Influence des structures turbulentes sur la vitesse de chute de particules solides (application au transport sédimentaire)

Le transport sédimentaire apparaît comme un des processus fondamentaux de l'évolution des zones littorales sableuses et des estuaires. Cependant, la complexité des problèmes hydrodynamique qui en sont la source, tels que la turbulence des écoulements réels ou la dynamique des suspensions, encore mal connus du fait de leur diversité, rend son étude délicate. Nous nous proposons ici de considérer le phénomène de chute seul en le dissociant si possible du phénomène de transport par l'utilisation d'une veine d'essai verticale. Nous procéderons ensuite à une étude systématique de la chute de particules isolées, de tailles et de densités variées, dans un écoulement dont on connaît les structures turbulentes.Cadiergue S., Michaux-Leblond N., Belorgey Michel. Influence des structures turbulentes sur la vitesse de chute de particules solides (application au transport sédimentaire). In: L'eau, l'homme et la nature. 24èmes journées de l'hydraulique. Congrès de la Société Hydrotechnique de France. Paris, 18-19-20 septembre 1996. 1996

Experimental investigation of volcanic particle aggregation in the absence of a liquid phase.

Understanding the dispersal and deposition of fine-grained silicate particles from volcanic plumes is key to interpreting ash fall deposits and predicting hazards for future eruptions. It is known that many of these particles fall incorporated into delicate, dry aggregates whose sedimentation characteristics have not been previously investigated. Here we present the results of laboratory experiments on aggregates of small, dry silicate particles produced by the fragmentation of pumice collected from the 18 May 1980 Mount St. Helens fall deposit. The aggregation process is driven by electrostatic charges naturally imparted to the particles during the fracture process. For particle fall distances of 1 m, images of the in-flight aggregates show that they commonly have irregular shapes and are up to 800 mm in size. Strobe photography was used to determine aggregate fall velocities and, by representing aggregates as falling spheres, suggested that they had densities of c. 100–200 kg m 3. Comparable densities were obtained from experiments where equivalent fall velocities were assumed for aggregates and single particles which had been transported similar distances within a horizontal airflow. These dispersal experiments produced bimodal particle size distributions, similar to those observed in the 18 May 1980 Mount St. Helens deposits, and suggest that the aggregates were composed mainly of particles <70 mm in diameter. Our experimental results are in agreement with aggregate size and density estimates previously used within several theoretical plume sedimentation models in order to explain some features of natural ash deposits