2,104 research outputs found
The evolution of energy in flow driven by rising bubbles
We investigate by direct numerical simulations the flow that rising bubbles
cause in an originally quiescent fluid. We employ the Eulerian-Lagrangian
method with two-way coupling and periodic boundary conditions. In order to be
able to treat up to 288000 bubbles, the following approximations and
simplifications had to be introduced: (i) The bubbles were treated as
point-particles, thus (ii) disregarding the near-field interactions among them,
and (iii) effective force models for the lift and the drag forces were used. In
particular, the lift coefficient was assumed to be 1/2, independent of the
bubble Reynolds number and the local flow field. The results suggest that large
scale motions are generated, owing to an inverse energy cascade from the small
to the large scales. However, as the Taylor-Reynolds number is only in the
range of 1, the corresponding scaling of the energy spectrum with an exponent
of -5/3 cannot develop over a pronounced range. In the long term, the property
of local energy transfer, characteristic of real turbulence, is lost and the
input of energy equals the viscous dissipation at all scales. Due to the lack
of strong vortices the bubbles spread rather uniformly in the flow. The
mechanism for uniform spreading is as follows: Rising bubbles induce a velocity
field behind them that acts on the following bubbles. Owing to the shear, those
bubbles experience a lift force which make them spread to the left or right,
thus preventing the formation of vertical bubble clusters and therefore of
efficient forcing. Indeed, when the lift is artifically put to zero in the
simulations, the flow is forced much more efficiently and a more pronounced
energy accumulates at large scales is achieved.Comment: 9 pages, 7 figure
Exciton diffusion in air-suspended single-walled carbon nanotubes
Direct measurements of the diffusion length of excitons in air-suspended
single-walled carbon nanotubes are reported. Photoluminescence microscopy is
used to identify individual nanotubes and to determine their lengths and chiral
indices. Exciton diffusion length is obtained by comparing the dependence of
photoluminescence intensity on the nanotube length to numerical solutions of
diffusion equations. We find that the diffusion length in these clean, as-grown
nanotubes is significantly longer than those reported for micelle-encapsulated
nanotubes.Comment: 4 pages, 4 figure
Crustal structure beneath the Trondelag Platform and adjacent areas of the Mid-Norwegian margin, as derived from wide-angle seismic and potential field data
The outer mid-Norwegian margin is characterized by strong breakup magmatism and has been extensively surveyed. The crustal structure of the inner continental shelf, however, is less studied, and its relation to the onshore geology, Caledonian structuring, and breakup magmatism remains unclear. Two Ocean Bottom Seismometer profiles were acquired across the Trøndelag Platform in 2003, as part of the Euromargins program. Additional-land stations recorded the marine shots. The P-wave data were modeled by ray-tracing, supported by gravity modeling. Older multi-channel seismic data allowed for interpretation of stratigraphy down to the top of the Triassic. Crystalline basement velocity is ~6 km s-1 onshore. Top basement is difficult to identify offshore, as velocities (5.3-5.7 km s-1) intermediate between typical crystalline crust and Mesozoic sedimentary strata appear 50-80 km from the coast. This layer thickens towards the Klakk-Ytreholmen Fault Complex and predates Permian and later structur-ing.
The velocities indicate sedimentary rocks, most likely Devonian. Onshore late- to post-Caledonian detachments have been proposed to extend offshore, based on the magnetic anomaly pattern. We do not find the expected correlation between upper basement velocity structure and detachments.
However, there is a distinct, dome-shaped lower-crustal body with a velocity of 6.6-7.0 km s-1. This is thickest under the Froan Basin, and the broad magnetic anomaly used to delineate the detachments correlates with this. The proposed offshore continuation of the detachments thus
appears- unreliable. While we find indications of high density and velocity (~7.2 km s-1) lower crust under the Rås Basin, similar to the proposed igneous underplating of the outer margin, this is poorly constrained near the end of our profiles. The gravity field indicates that this body may be continuous from the pre-breakup basement structures of the Utgard High to the Frøya High, suggesting that it could be an island arc or oceanic terrane-accreted during the Caledonian orogeny. Thus, we find no clear evidence of early Cenozoic igneous underplating of the inner part of the shelf
Gate-induced blueshift and quenching of photoluminescence in suspended single-walled carbon nanotubes
Gate-voltage effects on photoluminescence spectra of suspended single-walled
carbon nanotubes are investigated. Photoluminescence microscopy and excitation
spectroscopy are used to identify individual nanotubes and to determine their
chiralities. Under an application of gate voltage, we observe slight blueshifts
in the emission energy and strong quenching of photoluminescence. The
blueshifts are similar for different chiralities investigated, suggesting
extrinsic mechanisms. In addition, we find that the photoluminescence intensity
quenches exponentially with gate voltage.Comment: 4 pages, 4 figure
Evolution from Non-Fermi to Fermi Liquid Transport Properties by Isovalent Doping in BaFe2(As1-xPx)2 Superconductors
The normal-state charge transport is studied systematically in high-quality
single crystals of BaFe(AsP) (). By
substituting isovalent P for As, the spin-density-wave (SDW) state is
suppressed and the dome-shaped superconducting phase ( K)
appears. Near the SDW end point (), we observe striking linear
temperature () dependence of resistivity in a wide -range, and remarkable
low- enhancement of Hall coefficient magnitude from the carrier number
estimates. We also find that the magnetoresistance apparently violates the
Kohler's rule and is well scaled by the Hall angle as
. These non-Fermi liquid
transport anomalies cannot be attributed to the simple multiband effects. These
results capture universal features of correlated electron systems in the
presence of strong antiferromagnetic fluctuations.Comment: 4 pages, 4 figure
Resonant critical coupling of surface lattice resonances with fluorescent absorptive thin film
Surface lattice resonance supported on nanoparticle arrays is a promising
candidate in enhancing fluorescent effects in both absorption and emission. The
optical enhancement provided by surface lattice resonance is primarily through
the light confinement beyond the diffraction limit, where the nanoparticle
arrays can enhance light-matter interaction for increased absorption as well as
providing more local density of states for enhanced spontaneous emission. In
this work, we optimize the in-coupling efficiency to the fluorescent molecules
by finding the conditions to maximize the absorption, also known as the
critical coupling condition. We studied the transmission characteristics and
the fluorescent emission of a nanoparticle array embedded in an
index-matching layer with fluorescent dye at various concentrations. A modified
coupled-mode theory that describes the nanoparticle array was then derived and
verified by numerical simulations. With the analytical model, we analyzed the
experimental measurements and discovered the condition to critically couple
light into the fluorescent dye, which is demonstrated as the strongest
emission. This study presents a useful guide for designing efficient energy
transfer from excitation beam to the emitters, which maximizes the external
conversion efficiency.Comment: 26 pages, 10 figure
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