A novel internal waves generator

We present a new kind of generator of internal waves which has been designed for three purposes. First, the oscillating boundary conditions force the fluid particles to travel in the preferred direction of the wave ray, hence reducing the mixing due to forcing. Secondly, only one ray tube is produced so that all of the energy is in the beam of interest. Thirdly, temporal and spatial frequency studies emphasize the high quality for temporal and spatial monochromaticity of the emitted beam. The greatest strength of this technique is therefore the ability to produce a large monochromatic and unidirectional beam

Small-scale instabilities of an island wake flow in a rotating shallow-water layer

International audienceUnlike the standard two-dimensional Kármán street, the oceanic vortex streets which may occur behind isolated islands are affected by the earth's rotation and the vertical stratification of the thermocline. These effects induce a selective destabilisation of anticyclonic vorticity regions. Several experimental studies were devoted to the inertial instability, which induces transient and three-dimensional perturbations in a rotating fluid layer. However, these previous experiments correspond to a large or finite vertical h to horizontal L aspect ratio (α=h/L) while in an oceanic context this ratio is much smaller than unity (αsimilar, equals0.01). This vertical confinement induces a cutoff vertical scale for unstable perturbations. But, since dissipation preferentially damps smaller scales, the shallow-water aspect ratio α may become so small that no growth will occur. We present here the first experimental investigation of three-dimensional destabilizations of an island wake flow in a shallow-water configuration. These laboratory experiments where performed on the LEGI Coriolis Platform, with small aspect ratio (α=0.1) and large Reynolds numbers (Re=5000–35,000). We have shown that unstable three-dimensional perturbations occur when the island Rossby number View the MathML source is large enough (Ro>0.8) while the Reynolds number seems to control the duration of this transient instability. Qualitative dye visualisation reveals various types of passive tracer dispersion in the wake. Moreover, according to PIV measurements we have shown that, unlike experiments having large or finite aspect ratio (α≥1), the small-scale perturbations do not significantly reduce the local vorticity inside the unstable anticyclone. Hence, the shallow-water configuration (αmuch less-than1) seems to reduce the intensity and the impact of three-dimensional instabilities in the vortex street. Finally, for high Froude numbers, when the flow becomes supercritical and owing to the generation of large amplitude waves in the wake, the vortex street intensity is strongly reduced

Quantitative laboratory observations of internal wave reflection on ascending slopes

International audienceInternal waves propagate obliquely through a stratified fluid with an angle that is fixed with respect to gravity. Upon reflection on a sloping bed, striking phenomena are expected to occur close to the slope. We present here laboratory observations at moderately large Reynolds number. A particle image velocimetry (PIV) technique is used to provide time resolved velocity fields in large volumes. The generation of the second and third harmonic frequencies are clearly demonstrated in the impact zone. The mechanism for nonlinear wavelength selection is also discussed. Evanescent waves with frequency larger than the Brunt-Väisälä frequency are detected and experimental results agree very well with theoretical predictions. The amplitude of the different harmonics after reflection are also obtained

Inertial instability of von Karman street in a rotating shallow-water layer

The rotation alters the stability of 2D anticyclonic flow with respect to 3D perturbations. Experiments have shown that such instability induces a transient destabilization of anticyclonic vortices in von Kármán street, when w/f 5 000). We have shown that unstable 3D-perturbations occur for large enough Rossby number (Ro > 0.8) while the Reynolds number seems to control the duration of this transient instability. According to PIV measurements we have shown that, unlike the deepwater configuration, the small-scale perturbations do not reduce the local vorticity inside the unstable antiyclone. Finally, for high Rossby numbers, when the flow becomes supercritical (Fd > 1), due to the generation of high amplitude wave wake the vortex street intensity is strongly reduced

Dynamics of Convectively Driven Banded Jets in the Laboratory

The banded organization of clouds and zonal winds in the atmospheres of the outer planets has long fascinated observers. Several recent studies in the theory and idealized modeling of geostrophic turbulence have suggested possible explanations for the emergence of such organized patterns, typically involving highly anisotropic exchanges of kinetic energy and vorticity within the dissipationless inertial ranges of turbulent flows dominated (at least at large scales) by ensembles of propagating Rossby waves. The results from an attempt to reproduce such conditions in the laboratory are presented here. Achievement of a distinct inertial range turns out to require an experiment on the largest feasible scale. Deep, rotating convection on small horizontal scales was induced by gently and continuously spraying dense, salty water onto the free surface of the 13-m-diameter cylindrical tank on the Coriolis platform in Grenoble, France. A “planetary vorticity gradient” or “β effect” was obtained by use of a conically sloping bottom and the whole tank rotated at angular speeds up to 0.15 rad s−1. Over a period of several hours, a highly barotropic, zonally banded large-scale flow pattern was seen to emerge with up to 5–6 narrow, alternating, zonally aligned jets across the tank, indicating the development of an anisotropic field of geostrophic turbulence. Using particle image velocimetry (PIV) techniques, zonal jets are shown to have arisen from nonlinear interactions between barotropic eddies on a scale comparable to either a Rhines or “frictional” wavelength, which scales roughly as (β/Urms)−1/2. This resulted in an anisotropic kinetic energy spectrum with a significantly steeper slope with wavenumber k for the zonal flow than for the nonzonal eddies, which largely follows the classical Kolmogorov k−5/3 inertial range. Potential vorticity fields show evidence of Rossby wave breaking and the presence of a “hyperstaircase” with radius, indicating instantaneous flows that are supercritical with respect to the Rayleigh–Kuo instability criterion and in a state of “barotropic adjustment.” The implications of these results are discussed in light of zonal jets observed in planetary atmospheres and, most recently, in the terrestrial oceans.Engineering and Applied Science

Dynamics of convectively driven banded jets in the laboratory

The banded organization of clouds and zonal winds in the atmospheres of the outer planets has long fascinated observers. Several recent studies in the theory and idealized modeling of geostrophic turbulence have suggested possible explanations for the emergence of such organized patterns, typically involving highly anisotropic exchanges of kinetic energy and vorticity within the dissipationless inertial ranges of turbulent flows dominated (at least at large scales) by ensembles of propagating Rossby waves. The results from an attempt to reproduce such conditions in the laboratory are presented here. Achievement of a distinct inertial range turns out to require an experiment on the largest feasible scale. Deep, rotating convection on small horizontal scales was induced by gently and continuously spraying dense, salty water onto the free surface of the 13-m-diameter cylindrical tank on the Coriolis platform in Grenoble, France. A “planetary vorticity gradient” or “β effect” was obtained by use of a conically sloping bottom and the whole tank rotated at angular speeds up to 0.15 rad s−1. Over a period of several hours, a highly barotropic, zonally banded large-scale flow pattern was seen to emerge with up to 5–6 narrow, alternating, zonally aligned jets across the tank, indicating the development of an anisotropic field of geostrophic turbulence. Using particle image velocimetry (PIV) techniques, zonal jets are shown to have arisen from nonlinear interactions between barotropic eddies on a scale comparable to either a Rhines or “frictional” wavelength, which scales roughly as (β/Urms)−1/2. This resulted in an anisotropic kinetic energy spectrum with a significantly steeper slope with wavenumber k for the zonal flow than for the nonzonal eddies, which largely follows the classical Kolmogorov k−5/3 inertial range. Potential vorticity fields show evidence of Rossby wave breaking and the presence of a “hyperstaircase” with radius, indicating instantaneous flows that are supercritical with respect to the Rayleigh–Kuo instability criterion and in a state of “barotropic adjustment.” The implications of these results are discussed in light of zonal jets observed in planetary atmospheres and, most recently, in the terrestrial oceans

Laboratory modelling of atmospheric dynamical processes

International audienceWe discuss how atmospheric or oceanic flow processes can be reproduced at laboratory scale. The similarity conditions for the effects of density stratification and Earth rotation are first presented. It is argued that a large size is required to reproduce multi-scale turbulence with minimal viscous effects. Examples of results obtained on the large 'Coriolis' platform in Grenoble, 13 m in diameter, are described. The development of the baroclinic instability, the main source of perturbations in the mid-latitude atmosphere, is first presented. The second example is the driving of alternating azimuthal jets by small scale convective eddies, relevant for the zonal bands in the Giant planet atmosphere. Finally examples of topographic wakes in a stratified fluid and gravity currents are presented. It is argued that physical modelling must be used in close relationship with theory and numerical modelling, using advanced measurement and data assimilation techniques

Velocimétrie par images de particules 3 Dimensions-3 Composantes

International audienceLes techniques de mesures tridimensionnelles des vitesses par imagerie de particules sont en plein essor grâce notamment aux nouvelles caméras (fréquence, résolution et sensibilité) et aux sources lumineuses (puissance, fréquence). Après une présentation de la méthode de balayage de volume Scanning PIV, (SPIV) nous décrirons les algorithmes de traitement et de calibration. Nous finirons par une application sur des écoulements à grandes échelles (m3