The Influence of Particle Concentration on the Formation of Settling-Driven Gravitational Instabilities at the Base of Volcanic Clouds
| Autor: | Fries, Allan Alexis, Lemus, Jonathan, Jarvis, Paul, Clarke, Amanda B., Phillips, Jeremy C., Manzella, Irene, Bonadonna, Costanza |
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| Jazyk: | angličtina |
| Rok vydání: | 2021 |
| Předmět: | |
| Zdroj: | Fries, A, Lemus, J, Jarvis, P A, Clarke, A B, Phillips, J C, Manzella, I & Bonadonna, C 2021, ' The Influence of Particle Concentration on the Formation of Settling-Driven Gravitational Instabilities at the Base of Volcanic Clouds ', Frontiers in Earth Science, vol. 9, 640090 . https://doi.org/10.3389/feart.2021.640090 Frontiers in Earth Sciences, Vol. 9, No 640090 (2021) |
| ISSN: | 2296-6463 |
| DOI: | 10.3389/feart.2021.640090 |
| Popis: | Settling-driven gravitational instabilities observed at the base of volcanic ash clouds havethe potential to play a substantial role in volcanic ash sedimentation. They originate from anarrow, gravitationally unstable region called a Particle Boundary Layer (PBL) that forms atthe lower cloud-atmosphere interface and generates downward-moving ash fingers thatenhance the ash sedimentation rate. We use scaled laboratory experiments in combinationwith particle imaging and Planar Laser Induced Fluorescence (PLIF) techniques toinvestigate the effect of particle concentration on PBL and finger formation. Resultsshow that, as particles settle across an initial density interface and are incorporatedwithin the dense underlying fluid, the PBL grows below the interface as a narrow region ofsmall excess density. This detaches upon reaching a critical thickness, that scales with(v^2/g′)^1/3, where v is the kinematic viscosity and g′ is the reduced gravity of the PBL,leading to the formation of fingers. During this process, the fluid above and below theinterface remains poorly mixed, with only small quantities of the upper fluid phase beinginjected through fingers. In addition, our measurements confirm previous findings over awider set of initial conditions that show that both the number of fingers and their velocityincrease with particle concentration. We also quantify how the vertical particle mass fluxbelow the particle suspension evolves with time and with the particle concentration. Finally,we identify a dimensionless number that depends on the measurable cloud mass-loadingand thickness, which can be used to assess the potential for settling-driven gravitationalinstabilities to form. Our results suggest that fingers from volcanic clouds characterised byhigh ash concentrations not only are more likely to develop, but they are also expected toform more quickly and propagate at higher velocities than fingers associated with ash-poorclouds. |
| Databáze: | OpenAIRE |
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