I study explosive volcanic
eruptions, specifically the mixing of ash clouds with the surrounding air (entrainment). When an ash cloud entrains enough air, it
allows the plume to rise; if it is unable to ingest enough air, the ash cloud
will collapse. The stability of an ash
cloud is very important for assessing volcanic hazards. If a fully formed ash cloud develops, hazards
include roof collapse from falling ash and engine failure for planes if they
encounter ash cloud. If an ash cloud
collapses, it produces an ash flow like that which entombed Pompeii in 79 AD.
The traditional mixing hypothesis
is that air enters the ash cloud horizontally and in direct proportion to the
vertical velocity (Fig. 1). As the vertical speed
of the ash cloud decreases, mixing decreases.
Atmospheric scientists have been able to show that the entrainment model
for cumulus clouds is more complicated, thus prompting volcanologists to
believe that the entrainment model for volcanoes is not entirely correct.
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Figure 1. Schematic of volcanic eruption column (not to
scale) illustrating regions of the plume divided by motion type. Green arrows
indicate entrainment of ambient air into the column, proportional to vertical
velocity (blue arrows). If enough air is entrained in the gas-thrust region,
the plume will rise buoyantly as sketched here. If not enough air is ingested,
the relatively dense eruption material will collapse and flow down the sides of
the volcano as a dangerous pyroclastic density current. I am working to
establish a more accurate entrainment model than the historic one illustrated
by the arrows here, focusing on eddies (rotational flow) and mixing scales in
the various regions of the plume.
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I plan on conducting experiments
on analogue eruption columns produced by injecting colored salt water into a
large tank of water. Through observing
and measuring the controlled, scaled eruption to determine how the two fluids
mix, I hope to be able to map the different speeds and directions the cloud
travels as it mixes with the water.
I will analyze video taken of the
analogue eruptions, using a modified version of FlowJ, software that tracks the
change in position of the pixels in the video to determine their speeds. I will
also use a technique called particle image velocimetry, in which a laser sheet
in the tank illuminates the motion of small glass beads in the eruption column. Post-processing of the images showing the
illuminated particles allow us to calculate the velocities and generate a 2D
vector field of the flow in the column. Using infrasound, an acoustic technique
similar to sonar, I will record sub-audible sounds produced by the cloud as it
mixes turbulently with the ambient fluid.
The frequencies of noise emitted will be mapped back to mixing eddy
structures and velocities in the column.
From the data collected from all three methods, I hope to create a
mathematical function describing entrainment throughout the eruption plume.
It is important to collect
velocity data using the various methods to create a fully formed velocity
field. By combining the FlowJ output and
particle image velocimetery data, I can map both the surficial and interior
velocities. Infrasound is a relatively new,
innovative method for volcanic monitoring that I plan to further develop. I
hope to develop this method of volcanic ash cloud monitoring because 1) it is fairly
inexpensive 2) it allows scientists to monitor volcanoes from a safe distance
instantaneously during an eruption, and 3) it does not require daylight or
limited fly-bys to gather useable data.
To validate this method, I will compare the results of the infrasound
study to the video and particle image velocimetry studies to identify links
between eddy mixing scales and infrasonic frequencies.
The results of this study can be
used in eruption simulations to track ash dispersion and deposition. This information can be used by volcanic
monitoring agencies for hazard assessment and response.
