Size of snow particles in a powder-snow avalanche

The size of the snow particles involved in a powder-snow avalanche is a key parameter of the local dynamic of the flow. An experimental device has been realized to collect snow particles within powder-snow avalanches. Snow particles have been captured in the powder-snow part of an avalanche triggered artificially on the experimental test site of the vallée de la Sionne. The collected particles have been photographed and the pictures digitized. An image analysis tool to evaluate the size of the collected particles have been developed for the purpose of this study. The obtained order of magnitude is 0.2mm

Wake behind contaminated bubbles in a solid-body rotating flow

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Ultimate regime in Rayleigh-Bénard convection: The role of plates

International audienceThe ultimate regime of convection, long ago predicted by Kraichnan ͓Phys. Fluids 5, 1374 ͑1962͔͒, could be called elusive because some apparently equivalent experiments showed it while others did not, with no apparent reasons for this discrepancy. In this paper, we propose a model which accounts for the finite heat conductivity and heat capacity of real active boundaries. Bad thermal characteristics of the plates can explain differences between various experiments, in agreement with recent numerical simulations

An experimental study of particle-driven gravity currents on steep slopes with entrainment of particles

International audienceResults of laboratory experiments are presented in which a finite suspension of sawdust particles was released instantaneously into a rectangular channel immersed in a water tank. Two kinds of gravity currents were studied: currents with or without entrainment of particles from the bed. Experiments were repeated for two slopes: 30° and 45°. We observed that the velocity of the front was significantly in-creased as particle entrainment occurred. In addition, our experiments showed that the front kept a quasi-constant velocity for both runs. This might suggest that the flow regime corresponded to the "slumping regime" or "adjustment phase" described earlier by Huppert and Simpson (1980)

Sediment-entraining suspension clouds: a model of powder-snow avalanches

International audienceA dense cloud model of avalanches is presented which includes large density difference effects as well as sediment entrainment along the path of the cloud. This model demonstrates the importance of sediment entrainment in the evolution of the front velocity. Without sediment entrainment the cloud first accelerates and then decelerates, a behaviour known from previous studies of cloud or thermal motions. With sediment entrainment the cloud is mostly in an accelerating state. The closure coefficients in the model concerning the cloud shape and air entrainment are obtained from laboratory experiments. These coefficients can be considered generic in the Boussinesq limit. A correction for inertial effects which need to be taken into account when applied to large density difference clouds such as avalanches, is proposed. An expression for the sediment entrainment coefficient is derived, taking into account the flow parameters and the sediment layer properties. The model predictions are in good agreement with recent measurement of the front velocity of a powder-snow avalanche. A presentation, in terms of dimensionless variables, of avalanche and laboratory Boussinesq cloud velocities shows clearly the similarities and differences between the two

Etude de la dynamique des avalanches de neige en aerosol

Powder snow avalanches are highly turbulent, dilute suspensions clouds moving down steep slopes due to their buoyancy. The motivation for the present research was, on the one hand, to improve our knowledge of the effect of snow entrainment, by the avalanche, from the snow layer and, on the other hand, to develop an improved theoretical model of avalanche dynamics. The theoretical model presented in this thesis includes large density difference effects and demonstrates the importance of snow entrainment on the avalanche velocity. The model is in good agreement with recent measurements of the front velocity of an avalanche. The parameters in the model, such as the shape and spatial growth rate of the clouds, were determined from laboratory experiments, conducted with saline and suspension clouds, in a flow regime close to that of avalanches. A theoretical extrapolation of these parameters (obtained in the Boussinesq limit) to large density difference clouds (avalanches) is proposed. Laboratory experiments with sediment entraining clouds indicate the trend sediment entrainment has on the avalanche velocity. A presentation in terms of dimensionless variables of avalanche velocities and that of laboratory clouds shows clearly the similarities and differences between the two.Les avalanches de neige en aérosol sont des bouffées de suspensions diluées de particules, très turbulentes qui descendent, de par leur poids, sur des pentes fortement inclinées. La motivation de la présente recherche était d'une part, d'améliorer notre connaissance de l'entraînement de neige du manteau neigeux par l'avalanche et d'autre part de développer un modèle théorique amélioré de la dynamique des avalanches. Le modèle théorique présenté dans cette thèse prend en compte les effets dus à la forte différence de densité entre l'avalanche et l'extérieur et démontre l'importance de l'entraînement de neige sur la vitesse de l'avalanche. Le modèle est en accord avec des mesures récentes de la vitesse de front d'une avalanche. Les paramètres du modèle (forme et croissance des bouffées) ont été déterminés à partir des expériences d'écoulements de laboratoire (d'eau salée ou de suspensions) dans un régime proche de celui des avalanches. Une extrapolation théorique de ces paramètres (obtenus en régime de Boussinesq) aux bouffées de forte densité (avalanches) est proposée. Les expériences de laboratoire avec des bouffées entraînant des particules indiquent l'influence que l'entraînement de particules a sur la vitesse de l'avalanche. Une représentation en variables adimensionnelles des vitesses des avalanches et des bouffées de laboratoire montre clairement les ressemblances et différences entre les deux types d'écoulements

A study of the size of snow particles in powder snow avalanches

International audienceIn this work, we study the size of the particles involved in a powder snow avalanche phenomenon. To determine these sizes, we study all the phenomena the particles have to face before arriving in the "body" of the avalanche. We study the boundary layer which is at the bottom of the avalanche. We determine, with the help of experimental data, the range of size of the particles that can be entrained by the avalanche. We then examine the possibility for these particles to reach the top of the boundary layer, and so to take part in the avalanche. Our final result is that the more frequent particles suspended in a powder snow avalanche have a size lower than 200 micrometers

Etude de la dynamique des avalanches de neige en aerosol

Powder snow avalanches are highly turbulent, dilute suspensions clouds moving down steep slopes due to their buoyancy. The motivation for the present research was, on the one hand, to improve our knowledge of the effect of snow entrainment, by the avalanche, from the snow layer and, on the other hand, to develop an improved theoretical model of avalanche dynamics. The theoretical model presented in this thesis includes large density difference effects and demonstrates the importance of snow entrainment on the avalanche velocity. The model is in good agreement with recent measurements of the front velocity of an avalanche. The parameters in the model, such as the shape and spatial growth rate of the clouds, were determined from laboratory experiments, conducted with saline and suspension clouds, in a flow regime close to that of avalanches. A theoretical extrapolation of these parameters (obtained in the Boussinesq limit) to large density difference clouds (avalanches) is proposed. Laboratory experiments with sediment entraining clouds indicate the trend sediment entrainment has on the avalanche velocity. A presentation in terms of dimensionless variables of avalanche velocities and that of laboratory clouds shows clearly the similarities and differences between the two.Les avalanches de neige en aérosol sont des bouffées de suspensions diluées de particules, très turbulentes qui descendent, de par leur poids, sur des pentes fortement inclinées. La motivation de la présente recherche était d'une part, d'améliorer notre connaissance de l'entraînement de neige du manteau neigeux par l'avalanche et d'autre part de développer un modèle théorique amélioré de la dynamique des avalanches. Le modèle théorique présenté dans cette thèse prend en compte les effets dus à la forte différence de densité entre l'avalanche et l'extérieur et démontre l'importance de l'entraînement de neige sur la vitesse de l'avalanche. Le modèle est en accord avec des mesures récentes de la vitesse de front d'une avalanche. Les paramètres du modèle (forme et croissance des bouffées) ont été déterminés à partir des expériences d'écoulements de laboratoire (d'eau salée ou de suspensions) dans un régime proche de celui des avalanches. Une extrapolation théorique de ces paramètres (obtenus en régime de Boussinesq) aux bouffées de forte densité (avalanches) est proposée. Les expériences de laboratoire avec des bouffées entraînant des particules indiquent l'influence que l'entraînement de particules a sur la vitesse de l'avalanche. Une représentation en variables adimensionnelles des vitesses des avalanches et des bouffées de laboratoire montre clairement les ressemblances et différences entre les deux types d'écoulements