Journal Article

The Effect of Conduit Walls Roughness on Volcanic Jets and Their Seismo‐Acoustic Radiation: An Experimental Investigation

La Spina, Giuseppe, Scarlato, Piergiorgio, Peña Fernández, Juan José, Sesterhenn, Jörn, Spina, Laura, Morgavi, Daniele, Pennacchia, Francesco, Taddeucci, Jacopo

2023

Overview

To explore the effect of conduit roughness on volcanic jet dynamics and on the related seismo‐acoustic radiation we performed a series of shock‐tube experiments using pipes with variable inner surface fractal dimension D. Variable starting pressure produced subsonic to supersonic jets visualized using high‐speed shadowgraph and recorded with an array of accelerometers and microphones. At all starting pressures, increasing D increases the energy transfer from the gas to the conduit walls, decreasing the jet exit velocity (Mach number) and, for supersonic cases, the related shock‐cell spacing, and increasing the seismic to acoustic radiation amplitude ratio. The roughness‐induced changes in jet velocity and turbulence affect the dominant sources of the jet noise and modulates the spectral properties of the acoustic signals. From our study we show that conduit wall roughness is an important and yet largely neglected factor in the dynamics of explosive volcanic eruptions and their monitored geophysical signals. Plain Language Summary Volcanoes are amongst the most fascinating and mysterious subjects of science, for they allow no direct observation of what is happening within the conduit during eruptive activity. Indirect observations (such as measurements of the sound and vibration accompanying eruptions) are routinely performed for monitoring and research purposes. Laboratory studies mimicking the eruptive processes in small‐scale devices, are of great support for correctly interpreting such data. As such, we investigated the effect of the irregularity of conduit surface, amongst the most relevant and poorly known variables characterizing the eruptive processes, on volcanic jets and on their seismic and acoustic signals. We performed a series of laboratory experiments using conduits with different roughness of the internal surface and various starting pressures. Microphones and accelerometers, capable of measuring conduit sounds and vibration, respectively, in synch with high‐speed camera were used to constrain the characteristics of the generated subsonic and supersonic jets. Results show that conduit roughness controls: (a) the relative amplitude of seismic and acoustic signals; (b) the velocity, turbulence and properties of the sound of these jets. Our results will shed light on the link between observation at the surface and dynamic evolution of conduit geometry at depth. Key Points Elastic radiation of analogue experimental volcanic subsonic to supersonic jets was compared against high‐speed shadowgraph analysis We quantified variation in the spectral features, energy partitioning and jet structure due to differences in conduit roughness Different behavior in subsonic versus supersonic regime was observed due to distinct dominating noise sources at different roughness

• Publisher: John Wiley & Sons, Inc,Wiley
• Subject: Accelerometers / Acoustic properties / Acoustics / Amplitude / Amplitudes / Energy transfer / Eruptions / experimental volcanology / fractal conduits / Fractal geometry / high‐speed imaging / Jets / Laboratory experimentation / Laboratory experiments / Mach number / Microphones / Radiation / Roughness / seismo‐acoustic signals / Signal monitoring / Sound waves / supersonic jet flow / Turbulence / Velocity / Vibration / Vibration measurement / Vibration monitoring / Volcanic activity / Volcanic eruptions / volcanic jet