292 research outputs found
On role of symmetries in Kelvin wave turbulence
E.V. Kozik and B.V. Svistunov (KS) paper "Symmetries and Interaction
Coefficients of Kelvin waves", arXiv:1006.1789v1, [cond-mat.other] 9 Jun 2010,
contains a comment on paper "Symmetries and Interaction coefficients of Kelvin
waves", V. V. Lebedev and V. S. L'vov, arXiv:1005.4575, 25 May 2010. It relies
mainly on the KS text "Geometric Symmetries in Superfluid Vortex Dynamics}",
arXiv:1006.0506v1 [cond-mat.other] 2 Jun 2010. The main claim of KS is that a
symmetry argument prevents linear in wavenumber infrared asymptotics of the
interaction vertex and thereby implies locality of the Kelvin wave spectrum
previously obtained by these authors. In the present note we reply to their
arguments. We conclude that there is neither proof of locality nor any
refutation of the possibility of linear asymptotic behavior of interaction
vertices in the texts of KS
Universal Model of Finite-Reynolds Number Turbulent Flow in Channels and Pipes
In this Letter we suggest a simple and physically transparent analytical
model of the pressure driven turbulent wall-bounded flows at high but finite
Reynolds numbers Re. The model gives accurate qualitative description of the
profiles of the mean-velocity and Reynolds-stresses (second order correlations
of velocity fluctuations) throughout the entire channel or pipe in the wide
range of Re, using only three Re-independent parameters. The model sheds light
on the long-standing controversy between supporters of the century-old log-law
theory of von-K\`arm\`an and Prandtl and proposers of a newer theory promoting
power laws to describe the intermediate region of the mean velocity profile.Comment: 4 pages, 6 figs, re-submitted PRL according to referees comment
Analytical Model of the Time Developing Turbulent Boundary Layer
We present an analytical model for the time-developing turbulent boundary
layer (TD-TBL) over a flat plate. The model provides explicit formulae for the
temporal behavior of the wall-shear stress and both the temporal and spatial
distributions of the mean streamwise velocity, the turbulence kinetic energy
and Reynolds shear stress. The resulting profiles are in good agreement with
the DNS results of spatially-developing turbulent boundary layers at momentum
thickness Reynolds number equal to 1430 and 2900. Our analytical model is, to
the best of our knowledge, the first of its kind for TD-TBL.Comment: 5pages, 9 figs, JETP Letters, submitte
Locality and stability of the cascades of two-dimensional turbulence
We investigate and clarify the notion of locality as it pertains to the
cascades of two-dimensional turbulence. The mathematical framework underlying
our analysis is the infinite system of balance equations that govern the
generalized unfused structure functions, first introduced by L'vov and
Procaccia. As a point of departure we use a revised version of the system of
hypotheses that was proposed by Frisch for three-dimensional turbulence. We
show that both the enstrophy cascade and the inverse energy cascade are local
in the sense of non-perturbative statistical locality. We also investigate the
stability conditions for both cascades. We have shown that statistical
stability with respect to forcing applies unconditionally for the inverse
energy cascade. For the enstrophy cascade, statistical stability requires
large-scale dissipation and a vanishing downscale energy dissipation. A careful
discussion of the subtle notion of locality is given at the end of the paper.Comment: v2: 23 pages; 4 figures; minor revisions; resubmitted to Phys. Rev.
Variable damping and coherence in a high-density magnon gas
We report on the fast relaxation behavior of a high-density magnon gas
created by a parametric amplification process. The magnon gas is probed using
the technique of spin-wave packet recovery by parallel parametric pumping.
Experimental results show a damping behavior which is in disagreement with both
the standard model of exponential decay and with earlier observations of
non-linear damping. In particular, the inherent magnon damping is found to
depend upon the presence of the parametric pumping field. A phenomenological
model which accounts for the dephasing of the earlier injected magnons is in
good agreement with the experimental data
Super Stability of Laminar Vortex Flow in Superfluid 3He-B
Vortex flow remains laminar up to large Reynolds numbers (Re~1000) in a
cylinder filled with 3He-B. This is inferred from NMR measurements and
numerical vortex filament calculations where we study the spin up and spin down
responses of the superfluid component, after a sudden change in rotation
velocity. In normal fluids and in superfluid 4He these responses are turbulent.
In 3He-B the vortex core radius is much larger which reduces both surface
pinning and vortex reconnections, the phenomena, which enhance vortex bending
and the creation of turbulent tangles. Thus the origin for the greater
stability of vortex flow in 3He-B is a quantum phenomenon. Only large flow
perturbations are found to make the responses turbulent, such as the walls of a
cubic container or the presence of invasive measuring probes inside the
container.Comment: 4 pages, 6 figure
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