The concentrations of fluid and solid partices are the governing variables when attempting to determine the flow range and potential hazard. It is assumed that these flows are stratified so that the basal layer constitutes a granular flow regime, and the density-driven buoyant material is composed from granular to fluidized particle transitions. By using these flow regimes as a basis for particle related interactions in PDCs, we can begin to model and undestand the underlying physics taking place; primarily the particle-particle dominated (lower) part of the PDCs using applications of the granular flow theory (Campbell, 1990; Iverson and Vallance, 2001; Dartevelle,2004). Detailed field observations, sedimentological studies of PDC-related deposits, controlled labarotory experiements, and physical models, the framework for understanding the processes controlling the dynamic behavior of PDCs can be built upon the following elements.
-Fluid dynamics: The propagation and emplacement of PDCs is a fluid process that can be described by the laws of continuum mechanics. PDCs can be described as high-tempreature multiphase flows of gases and suspended solid particles.
-Buoyancy: PDCs are driven by their (negative) density contrast with the surrounding atmosphere.
- Sedimentation/deposition: In typical natural regimes, sedimentation (i.e., particle decoupling and settling) leads to stratification of the current and deposition of particles. This results in multiphase layers which carry different physical interactions.
- Flow regime: In the basal concentrated layer, deposition of particles can be controlled by granular phenomena. Transition from granular to fluidized to collisional and kinetic regimes is one of the key aspects of PDC dynamics.
- Turbulence: In the upper layer, fluid turbulence controls the entrainment and heating of atmospheric air. Gas-particle heat exchange is one of the controlling processes.
- Buoyancy reversal: Entrainment of air and deposition of particles lower the average density of the current, which eventually reverse its buoyancy and lifts off, stopping its horizontal propagation.
- Topography: Interaction with topography can control the dynamics of PDC in different ways: hydraulic effects (associated to changes in slope, current height and width), stratified flow effects (blocking and modification of the vertical flow profile) and flow diversion and decoupling (through overbank/avulsion and surge detachment).
Understanding of the physical processes controlling the behavior of PDCs allows us to formulate a mathmatical relationship between specific variables through the laws of fluid mechanics. For granular based flows, explaining particle interactions at this magnitude can still be hard to resolve. Pyroclastic density currents, consisting of multiphase interactions, specifically the transition from granular to fluidized properties is the main chellenge of study.