HR: 1330h
AN: V52B-0426    [PDF]
TI: Source model of volcanic tremor: two-phase flow instability in a pipe-valve system
AU: * Fujita, E
EM: fujita@bosai.go.jp
AF: National Research Institute for Earth Science and Disaster Prevention, Tennodai 3-1, Tsukuba, 305-0006 Japan
AB: Volcanic tremor at a shallow depth beneath the volcano is inferred to link to hydrothermal activities powered by heat supply from magma. In this study, we developed numerical simulations of the instabilities of the water-steam two-phase flow in a pipe-valve system and considered the source mechanism of volcanic tremor. The experiments of two-phase flow by Veziroglu and Lee [1968] revealed the two kinds of oscillating modes, density wave oscillation with the period of a few seconds and pressure drop oscillation with the period of dozens of seconds. These modes were mainly controlled by the pressure difference between inlet and outlet, flux rate of fluid and heat supply rate. Especially, the former mode appears when the flux rate is small and the latter does when the pressure difference and heat supply rate are larger. We performed some preliminary numerical simulation of these oscillations in water-steam flow in a cylindrical conduit. As an example, we assume the flow in conduit of 4 m length with the valves at inlet and outlet with the conditions of non-slip at the wall. As initial conditions, the inlet and outlet pressures are fixed to be 1.2E5 Pa and 1.0E5 Pa, respectively, water temperature of 370 K, heat supply of 1.0E6 - 2.0E7W/m3. The friction except the valve area is assumed to be 1000kg/m3. After the heating condition becomes stable, we shut the valve at the outlet and detect the significant oscillation. In case of the heat supply of 1.1E7W/m3, density drop oscillation with the period of 0.16s has appeared. In this model, the oscillation originates from the density change due to vaporization, and its information arrives at the outlet with the velocity of two-phase flow. The cycle of heating and boiling controls the interval of the tremor occurrence and the period is determined by the length of the pipe and the flow velocity. The shut of valve physically corresponds to geometrical narrowing, choking, and non-linear effect of flow and/or surrounding medium.
DE: 7280 Volcano seismology (8419)
DE: 8414 Eruption mechanisms
DE: 8419 Eruption monitoring (7280)
DE: 8424 Hydrothermal systems (8135)
DE: 8434 Magma migration
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2003 Fall Meeting