203 research outputs found
PSI to turbulence during internal wave beam refraction through the upper ocean pycnocline
A numerical study based on large eddy simulation (LES) is performed to investigate the nonlinear interaction of a semidiurnal (M2) internal wave beam with an upper ocean pycnocline. During refraction through the pycnocline, the wave beam undergoes parametric subharmonic instability (PSI) with formation of waves with (1/2)M2 frequency. The three-dimensional LES enables new results that quantify the route to turbulence through PSI. The subharmonic waves generated from PSI have an order of magnitude smaller vertical scale and are susceptible to wave breaking. Convective instability initiates transition to turbulence, while shear production maintains it. Turbulence at points in the subharmonic wave paths is modulated at (1/2)M2 frequency. The beam suffers substantial degradation owing to PSI, reflected harmonics and ducted waves so that only about 30% of the incoming energy is transported by the main reflected beam.We are pleased to acknowledge
support through ONR N000140910287,
program manager Terri Paluszkiewicz,
ARC DECRA Fellowship DE140100089
for B.G., and helpful discussions with
Shaun Johnston, SIO
Transport by deep convection in basin-scale geostrophic circulation: Turbulence-resolving simulations
Direct numerical simulations are used to investigate the nature of fully resolved small-scale convection and its role in large-scale circulation in a rotating -plane rectangular basin with imposed surface temperature difference. The large-scale circulation has a horizontal geostrophic component and a deep vertical overturning. This paper focuses on convective circulation with no wind stress, and buoyancy forcing sufficiently strong to ensure turbulent convection within the thermal boundary layer (horizontal Rayleigh numbers ). The dynamics are found to depend on the value of a convective Rossby number, Ro_{\unicode[STIX]{x0394}T}, which represents the strength of buoyancy forcing relative to Coriolis forces. Vertical convection shifts from a mean endwall plume under weak rotation (Ro_{\unicode[STIX]{x0394}T}>10^{-1}) to âopen oceanâ chimney convection plus mean vertical plumes at the side boundaries under strong rotation (Ro_{\unicode[STIX]{x0394}T}<10^{-1}). The overall heat throughput, horizontal gyre transport and zonally integrated overturning transport are then consistent with scaling predictions for flow constrained by thermal wind balance in the thermal boundary layer coupled to vertical advectionâdiffusion balance in the boundary layer. For small Rossby numbers relevant to circulation in an ocean basin, vertical heat transport from the surface layer into the deep interior occurs mostly in âopen oceanâ chimney convection while most vertical mass transport is against the side boundaries. Both heat throughput and the mean circulation (in geostrophic gyres, boundary currents and overturning) are reduced by geostrophic constraints.</jats:p
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Geostrophic and chimney regimes in rotating horizontal convection with imposed heat flux
Convection in a rotating rectangular basin with differential thermal forcing at one horizontal boundary is examined using laboratory experiments. The experiments have an imposed heat flux boundary condition, are at large values of the flux Rayleigh number ( based on the box length ), use water with Prandtl number and have a small depth to length aspect ratio. The results show the conditions for transition from non-rotating horizontal convection governed by an inertialâbuoyancy balance in the thermal boundary layer, to circulation governed by geostrophic flow in the boundary layer. The geostrophic balance constrains mean flow and reduces the heat transport as Nusselt number , where is the convective Rossby number, is the imposed buoyancy flux and is the Coriolis parameter. Thus flow in the geostrophic boundary layer regime is governed by the relative roles of horizontal convective accelerations and Coriolis accelerations, or buoyancy and rotation, in the boundary layer. Experimental evidence suggests that for more rapid rotation there is another transition to a regime in which the momentum budget is dominated by fluctuating vertical accelerations in a region of vortical plumes, which we refer to as a âchimneyâ following related discussion of regions of deep convection in the ocean. Coupling of the chimney convection in the region of destabilising boundary flux to the diffusive boundary layer of horizontal convection in the region of stabilising boundary flux gives heat transport independent of rotation in this âinertial chimneyâ regime, and the new scaling . Scaling analysis predicts the transition conditions observed in the experiments, as well as a further âgeostrophic chimneyâ regime in which the vertical plumes are controlled by local geostrophy. When Ro<10^{-1}, the convection is also observed to produce a set of large basin-scale gyres at all depths in the time-averaged flow.</jats:p
Wave Motion due to a Ring Source in Two Superposed Fluids Covered by a Thin Elastic Plate
The problem of wave generation by a horizontal ring of wave sources of the same time-dependent strength present in any one layer of a two-layer fluid is investigated here. The upper fluid is of finite height above the interface and is covered by a floating thin infinite elastic plate (modeling a thin sheet of ice) while the lower fluid extends infinitely downwards. Assuming linear theory, the problem is formulated as an initial value problem and the Laplace transform in time is employed to solve it. For time-harmonic source strength, the asymptotic representations of the potential functions describing the motion in the two layers for large time and distance are derived. In these representations, the two different coefficients for each of the surface and interface wave modes have the same numerical values although it has not been possible to prove their equivalence analytically. This shows that the steady-state analysis of the potential functions produces outgoing progressive waves at the surface and at the interface. The forms of the surface and interface waves are depicted graphically for different values of the flexural rigidity of the elastic plate and the ring source being submerged in the lower or upper layer
Absorbing Phase Transition in a Four State Predator Prey Model in One Dimension
The model of competition between densities of two different species, called
predator and prey, is studied on a one dimensional periodic lattice, where each
site can be in one of the four states say, empty, or occupied by a single
predator, or occupied by a single prey, or by both. Along with the pairwise
death of predators and growth of preys, we introduce an interaction where the
predators can eat one of the neighboring prey and reproduce a new predator
there instantly. The model shows a non-equilibrium phase transition into a
unusual absorbing state where predators are absent and the lattice is fully
occupied by preys. The critical exponents of the system are found to be
different from that of the Directed Percolation universality class and they are
robust against addition of explicit diffusion.Comment: 10 pages, 6 figures, to appear in JSTA
Sum Rules for Multi-Photon Spectroscopy of Ions in Finite Symmetry
Models describing one- and two-photon transitions for ions in crystalline
environments are unified and extended to the case of parity-allowed and parity-
forbidden p-photon transitions. The number of independent parameters for
characterizing the polarization dependence is shown to depend on an ensemble of
properties and rules which combine symmetry considerations and physical models.Comment: 16 pages, Tex fil
Tissue factor-positive monocytes in children with sickle cell disease: correlation with biomarkers of haemolysis
Tissue Factor (TF) initiates thrombin generation, and whole blood TF (WBTF) is elevated in sickle cell disease (SCD). We sought to identify the presence of TF-positive monocytes in SCD and their relationship with the other coagulation markers including WBTF, microparticle-associated TF, thrombin-antithrombin (TAT) complexes and D-dimer. Whether major SCD-related pathobiological processes, including haemolysis, inflammation and endothelial activation, contribute to the coagulation abnormalities was also studied. The cohort comprised children with SCD (18 HbSS, 12 HbSC, mean age 3.6 years). We demonstrated elevated levels of TF-positive monocytes in HbSS, which correlated with WBTF, TAT and D-Dimer (p=0.02 to p=0.0003). While TF-positive monocytes, WBTF, TAT and D-dimer correlated with several biomarkers of haemolysis, inflammation and endothelial activation in univariate analyses, in multiple regression models the haemolytic markers (reticulocytes and lactate dehydrogenase) contributed exclusively to the association with all four coagulant markers evaluated. The demonstration that haemolysis is the predominant operative pathology in the associated perturbations of coagulation in HbSS at a young age provides additional evidence for the early use of therapeutic agents, such as hydroxycarbamide to reduce the haemolytic component of this disease
Alignment, Morphology and Defect Control of Vertically Aligned ZnO Nanorod Array: Competition between âSurfactantâ and âStabilizerâ Roles of the Amine Species and Its Photocatalytic Properties
Experiments on Ladders Reveal a Complex Interplay between a Spin-Gapped Normal State and Superconductivity
In recent years, the study of ladder materials has developed into a
well-established area of research within the general context of Strongly
Correlated Electrons. This effort has been triggered by an unusual
cross-fertilization between theory and experiments. In this paper, the main
experimental results obtained in the context of ladders are reviewed from the
perspective of a theorist. Emphasis is given to the many similarities between
the two-dimensional high- cuprates and the two-leg ladder compounds,
including SrCaCuO (14-24-41) which has a
superconducting phase at high pressure and a small hole density. Examples of
these similarities include regimes of linear resistivity vs temperature in
metallic ladders and a normal state with spin-gap or pseudogap characteristics.
Some controversial results in this context are also discussed. It is remarked
that the ladder 14-24-41 is the first superconducting copper-oxide material
with a non-square-lattice layered arrangement, and certainly much can be
learned from a careful analysis of this compound. A short summary of the main
theoretical developments in this field is also included, as well as a brief
description of the properties of non-copper-oxide ladders. Suggestions by the
author on possible experiments are described in the text. Overall, it is
concluded that the enormous experimental effort carried out on ladders has
already unveiled quite challenging and interesting physics that adds to the
rich behavior of electrons in transition-metal-oxides, and in addition
contributes to the understanding of the two-dimensional cuprates. However,
still considerable work needs to be carried out to fully understand the
interplay between charge and spin degrees of freedom in these materials.Comment: Latex, 22 pages, with 41 GIF figures attache
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