AGGREGATION OF POLYDISPERSE VOLCANIC ASH PARTICLES UNDER TURBULENT AND MOIST CONDITIONS

Mir Md Mushfique Mahmood

doi:10.7273/000007416

Volcanic eruptions may result in ash fallout with severe negative impacts on health, infrastructure, environment, and aviation. Volcanic ash transport and deposition (VATD) poses the most significant hazard in the case of eruption events. To mitigate these hazards and model ash transport and fallout accurately, comprehensive understanding the physical aggregation process of ash particles within volcanic plumes is of paramount interest. Even though there is a wealth of evidence in field observations supporting the aggregation of fine ash particles in water-rich volcanic plumes, most commonly employed ash transport codes neglect this agglomeration process. Ash particles finer than 125 μm largely settle out of the atmosphere as aggregates. As these aggregates are larger than single particles, they fall out of the atmosphere faster. Overlooking this premature fallout lead to systematic error in the prediction of ash dispersal. This study experimentally investigates two crucial factors that affect ash particle aggregation: 1. Liquid film coating on the particles in moist environments, as is expected in volcanic plumes with high water content; 2. The presence of turbulence which is ubiquitous inside volcanic plumes. The foremost objective of the work is to analyze and characterize the roles of these conditions in particle aggregation. A novel turbulence tower is used to simulate environments found inside volcanic plumes. This octagonal chamber employs carefully arrayed synthetic jet actuators to produce a nearly isotropic, homogeneous column of turbulence along the central axis of the tower. Tests are conducted under maximum turbulence, which has a Reynolds number, Reλ ≈ 240, with a corresponding Taylor microscale of 4.9 mm, at peak jet power (29 W), and under no turbulence conditions. Pre-chilled particles are allowed to fall through the tower under both turbulence intensities for a range of relative humidity levels, directly related to the particle liquid film thickness. Monodisperse solid glass spheres of̴ 40 μm diameter are used as ash particle analogs. Highly polydisperse volcanic ash collected from the 1980 eruption of Mount St. Helens are also tested extensively. For solid glass particles, the RH levels tested are 55%, 63%, 68%, 78%, 85%, and 95%. Two sets of tests are done at 55% and 63% and the data extracted are averaged. Volcanic ash particles are tested at 58%, 60%, 67%, 70%, 75%, 78%, 82%, 87%, 90%, and 93% RH levels. Aggregate samples are collected from these different conditions and microscopically imaged. These images are analyzed with a MATLAB image processing pipeline to extract information about the aggregation response of particles as functions of RH and turbulence levels. The results obtained demonstrate that monodisperse particle aggregation behavior adhere somewhat strictly to an upward trend for rising turbulence and humidity conditions. With the exception of a few unexpected surges and drops, increasing moisture content and turbulence lead to increasingly larger aggregate areas and perimeters and decreasing circularities. The probability distribution of aggregate sizes is also plotted. In significant contrast, it is observed that the aggregation behavior of polydisperse volcanic ash particles is complex and erratic, exhibiting stochastic sticking response. Only but a very weak upward trend can be discerned as relative humidity levels rise. The effect of turbulence on aggregation is even less obvious and appears seemingly random. Some suggestions for physically explaining these extreme fluctuations are ventured. Some of the limitations of this work are addressed in the conclusion section. It provides a reasonable framework for further studies attempting to quantify the effects of moisture and turbulence on volcanic ash particle aggregation behavior. The data obtained from this investigation and from similar experiments in the future may be implemented to refine existing ash transport codes to account for particle aggregation as a function of particle liquid coating thickness and turbulence.

AGGREGATION OF POLYDISPERSE VOLCANIC ASH PARTICLES UNDER TURBULENT AND MOIST CONDITIONS

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