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Bulletin of Volcanology

, 81:5 | Cite as

Recognizing unsteadiness in the transport systems of dilute pyroclastic density currents

  • Benjamin J. AndrewsEmail author
Research Article
  • 122 Downloads

Abstract

Laboratory density currents generated with unsteady source conditions provide insight into the distances and timescales over which unsteadiness at the vent should persist in natural dilute pyroclastic density currents (PDCs). The laboratory experiments comprise heated 20-μm talc particles turbulently suspended in air and introduced as density currents into an 8.5 × 6 × 2.6 m air-filled chamber. The densimetric and thermal Richardson, Froude, Stokes, and settling numbers are all similar to those of natural dilute PDCs. Although the experiments’ Reynolds numbers are several orders of magnitude less than natural PDCs, the experiments are fully turbulent and are thus dynamically similar to some dilute natural PDCs. “Unsteadiness” in the experiments is generated by adding two pauses of duration t to the eruption (e.g., a 100 s experiment comprising three ~ 33 s pulses separated by two pauses of t = 10 s). Propagation distance of the leading head is proportional to the square root of time; positions of the trailing pulses have similar time dependence, but they generally travel slightly faster than, and thus catch and merge with the leading current. Trailing pulses are more easily distinguished from the body of the preceding current when t is large. Analysis of turbulent structures through space and time shows that unsteadiness at the eruption source can be distinguished from turbulent fluctuations in the currents when t is greater than the integral turbulent timescale of the current body τbody, a statistical measure of the characteristic timescale of unsteadiness within a turbulent flow. The distances over which unsteadiness in the transport system persist scale with t/τbody and the ratio of the thermal Richardson numbers of the trailing and leading pulses. As t/τbody increases from ~ 2 to ~ 4, trailing pulses remain distinct over an increasing fraction of the lead runout distance. When subsequent pulses have higher RiT than the leading current, they coalesce with the leading pulse at more proximal distances, but when RiT is much lower, they remain distinct over distances similar to that of the leading head even when t is similar to τbody. For currents that have depositional timescales comparable to or shorter than τbody, deposits should be expected to preserve a record of unsteadiness over the distances that unsteady pulses persist.

Keywords

Pyroclastic density current Pyroclastic flow Pyroclastic surge Turbulence Experimental volcanology 

Notes

Acknowledgements

The author wishes to acknowledge T. Gooding for assistance with many aspects of the experimental volcanology laboratory. K. Befus provided very helpful discussions regarding the interpretation of the experimental results. Thoughtful and helpful comments by O. Roche, R.J. Brown, and an anonymous reviewer improved and clarified this manuscript.

Funding information

The Smithsonian National Museum of Natural History supported this research through its Small Grants Program. This research was also supported by a grant from the National Science Foundation (EAR-1447480) to the author.

Supplementary material

445_2018_1266_MOESM1_ESM.pdf (244 kb)
ESM 1 Schematic of the experimental apparatus located at the Smithsonian Museum Support Center in Suitland, Maryland, USA. Experimental currents are generated by loading (heated) 20-μm talc powder onto a conveyor belt that feeds the powder down a chute and into the “tank.” The inside of the tank measures 8.5 × 6 × 2.6 m. Experiments are illuminated with red, green, and blue laser sheets. Temperature was monitored by an array of 0.001″ K-type thermocouples mounted at heights of 5 and 30 cm. (PDF 243 kb)
445_2018_1266_MOESM2_ESM.avi (26.1 mb)
ESM 2 Movies of experimental density currents showing the streamwise vertical plane of the currents along the centerline of the tank. The color has been adjusted to the Matlab “hot” color scheme. 20150428-07, steady current; 20150428-03, 3.1 s pauses; 20150428-05, 6.2 s pauses. All movies have been sped up by a factor of 3 (i.e., 10 s in the movie corresponds to 30 s of observation). (AVI 26735 kb)
445_2018_1266_MOESM3_ESM.avi (20.3 mb)
ESM 3 (AVI 20802 kb)
445_2018_1266_MOESM4_ESM.avi (25.3 mb)
ESM 4 (AVI 25939 kb)

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Copyright information

© This is a U.S. government work and not under copyright protection in the U.S.; foreign copyright protection may apply 2019

Authors and Affiliations

  1. 1.Global Volcanism ProgramNational Museum of Natural History, Smithsonian InstitutionWashingtonUSA

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