Wind Entrainment in Jets with Reversing Buoyancy: Implications for Volcanic Plumes

Abstract

Explosive volcanic eruptions commonly undergo a transition from stable plume to collapsing fountain with associated destructive pyroclastic density currents. A major goal in physical volcanology is to predict quantitatively the limit between the flow regimes as a function of the source and environmental conditions. Atmospheric winds influence the dynamics and stability of the column causing bending and enhancing turbulent air entrainment. However, the predictions made with 1-D models of volcanic plumes accounting for the presence of wind strongly depend on the wind entrainment coefficient , a parameter whose value varies in the literature. Here we present a new theoretical model to identify an analytical criterion for column collapse in windy conditions. We then present new laboratory experiments on turbulent jets with reversing buoyancy rising in a crossflow in order to better constrain. Our results show that a single value of = 0.5 can be used to describe the behavior of laboratory jets with arbitrary buoyancy. The results allow us to parameterize our 1-D model of volcanic plumes PPM and to show the crucial importance of wind gradient and profile on volcanic column dynamics through the use of the 1991 Mt. Hudson eruption as a case study. Finally, we propose a new transition diagram between the stable plume and collapsing fountain regimes, as a function of wind speed and mass discharge rate only, which can be used for the rapid assessment of major hazards during an explosive eruption. Since the 1970s, numerous 1-D, 2-D, and 3-D theoretical models provided a good knowledge of the overall physics of these eruptions and have yielded a better understanding of the conditions leading to column collapse (

Physical Characterization of Long‐Lasting Hybrid Eruptions: The 2021 Tajogaite Eruption of Cumbre Vieja (La Palma, Canary Islands)

“…In contrast, during days of trade winds the direction was mostly SW, as shown for 9 December (during sedimentation of UU; Figure 11). Finally, as expected from buoyant plume theory (e.g., Degruyter & Bonadonna, 2012Michaud-Dubuy et al, 2020;Woodhouse et al, 2013), the variations in plume height (Figure 2) seem to be dominated more by the characteristics of wind rather than by the short-term MER oscillations. This is suggested by the fact that deposit-derived MER for the tephra blanket is relatively constant across all units and layers (Table 3) and that a clear correlation is observed between wind type and plume height.…”

A Lagrangian Stochastic Model of a Volcanic Eruption Column

“…(2010) suggested that this additive entrainment assumption be an l m ‐norm: where Δ u ∥ and Δ u ⊥ are the components of the relative velocity parallel to and perpendicular to the plume axis, respectively, α and β are the entrainment coefficients associated with each entrainment mechanism and m ≥ 1 is a tunable parameter. Throughout this study we take α = 0.1, β = 0.5, and m = 3/2: these values are consistent with previous laboratory and numerical studies of purely buoyant plumes (e.g., Aubry, Carazzo, & Jellinek, 2017; Briggs, 1984; Devenish, Rooney, & Thomson, 2010; Devenish, Rooney, Webster, et al., 2010; Hoult & Weil, 1972; McNeal et al., 2019), with laboratory studies of jets with reversing buoyancy (Michaud‐Dubuy et al., 2020) and with observations of volcanic eruptions (Aubry & Jellinek, 2018).…”

A Model for Buoyant Tephra Plumes Coupled to Lava Fountains With an Application to the 29th of August 2011 Paroxysmal Eruption at Mount Etna, Italy

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