Resurrecting Dead-water Phenomenon

We revisit experimental studies performed by Ekman on dead-water using modern techniques in order to present new insights on this peculiar phenomenon. We extend its description to more general situations such as a three-layer fluid or a linearly stratified fluid in presence of a pycnocline, showing the robustness of dead-water phenomenon. We observe large amplitude nonlinear internal waves which are coupled to the boat dynamics, and we emphasize that the modeling of the wave-induced drag requires more analysis, taking into account nonlinear effects

Dead Waters: Large amplitude interfacial waves generated by a boat in a stratified fluid

We present fluid dynamics videos of the motion of a boat on a two-layer or three-layer fluid. Under certain specific conditions, this setup generates large amplitude interfacial waves, while no surface waves are visible. The boat is slowed down leading to a peristaltic effect and sometimes even stopped: this is the so-called dead water phenomenon

Particles, Drops, and Bubbles Moving Across Sharp Interfaces and Stratified Layers

International audienceRigid or deformable bodies moving through continuously stratified layers or across sharp interfaces are involved in a wide variety of geophysical and engineering applications, with both miscible and immiscible fluids. In most cases, the body moves while pulling a column of fluid, in which density and possibly viscosity differ from those of the neighboring fluid. The presence of this column usually increases the fluid resistance to the relative body motion, frequently slowing down its settling or rise in a dramatic manner. This column also exhibits specific dynamics that depend on the nature of the fluids and on the various physical parameters of the system, especially the strength of the density/viscosity stratification and the relative magnitude of inertia and viscous effects. In the miscible case, as stratification increases, the wake becomes dominated by the presence of a downstream jet, which may undergo a specific instability. In immiscible fluids, the viscosity contrast combined with capillary effects may lead to strikingly different evolutions of the column , including pinch-off followed by the formation of a drop that remains attached to the body, or a massive fragmentation phenomenon. This review discusses the flow organization and its consequences on the body motion under a wide range of conditions, as well as potentialities and limitations of available models aimed at predicting the body and column dynamics

Dynamique de suspension dans un fluide linéairement stratifie

Nous étudions la dynamique à bas nombre de Reynolds d'une suspension de particules non-Browniennes initialement réparties de façon uniforme dans un fluide visqueux linéairement stratifié en sel. Une caméra permet de visualiser dans un plan vertical, grâce à deux sources lumineuses (laser et lumière blanche) synchronisées, l'évolution temporelle de la répartition spatiale et du champ de vitesses des particules. Nous quantifions les valeurs moyennes et fluctuantes de ces quantités pour différentes stratifications, et les comparons à des mesures similaires réalisées en fluide homogène

Internal tide generation from isolated seamounts and continental shelves

We model linear, inviscid non-hydrostatic internal tides generated by the interaction of a barotropic tide with variable topography in two dimensions. We first derive an asymptotic solution for the nonuniform barotropic flow over the topography that serves as forcing for the baroclinic equations. The resulting internal-tide generation problem is reformulated as a Coupled-Mode System (CMS) by means of a series decomposition of the baroclinic stream function in terms of vertical basis functions. We solve this CMS numerically and also provide a method for estimating the sea-surface signature of internal tides. We consider several seamounts and shelf profiles and perform calculations for a wide range of (topographic) heights and slopes. For subcritical topographies, the energy flux as a function of height exhibits local maxima, separated by cases of weakly- or even non-radiating topographies. For supercritical topographies, the energy flux generally increases with height and criticality. Our calculations agree with the Weak Topography Approximation only for very small heights. Perhaps more surprisingly, they agree with the Knife Edge model only for moderately supercritical topographies. We also compare the effect of the adjusted barotropic tide on the energy flux and the local properties of the baroclinic field with other semi-analytical methods based on a uniform barotropic tide. We observe significant differences in the flow field near the topographies only

New (Internal) Wave Generation - Laboratory Experiments

In this fluid dynamics video, we demonstrate the experimental generation of various internal wave fields using a novel wave generator. Specifically, uni-directional internal wave beams and vertical modes 1 and 2 are generated and visualized using Particle Image Velocimetry. Further details and analysis of these experiments can be found in [1].Comment: 2 pages, no figures, article linked to a fluid dynamics video submitted to the Gallery of Fluid Motion, APS DFD 200

Particles, Drops, and Bubbles Moving Across Sharp Interfaces and Stratified Layers

Rigid or deformable bodies moving through continuously stratified layers or across sharp interfaces are involved in a wide variety of geophysical and engineering applications, with both miscible and immiscible fluids. In most cases, the body moves while pulling a column of fluid, in which density and possibly viscosity differ from those of the neighboring fluid. The presence of this column usually increases the fluid resistance to the relative body motion, frequently slowing down its settling or rise in a dramatic manner. This column also exhibits specific dynamics that depend on the nature of the fluids and on the various physical parameters of the system, especially the strength of the density/viscosity stratification and the relative magnitude of inertia and viscous effects. In the miscible case, as stratification increases, the wake becomes dominated by the presence of a downstream jet, which may undergo a specific instability. In immiscible fluids, the viscosity contrast combined with capillary effects may lead to strikingly different evolutions of the column, including pinch-off followed by the formation of a drop that remains attached to the body, or a massive fragmentation phenomenon. This review discusses the flow organization and its consequences on the body motion under a wide range of conditions, as well as potentialities and limitations of available models aimed at predicting the body and column dynamics

Sedimentation of disks in a linearly stratified fluid

We consider the unbounded settling dynamics of a circular disk of diameter d and finite thickness h evolving with a vertical speed U in a linearly stratified fluid of kinematic viscosity v and diffusivity k of the stratifying agent, at moderate Reynolds numbers (Re=Ud/v ). The influence of the disk geometry (diameter d and aspect ratio x = d/h ) and of the stratified environment (buoyancy frequency N, viscosity and diffusivity) are experimentally and numerically investigated. Three regimes for the settling dynamics have been identified for a disk reaching its gravitational equilibrium level. The disk first falls broadside-on, experiencing an enhanced drag force that can be linked to the stratification. A second regime corresponds to a change of stability for the disk orientation, from broadside-on to edgewise settling. This occurs when the non-dimensional velocity U/√vN becomes smaller than some threshold value. Uncertainties in identifying the threshold value is discussed in terms of disk quality. It differs from the same problem in a homogeneous fluid which is associated with a fixed orientation (at its initial value) in the Stokes regime and a broadside-on settling orientation at low, but finite Reynolds numbers. Finally, the third regime corresponds to the disk returning to its broadside orientation after stopping at its neutrally buoyant level