460 research outputs found
Translational and rotational dynamics of a large buoyant sphere in turbulence
We report experimental measurements of the translational and rotational dynamics of a large buoyant sphere in isotropic turbulence. We introduce an efficient method to simultaneously determine the position and (absolute) orientation of a spherical body from visual observation. The method employs a minimization algorithm to obtain the orientation from the 2D projection of a specific pattern drawn onto the surface of the sphere. This has the advantages that it does not require a database of reference images, is easily scalable using parallel processing, and enables accurate absolute orientation reference. Analysis of the sphere’s translational dynamics reveals clear differences between the streamwise and transverse directions. The translational autocorrelations and PDFs provide evidence for periodicity in the particle’s dynamics even under turbulent conditions. The angular autocorrelations show weak periodicity. The angular accelerations exhibit wide tails, however without a directional dependence
Wake-Driven Dynamics of Finite-Sized Buoyant Spheres in Turbulence
Particles suspended in turbulent flows are affected by the turbulence and at
the same time act back on the flow. The resulting coupling can give rise to
rich variability in their dynamics. Here we report experimental results from an
investigation of finite-sized buoyant spheres in turbulence. We find that even
a marginal reduction in the particle's density from that of the fluid can
result in strong modification of its dynamics. In contrast to classical spatial
filtering arguments and predictions of particle models, we find that the
particle acceleration variance increases with size. We trace this reversed
trend back to the growing contribution from wake-induced forces, unaccounted
for in current particle models in turbulence. Our findings highlight the need
for improved multi-physics based models that account for particle wake effects
for a faithful representation of buoyant-sphere dynamics in turbulence.Comment: 5 pages, 4 figures,
http://journals.aps.org/prl/abstract/10.1103/PhysRevLett.115.12450
Homology and K--Theory Methods for Classes of Branes Wrapping Nontrivial Cycles
We apply some methods of homology and K-theory to special classes of branes
wrapping homologically nontrivial cycles. We treat the classification of
four-geometries in terms of compact stabilizers (by analogy with Thurston's
classification of three-geometries) and derive the K-amenability of Lie groups
associated with locally symmetric spaces listed in this case. More complicated
examples of T-duality and topology change from fluxes are also considered. We
analyse D-branes and fluxes in type II string theory on with torsion flux and demonstrate in details
the conjectured T-duality to with no flux. In the
simple case of , T-dualizing the circles reduces to
duality between with
flux and with no flux.Comment: 27 pages, tex file, no figure
Influence of cataract and small incision cataract surgery on the macular thickness measurements: an optical coherence tomography-based study
Background: Optical Coherence Tomography (OCT) is a noninvasive imaging technique for analysing retinal architecture. It is a common investigation for various optic disc and macular diseases like glaucoma and diabetic macular oedema nowadays. OCT image quality is affected by many factors especially media opacity due to cataract. This study was done to compare macular thickness measurements by OCT in the presence of cataract and after removal of the cataract by Small Incision Cataract Surgery (SICS).Methods: A prospective observational study was designed which included 99 eyes of 99 patients with no optic disc and retinal pathology who underwent uncomplicated small incision cataract surgery. Routine ophthalmological evaluation including scans using macular analysis protocols of Cirrus HD OCT were done on the first visit to outpatient department and repeated on the day of surgery, one week and three weeks after surgery. The difference between the visits were analysed by Student’s t-test for paired samples.Results: The best corrected visual acuity and signal strength of OCT scans improved significantly after surgery. Among the macular parameters the temporal inner, nasal inner and nasal outer area thicknesses showed significant improvement from preoperative to postoperative values. The foveal thickness, nasal inner and nasal outer thicknesses changed significantly between two postoperative visits.Conclusions: The presence of cataract and small incision cataract surgery affects the macular measurements performed with Cirrus HD OCT. This should be taken into consideration while managing macular diseases like diabetic macular oedema
Anti-ferromagnetic ordering in arrays of superconducting pi-rings
We report experiments in which one dimensional (1D) and two dimensional (2D)
arrays of YBa2Cu3O7-x-Nb pi-rings are cooled through the superconducting
transition temperature of the Nb in various magnetic fields. These pi-rings
have degenerate ground states with either clockwise or counter-clockwise
spontaneous circulating supercurrents. The final flux state of each ring in the
arrays was determined using scanning SQUID microscopy. In the 1D arrays,
fabricated as a single junction with facets alternating between alignment
parallel to a [100] axis of the YBCO and rotated 90 degrees to that axis,
half-fluxon Josephson vortices order strongly into an arrangement with
alternating signs of their magnetic flux. We demonstrate that this ordering is
driven by phase coupling and model the cooling process with a numerical
solution of the Sine-Gordon equation. The 2D ring arrays couple to each other
through the magnetic flux generated by the spontaneous supercurrents. Using
pi-rings for the 2D flux coupling experiments eliminates one source of disorder
seen in similar experiments using conventional superconducting rings, since
pi-rings have doubly degenerate ground states in the absence of an applied
field. Although anti-ferromagnetic ordering occurs, with larger negative bond
orders than previously reported for arrays of conventional rings, long-range
order is never observed, even in geometries without geometric frustration. This
may be due to dynamical effects. Monte-Carlo simulations of the 2D array
cooling process are presented and compared with experiment.Comment: 10 pages, 15 figure
Kinematics and dynamics of freely rising spheroids at high Reynolds numbers
We experimentally investigate the effect of geometrical anisotropy for
buoyant ellipsoidal particles rising in a still fluid. All other parameters,
such as the Galileo number and the particle density ratio
are kept constant. The geometrical aspect ratio, ,
of the particle is varied systematically from = 0.2 (oblate) to 5
(prolate). Based on tracking all degrees of particle motion, we identify six
regimes characterised by distinct rise dynamics. Firstly, for , increased rotational dynamics are observed and the particle flips
over semi-regularly in a "tumbling"-like motion. Secondly, for oblate particles
with , planar regular "zig-zag" motion is observed,
where the drag coefficient is independent of . Thirdly, for the most
extreme oblate geometries () a "flutter"-like behaviour is
found, characterised by precession of the oscillation plane and an increase in
the drag coefficient. For prolate geometries, we observed two coexisting
oscillation modes that contribute to complex trajectories: the first is related
to oscillations of the pointing vector and the second corresponds to a motion
perpendicular to the particle's symmetry axis. We identify a "longitudinal"
regime (), where both modes are active and a different
one, the "broadside"-regime (), where only the second mode is
present. Remarkably, for the most prolate particles (), we observe an
entirely different "helical" rise with completely unique features.Comment: 46 pages, 20 figure
T-Duality as a Duality of Loop Group Bundles
Representing the data of a string compactified on a circle in the background
of H-flux in terms of the geometric data of a principal loop group bundle, we
show that T-duality in type II string theory can be understood as the
interchange of the momentum and winding homomorphisms of the principal loop
group bundle, thus giving rise to a new interpretation of T-duality.Comment: 8 pages, latex 2e, new reference added, J.Phys.A: Fast Track
Publications (to appear
Orthorhombically Mixed s and d Wave Superconductivity and Josephson Tunneling
The effect of orthorhombicity on Josephson tunneling in high T
superconductors such as YBCO is studied for both single crystals and highly
twinned crystals. It is shown that experiments on highly twinned crystals
experimentally determine the symmetry of the superconducting twin boundaries
(which can be either even or odd with respect to a reflection in the twinning
plane). Conversely, Josephson experiments on highly twinned crystals can not
experimentally determine whether the superconductivity is predominantly
-wave or predominantly -wave. The direct experimental determination of
the order-parameter symmetry by Josephson tunneling in YBCO thus comes from the
relatively few experiments which have been carried out on untwinned single
crystals.Comment: 5 pages, RevTeX file, 1 figure available on request
([email protected]
Effective descriptions of branes on non-geometric tori
We investigate the low-energy effective description of non-geometric
compactifications constructed by T-dualizing two or three of the directions of
a T^3 with non-vanishing H-flux. Our approach is to introduce a D3-brane in
these geometries and to take an appropriate decoupling limit. In the case of
two T-dualities, we find at low energies a non-commutative T^2 fibered
non-trivially over an S^1. In the UV this theory is still decoupled from
gravity, but is dual to a little string theory with flavor. For the case of
three T-dualities, we do not find a sensible decoupling limit, casting doubt on
this geometry as a low-energy effective notion in critical string theory.
However, by studying a topological toy model in this background, we find a
non-associative geometry similar to one found by Bouwknegt, Hannabuss, and
Mathai.Comment: 22 pages, 4 figures, references adde
T-duality and Differential K-Theory
We give a precise formulation of T-duality for Ramond-Ramond fields. This
gives a canonical isomorphism between the "geometrically invariant" subgroups
of the twisted differential K-theory of certain principal torus bundles. Our
result combines topological T-duality with the Buscher rules found in physics.Comment: 23 pages, typos corrected, submitted to Comm.Math.Phy
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