G. H. Bagheri, C. Bonadonna, I. Manzella, J. Phillips, P. Haas

Tephra is defined as all particles ejected during explosive eruptions irrespective to size, shape and composition. Tephra hazards include disruption to aviation, water contamination, collapse of buildings and health problems. Forecasting tephra dispersal during and after a volcanic eruption is a critical part of risk reduction and long-term hazard-assessment studies. A wide range of analytical and numerical models have been developed during the last 20 years to forecast tephra dispersion and yet there are some important issues that still need to be addressed. As an example, volcanic particles are typically highly irregular and porous (figure 1) and existing models either approximate shape of volcanic particles as spheres or need some morphological information of the particles which are complex to measure, e.g. particle surface area.

Figure 1. Volcanic particles are typically irregular and porous. (Image courtesy of T. Kircher and the Alaska Volcano Observatory)

Another critical issue that still needs to be investigated in detail is particle aggregation (figure 2). In fact, aggregation of volcanic particles is a fundamental process which typically occurs in ash-rich volcanic eruptions for particles with diameters less than about 100 μm and results into premature fallout of fine ashes.

Figure 2. Ash particles typically fall as aggregates of various nature, e.g. structureless pellet (left) and broken ash cluster (right) from the 2010 Eyjafjallajökull eruption (Iceland); (Image courtesy of Costanza Bonadonna).

In order to improve our understanding of tephra dispersal and sedimentation, it is necessary to obtain experimental results with real volcanic particles in a controlled laboratory environment. As a result, a 4-meter-high vertical wind tunnel (figure 3) has been built by the Volcanology group of the University of Geneva in collaboration with the Groupe de compétence en mécanique des fluidesetprocédésénergétiques (CMEFE).

Figure 3. The vertical wind tunnel.

The tunnel is designed to suspend particles with different shapes and sizes in its test section (divergence) in order to study the aero-dynamical behavior of volcanic particles and their collision and aggregation. Figure 4 shows components of the vertical wind tunnel.

Figure 4. Components of the wind tunnel

A high speed camera is used to record particle movement, collision and aggregation in the wind tunnel. A software is developed to analyze images recorded by the camera. This software is written in FORTRAN language which can be used for statistical analysis of particle behavior in the tunnel. Figure 5 shows an example of analysis performed by our particle tracker software to measure terminal velocity of one volcanic particle in the tunnel.

Figure 5. Analyzing terminal velocity with the particle tracker software. locations of the particle inside the divergence, colors show the speed of air (Vair) and VP is the speed of particle inside the divergence (left); frequency histogram of air speed at the particle locations (right).