The Multiphase Characterization of Volcanic Phenomena: A Comparison of Numerical and Experimental Approaches

Volcanic phenomena are inherently multiphase with emergent heterogeneity resulting from the interaction of ash, lithics and a turbulent gas phase. Our objective is to present a modeling approach that can be applied to both active eruptions as well as a guide to the interpretation of volcanic deposits. In order to interpret deposits and understand the connection to their parent flows, the physics of particle-particle interaction and multiphase turbulence must be explicitly addressed. Additionally, comparison of experimental and numerical results with nature requires similarity of continuous (fluid-based) and lagrangian (particle-based) dimensionless groups. We will review current numerical and experimental approaches to volcanic phenomena and discuss their benefits and limitations in light of known scaling challenges of multiphase flow (Burgisser et al., 2005). For example, open questions regarding multiphase turbulence and particle-particle interaction will be discussed. We will exemplify one particular modeling approach that combines aspects of both eulerian and lagrangian descriptions for the dispersal of a range of different clast sizes. This hybrid modeling approach will be compared to experimental and field deposits from the Kos Plateau Tuff. Emphasis will be given to the example of particle laden gravity currents (i.e. pyroclastic flows) and flow transformations resulting from particle sorting and segregation. Particle-particle interaction and particle-boundary interaction, as well as particle segregation, can result in significantly different flow structure and runout distances compared to the assumption of a homogeneous fluid defined by a mixture density. In particular, the simulations emphasize that a saltating and frictional bedload is likely to develop in many large-scale pyroclastic flows and can result in much greater mass transport than the completely suspended load assumption would predict. [Burgisser, A., G. W. Bergantz & R. E. Breidenthal (2005). JVGR 141(3-4), 245-265.]