paraprofessional catalogers as they are perceived within the cataloging community. A survey was sent to all catalogers in member libraries of the Association of Research Libraries. In presenting these results, the authors consider whether a difference still exists between professional and paraprofessional catalogers beyond the master of library and information science degree and, if so, the nature of any such difference. In the process, the authors also examine issues such as whether catalogers feel that their work is valued and how cataloging work is evaluated
Bringing Rare Books to Light: The State of the Profession
This article considers changing approaches to rare book cataloging in response to the recent focus on “hidden collections” in special collections departments of academic libraries. The authors analyze the results of a survey of rare book cataloging professionals regarding reactions to the hidden collections discourse, with a particular emphasis on changing policies and practices. A case study of backlog reduction efforts in the rare book unit of Morris Library, Southern Illinois University Carbondale, is also presented
Black-hole quasinormal modes and scalar glueballs in a finite-temperature AdS/QCD model
We use the holographic AdS/QCD soft-wall model to investigate the spectrum of
scalar glueballs in a finite temperature plasma. In this model, glueballs are
described by a massless scalar field in an AdS_5 black hole with a dilaton
soft-wall background. Using AdS/CFT prescriptions, we compute the boundary
retarded Green's function. The corresponding thermal spectral function shows
quasiparticle peaks at low temperatures. We also compute the quasinormal modes
of the scalar field in the soft-wall black hole geometry. The temperature and
momentum dependences of these modes are analyzed. The positions and widths of
the peaks of the spectral function are related to the frequencies of the
quasinormal modes. Our numerical results are found employing the power series
method and the computation of Breit-Wigner resonances.Comment: Revision: Results unchanged. More discussions on the model and on the
results. References added. 28 pages, 7 figures, 5 table
Strongly bound mesons at finite temperature and in magnetic fields from AdS/CFT
We study mesons in N=4 super Yang-Mills theory with fundamental flavors added
at large 't Hooft coupling using the gauge/gravity correspondence. High-spin
mesons are well described by using semiclassical string configurations. We
determine the meson spectrum at finite temperature and in a background magnetic
field.Comment: 15 pages, 11 figures; v2: references adde
Late-Time Tails of Wave Propagation in Higher Dimensional Spacetimes
We study the late-time tails appearing in the propagation of massless fields
(scalar, electromagnetic and gravitational) in the vicinities of a
D-dimensional Schwarzschild black hole. We find that at late times the fields
always exhibit a power-law falloff, but the power-law is highly sensitive to
the dimensionality of the spacetime. Accordingly, for odd D>3 we find that the
field behaves as t^[-(2l+D-2)] at late times, where l is the angular index
determining the angular dependence of the field. This behavior is entirely due
to D being odd, it does not depend on the presence of a black hole in the
spacetime. Indeed this tails is already present in the flat space Green's
function. On the other hand, for even D>4 the field decays as t^[-(2l+3D-8)],
and this time there is no contribution from the flat background. This power-law
is entirely due to the presence of the black hole. The D=4 case is special and
exhibits, as is well known, the t^[-(2l+3)] behavior. In the extra dimensional
scenario for our Universe, our results are strictly correct if the extra
dimensions are infinite, but also give a good description of the late time
behaviour of any field if the large extra dimensions are large enough.Comment: 6 pages, 3 figures, RevTeX4. Version to appear in Rapid
Communications of Physical Review
Meson Spectra and Magnetic Fields in the Sakai-Sugimoto Model
We continue our study of the dynamics of the flavour sector of the
Sakai-Sugimoto model in the presence of an external magnetic field, uncovering
several features of the meson spectrum at high and low temperatures. We employ
both analytical and numerical methods to study the coupled non-linear equations
that result from the gravity dual.Comment: 44 pages, multiple figure
Cosmic F- and D-strings
Macroscopic fundamental and Dirichlet strings have several potential
instabilities: breakage, tachyon decays, and confinement by axion domain walls.
We investigate the conditions under which metastable strings can exist, and we
find that such strings are present in many models. There are various
possibilities, the most notable being a network of (p,q) strings. Cosmic
strings give a potentially large window into string physics.Comment: 27 pages, 5 figures; v. 5: JHEP style, added comments in section 2.
Magnetic phases and reorientation transitions in antiferromagnetically coupled multilayers
In antiferromagnetically coupled superlattices grown on (001) faces of cubic
substrates, e.g. based on materials combinations as Co/Cu, Fe/Si, Co/Cr, or
Fe/Cr, the magnetic states evolve under competing influence of bilinear and
biquadratic exchange interactions, surface-enhanced four-fold in-plane
anisotropy, and specific finite-size effects. Using phenomenological
(micromagnetic) theory, a comprehensive survey of the magnetic states and
reorientation transitions has been carried out for multilayer systems with even
number of ferromagnetic sub-layers and magnetizations in the plane. In
two-layer systems (N=2) the phase diagrams in dependence on components of the
applied field in the plane include ``swallow-tail'' type regions of
(metastable) multistate co-existence and a number of continuous and
discontinuous reorientation transitions induced by radial and transversal
components of the applied field. In multilayers (N \ge 4) noncollinear states
are spatially inhomogeneous with magnetization varying across the multilayer
stack. For weak four-fold anisotropy the magnetic states under influence of an
applied field evolve by a complex continuous reorientation into the saturated
state. At higher anisotropy they transform into various inhomogeneous and
asymmetric structures. The discontinuous transitions between the magnetic
states in these two-layers and multilayers are characterized by broad ranges of
multi-phase coexistence of the (metastable) states and give rise to specific
transitional domain structures.Comment: Manuscript 34 pages, 14 figures; submitted for publicatio
Thermodynamics of Gauss-Bonnet black holes revisited
We investigate the Gauss-Bonnet black hole in five dimensional anti-de Sitter
spacetimes (GBAdS). We analyze all thermodynamic quantities of the GBAdS, which
is characterized by the Gauss-Bonnet coupling and mass , comparing with
those of the Born-Infeld-AdS (BIAdS), Reissner-Norstr\"om-AdS black holes
(RNAdS), Schwarzschild-AdS (SAdS), and BTZ black holes. For we cannot
obtain the black hole with positively definite thermodynamic quantities of
mass, temperature, and entropy because the entropy does not satisfy the
area-law. On the other hand, for , we find the BIAdS-like black hole,
showing that the coupling plays the role of pseudo-charge. Importantly, we
could not obtain the SAdS in the limits of , which means that the GBAdS
is basically different from the SAdS. In addition, we clarify the connections
between thermodynamic and dynamical stability. Finally, we also conjecture that
if a black hole is big and thus globally stable, its quasinormal modes may take
analytic expressions.Comment: 22 pages, 8 figures, version to appear in EPJ
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