IAVCEI 2013 Scientific Assembly - July 20 - 24, Kagoshima, Japan
Forecasting Volcanic Activity - Reading and translating the messages of nature for society

4W_3E-P8

Date/Time: July 24 Poster

Effects of subgrid-scale turbulent models on spatial structure in large-eddy simulation ofan eruption column
Hitoshi Suto, Yasuo Hattori, Kiyoshi ToshidaCentral Research Institute of Electric Power Industry, JAPANE-mail: suto@criepi.denken.or.jp

In the transport of volcanic ash in an explosive eruption, the turbulence is one of the important elements which
determine the eruption styles such as an eruption column and a pyroclastic flow.

The conventional steady

one-dimensional models for an eruption column assume that the effect of turbulence is represented as a constantin a theoretical formula, although it is indicated that the assumption has measurable uncertainty (Suzuki et al. 2005,
Ishimune 2006). In recent years, unsteady three-dimensional eruption column models, which represent turbulence
more directly, have been developed. However, the turbulence modeling in the transports of gas and pyroclastic
materials are left unclarified. For instance, subgrid-scale (SGS) turbulence stresses are differently modeled in
Neri et al. (2007) and in Suzuki and Koyaguchi (2010), although the contribution of SGS models to simulations ofreal-scale eruption column and the consequences of different SGS models applied are not well understood. In thisresearch, the unsteady three-dimensional simulation code for an eruption column is developed. The effects of SGS
models are discussed through the comparisons between the simulation results using two kinds of the models.
In this code, the model concept of Neri et al. (2007) is adopted, which is advantageous in its applicability to various
volcanic styles and to the transport of pyroclastic materials of various sizes. The large-eddy simulation (LES)
technique based on a SGS model is adopted as expression of turbulence. The gas component and pyroclastic
materials of various diameters are classified into two or more phases (the multi-fluid approximation). The SGS
model is a part of the submodels in the stress term and the diffusion term in the basic equations. As the SGS
models, the Smagorinsky model, which is generally used, and the Yuu model, in which the effect of SGS drag isconsidered (Yuu et al. 2001), are adopted for the gas phase, and the equation proposed by Hinze, in which therelation between the relaxation time of particles and the temporal duration of vortex is considered (Hinze 1975), forthe particle phases.
The following results were obtained from the LES results for an eruption column and laboratory simple flows: (1)When fine grid systems that enable the capture of the major spectral bands in a turbulence field are used in thesimulation, the LES results show general agreement with those of the existing experiments and the theoreticalsolution in laboratory simple flows, and qualitative agreement of cloud shapes with those of the one-dimensionalmodel in an eruption column. (2) The SGS models for gas and particle phases cause variations in eruption cloudshape. Possible causes of the variations are the strong dependence of the whole diffusion process in the LES ofan eruption column on the SGS model used and the high sensitivity of the eruption cloud shape to the strength of
turbulent diffusion.

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