525 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
Traces of Thermalization from Transverse Momentum Fluctuations in Nuclear Collisions
Scattering of particles produced in Au+Au collisions at RHIC can wrestle the
system into a state near local thermal equilibrium. I illustrate how
measurements of the centrality dependence of the mean transverse momentum and
its fluctuations can exhibit this thermalization.Comment: 4 pages, 2 eps figures, final version to appear in PR
Quantifying transport within a two-cell microdroplet induced by circular and sharp channel bends
Abstract not availableSanjeeva Balasuriya
Heavy-quark azimuthal momentum correlations as a sensitive probe of thermalization
In high-energy nuclear collisions the degree of thermalization at the
partonic level is a key issue. Due to their large mass, heavy quarks and their
possible participation in the collective flow of the QCD-medium constitute a
powerful probe for thermalization. We present studies with PYTHIA for p+p
collisions at the top LHC energy of = 14 TeV applying the
two-particle transverse momentum correlator
to pairs of heavy-quark hadrons and their semi-leptonic decay products as a
function of their relative azimuth. Modifications or even the complete absence
of initially existing correlations in Pb+Pb collisions might indicate
thermalization at the partonic level.Comment: 7 pages, 5 figs.; accepted for publication in Nucl. Phys.
Quantifying transport within a two-cell microdroplet induced by circular and sharp channel bends
Thermodynamic limit and semi--intensive quantities
The properties of statistical ensembles with abelian charges close to the
thermodynamic limit are discussed. The finite volume corrections to the
probability distributions and particle density moments are calculated. Results
are obtained for statistical ensembles with both exact and average charge
conservation. A new class of variables (semi--intensive variables) which differ
in the thermodynamic limit depending on how charge conservation is implemented
in the system is introduced. The thermodynamic limit behavior of these
variables is calculated through the next to leading order finite volume
corrections to the corresponding probability density distributions.Comment: 11 pages, 2 figures In v2 figures are added and corresponding
editorial changes are done. Paper will be published in Journal of Physics
Ascending aortic aneurysm and aortic valve dysfunction in bicuspid aortic valve disease
BACKGROUND: The relationship of aortic valve dysfunction and ascending aortic aneurysm is unclear in adults with bicuspid aortic valve disease. METHODS: We retrospectively studied 134 consecutive out-patients (98 men, 36 women aged 43+/-18years) with bicuspid aortic valve disease. To investigate the relationship of ascending aortic aneurysm and aortic valve dysfunction we exclusively considered severe pathologies that required treatment by surgical or percutaneous intervention. RESULTS: Of 134 patients, 39 had aortic valve dysfunction without concomitant ascending aortic aneurysm which had been treated previously with isolated valve surgery or percutaneous valvuloplasty comprising 25 patients with aortic stenosis (19%) and 14 patients with aortic regurgitation (10%). Conversely, 26 patients had ascending aortic aneurysm which had been treated previously with aortic surgery (19%). Of these, ascending aortic aneurysm was associated with severe aortic stenosis in 13 patients and with severe aortic regurgitation in 7 patients, whereas aneurysm was unrelated to severe aortic valve dysfunction in the remaining 6 patients including 2 without any degree of aortic valve dysfunction. The maximal aortic diameters were similar at the time of aortic surgery irrespective of presence of severe aortic valve dysfunction (P=.527). Other characteristics of patients with ascending aortic aneurysm were also similar irrespective of presence or type of aortic valve dysfunction. CONCLUSION: The majority of patients with bicuspid aortic valve disease exhibit ascending aortic aneurysm in conjunction with severe aortic valve dysfunction. However, in our study 6 of 134 (5%) of persons with bicuspid aortic valve disease developed ascending aortic aneurysm without aortic valve dysfunction
Measurement of event-by-event transverse momentum and multiplicity fluctuations using strongly intensive measures and in nucleus-nucleus collisions at the CERN Super Proton Synchrotron
Results from the NA49 experiment at the CERN SPS are presented on
event-by-event transverse momentum and multiplicity fluctuations of charged
particles, produced at forward rapidities in central Pb+Pb interactions at beam
momenta 20, 30, 40, 80, and 158 GeV/c, as well as in systems of
different size (, C+C, Si+Si, and Pb+Pb) at 158 GeV/c. This publication
extends the previous NA49 measurements of the strongly intensive measure
by a study of the recently proposed strongly intensive measures of
fluctuations and . In the explored kinematic
region transverse momentum and multiplicity fluctuations show no significant
energy dependence in the SPS energy range. However, a remarkable system size
dependence is observed for both and , with the
largest values measured in peripheral Pb+Pb interactions. The results are
compared with NA61/SHINE measurements in collisions, as well as with
predictions of the UrQMD and EPOS models.Comment: 12 pages, 14 figures, to be submitted to PR
- …