101 research outputs found
Excitation and resonant enhancement of axisymmetric internal wave modes
To date, axisymmetric internal wave fields, which have relevance to
atmospheric internal wave fields generated by storm cells and oceanic
near-inertial wave fields generated by surface storms, have been experimentally
realized using an oscillating sphere or torus as the source. Here, we use a
novel wave generator configuration capable of exciting axisymmetric internal
wave fields of arbitrary radial form to generate axisymmetric internal wave
modes. After establishing the theoretical background for axisymmetric mode
propagation, taking into account lateral and vertical confinement, and also
accounting for the effects of weak viscosity, we experimentally generate and
study modes of different order. We characterize the efficiency of the wave
generator through careful measurement of the wave amplitude based upon group
velocity arguments. This established, we investigate the ability of vertical
confinement to induce resonance, identifying a series of experimental resonant
peaks that agree well with theoretical predictions. In the vicinity of
resonance, the wave fields undergo a transition to non-linear behaviour that is
initiated on the central axis of the domain and proceeds to erode the wave
field throughout the domain.Comment: 15 pages, 9 figure
Experimental study of parametric subharmonic instability for internal waves
Internal waves are believed to be of primary importance as they affect ocean
mixing and energy transport. Several processes can lead to the breaking of
internal waves and they usually involve non linear interactions between waves.
In this work, we study experimentally the parametric subharmonic instability
(PSI), which provides an efficient mechanism to transfer energy from large to
smaller scales. It corresponds to the destabilization of a primary plane wave
and the spontaneous emission of two secondary waves, of lower frequencies and
different wave vectors. Using a time-frequency analysis, we observe the time
evolution of the secondary waves, thus measuring the growth rate of the
instability. In addition, a Hilbert transform method allows the measurement of
the different wave vectors. We compare these measurements with theoretical
predictions, and study the dependence of the instability with primary wave
frequency and amplitude, revealing a possible effect of the confinement due to
the finite size of the beam, on the selection of the unstable mode
Experimental observation of a strong mean flow induced by internal gravity waves
We report the experimental observation of a robust horizontal mean flow
induced by internal gravity waves. A wave beam is forced at the lateral
boundary of a tank filled with a linearly stratified fluid initially at rest.
After a transient regime, a strong jet appears in the wave beam, with
horizontal recirculations outside the wave beam. We present a simple physical
mechanism predicting the growth rate of the mean flow and its initial spatial
structure. We find good agreement with experimental results
Axisymmetric internal wave transmission and resonance in non-linear stratifications
To date, the influence of non-linear stratifications and two layer
stratifications on internal wave propagation has been studied for
two-dimensional wave fields in a cartesian geometry. Here, we use a novel wave
generator configuration to investigate transmission in non-linear
stratifications of axisymmetric internal wave. Two configurations are studied,
both theoretically and experimentally. In the case of a free incident wave, a
transmission maximum is found in the vicinity of evanescent frequencies. In the
case of a confined incident wave, resonant effects lead to enhanced
transmission rates from an upper layer to layer below. We consider the
oceanographic relevance of these results by applying them to an example oceanic
stratification, finding that there can be real-world implications.Comment: 21 pages, 15 figure
Première expérience sur la plateforme tournante PERPET : convection pénétrative en rotation
Notre laboratoire s'est doté récemment d'une plate-forme tournante de 2 m de diamètre (PERPET, Plate-forme d'Études en Rotation des Phénomènes et Écoulements de la Terre), pouvant tourner jusqu'à 1 tour/s tout en portant une charge utile de 750 kg. Elle est équipée d'un contacteur tournant permettant à la fois la transmission de la puissance électrique nécessaire aux expériences embarquées et la transmission des signaux mesurés. Cette plate-forme a été conçue en concertation avec plusieurs équipes sur la région lyonnaise, afin qu'elle puisse servir non seulement aux desseins des équipes de notre Laboratoire mais aussi à d'autres projets. En collaboration avec Thierry Alboussière et Stéphane Labrosse, du Laboratoire de Géologie de Lyon, une première campagne d'expériences a déjà été effectuée sur la plate-forme, depuis son installation en janvier 2013. Elle concerne une étude de l'influence de la rotation sur le phénomène de convection pénétrante, qui existe probablement dans le noyau liquide terrestre et que l'on peut reproduire en refroidissant de l'eau par le bas en-dessous de 4°C. En effet l'eau présente un maximum de densité à cette température et par conséquent, la région entre 0 et 4°C sera convective, alors que la région audessus de 4°C sera stratifiée de manière stable. Dans un tel dispositif, on peut donc étudier la pénétration de la région convective dans la région stratifiée. Les premiers résultats de cette expérience montrent une forte influence de la rotation sur le phénomène de convection : l'organisation habituelle en grands rouleaux est remplacée, lors du cas avec rotation, par des colonnes montantes et descendantes à plus petite échelle. Une étude plus précise de l'évolution des températures, que nous avons mesurées dans la cuve sur une ligne verticale, et du front convectif, permettra d'obtenir des informations plus quantitative sur le rôle de la rotation dans ce phénomène
Resonant Triad Instability in Stratified Fluids
Internal gravity waves contribute to fluid mixing and energy transport, not
only in oceans but also in the atmosphere and in astrophysical bodies. We
provide here the first experimental measurement of the growth rate of a
resonant triad instability (also called parametric subharmonic instability)
transferring energy to smaller scales where it is dissipated. We make careful
and quantitative comparisons with theoretical predictions for propagating
vertical modes in laboratory experiments
An iterative study of time independent induction effects in magnetohydrodynamics
International audienceWe introduce a new numerical approach to study magnetic induction in flows of an electrically conducting fluid submitted to an external applied field B-0. In our procedure the induction equation is solved iteratively in successive orders of the magnetic Reynolds number Rm. All electrical quantities such as potential, currents, and fields are computed explicitly with real boundary conditions. We validate our approach on the well known case of the expulsion of magnetic field lines from large scale eddies. We then apply our technique to the study of the induction mechanisms in the von Karman flows generated in the gap between coaxial rotating disks. We demonstrate how the omega and alpha effects develop in this flow, and how they could cooperate to generate a dynamo in this homogeneous geometry. We also discuss induction effects that specifically result from boundary conditions
Mixing in stratified gravity currents: Prandtl mixing length
Shear-induced vertical mixing in a stratified flow is a key ingredient of
thermohaline circulation. We experimentally determine the vertical flux of
momentum and density of a forced gravity current using high-resolution velocity
and density measurements. A constant eddy viscosity model provides a poor
description of the physics of mixing, but a Prandtl mixing length model
relating momentum and density fluxes to mean velocity and density gradients
works well. For and , the mixing
lengths are fairly constant, about the same magnitude, comparable to the
turbulent shear length.Comment: 4 pages, 4 figures, accepted in PRL, February 200
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