Experimental analysis of Taylor bubble regimes using kymography: a tool for understanding bubble ascent dynamics in open-vent volcanic conduits

Abstract

Taylor bubbles, or gas slugs, are elongated gas pockets that drive discrete and cyclic Strombolian explosions. To understand the surface dynamics of such eruptions, it is essential to first characterize the subsurface flow behaviour within the shallow (< 1 km) volcanic plumbing system. This can be achieved experimentally by simulating Taylor bubble flow under conditions that are mathematically scalable to volcanic conduit settings. This paper presents a novel application of kymography-an existing visual analysis technique-for measuring isolated or continuous Taylor bubble flow experimentally in vertical cylindrical pipes. Kymographs condense thousands of frames of experimental footage into a single space-time image, enabling efficient analysis of flow dynamics. The method utilises open-source software (ImageJ), affordable experimental equipment, and straightforward calibration, making it both cost-effective and widely accessible. Here, we illustrate the value of incorporating kymography to simplify and enhance data retrieval from complex two-phase fluid problems which provide a rigorous first-order understanding of the flow processes governing surface eruption dynamics exhibited by open-vent basaltic volcanoes. We show that kymography serves as a valuable and effective visual analysis tool for the experimental measurement of gas volume fraction, gas and liquid slug velocities, bubble length and diameter, falling film thickness, bubble and coalescence event counts, and to indicate steady-state ascent. In a volcanic conduit, these parameters have important implications for flow stability, interaction dynamics, overpressure development, and the volume of gas released at burst, which ultimately aids our ability to understand and predict eruption style, periodicity, repose, and explosivity level.