299 research outputs found
On the decrease of intermittency in decaying rotating turbulence
The scaling of the longitudinal velocity structure functions, , is analyzed up to order in a
decaying rotating turbulence experiment from a large Particle Image Velocimetry
(PIV) dataset. The exponent of the second-order structure function, ,
increases throughout the self-similar decay regime, up to the Ekman time scale.
The normalized higher-order exponents, , are close to those
of the intermittent non-rotating case at small times, but show a marked
departure at larger times, on a time scale ( is the
rotation rate), although a strictly non-intermittent linear law is not reached.Comment: 5 pages, 5 figures. In revision for Phys. Fluids Letter
Experimental and numerical study of mean zonal flows generated by librations of a rotating spherical cavity
International audienceWe study both experimentally and numerically the steady zonal flow generated by longitudinal librations of a spherical rotating container. This study follows the recent weakly nonlinear analysis of Busse (2010), developed in the limit of small libration frequency - rotation rate ratio, and large libration frequency - spin-up time product. Using PIV measurements as well as results from axisymmetric numerical simulations, we confirm quantitatively the main features of Busse's analytical solution: the zonal flow takes the form of a retrograde solid body rotation in the fluid interior, which does not depend on the libration frequency nor on the Ekman number, and which varies as the square of the amplitude of excitation. We also report the presence of an unpredicted prograde flow at the equator near the outer wall
Campagne CORAIL 1 de chalutage exploratoire aux ßles Chesterfield et à Lansdowne ("N.O. ALIS" - 15 août au 4 septembre 1988)
Au cours de la campagne de chalutage exploratoire CORAIL 1 le N.O. ALIS a rĂ©alisĂ© 55 traits de chalut aux Ăźles Chesterfield, Bellona et sur le banc de Lansdowne. La proportion de fonds chalutables n'excĂšde pas 30 % des fonds de plus de 50 m et est nulle entre 0 et 50 m. Au total 123 espĂšces ont Ă©tĂ© collectĂ©es dont trois pourraient ĂȘtre nouvelles. Les affinitĂ©s des peuplements ichthyologiques rencontrĂ©s sont plus importantes avec la Nouvelle-CalĂ©donie qu'avec l'Australie. Les densitĂ©s (8 Ă 35 poissons/ha) et les biomasses (0.4 Ă 2.6 kg/ha) estimĂ©es sont trĂšs faibles, de 8 Ă 60 fois moindres que ce qui a Ă©tĂ© observĂ© en Nouvelle CalĂ©donie. Les frĂ©quences de longueur des principales espĂšces sont du mĂȘme ordre que celles observĂ©es en Nouvelle-CalĂ©donie Ă des Ă©poques similaires. La taille maximale des espĂšces commerciales est supĂ©rieure Ă celles notĂ©es en Nouvelle-CalĂ©donie. (RĂ©sumĂ© d'auteur
From granular collapses to shallow water waves: A predictive model for tsunami generation
In this article, we present a predictive model for the amplitude of impulse
waves generated by the collapse of a granular column into a water layer. The
model, which combines the spreading dynamics of the grains and the wave
hydrodynamics in shallow water, is successfully compared to a large dataset of
laboratory experiments, and captures the influence of the initial parameters
while giving an accurate prediction. Furthermore, the role played on the wave
generation by two key dimensionless numbers, i.e., the global Froude number and
the relative volume of the immersed deposit, is rationalized. These results
provide a simplified, yet comprehensive, physical description of the generation
of tsunami waves engendered by large-scale subaerial landslides, rockfalls, or
cliff collapses in a shallow water
Vortex rings generated by a translating disk from start to stop
In this article, we investigate experimentally and numerically the time
evolution of vortex rings generated by the translation of a rigid disk in a
fluid initially at rest and submitted to an acceleration followed by a
deceleration. The size of the disk and its motion in terms of stroke length and
travel time are varied as control parameters. The start-up vortex ring created
in the near wake of the disk is characterized experimentally by PIV, and the
measurements agree quantitatively with axisymmetric numerical simulations
performed with the Basilisk flow solver. The maximum radius and circulation of
the annular vortex and its dynamics are shown to follow different power laws
with the control parameters. The modeling adapted from Wedemeyer's
two-dimensional theoretical calculations [E. Wedemeyer, Ausbildung eines
Wirbelpaares an den Kanten einer Platte, Ingenieur-Archiv 30, (1961)] captures
the observed scaling laws. Besides, after the disk stops, a secondary
``stopping" vortex ring is generated, which is shown to affect the motion of
the main vortex ring
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