10:45 AM - 12:15 PM
[SVC29-P04] Numerical study of the relationship conduit geometry and flow Satibility
Keywords:Conduit flow model, Conduit deformation, Temporal change of eruption
A volcanic conduit, where the various processes that characterize the eruptive style and explosivity occur, is a magma pathway between a magma chamber and a crater. Previous studies of conduit flow modeling have revealed complex conduit flow dynamics assuming the conduit is rigid. However, the existence of lithic fragments in the pyroclasts and estimation by the miclolite-decomprassion rate meter has recently suggested that the conduit is deformable. The previous numerical studies also suggested that the viscoelastic deformation driven by the overpressure is involved in the damped oscillation of lava extrusion rate on effusive eruptions (Maeda,2000; Chen et al., 2018) and the conduit erosion by gas-pyroclastic flow is involved in the increasing the discharge rate toward the end of explosive eruptions (Wilson, 1980; Massaro et al., 2018). It has been suggested that the eruption rate increases toward the end of the explosive eruption.
In order to discuss the dynamics of the temporal change of eruptions, it is necessary to consider the effects of conduit deformation on the dynamics of conduit flows. In this study, numerical simulations were performed to understand the stability and temporal change of conduit flow when the shape of the conduit is changed.
The 1-D steady conduit flow model based on Kozono and Koyaguchi (2009) has the following assumptions: the magma is a multiphase fluid consisting of H2O-melt-crystal and considers depressurization equilibrium vesiculation and crystallization in the ascending magma. The vertical outgassing, fragmentation, and associated changes in magma flow patterns are also incorporated to represent the transition between effusive and explosive eruptions. Under these assumptions, the model was solved numerically as the two-point boundary value problem with fixed conduit length. The pressure at its lower end (chamber pressure) is the atmospheric pressure. The boundary condition at its upper end is the atmospheric pressure or the choking condition. In order to systematically investigate the change of the conduit flow when the conduit geometry changes, the change of the discharge rate q is investigated given its radius a is changed while the chamber pressure is fixed. For simplicity, the conduit is assumed to be cylindrical in this study.
As a result, under the conditions of constant chamber pressure, a negative resistance (dq/da < 0) region was found to exist between the relationship between the conduit radius and the discharge rate of a perfect cylindrical conduit. This result suggests that a slight decrease (increase) in the radius might cause an increase (decrease) in the discharge rate and a sudden flow style change due to accelerated conduit deformation processes, for example, viscoelastic deformation caused by overpressure and conduit wall erosion. The radius change causes the transition from an effusive to an explosive eruption, which might drive the effects of conduit deformation processes because of the discontinuous change in the discharge rate. The frictional force between magma and the conduit wall varies depending on the cross-section of the conduit. Therefore, it is necessary to consider the effect of a dike-shaped conduit with a large aspect ratio on the stability of the conduit flow.
In order to discuss the dynamics of the temporal change of eruptions, it is necessary to consider the effects of conduit deformation on the dynamics of conduit flows. In this study, numerical simulations were performed to understand the stability and temporal change of conduit flow when the shape of the conduit is changed.
The 1-D steady conduit flow model based on Kozono and Koyaguchi (2009) has the following assumptions: the magma is a multiphase fluid consisting of H2O-melt-crystal and considers depressurization equilibrium vesiculation and crystallization in the ascending magma. The vertical outgassing, fragmentation, and associated changes in magma flow patterns are also incorporated to represent the transition between effusive and explosive eruptions. Under these assumptions, the model was solved numerically as the two-point boundary value problem with fixed conduit length. The pressure at its lower end (chamber pressure) is the atmospheric pressure. The boundary condition at its upper end is the atmospheric pressure or the choking condition. In order to systematically investigate the change of the conduit flow when the conduit geometry changes, the change of the discharge rate q is investigated given its radius a is changed while the chamber pressure is fixed. For simplicity, the conduit is assumed to be cylindrical in this study.
As a result, under the conditions of constant chamber pressure, a negative resistance (dq/da < 0) region was found to exist between the relationship between the conduit radius and the discharge rate of a perfect cylindrical conduit. This result suggests that a slight decrease (increase) in the radius might cause an increase (decrease) in the discharge rate and a sudden flow style change due to accelerated conduit deformation processes, for example, viscoelastic deformation caused by overpressure and conduit wall erosion. The radius change causes the transition from an effusive to an explosive eruption, which might drive the effects of conduit deformation processes because of the discontinuous change in the discharge rate. The frictional force between magma and the conduit wall varies depending on the cross-section of the conduit. Therefore, it is necessary to consider the effect of a dike-shaped conduit with a large aspect ratio on the stability of the conduit flow.