Modelling the rapid near-surface expansion of gas slugs in low-viscosity magmas

Abstract

The ascent of large gas bubbles (slugs) in vertical cylindrical conduits and low-viscosity magmas is simulated using 1D mathematical and 3D computational fluid dynamic (CFD) models. Following laboratory evidence, the 1D model defines a constant rise velocity for the slug base and allows gas expansion to accelerate the slug nose through the overlying fluid during ascent. The evolution of rapidly expanding gas slugs observed in laboratory experiments is reproduced well and, at volcano scales, predicts at-surface overpressures of several atmospheres without requiring any initial overpressure at depth. The near-surface dynamics increase slug nose velocities through the overlying magma by a factor of c. 2.5 and the gas expansion results in pre-burst magma surface velocities of c. 35 m s−1. To examine pressure distributions and the forces exerted on a conduit, 3D CFD simulations were carried out. At volcano scales, the vertical single forces during final slug ascent to the surface are c. 106 N, two orders of magnitude smaller than those associated with very-long-period seismic events at Stromboli. This supports a previous interpretation of these events in which they are generated by gas slugs flowing through changes in conduit geometry, rather than being the direct result of slug eruption processes.

Recycled ejecta modulating Strombolian explosions

“…These changes occurred in the timescale of hundreds of seconds, which is comparable to the timescale of pre-explosion conduit pressurization recorded by ground motion (Genco and Ripepe 2010), and is also compatible with time scales of bubble rise and growth in a basaltic magma (Nishimura 2009). The accelerating trend of inflation of the debris that we observed in the seconds before an explosion also matched the surface motion of a liquid column hosting a rising, pressurized gas slug (James et al 2008(James et al , 2009. We conclude that, before an explosion, the vent-filling debris is pushed upward by the magma head which, in turn, is rising under the effect of the ascending and expanding gas slug.…”

“…Vergniolle et al 1996;Chouet et al 2003Chouet et al , 2008Marchetti and Ripepe 2005), experimental studies (e.g. James et al 2004James et al , 2006James et al , 2008Lane et al 2013), and field observations (e.g. Chouet et al 1974;Blackburn et al 1976;Ripepe et al 1993Ripepe et al , 2005Patrick et al 2007;Harris et al 2012;Taddeucci et al 2012a, b;Gaudin et al 2014;Bombrun et al 2015).…”

The existence and behaviour of large diameter Taylor bubbles

“…The majority of the literature circumvents explicitly calculating the surface's behaviour by means of methods such as the Volume of Fluid approach (e.g. [21,8]). More recently a number of papers have begun explicitly tracking the surface [10,17].…”

Global Weak Solutions for a Gas-Liquid Model with External Forces and General Pressure Law

“…In particular, for the choices (20) and (23) with 7 = 1, we see that (30) corresponds to solving a second order polynomial which has a unique physical relevant solution. More generally, for 7 > 1 the existence and uniqueness of solutions leading to a well-defined pressure p require finer investigations.…”

“…Strong gas slug expansion is possible near the surface, and transition between two-phase and single-jjhase regions typically will occur. This type of flow is highly relevant for gas-kick scenarios, which ultimately can lead to blowout [1], as well as for the study of volcanic eruption mechanisms [20]. (B) Second, we provide mathematical analysis of a simplified gas-liquid model similar to (2) but with two important extensions relevant for the simulation cases demonstrated in (A): (i) inclusion of a frictional force term and gravity term (compare (14) with (7)); and (ii) use of a general equation of state for the gas phase.…”