27 research outputs found
Impact of dissipation on the energy spectrum of experimental turbulence of gravity surface waves
We discuss the impact of dissipation on the development of the energy
spectrum in wave turbulence of gravity surface waves with emphasis on the
effect of surface contamination. We performed experiments in the Coriolis
facility which is a 13-m diameter wave tank. We took care of cleaning surface
contamination as well as possible considering that the surface of water exceeds
100~m. We observe that for the cleanest condition the frequency energy
spectrum shows a power law decay extending up to the gravity capillary
crossover (14 Hz) with a spectral exponent that is increasing with the forcing
strength and decaying with surface contamination. Although slightly higher than
reported previously in the literature, the exponent for the cleanest water
remains significantly below the prediction from the Weak Turbulence Theory. By
discussing length and time scales, we show that weak turbulence cannot be
expected at frequencies above 3 Hz. We observe with a stereoscopic
reconstruction technique that the increase with the forcing strength of energy
spectrum beyond 3~Hz is mostly due to the formation and strenghtening of bound
waves.Comment: accepted for publication in Physical Review Fluid
From internal waves to turbulence in a stably stratified fluid
We report on the statistical analysis of stratified turbulence forced by
large-scale waves. The setup mimics some features of the tidal forcing of
turbulence in the ocean interior at submesoscales. Our experiments are
performed in the large-scale Coriolis facility in Grenoble which is 13 m in
diameter and 1 m deep. Four wavemakers excite large scale waves of moderate
amplitude. In addition to weak internal wave turbulence at large scales, we
observe strongly nonlinear waves, the breaking of which triggers intermittently
strong turbulence at small scales. A transition to strongly nonlinear
turbulence is observed at smaller scales. Our measurements are reminiscent of
oceanic observations. Despite similarities with the empirical Garrett & Munk
spectrum that assumes weak wave turbulence, our observed energy spectra are
rather be attributed to strongly nonlinear internal waves.Comment: accepted for publication in Physical Review Letter
Generation of weakly nonlinear turbulence of internal gravity waves in the Coriolis facility
We investigate experimentally stratified turbulence forced by waves.
Stratified turbulence is present in oceans and it is expected to be dominated
by nonlinear interaction of internal gravity waves as described by the Garrett
& Munk spectrum. In order to reach turbulent regimes dominated by
stratification we use the Coriolis facility in Grenoble (France) which large
size enables us to reach regimes with both low Froude number and large Reynolds
number. Stratification is obtained by using vertically linearly varying salt
concentration and we force large scale waves in a m
domain. We perform time-resolved PIV to probe the space-time structure of the
velocity field. We observe a wide band spectrum which is made of waves.
Discrete modes are observed due to the square shape of the flow container as
well as a continuum part which appears consistent with an axisymmetric
superposition of random weakly nonlinear waves. Our observations support the
interpretation of turbulence of a strongly stratified fluid as wave turbulence
of internal waves although our spectrum is quite different from the Garrett &
Munk spectrum. Weak turbulence proceeds down to a small cutoff length scale
(the buoyancy wavelength) at which a transition to more strongly nonlinear
turbulence is expected.Comment: accepted for publication in Physical Review Fluid
The Dynamics of Bi-Directional Exchange Flows:Implications for Morphodynamic Change within Estuaries
The dynamics of bi-directional exchange flows::implication for morphodynamic change within estuaries and sea straits
Environmental and geophysical flows, including dense bottom gravity currents in the ocean and buoyancy-driven exchange flows in marginal seas,
are strongly controlled by topographic features.
These are known to exert significant influence on both internal mixing and secondary circulations generated by these flows.
In such cases, uni-directional or bi-directional exchange flows develop when horizontal density differences
and/or pressure gradients are present between adjacent water bodies connected by a submerged channel.
The flow dynamics of the dense lower layer depend primarily on the volumetric flux and channel cross-sectional shape,
while the stratified interfacial flow mixing characteristics, leading to fluid entrainment/detrainment,
are also dependent on the buoyancy flux and motion within the upper (lower density) water mass.
For submerged channels that are relatively wide compared to the internal Rossby radius of deformation,
Earth rotation effects introduce geostrophic adjustment of these internal fluid motions,
which can suppress turbulent mixing generated at the interface and result in the development of Ekman layers that induce secondary,
cross-channel circulations, even within straight channels.
Moreover, recent studies of dense, gravity currents generated in rotating and non-rotating systems,
respectively, indicated that the V-shaped channel topography had a strong influence on both flow distribution
and associated interfacial mixing characteristics along the channel.
However, such topographic controls on the interfacial mixing and secondary circulations generated by bi-directional exchange flows
are not yet fully understood and remain to be investigated thoroughly in the laboratory.
Also the effect of mobile bed for bi-directional exchange flows generated in deformable channels along with the physical interactions
between the lower dense water flow and the erodible bed sediments
will have a strong influence in (re-)shaping the overall channel bed topography (i.e. bed morphodynamics).
Consequently, the resulting temporal changes in cross-sectional channel bathymetry (i.e. through erosion and deposition processes)
would also be expected to have associated feedbacks on transverse asymmetries in the bi-directional exchange flow structure,
as well as on the internal flow stability
Influence of Coriolis Force Upon Bottom Boundary Layers in a LargeâScale Gravity Current Experiment: Implications for Evolution of Sinuous DeepâWater Channel Systems
Oceanic density currents in many deepâwater channels are strongly influenced by the Coriolis force. The dynamics of the bottom boundary layer in large geostrophic flows and low Rossby number turbidity currents are very important for determining the erosion and deposition of sediment in channelized contourite currents and many largeâscale turbidity currents. However, these bottom boundary layers are notoriously difficult to resolve with oceanic field measurements or in previous smallâscale rotating laboratory experiments. We present results from a large, 13âm diameter, rotating laboratory platform that is able to achieve both stratified and highly turbulent flows in regimes where the rotation is sufficiently rapid that the Coriolis force can potentially dominate. By resolving the dynamics of the turbulent bottom boundary in straight and sinuous channel sections, we find that the Coriolis force can overcome centrifugal force to switch the direction of nearâbed flows in channel bends. This occurs for positive Rossby numbers less than +0.8, defined as RoR = /Rf, where is the depth and timeâaveraged velocity, R is the radius of channel curvature, and f is the Coriolis parameter. Density and velocity fields decoupled in channel bends, with the densest fluid of the gravity current being deflected to the outer bend of the channel by the centrifugal force, while the location of velocity maximum shifted with the Coriolis force, leading to asymmetries between leftâ and rightâturning bends. These observations of Coriolis effects on gravity currents are synthesized into a model of how sedimentary structures might evolve in sinuous turbidity current channels at various latitudes
Laboratory experiments reveal intrinsic self-sustained oscillations in ocean relevant rotating fluid flows
Several ocean Western Boundary Currents (WBCs) encounter a lateral gap along their path. Examples are the Kuroshio Current penetrating into the South China Sea through the Luzon Strait and the Gulf of Mexico Loop Current leaping from the Yucatan peninsula to Florida as part of the Gulf Stream system. Here, we present results on WBC relevant flows, generated in the worldâs largest rotating platform, where the Earthâs sphericity necessary to support WBCs is realized by an equivalent topographic effect. The fluid is put in motion by a pump system, which produces a current that is stationary far from the gap. When the jet reaches the gap entrance, time-dependent patterns with complex spatial structures appear, with the jet leaking, leaping or looping through the gap. The occurrence of these intrinsic self-sustained periodic or aperiodic oscillations depending on current intensity is well known in nonlinear dynamical systems theory and occurs in many real systems. It has been observed here for the first time in real rotating fluid flows and is thought to be highly relevant to explain low-frequency variability in ocean WBCs
Plate-forme tournante instrumentée pour la dynamique des fluides de l'environnement
International audienc
Identifying four-wave-resonant interactions in a surface gravity wave turbulence experiment
International audienc
Experimental observations of internal wave turbulence transition in a stratified fluid
International audienc