HR: 0800h
AN: V31A-0288    [Abstracts]
TI: A tephra-dispersal model based on 3-D simulations of eruption clouds and experiments on particle settling in turbulent flow
AU: * Koyaguchi, T
EM: tak@eri.u-tokyo.ac.jp
AF: Earthquake Research Institute, University of Tokyo, 1-1-1, Yayoi, Bunkyo-ku, Tokyo, 113- 0032, Japan
AU: Ochiai, K
EM: VEB04532@nifty.com
AF: Earthquake Research Institute, University of Tokyo, 1-1-1, Yayoi, Bunkyo-ku, Tokyo, 113- 0032, Japan
AU: Suzuki, Y J
EM: yujiros@jamstec.go.jp
AF: Japan Agency for Marine-Earth Science and Technology, 3173-25 Showa-machi, Kanazawa-ku, Yokohama, 236-0001, Japan
AB: During an explosive volcanic eruption, hot volcanic gases and pyroclasts are ejected from the volcanic vent into the atmosphere, and the mixture of the ejected materials and the air buoyantly rises as an eruption column. After the eruption column reaches the neutral buoyancy level, it flows horizontally to form an umbrella cloud. Pyroclasts generated by the eruption fall out from the umbrella cloud to the ground surface. In previous tephra-dispersal models, it is assumed that (1) pyroclasts are homogeneously distributed in the umbrella cloud because of turbulence, and (2) they fall out at their terminal velocities from the bottom of the umbrella cloud where turbulence diminishes [e.g., Koyaguchi and Ohno, 2001]. The first assumption is appropriate only when turbulent intensity is sufficiently strong relative to the terminal velocities of particles. Here, we attempt to establish a generalized model in which the relationship between the turbulent intensity and the terminal velocity is taken into account on the basis of 3-dimensional (3-D) numerical simulations of eruption clouds and a series of laboratory experiments on particle settling in turbulent flow. The numerical model is designed to simulate the fluid dynamical features of the eruption cloud, such as column height and laterally spreading umbrella cloud as a function of vent conditions such as magma discharge rate. The model correctly reproduces the turbulent mixing as well as the density of the eruption cloud as a function of mixing ratio by applying 3-D coordinates, high order accuracy calculation schemes, and sufficiently fine grid sizes. From the 3-D simulations, we determined the turbulent intensity in the eruption clouds. Laboratory experiments of particle settling in turbulent flow are performed focusing on the effects of turbulent intensity on the process of particle settling. In the experiments spherical glass-bead particles are mixed in stirred water with variable turbulent intensity, and the spatial distribution and the temporal evolution of the particle concentration are measured. The experimental results suggest that, when the root-mean-square (rms) of velocity fluctuation in the fluid is much greater than the particle terminal velocity, the particles are homogeneously distributed in the fluid, while they settle at their terminal velocities from the bottom of the fluid. On the other hand, when the rms of velocity fluctuation is smaller than the particle terminal velocity, the particle concentration increases toward the bottom of the fluid during settling process, which substantially increases the rate of particle settling. The above results of numerical simulations and laboratory experiments imply that small pyroclasts (less than 1/8 mm in diameter) are distributed homogeneously throughout the umbrella cloud, whereas relatively large pyroclasts (more than a few mm in diameter) tend to concentrate around the bottom of the umbrella cloud. The generalized tephra-dispersal model in which the gradient of particle concentration is taken into consideration better explains the granulometric data of the deposits of Pinatubo 1991 eruption.
DE: 8404 Volcanoclastic deposits
DE: 8414 Eruption mechanisms and flow emplacement
DE: 8428 Explosive volcanism
DE: 8445 Experimental volcanism
DE: 8485 Remote sensing of volcanoes
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2007 Fall Meeting