Ballottement

Résumé

Le ballottement ou ballotement désigne les petits mouvements d'un liquide contenu dans un réservoir soumis à un mouvement accéléré et présentant une surface libre. Lorsque le réservoir est soumis à des vibrations extérieures, celles-ci donnent naissance à des vagues se propageant à la surface du liquide. Hors cas particuliers (parois spécialement construites pour absorber les ondes), les vagues se réfléchissent sur les parois et forment des ondes stationnaires, ce qui distingue le ballottement du mouvement général des vagues en milieu ouvert. Lorsqu'on considère les mouvements du réservoir dans son ensemble (sa structure et le liquide qu'il contient), le ballottement du liquide conduit donc à l'apparition de résonances, appelées modes de ballottement. Ces modes de déformation de la surface libre doivent être distingués des modes de cavité acoustique du liquide dans le réservoir en ce qu'il s'agit de modes à beaucoup plus basse fréquence que les modes acoustiques proprement dits. L'

Source officielle

https://fr.wikipedia.org/wiki/Ballottement

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Publications associées (12)

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Personnes associées (3)

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Cours associés (3)

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Séances de cours associées (4)

Séances de cours associées

Chargement

Two-phase flow phenomena in hydraulic turbines and pump-turbines operating in synchronous condenser mode

Elena Vagnoni

Hydraulic turbines and pump-turbines are often required to operate in particular conditions at which they consume active power. To keep the power consumption at a minimum, it is necessary to dewater the runner to reduce the mechanical torque. One of these operating conditions is the synchronous condenser mode for the reactive power regulation of the grid. This operating mode requires the guide vanes closure and the lowering of the tail water below the runner by injecting pressurized air in the machine. Due to the air-water mixing, torque instabilities and air losses are observed in this operating mode causing the increase of the power consumption. In this thesis, the two-phase flow phenomena characterizing the operation in synchronous condenser mode are experimentally investigated; additionally, a theoretical framework is proposed to analytically describe and predict the observed flow phenomena and the corresponding power and air losses. These research challenges are addressed by means of four different contributions. First, the development of the sloshing motion of the air-water free-surface below the runner is qualitatively and quantitatively described by using high-speed visualization, pressure fluctuations measurements and automated image processing methods. Second, the development of an air-water ring in the vaneless gap between the runner and the guide vanes is studied by means of image processing and pressure fluctuations measurements to assess both the velocity and the pressure flow field. The pressure measurements in the vaneless gap make apparent a fluctuation due to the interaction of the rotating and stationary components and of the rotating two-phase flow. This interaction is revealed by the development of the Fourier series applied to the harmonic components of the system and the modal decomposition of the pressure signal. Third, the influence of the two-phase flow phenomena on the torque swings is investigated. Measurements of the resisting mechanical torque transmitted through the coupling of the runner and the shaft and corresponding to the absorbed mechanical power of the runner in the hydraulic turbine are performed; in addition, a study on the influencing operating parameters allows building an empirical predictive model of the torque. Finally, a study on the oxygen mass transfer in the water volume is performed by measuring the oxygen concentration in the spiral case and in the draft tube of the hydraulic turbine. A theoretical framework is developed on the equations governing the diffusion of oxygen in water and the diffusion coefficients are computed and validated with experimental results. Moreover, this study allows quantifying the air losses due to the diffusion of air in water and to model the diffusion process as a function of the operating parameters of the machine. The results allow the understanding of the two-phase flow mechanism in a turbine or pump-turbine operating in synchronous condenser mode causing instabilities and air losses. Furthermore, the predictive model of the power losses due to both torque and air consumption represents a novel achievement for the study of the two-phase flow in a turbine operating in condenser mode or, more in general, in dewatered condition, and for the control of the machine behavior and consumption on the full scale prototype.

EPFL2018

Chargement

Be it to aerate a glass of wine before tasting, to accelerate a chemical reaction or to cultivate cells in suspension, the “swirling” (or orbital shaking) of a container ensures good mixing and gas exchange in a simple and intuitive way. Despite being used in such a large variety of applications, the flow generated in a container subject to orbital shaking is far from be- ing understood, and presents a richness of patterns and behaviours which has not yet been reported. While orbital shaken cell cultures are very efficient and productive at small scale, their increase in scale is hampered by several issues, some of which are thought to have their origin in the motion of the liquid medium. The present research remedies to this situation, charting the evolution of the wave behaviour with the operating parameters, highlighting the importance of the wave regimes and assessing their mixing efficiency. We present here a mathematical solution, based on the potential hypothesis and on techniques used in sloshing dynamics, predicting the shape of the free surface and the liquid motion. The validity and the limits of this model were assessed by comparison with a very large number of free surface measurement, obtained using a specifically developed automated acquisition system, and with non intrusive velocity measurements of several shaking configurations. A large variety of wave patterns (i.e. free surface shapes) were identified, ranging from single and multiple crested waves to breaking waves and waves having a shape constantly changing as they rotate. Our research revealed the importance of free surface natural modes and their sub-harmonics in the behaviour of the waves. From the results of the potential model and the measurements, we identified four dimensionless groups ensuring hydrodynamic similarity of the flow be- tween different scales. Moreover, we were able to identify the most efficient waves in terms of mixing, and to suggest optimal ranges of the operating parameters to enhance the mixing and oxygenation of the cell cultures.

EPFL2013

Chargement

Foam on troubled water: Capillary induced finite-time arrest of sloshing waves

Pierre-Thomas Paul Brun, Benjamin Jérémy Dollet, François Gallaire, Francesco Viola

Interfacial forces exceed gravitational forces on a scale small relative to the capillary length-two millimeters in the case of an air-water interface-and therefore dominate the physics of sub-millimetric systems. They are of paramount importance for various biological taxa and engineering processes where the motion of a liquid meniscus induces a viscous frictional force that exhibits a sublinear dependence in the meniscus velocity, i.e., a power law with an exponent smaller than one. Interested in the fundamental implications of this dependence, we use a liquid-foam sloshing system as a prototype to exacerbate the effect of sublinear friction on the macroscopic mechanics of multi-phase flows. In contrast to classical theory, we uncover the existence of a finite-time singularity in our system yielding the arrest of the fluid's oscillations. We propose a minimal theoretical framework to capture this effect, thereby amending the paradigmatic damped harmonic oscillator model. Our results suggest that, although often not considered at the macroscale, sublinear capillary forces govern the friction at liquid-solid and liquid-liquid interfaces. Published by AIP Publishing.

Amer Inst Physics2016

Chargement

Two-phase flow phenomena in hydraulic turbines and pump-turbines operating in synchronous condenser mode

Elena Vagnoni

EPFL2018

EPFL2013