CSDMS 2016 annual meeting poster AmeliaWinner
Investigating bed erosion from pyroclastic density currents
Kenneth Ferrier, Georgia Tech Atlanta Georgia, United States. ferrier@gatech.edu
Josef Dufek, Georgia Tech Atlanta Georgia, United States. dufek@gatech.edu
Abstract:
Pyroclastic density currents (PDCs) are ground-hugging mixtures of hot gas and rock that can reach temperatures > 800 and speeds of 200 m/s. These flows are capable of eroding and entraining the underlying bed material into the flow, which can strongly influence flow momentum, runout distance, and hazards associated with PDCs. However, the mechanism of erosion remains poorly constrained, with proposed mechanisms including under-pressure following the head of the fluidized current, force chain enhanced stresses at the bed, and discrete particle impacts and friction. The interactions between PDCs and the bed have been difficult to observe in the field, as their infrequent occurrence, opacity, and hostile environment make real-time measurement difficult. This study is aimed at obtaining a better understanding of the interactions between PDCs and the bed through lab experiments. Our experimental apparatus consists of a rotating cylindrical flume of radius 22 cm, within which gas-rich granular material flows along the interior of the cylinder as it rotates. The grain size and speed of the drum in this gas-particle mixture can be varied to examine variable degrees of fluidization of the mixture. By using a rotating cylinder, we are able to simulate long-duration flows, allowing us to observe impact and sliding forces at the bed as well as bed erosion rates over timescales comparable to the flow duration of natural PDCs. To measure the distribution and evolution of forces imparted by the flow on the bed, we constructed a cylindrical insert with a non-erodible bed in which we embedded force sensor arrays parallel and perpendicular to the direction of flow. To measure the erosion of the bed by the flow, we constructed a second cylindrical insert with a concrete erodible bed that can be removed from the flume and weighed before and after experiments. To measure the forces felt by the particles in the flow, we added “smart particles” 25 to 50 mm in diameter to the flow. Each smart particle contains a three-axis accelerometer and a micro SD card enclosed in a spherical plastic casing, and possesses a density similar to that of the pumice in the experimental flow. Each smart particle also contains a three-axis magnetometer which permits its location to be tracked by means of a unique applied magnetic field. Ultimately, data from these experiments will provide a robust basis for the sensitivity of bed erosion from bed and flow parameters such as bed roughness, particle size, degree of fluidization, and the distribution of forces imparted by the flow on the bed.
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