This pilot study project addresses the general problem of reducing volcanic ash impact by determining the viability of using novel instrumentation to determine the transport properties of fine volcanic ash in the atmosphere. Volcanic ash is known to present hazards to aviation, infrastructure, agriculture, and human and animal health. With the emergence of aviation in the last 50 years as a key component of global travel and transport, the importance of understanding how long ash is suspended in the atmosphere, and how far it is transported has taken on greater importance. Airborne ash abrades the exteriors of aircraft, enters modern jet engines and melts while coating the interior parts thus causing damage and failure. For example, the 2010 Eyjafjallajökull eruption in Iceland was the most disruptive event in aviation history, with billions of dollars of losses to the aviation industry and global economy. Much of this was unnecessary and better knowledge of the transport of fine ash could minimize such losses in the future. However, present understanding of ash transportation can only account for general air movements, but cannot fully address how much or how long ash remains in the atmosphere, and how much falls out as it travels downwind. To address this lacking, this project focuses on the interaction between ash and atmospheric air by performing experiments of ash flow in a special scientific wind tunnel designed to simulate slow atmospheric currents.
The time fine ash stays in the atmosphere depends on its terminal velocity (under the influence of gravity), but current formulations for this are based on raindrops that are relatively large and quasi-spherical, rendering them inapplicable to fine ash, which is smaller (<60 μm), non-spherical, and can have complex surface and internal structure. As a result, it is not presently possible to accurately predict the removal rates of fine particles from the volcanic ash clouds that pose aviation and other hazards. To provide observational data to resolve this problem, the novel facilities at UNH and Lehigh University are being used in this pilot study to design experiments for measuring terminal velocities of fine ash with a range of sizes and shapes. The new Flow Physics Facility (FPF) at UNH is the largest low turbulence slow flow wind tunnel in the world designed for academic research. Now, for the first time, it is being used to analyze the aerodynamic properties of fine ash particles in both laminar and turbulent conditions. The Center for Optical Technologies at Lehigh includes state of the art SEMs (stereo and mono) that provide the means for characterizing the shapes and sizes of fine ash to be used in the wind tunnel (FPF). The results of this pilot study will set the stage for subsequent empirical formulations for terminal velocities of the two types of ash particles (simple and compound) that have recently emerged from a previous NSF-supported study of volcanic ash morphology. This will lead to an understanding of the fundamental physics that controls the aerodynamics of volcanic ash in the atmosphere (altitude range from 150 to 1000 mb).
Volcanic ash erupted into the atmosphere poses hazards to aviation, agriculture, and human health. This was recently demonstrated in the Icelandic eruption of 2010 that disrupted air traffic all over Europe. It turned out that much of the grounding of planes was unessessary. As such, we need to better understand how volcanic ash is transported so we can avoid hazards while not shutting down the global economy. Computer models of ash transport in the atmophere, driven by large-scale wind patterns, have always assumed that ash particles are like little spheres, and should fall out of the sky at a speed determined by "Stokes Law", a general relation for the terminal velocity of a solid sphere. However, volcanic ash particles are neither solid nor spherical, and this project demonstrated how different shapes fall at a range of speeds. In a unique wind tunnel at UNH, large popullations of tiny particles were dropped in still air to see if shape matters. We discovered that spheres fall as Stoke's law would expect. Flat flakes, however, fall with a broad distribution of velocities. Some of them even rise up in air flow caused by aerodynamic drag of dense clusters of falling particles. Real volcanic ash particles have a range between spheres and flakes. Further, the non-spherical shape of of ash causes it to diffuse sideways, and thus spread would spread out across an atmospheric wind current more than would be expected for spheres. This would spread volcanic ash hazards farther across the stream and less downstream. However, the shape and rough surface of the ash particles also enables them all to stay aloft longer than spheres would do, thus extending the range of volcanic hazards in all directions.
