Vortex rings and acoustic signatures illuminate vent diameter and jet velocity of Strombolian explosions
Abstract
Volcanic jets are frequent among a variety of explosive activity styles, including the Strombolian one, and their dynamics can be used to infer important eruption properties. In 2018 we documented jet-forming explosions at Stromboli volcano (Italy) by using: 1) a high-speed camera zooming on the vent and filming at 500 frames per second in the visible light; 2) a camera recording a broader field of view at 200 frames per second in the thermal infrared; 3) a microphone recording at a rate of 20 kHz in the audible to infrasonic range; 4) multiple acoustic arrays recording at 200 Hz as well as 10-10kHz. Our focus was on the aeroacoustic signatures of the explosions and the formation of vortex rings. These are toroidal vortices generated at the noozle by a jet (in this case the gas-pyroclast mixture) entering into a still fluid (in this case the Earth atmosphere).
All explosions were characterized by the formation of one or more vortex ring at the beginning of distinct jet phases. The rise and expansion rates of the vortex rings are in agreement with literature experiments and theory. The emergence of vortex rings was accompanied by a typical acoustic wave with similar emergence rate and an amplitude linearly proportional with the rise velocity of the vortex rings. Relating the growth rate of the observed vortex rings with an empirical prediction based on numerical and experimental results allowed an estimate of the vent diameter. The acoustic signal of the explosions also includes an audible component related to jet noise. Part of this component, mostly around 1 kHz in frequency, is related to the broadband shock noise generated by the interaction between generated by the interaction between the shock waves and the vortices of the shear layer in supersonic jets. Scaling the peak frequency of the broadband shock noise by the vent diameter and the sound velocity in air provides the characteristic Helmholtz number of the jet at a given time, which is related to the shock cell length and then to the Mach number inside the jet and the driving pressure ratio (pressure in the conduit before explosion divided by the atmospheric pressure). In the investigated jets the pressure ratio is about 3, and Mach number ranges 1.1 - 1.7, which, depending on the sound velocity in the eruptive mixture, provide jet velocities up to about 1200 m/s.- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2019
- Bibcode:
- 2019AGUFM.V44B..04T
- Keywords:
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- 8414 Eruption mechanisms and flow emplacement;
- VOLCANOLOGY;
- 8419 Volcano monitoring;
- VOLCANOLOGY;
- 8485 Remote sensing of volcanoes;
- VOLCANOLOGY;
- 8494 Instruments and techniques;
- VOLCANOLOGY