Research on the Occurrence and Motion of the Pyroclastic Flow

点击翻译按钮获取中文摘要

There have been no reliable method to predict the hazardous area due to onslaught of the pyroclastic flow at the foot of an active volcano. This study considers that the self-excited fluidization of the pyroclastic materials due to upward movement of the gas emitted from the material itself plays an essential role. To imitate the real pyroclastic flow in the laboratory experimental flume, some quantity of the sodium bi-carbonate is mixed with sand heated up to 300ﾟC and poured into the flume. That experimental material flows in the flume violently emitting H_2O and CO_2 gas thereby making the main body and hot ash cloud layr. This flow resembles very much with the actual pyroclastic flow observed at the Unzen volcano.Theoretical considerations on the mechanics of the motion of the main body assumes that the main body comprises the base layr and the fluidized layr on it. In the base layr particles are sustained by the contact force between the particles and the energy is consumed mainly by the interparticle friction, whereas in the fluidized layr particles are sustained by the up-lifting force of the upward gas flow and the large scale turbulent mixing of the particles and gas is the main cause of energy consumption. The formulae to predict the thickness, velocity distributions in the base asa well as the fluidized layrs and the resistance to flow of the main body are obtained. Those formulae can explain the behaviors of the actual pyroclastic flow.The mechanism of deposition of the flow is also discussed theoretically as well as experimentally. Thus, the computer simulation of the pyroclastic flow appeared at the Unzen volcano is made possible. The results of the simulations well explain the real situation.