708 research outputs found
An experimental study of boundary-layer transition induced vibrations on a hydrofoil
This paper aims at characterizing experimentally laminar to turbulent transition induced vibrations. Here, the transition is known to be triggered by a Laminar Separation Bubble that results from a laminar separation of the boundary-layer flow on a hydrofoil. In this study we consider two NACA66312 (Mod) laminar hydrofoils at low angles of incidence (mostly 2° and 4°) and Reynolds numbers ranging from Re=450 000 to 1 200 000, in order to get transitional regimes. The first hydrofoil, made of steel (E=2.1Ă1011 Pa), is referred to as the rigid hydrofoil, although it is seen to vibrate under the action of the LSB. To better understand the possible interaction between the flow and the foil vibrations, vibration measurements are repeated using a flexible hydrofoil (E=3Ă109 Pa) of same geometry (under zero loading) and in close configurations. The experiments are carried out at the French Naval Academy Research Institute (IRENav, France). Wall pressure and flow velocity measurements enable a characterization of the laminar separation bubble and the identification of a vortex shedding at a given frequency. It is hence shown that the boundary-layer transition induces important foil vibrations, whose characteristics in terms of frequency and amplitude depend on the vortex shedding frequency, and can be coupled with natural frequencies of the hydrofoils
From multifragmentation to supernovae and neutron stars
The thermodynamics properties of globally neutral dense stellar matter are
analyzed both in terms of mean field instabilities and structures beyond the
mean field. The mean field response to finite wavelenght fluctuations is
calculated with the realistic Sly230a effective interaction. A Monte Carlo
simulation of a schematic lattice Hamiltonian shows the importance of
calculations beyond the mean field to calculate the phase diagram of stellar
matter. The analogies and differences respect to the thermodynamics of nuclear
matter and finite nuclei are stressed.Comment: To be published in Acta Phys. Hung.
Core-crust transition in neutron stars: predictivity of density developments
The possibility to draw links between the isospin properties of nuclei and
the structure of compact stars is a stimulating perspective. In order to pursue
this objective on a sound basis, the correlations from which such links can be
deduced have to be carefully checked against model dependence. Using a variety
of nuclear effective models and a microscopic approach, we study the relation
between the predictions of a given model and those of a Taylor density
development of the corresponding equation of state: this establishes to what
extent a limited set of phenomenological constraints can determine the
core-crust transition properties. From a correlation analysis we show that a)
the transition density is mainly correlated with the symmetry energy
slope , b) the proton fraction with the symmetry energy and
symmetry energy slope defined at saturation density, or, even better,
with the same quantities defined at fm, and c) the transition
pressure with the symmetry energy slope and curvature
defined at fm
Galclaim: A tool to identify host galaxy of astrophysical transient sources
The Galclaim software is designed to identify association between
astrophysical transient sources and host galaxy by computing the probability of
chance alignment. It is distributed as an open source Python software. It is
already used to identify, confirm or reject host galaxy candidates of GRBs and
to validate or invalidate transient candidates in astrophysical observations.
Such tools are also very useful to characterise archived transient candidates
in large sky survey telescopes
Cavity induced vibration of flexible hydrofoils
The objective of this work is to investigate the influence of cavity-induced vibrations on the dynamic response and stability of a NACA66 hydrofoil at 8° angle of attack at Re=750 000 via combined experimental measurements and numerical simulations. The rectangular, cantilevered hydrofoil is assumed to be rigid in the chordwise direction, while the spanwise bending and twisting deformations are represented using a two-degrees-of-freedom structural model. The multiphase flow is modeled with an incompressible, unsteady Reynolds Averaged NavierâStokes solver with the kâÏ Shear Stress Transport (SST) turbulence closure model, while the phase evolutions are modeled with a mass-transport equation based cavitation model. The numerical predictions are compared with experimental measurements across a range of cavitation numbers for a rigid and a flexible hydrofoil with the same undeformed geometries. The results showed that foil flexibility can lead to: (1) focusing â locking â of the frequency content of the vibrations to the nearest sub-harmonics of the foilŚłs wetted natural frequencies, and (2) broadening of the frequency content of the vibrations in the unstable cavitation regime, where amplifications are observed in the sub-harmonics of the foil natural frequencies. Cavitation was also observed to cause frequency modulation, as the fluid density, and hence fluid induced (inertial, damping, and disturbing) forces fluctuated with unsteady cavitation.The authors gratefully acknowledge Ms. Kelly Cooper (program manager) and the Office of Naval Research (ONR), for their financial support through Grant nos. N00014-11-1-0833 and N0014-12-C-0585, as well as ONR Global and Dr. Woei-Min Lin (program manager) through grant no. N62909-12-1-7076
Giant Pulsar Glitches and the Inertia of Neutron-Star Crusts
Giant pulsar frequency glitches as detected in the emblematic Vela pulsar
have long been thought to be the manifestation of a neutron superfluid
permeating the inner crust of a neutron star. However, this superfluid has been
recently found to be entrained by the crust, and as a consequence it does not
carry enough angular momentum to explain giant glitches. The extent to which
pulsar-timing observations can be reconciled with the standard vortex-mediated
glitch theory is studied considering the current uncertainties on dense-matter
properties. To this end, the crustal moment of inertia of glitching pulsars is
calculated employing a series of different unified dense-matter equations of
state.Comment: 11 pages, 6 figures, submitted to PR
Nuclear symmetry energy and core-crust transition in neutron stars: a critical study
The slope of the nuclear symmetry energy at saturation density is pointed
out as a crucial quantity to determine the mass and width of neutron-star
crusts. This letter clarifies the relation between and the core-crust
transition. We confirm that the transition density is soundly correlated with
despite differences between models, and we propose a clear understanding of
this correlation based on a generalised liquid drop model. Using a large number
of nuclear models, we evaluate the dispersion affecting the correlation between
the transition pressure and . From a detailed analysis it is shown
that this correlation is weak due to a cancellation between different terms.
The correlation between the isovector coefficients and plays
a crucial role in this discussion
Isospin fractionation : equilibrium versus spinodal decomposition
This paper focuses on the isospin properties of the asymmetric nuclear-matter
liquid-gas phase transition analyzed in a mean-field approach, using Skyrme
effective interactions. We compare two different mechanisms of phase separation
for low-density matter: equilibrium and spinodal decomposition. The isospin
properties of the phases are deduced from the free-energy curvature, which
contains information both on the average isospin content and on the system
fluctuations. Some implications on experimentally accessible isospin
observables are presented
Cluster formation in asymmetric nuclear matter: semi-classical and quantal approaches
The nuclear-matter liquid-gas phase transition induces instabilities against
finite-size density fluctuations. This has implications for both
heavy-ion-collision and compact-star physics. In this paper, we study the
clusterization properties of nuclear matter in a scenario of spinodal
decomposition, comparing three different approaches: the quantal RPA, its
semi-classical limit (Vlasov method), and a hydrodynamical framework. The
predictions related to clusterization are qualitatively in good agreement
varying the approach and the nuclear interaction. Nevertheless, it is shown
that i) the quantum effects reduce the instability zone, and disfavor
short-wavelength fluctuations; ii) large differences appear bewteen the two
semi-classical approaches, which correspond respectively to a collisionless
(Vlasov) and local equilibrium description (hydrodynamics); iii) the
isospin-distillation effect is stronger in the local equilibrium framework; iv)
important variations between the predicted time-scales of cluster formation
appear near the borders of the instability region.Comment: 27 pages, 11 figures, Submitted to Nuclear Physics A, Nuclear Physics
A In press (2008
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