1. 5-10-5-3-20-1 is the material thrown out by a
volcano.
Answers
Answer:
Most of the current ash transport and dispersion models neglect particle-fluid (two-way) and particle-fluid plus particle-particle (four-way) reciprocal interactions during particle fallout from volcanic plumes. These interactions, a function of particle concentration in the plume, could play an important role, explaining, for example, discrepancies between observed and modelled ash deposits. Aiming at a more accurate prediction of volcanic ash dispersal and sedimentation, the settling of ash particles at particle volume fractions (ϕp) ranging 10−7-10−3 was performed in laboratory experiments and reproduced by numerical simulations that take into account first the two-way and then the four-way coupling. Results show that the velocity of particles settling together can exceed the velocity of particles settling individually by up to 4 times for ϕp ~ 10−3. Comparisons between experimental and simulation results reveal that, during the sedimentation process, the settling velocity is largely enhanced by particle-fluid interactions but partly hindered by particle-particle interactions with increasing ϕp. Combining the experimental and numerical results, we provide an empirical model allowing correction of the settling velocity of particles of any size, density, and shape, as a function of ϕp. These corrections will impact volcanic plume modelling results as well as remote sensing retrieval techniques for plume parameters.
Volcanic ash is injected in the atmosphere during explosive eruptions and, dispersed by wind and eventually deposited on the ground, may cause health, social, and economic disruption during and after an eruption1. Where, when, and how ash from an eruption may impact human activities is currently forecast by using numerical simulations of ash dispersal. Current simulations incorporate wind advection, atmospheric diffusion, particle aggregation, and simplified sedimentation models that consider the terminal velocity of particles as if settling individually2,3. To overcome this last simplification, here we describe joint experimental and numerical simulations investigating the effect of particle volume fraction on the settling velocity of volcanic ash particles in suspensions.
Complexities of the volcanic cloud-atmosphere system and large variations in the size, density, shape and concentration of erupted particles result in large variations in ash settling dynamics and corresponding settling velocity. The terminal settling velocity of a single volcanic ash particle is usually derived analytically and/or experimentally, and previous studies focussed primarily on the effect of particle size, shape, and density, and of atmospheric properties on this parameter4,5,6,7,8,9,10,11,12. This approach considers the settling of an individual particle in a still fluid, ignoring the presence of neighbouring particles. Similarly, the effect of particle volume fraction (ϕp) on the settling velocity of individual (i.e., not aggregated) particles is not included in most numerical models of ash dispersal13,14,15,16,17,18,19,20 which are mostly concerned with the fate of volcanic ash in the dilute, medium-distal regions of the plume. These models assume particles are only affected by the drag due to the local velocity of the carrier flow (one-way coupling), ignoring the effect of particle motion on the flow itself (two-way coupling) and inter-particle collisions (four-way coupling). This approach is justified as long as ϕp is less than 10–6, below which negligible alteration to the structure of turbulence by particles occurs21,22. With increasing ϕp in the fluid, however, two-way and four-way coupling effects are expected to play an increasingly important role on the settling velocity of particles23,24,25,26,27,28. Within a few hundreds of kilometres from the vent, ϕp in the plume can be as high as 10−5, and up to 10−3 in more proximal regions29,30,31. The observations and modelling of localized regions of instability, including particle-rich ‘fingers’ or ‘scalloped umbrella’32,33, suggest that the dynamics of particle sedimentation is strongly governed by several factors mostly related to particle concentration34.