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 is known to present hazards to infrastructure, agriculture, and human and animal health (Figure 1). In particular, 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. It also enters modern jet engines where it melts, coating interior engine parts thus causing damage and potential failure. The amount of time fine ash stays in the atmosphere depends on its terminal velocity. Existing models are based on relatively large, quasi-spherical particles characterized by Stokes velocities. Ash particles violate the various assumptions upon which Stokes flow and associated models are based. They are non-spherical and can have complex surface and internal structure. This suggests that particle shape may be one reason that models fail to accurately predict removal rates of fine particles from the volcanic ash clouds. The present research addresses problems to better parameterize predictive models for particle concentration in volcanic ash clouds as they travel far distances with the atmospheric currents and often circumnavigate the Globe several times. To better characterize the transportability of volcanic ash, we have conducted a study that includes a physical model for fine volcanic ash terminal velocities, diffusion, and boundary layer dispersion in atmospheric flows. It accounts for actual volcanic ash size distributions, complex ash particle geometry, and geometry variability. The fundamental hypothesis being tested is that particle shape irreducibly impacts the fate and transport properties of fine volcanic ash. Experiments are thus being conducted at the Flow Physics Facility (FPF) at UNH which is the largest low turbulence slow flow wind tunnel in the world designed for academic research. Still Air Drop Experiments (SADE) determine horizontal dispersion properties of fine ash as a function of size and shape. SADE is conducted in an unconstrained open space using a homogenized mix of source particles, with dispersion and sedimentation dynamics that are measured by specifically designed laser PIV (particle image velocimetry) and by Scanning Electron Microscopic (SEM) study of the ash particles in the area of deposition. In addition, numerical models have been developed for Wind Tunnel Dispersion Experiments (WTDE) that release ash into air of known velocity from a prescribed height to determine the dependence of transport on particle shape and size in a mixed population, as found under natural conditions.
