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 $\approx 0.08$ and $Re_\lambda \approx 100$, 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