5,560 research outputs found
Modeling the elastic transmission of tidal stresses to great distances inland in channelized ice streams
Geodetic surveys suggest that ocean tides can modulate the motion of Antarctic ice streams, even at stations many tens of kilometers inland from the grounding line. These surveys suggest that ocean tidal stresses can perturb ice stream motion at distances about an order of magnitude farther inland than tidal flexure of the ice stream alone. Recent models exploring the role of tidal perturbations in basal shear stress are primarily one- or two-dimensional, with the impact of the ice stream margins either ignored or parameterized. Here, we use two- and three-dimensional finite-element modeling to investigate transmission of tidal stresses in ice streams and the impact of considering more realistic, three-dimensional ice stream geometries. Using Rutford Ice Stream as a real-world comparison, we demonstrate that the assumption that elastic tidal stresses in ice streams propagate large distances inland fails for channelized glaciers due to an intrinsic, exponential decay in the stress caused by resistance at the ice stream margins. This behavior is independent of basal conditions beneath the ice stream and cannot be fit to observations using either elastic or nonlinear viscoelastic rheologies without nearly complete decoupling of the ice stream from its lateral margins. Our results suggest that a mechanism external to the ice stream is necessary to explain the tidal modulation of stresses far upstream of the grounding line for narrow ice streams. We propose a hydrologic model based on time-dependent variability in till strength to explain transmission of tidal stresses inland of the grounding line. This conceptual model can reproduce observations from Rutford Ice Stream
Distribution of "level velocities" in quasi 1D disordered or chaotic systems with localization
The explicit analytical expression for the distribution function of
parametric derivatives of energy levels ("level velocities") with respect to a
random change of scattering potential is derived for the chaotic quantum
systems belonging to the quasi 1D universality class (quantum kicked rotator,
"domino" billiard, disordered wire, etc.).Comment: 11 pages, REVTEX 3.
Polaritons and Pairing Phenomena in Bose--Hubbard Mixtures
Motivated by recent experiments on cold atomic gases in ultra high finesse
optical cavities, we consider the problem of a two-band Bose--Hubbard model
coupled to quantum light. Photoexcitation promotes carriers between the bands
and we study the non-trivial interplay between Mott insulating behavior and
superfluidity. The model displays a global U(1) X U(1) symmetry which supports
the coexistence of Mott insulating and superfluid phases, and yields a rich
phase diagram with multicritical points. This symmetry property is shared by
several other problems of current experimental interest, including
two-component Bose gases in optical lattices, and the bosonic BEC-BCS crossover
problem for atom-molecule mixtures induced by a Feshbach resonance. We
corroborate our findings by numerical simulations.Comment: 4 pages, 3 figure
An experimental investigation of two large annular diffusers with swirling and distorted inflow
Two annular diffusers downstream of a nacelle-mounted fan were tested for aerodynamic performance, measured in terms of two static pressure recovery parameters (one near the diffuser exit plane and one about three diameters downstream in the settling duct) in the presence of several inflow conditions. The two diffusers each had an inlet diameter of 1.84 m, an area ratio of 2.3, and an equivalent cone angle of 11.5, but were distinguished by centerbodies of different lengths. The dependence of diffuser performance on various combinations of swirling, radially distorted, and/or azimuthally distorted inflow was examined. Swirling flow and distortions in the axial velocity profile in the annulus upstream of the diffuser inlet were caused by the intrinsic flow patterns downstream of a fan in a duct and by artificial intensification of the distortions. Azimuthal distortions or defects were generated by the addition of four artificial devices (screens and fences). Pressure recovery data indicated beneficial effects of both radial distortion (for a limited range of distortion levels) and inflow swirl. Small amounts of azimuthal distortion created by the artificial devices produced only small effects on diffuser performance. A large artificial distortion device was required to produce enough azimuthal flow distortion to significantly degrade the diffuser static pressure recovery
Non-universal corrections to the level curvature distribution beyond random matrix theory
The level curvature distribution function is studied beyond the random matrix
theory for the case of T-breaking perturbations over the orthogonal ensemble.
The leading correction to the shape of the level curvature distribution is
calculated using the nonlinear sigma-model. The sign of the correction depends
on the presence or absence of the global gauge invariance and is different for
perturbations caused by the constant vector-potential and by the random
magnetic field. Scaling arguments are discussed that indicate on the
qualitative difference in the level statistics in the dirty metal phase for
space dimensionalities .Comment: 4 pages, Late
Topological universality of level dynamics in quasi-one-dimensional disordered conductors
Nonperturbative, in inverse Thouless conductance 1/g, corrections to
distributions of level velocities and level curvatures in quasi-one-dimensional
disordered conductors with a topology of a ring subject to a constant vector
potential are studied within the framework of the instanton approximation of
nonlinear sigma-model. It is demonstrated that a global character of the
perturbation reveals the universal features of the level dynamics. The
universality shows up in the form of weak topological oscillations of the
magnitude ~ exp(-g) covering the main bodies of the densities of level
velocities and level curvatures. The period of discovered universal
oscillations does not depend on microscopic parameters of conductor, and is
only determined by the global symmetries of the Hamiltonian before and after
the perturbation was applied. We predict the period of topological oscillations
to be 4/(pi)^2 for the distribution function of level curvatures in orthogonal
symmetry class, and 3^(1/2)/(pi) for the distribution of level velocities in
unitary and symplectic symmetry classes.Comment: 15 pages (revtex), 3 figure
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