304 research outputs found
Slug genesis in cylindrical pipe flow
International audienceTransition to uniform turbulence in cylindrical pipe flow occurs experimentally via the spatial expansion of isolated coherent structures called slugs, triggered by localized finite-amplitude disturbances. We study this process numerically by examining the preferred route in phase space through which a critical disturbance initiates a slug. This entails first identifying the relative attractor - edge state - on the laminar-turbulent boundary in a long pipe and then studying the dynamics along its low-dimensional unstable manifold, leading to the turbulent state. Even though the fully turbulent state delocalizes at Re ˜ 2300, the edge state is found to be localized over the range Re = 2000-6000, and progressively reduces in both energy and spatial extent as Re is increased. A key process in the genesis of a slug is found to be vortex shedding via a Kelvin-Helmholtz mechanism from wall-attached shear layers quickly formed at the edge state's upstream boundary. Whether these shedded vortices travel on average faster or slower downstream than the developing turbulence determines whether a puff or a slug (respectively) is formed. This observation suggests that slugs are out-of-equilibrium puffs which therefore do not co-exist with stable puffs. © 2010 Cambridge University Press
Halo phenomenon in finite many-fermion systems. Atom-positron complexes and large-scale study of atomic nuclei
The analysis method proposed in Ref. \cite{rotival07a} is applied to
characterize halo properties in finite many-fermion systems. First, the
versatility of the method is highlighted by applying it to light and
medium-mass nuclei as well as to atom-positron and ion-positronium complexes.
Second, the dependence of nuclear halo properties on the characteristics of the
energy density functional used in self-consistent Hartree-Fock-Bogoliubov
calculations is studied. It is found that (a) the low-density behavior of the
pairing functional and the regularization/renormalization scheme must be chosen
coherently and with care to provide meaningful predictions, (b) the impact of
pairing correlations on halo properties is significant and is the result of two
competing effects, (c) the detailed characteristics of the pairing functional
has however only little importance, (d) halo properties depend significantly on
any ingredient of the energy density functional that influences the location of
single-particle levels; i.e. the effective mass, the tensor terms and the
saturation density of nuclear matter. The latter dependencies give insights to
how experimental data on medium-mass drip-line nuclei can be used in the
distant future to constrain some characteristics of the nuclear energy density
functional. Last but not least, large scale predictions of halos among all
spherical even-even nuclei are performed using specific sets of particle-hole
and particle-particle energy functionals. It is shown that halos in the ground
state of medium-mass nuclei will only be found at the very limit of neutron
stability and for a limited number of elements.Comment: 24 Pages, 32 Figures. Accepted for publication in Phys. Rev. C
back-to back with first part (nucl-th/0702050
Highly-symmetric travelling waves in pipe flow
The recent theoretical discovery of finite-amplitude travelling waves in pipe
flow has re-ignited interest in the transitional phenomena that Osborne
Reynolds studied 125 years ago. Despite all being unstable, these waves are
providing fresh insight into the flow dynamics. Here we describe two new
classes of highly-symmetric travelling waves (possessing rotational,
shift-&-reflect and mirror symmetries) and report a new family of
mirror-symmetric waves which is the first found in pipe flow not to have
shift-&-reflect symmetry. The highly-symmetric waves appear at lower Reynolds
numbers than the originally-discovered non-mirror-symmetric waves found by
Faisst & Eckhardt 2003 and Wedin & Kerswell 2004 and have much higher wall
shear stresses. The first M-class comprises of the various
discrete-rotationally-symmetric analogues of the mirror-symmetric wave found in
Pringle & Kerswell (2007) and have a distinctive double layer structure of fast
and slow streaks across the pipe radius. The second N-class has the more
familiar separation of fast streaks to the exterior and slow streaks to the
interior and looks the precursor to the class of non-mirror-symmetric waves
already known.Comment: 16 pages, 8 figures, for Phil Trans theme issue on pipe flo
Transition in pipe flow: the saddle structure on the boundary of turbulence
The laminar-turbulent boundary S is the set separating initial conditions
which relaminarise uneventfully from those which become turbulent. Phase space
trajectories on this hypersurface in cylindrical pipe flow look to be chaotic
and show recurring evidence of coherent structures. A general numerical
technique is developed for recognising approaches to these structures and then
for identifying the exact coherent solutions themselves. Numerical evidence is
presented which suggests that trajectories on S are organised around only a few
travelling waves and their heteroclinic connections. If the flow is suitably
constrained to a subspace with a discrete rotational symmetry, it is possible
to find locally-attracting travelling waves embedded within S. Four new types
of travelling waves were found using this approach.Comment: 24 pages, 14 figures. Accepted, Jou. Fluid Mec
Pattern fluctuations in transitional plane Couette flow
In wide enough systems, plane Couette flow, the flow established between two
parallel plates translating in opposite directions, displays alternatively
turbulent and laminar oblique bands in a given range of Reynolds numbers R. We
show that in periodic domains that contain a few bands, for given values of R
and size, the orientation and the wavelength of this pattern can fluctuate in
time. A procedure is defined to detect well-oriented episodes and to determine
the statistics of their lifetimes. The latter turn out to be distributed
according to exponentially decreasing laws. This statistics is interpreted in
terms of an activated process described by a Langevin equation whose
deterministic part is a standard Landau model for two interacting complex
amplitudes whereas the noise arises from the turbulent background.Comment: 13 pages, 11 figures. Accepted for publication in Journal of
statistical physic
Pairing correlations. Part 1: description of odd nuclei in mean-field theories
In order to extract informations on pairing correlations in nuclei from
experimental mass differences, the different contributions to odd-even mass
differences are investigated within the Skyrme HFB method. In this first paper,
the description of odd nuclei within HFB is discussed since it is the key point
for the understanding of the above mentioned contributions. To go from an even
nucleus to an odd one, the advantage of a two steps process is demonstrated and
its physical content is discussed. New results concerning time-reversal
symmetry breaking in odd-nuclei are also reported.
PACS: 21.10Dr; 21.10.Hw; 21.30.-x.
Keywords: Mean-field theories; Pairing correlations; odd nuclei;Comment: 34 pages, 8 figures. Submitted to Phys. Rev.
On the decay of turbulence in plane Couette flow
The decay of turbulent and laminar oblique bands in the lower transitional
range of plane Couette flow is studied by means of direct numerical simulations
of the Navier--Stokes equations. We consider systems that are extended enough
for several bands to exist, thanks to mild wall-normal under-resolution
considered as a consistent and well-validated modelling strategy. We point out
a two-stage process involving the rupture of a band followed by a slow
regression of the fragments left. Previous approaches to turbulence decay in
wall-bounded flows making use of the chaotic transient paradigm are
reinterpreted within a spatiotemporal perspective in terms of large deviations
of an underlying stochastic process.Comment: ETC13 Conference Proceedings, 6 pages, 5 figure
On the equivalence of pairing correlations and intrinsic vortical currents in rotating nuclei
The present paper establishes a link between pairing correlations in rotating
nuclei and collective vortical modes in the intrinsic frame. We show that the
latter can be embodied by a simple S-type coupling a la Chandrasekhar between
rotational and intrinsic vortical collective modes. This results from a
comparison between the solutions of microscopic calculations within the HFB and
the HF Routhian formalisms. The HF Routhian solutions are constrained to have
the same Kelvin circulation expectation value as the HFB ones. It is shown in
several mass regions, pairing regimes, and for various spin values that this
procedure yields moments of inertia, angular velocities, and current
distributions which are very similar within both formalisms. We finally present
perspectives for further studies.Comment: 8 pages, 4 figures, submitted to Phys. Rev.
Nonlinear description of transversal motion in a laminar boundary layer with streaks
The nonlinear streamwise growth of a spanwise periodic array of steady streaks in a flat plate boundary layer is numerically computed using the well known Reduced Navier-Stokes formulation. It is found that the flow configuration changes substantially when the amplitude of the streaks grows and the nonlinear effects come into play. The transversal motion (in the wall normal-spanwise plane), which is normally not considered, becomes non-negligible in the nonlinear regime, and it strongly distorts the streamwise velocity profiles, which end up being quite different from those predicted by the linear theory. We analyze in detail the resulting flow patterns for the nonlinearly saturated streaks, and compare them with available experimental results
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