Flottabilité

Résumé

vignette|Forces impliquées dans la flottabilité : l'objet flotte parce que la portance (dirigée vers le haut) équilibre le poids (dirigé vers le bas). Dans un fluide (gaz ou liquide), les corps sont soumis à la poussée d'Archimède. Les corps ont une flottabilité différente selon leur masse volumique et donc leur densité. La flottabilité est la poussée verticale, dirigée de bas en haut, qu'un fluide (gaz ou liquide) exerce sur un volume immergé. La flottabilité agit toujours dans la direction opposée à la gravité. Selon la différence entre la poussée d'Archimède P et le poids réel P, on distingue les corps de flottabilité :

positive (P > P) : l'objet remonte ;
nulle (P = P) : l'objet flotte entre deux eaux ;
négative (P < P) : l'objet coule.

Expression triviale Si un objet est plus léger que l'eau qu'il déplace, il flotte, s'il est plus lourd il coule. Un objet creux ou concave (par exemple un bateau en acier) peut déplacer un volume d'eau de masse supérieure à la

Source officielle

https://fr.wikipedia.org/wiki/Flottabilit%C3%A9

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La masse volumique d'une substance, aussi appelée volumique de masse, est une grandeur physique qui caractérise la masse de cette substance par unité de volume. C'est l'inverse du volume massique.

Une force modélise, en physique classique, une action mécanique exercée sur un objet ou une partie d'un objet par un autre objet ou partie d'objet. L'ensemble des forces appliquées à un objet a pour

L'eau est une substance chimique constituée de molécules . Ce composé, très stable, mais aussi très réactif, est un excellent solvant à l'état liquide. Dans de nombreux contextes, le terme eau est e

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On the Dynamics of Magnetic Swimmers in Stokes Flow

Pauline Marie Rüegg-Reymond

In the past decade, the engineering community has conceived, manufactured and tested micro-swimmers, i.e. microscopic devices which can be steered in their intended environment. Foreseen applications range from microsurgery and targeted drug delivery to environmental decontamination. This thesis presents a mathematical analysis of the dynamics of rigid magnetic swimmers in a Stokes flow driven by an external uniform magnetic field that rotates steadily about its axis of rotation. The swimmer is assumed to be made of a permanent magnetic material and to be placed in a fluid that fills an infinite enveloping space. A specific swimmer is prescribed by its magnetic moment and mobility matrix. For a given swimmer, its dynamics depend on two parameters that can be changed during an experiment: the Mason number, related to the magnitude and angular speed of the magnetic field, and the conical angle between the magnetic field and its axis of rotation. As these two parameter vary, strikingly different regimes of responses occur. The swimmer's trajectory is entirely governed by its rotational dynamics: once its orientation dynamics are known, its position trajectory can be recovered. For neutrally buoyant swimmers, this work provides a complete classification of the steady states of the rotational dynamics, along with a study of non-steady solutions in the asymptotic limits of small and large Mason number, and small conical angle. Predicted out-of-equilibrium solutions are in good agreement with numerical simulations. Swimmer's trajectories corresponding to steady states and periodic solutions of the rotational dynamics are then recovered. Finally, the effect of buoyancy is taken into account, and the relative equilibria of swimmers with a different density than that of the fluid are determined when the axis of rotation of the magnetic field is aligned with gravity.

EPFL2019

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Hyperpycnal (negatively buoyant) river inflow into lakes and oceans often develops three-dimensional (3D) plunging flow patterns when laterally unconfined. To determine the 3D flow pattern characteristics, laboratory experiments of laterally unconfined plunging on a sloping bed were carried out using salinity to control the density difference. The experiments were complemented by numerical modeling based on a high-resolution computational fluid dynamics model. As is the case for confined plunging plumes, it was found that in unconfined plunging, the inflow densimetric Froude number at the river mouth and the bed slope of the receiving water body are the dominant control parameters. However, the results documented that the hydrodynamics of laterally unconfined plunging are fundamentally different: The hyperpycnal plume in unconfined configurations forms a triangle on the surface in the plunge zone due to its convergence near the surface and lateral spreading near the bottom. The triangular pattern extends further into the receiving water when the inflow densimetric Froude number increases or the bottom slope decreases. The unconfined entrainment coefficient, which quantifies the amount of ambient water entrained into the plunging plume, also increases with increasing inflow densimetric Froude number. In general, entrainment is much higher in unconfined than in confined plunging. The plunging densimetric Froude number takes a constant value of ∼0.5 in confined plunging, whereas it increases with increasing inflow densimetric Froude number and can be ≫ 1 in unconfined plunging. Complex patterns of secondary currents occur in the plunging plume. A low-velocity zone whose size increases with the inflow densimetric Froude number is observed near the centerline above the bed.

2022

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Bretherton's buoyant bubble

Wassim Dhaouadi, John Martin Kolinski

When a buoyant bubble is inserted into a closed capillary that is slightly smaller than the capillary length, it appears stuck; exactly why this is so is a puzzle that has remained unanswered over the past 50 years. Recent calculations suggest that the bubble's motion is critically dependent on the hydrodynamics of the surrounding liquid film; however, quantitative measurements of these dynamics are lacking. We provide direct measurements of the dynamics of the liquid film surrounding a “stuck” bubble, recorded using interference microscopy. The film slowly relaxes to a constant thickness, and is stabilized by disjoining pressure at long times. The film's stability at this thickness is demonstrated by recovery after applied thermal perturbations; thus, we confirm that Bretherton's buoyant bubble is not pinned at a contact line, but is instead ostensibly stuck by extraordinarily slow flow in the surrounding liquid film whose thickness is set by a balance of capillary stress and disjoining pressure.

2019

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