1,275 research outputs found
Helicity Transfer in Turbulent Models
Helicity transfer in a shell model of turbulence is investigated. We show
that a Reynolds-independent helicity flux is present in the model when the
large scale forcing breaks inversion symmetry. The equivalent in Shell Models
of the ``2/15 law'', obtained from helicity conservation in Navier-Stokes eqs.,
is derived and tested. The odd part of helicity flux statistic is found to be
dominated by a few very intense events. In a particular model, we calculate
analytically leading and sub-leading contribution to the scaling of triple
velocity correlation.Comment: 4 pages, LaTex, 2 figure
Extreme events in the dispersions of two neighboring particles under the influence of fluid turbulence
We present a numerical study of two-particle dispersion from point-sources in
3D incompressible Homogeneous and Isotropic turbulence, at Reynolds number Re
\simeq 300. Tracer particles are emitted in bunches from localized sources
smaller than the Kolmogorov scale. We report the first quantitative evidence,
supported by an unprecedented statistics, of the deviations of relative
dispersion from Richardson's picture. Deviations are due to extreme events of
pairs separating much faster than average, and of pairs remaining close for
long times. The two classes of events are the fingerprint of complete different
physics, the former being dominated by inertial subrange and large-scale
fluctuations, while the latter by the dissipation subrange. A comparison of
relative separation in surrogate white-in-time velocity field, with correct
viscous-, inertial- and integral-scale properties allows us to assess the
importance of temporal correlations along tracer trajectories.Comment: 5 pages, 6 figure
Helicity advection in Turbulent Models
Helicity transfer in a shell model of turbulence is investigated. In
particular, we study the scaling behavior of helicity transfer in a dynamical
model of turbulence lacking inversion symmetry. We present some
phenomenological and numerical support to the idea that Helicity becomes -at
scale small enough- a passively-advected quantity.Comment: 6 pages, 2 figures, contribution to the proceedings of the
conference: Disorder and Chaos, in honour of Giovanni Paladin, September
22-24, 1997, Rom
Universality in passively advected hydrodynamic fields: the case of a passive vector with pressure
Universality of statistical properties of passive quantities advected by
turbulent velocity fields at changing the passive forcing mechanism is
discussed. In particular, we concentrate on the statistical properties of an
hydrodynamic system with pressure. We present theoretical arguments and
preliminary numerical results which show that the fluxes of passive vector
field and of the velocity field have the same scaling behavior. By exploiting
such a property, we propose a way to compute the anomalous exponents of three
dimensional turbulent velocity fields. Our findings are in agreement within 5%
with experimental values of the anomalous exponents.Comment: 15 pages, 6 figure
Joint statistics of acceleration and vorticity in fully developed turbulence
We report results from a high resolution numerical study of fluid particles
transported by a fully developed turbulent flow. Single particle trajectories
were followed for a time range spanning more than three decades, from less than
a tenth of the Kolmogorov time-scale up to one large-eddy turnover time. We
present results concerning acceleration statistics and the statistics of
trapping by vortex filaments conditioned to the local values of vorticity and
enstrophy. We distinguish two different behaviors between the joint statistics
of vorticity and centripetal acceleration or vorticity and longitudinal
acceleration.Comment: 8 pages, 6 figure
Quasi-particle dephasing time in disordered d-wave superconductors
We evaluate the low-temperature cutoff for quantum interference 1/tf induced
in a d-wave superconductor by the diffusion enhanced quasiparticle interactions
in the presence of disorder. We carry out our analysis in the framework of the
non-linear sigma-model which allows a direct calculation of 1/tf, as the mass
of the transverse modes of the theory. Only the triplet amplitude in the
particle-hole channel and the Cooper amplitude with is pairing symmetry
contribute to 1/tf. We discuss the possible relevance of our results to the
present disagreement between thermal transport data in cuprates and the
localization theory for d-wave quasiparticles
Universality of anisotropic fluctuations from numerical simulations of turbulent flows
We present new results from a direct numerical simulation of a three
dimensional homogeneous Rayleigh-Benard system (HRB), i.e. a convective cell
with an imposed linear mean temperature profile along the vertical direction.
We measure the SO(3)-decomposition of both velocity structure functions and
buoyancy terms. We give a dimensional prediction for the values of the
anisotropic scaling exponents in this Rayleigh-Benard systems. Measured scaling
does not follow dimensional estimate, while a better agreement can be found
with the anisotropic scaling of a different system, the random-Kolmogorov-flow
(RKF). Our findings support the conclusion that scaling properties of
anisotropic fluctuations are universal, i.e. independent of the forcing
mechanism sustaining the turbulent flow.Comment: 4 pages, 3 figure
Lattice Boltzmann simulations of droplet dynamics in time-dependent flows
We study the deformation and dynamics of droplets in time-dependent flows
using 3D numerical simulations of two immiscible fluids based on the lattice
Boltzmann model (LBM). Analytical models are available in the literature, which
assume the droplet shape to be an ellipsoid at all times (P.L. Maffettone, M.
Minale, J. Non-Newton. Fluid Mech 78, 227 (1998); M. Minale, Rheol. Acta 47,
667 (2008)). Beyond the practical importance of using a mesoscale simulation to
assess ab-initio the robustness and limitations of such theoretical models, our
simulations are also key to discuss - in controlled situations - some relevant
phenomenology related to the interplay between the flow time scales and the
droplet time scales regarding the transparency transition for high enough shear
frequencies for an external oscillating flow. This work may be regarded as a
step forward to discuss extensions towards a novel DNS approach, describing the
mesoscale physics of small droplets subjected to a generic hydrodynamical
strain field, possibly mimicking the effect of a realistic turbulent flow on
dilute droplet suspensions
Coherence length in superconductors from weak to strong coupling
We study the evolution of the superconducting coherence length from
weak to strong coupling, both within a s-wave and a d-wave lattice model. We
show that the identification of with the Cooper-pair size
in the weak-coupling regime is meaningful only for a fully-gapped (e.g.,
s-wave) superconductor. Instead in the d-wave superconductor, where
diverges, we show that is properly defined as the
characteristic length scale for the correlation function of the modulus of the
superconducting order parameter. The strong-coupling regime is quite
intriguing, since the interplay between particle-particle and particle-hole
channel is no more negligible. In the case of s-wave pairing, which allows for
an analytical treatment, we show that is of order of the lattice
spacing at finite densities. In the diluted regime diverges, recovering
the behavior of the coherence length of a weakly interacting effective bosonic
system. Similar results are expected to hold for d-wave superconductors.Comment: 11 pages, 5 figures. Two appendices and new references adde
Shear-Improved Smagorinsky Model for Large-Eddy Simulation of Wall-Bounded Turbulent Flows
A shear-improved Smagorinsky model is introduced based on recent results
concerning shear effects in wall-bounded turbulence by Toschi et al. (2000).
The Smagorinsky eddy-viscosity is modified subtracting the magnitude of the
mean shear from the magnitude of the instantaneous resolved strain-rate tensor.
This subgrid-scale model is tested in large-eddy simulations of plane-channel
flows at two different Reynolds numbers. First comparisons with the dynamic
Smagorinsky model and direct numerical simulations, including mean velocity,
turbulent kinetic energy and Reynolds stress profiles, are shown to be
extremely satisfactory. The proposed model, in addition of being physically
sound, has a low computational cost and possesses a high potentiality of
generalization to more complex non-homogeneous turbulent flows.Comment: 10 pages, 6 figures, added some reference
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