Dispersal and air entrainment in unconfined dilute pyroclastic density currents
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Unconfined scaled laboratory experiments show that 3D structures control the behavior of dilute pyroclastic density currents (PDCs) during and after liftoff. Experiments comprise heated and ambient temperature 20 μm talc powder turbulently suspended in air to form density currents within an unobstructed 8.5 × 6 × 2.6-m chamber. Comparisons of Richardson, thermal Richardson, Froude, Stokes, and settling numbers and buoyant thermal to kinetic energy densities show good agreement between experimental currents and dilute PDCs. The experimental Reynolds numbers are lower than those of PDCs, but the experiments are fully turbulent; thus, the large-scale dynamics are similar between the two systems. High-frequency, simultaneous observation in three orthogonal planes shows that the currents behave very differently than previous 2D (i.e., confined) currents. Specifically, whereas ambient temperature currents show radial dispersal patterns, buoyancy reversal, and liftoff of heated currents focuses dispersal along narrow axes beneath the rising plumes. The aspect ratios, defined as the current length divided by a characteristic width, are typically 2.5–3.5 in heated currents and 1.5–2.5 in ambient temperature currents, reflecting differences in dispersal between the two types of currents. Mechanisms of air entrainment differ greatly between the two currents: entrainment occurs primarily behind the heads and through the upper margins of ambient temperature currents, but heated currents entrain air through their lateral margins. That lateral entrainment is much more efficient than the vertical entrainment, >0.5 compared to ∼0.1, where entrainment is defined as the ratio of cross-stream to streamwise velocity. These experiments suggest that generation of coignimbrite plumes should focus PDCs along narrow transport axes, resulting in elongate rather than radial deposits.
KeywordsPyroclastic density currents Experimental volcanology Entrainment Explosive volcanism
R. Dennen was instrumental in the construction of the experimental facility used in this research. T. Gooding provided technical insights regarding instrumentation of the facility. R. Dennen and G. Ramirez helped run many of the experiments presented in this paper. M. Manga provided helpful feedback on an early draft of this manuscript. Thorough and thoughtful comments by O. Roche and B. Brand improved this paper. This research was supported by funding from the Smithsonian Institution Grand Challenges program, the National Museum of Natural History Small Grants program, and the SI Competitive Grants Program for Science.
Oblique movies of ambient-temperature (20130625-1) and warm (20130716-4) currents. Currents have similar mass discharge (0.4 g/s) and durations (100 s), but different thermal energy to kinetic energy densities (0 and 2.2). Movies are sped up by a factor of 5. (AVI 80.6 MB)
Map projections of ambient-temperature (20130627-3) and warm (20130716-4) currents. Currents have similar mass discharge (∼0.1 g/s), durations >300 s, but different buoyant thermal to kinetic energy densities (0 and 2.5). Movies are sped up by a factor of 5. Streaking in the upper right corner of the movies is the result of imperfect background removal. (AVI 94.5 MB)
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