火砕流の流動機構に関する研究(2)

Abstract

A theory for the mechanics of the Merapi type pyroclastic flow is presented, in which the self-excited fluidization of the bed due to upward movement of the gas emitted from the pyroclastic material itself plays an essential role.The main body of the flow comprises the base layer and the fluidized layer on it. In the base layer particles are sustained by the contact force between the particles and the energy is consumed mainly by friction, whereas in the fluidized layer particles are sustained by the up-lifting force of the upward gas flow and the large scale turbulent mixing of the mixture of particles and the gas is the main cause of consumption of energy. The solids concentration in the main body is nearly uniform and the formulae to predict the thicknesses of the base as well as the fluidized layers, velocity distributions in the base as well as the fluidized layers and the resistance to flow of the main body are obtained. The laboratory flume experiments prove the theory is valid and that theory well predicts the velocity of the real pyroclastic flow observed at Unzen and Semeru volcanos.The solids concentration and the velocity distribution in the hot ash cloud layer which exists above the main body are also discussed.The process of deposition downstream of the change in slope is observed and some important characteristics are made clear.