Research on the Occurrence and Motion of the Pyroclastic Flow

目前还没有可靠的方法来预测活火山脚下火山碎屑流冲击的危险区域。本研究认为，火山碎屑物料本身所释放的气体向上运动所引起的自激流化起着至关重要的作用。为了模拟实验室实验水槽中真实的火山碎屑流动，将一定量的碳酸氢钠与加热至300℃的沙子混合，倒入水槽中。实验物质在水槽内剧烈流动，释放出H_2O和CO_2气体，从而形成主体和热灰云层。这种流动与在云仙火山观测到的实际火山碎屑流非常相似。对主体运动力学的理论考虑假定主体由基层和基层上的流化层组成。在基层中，颗粒由颗粒间的接触力维持，能量主要由颗粒间的摩擦消耗，而在流化层中，颗粒由向上气流的上升力维持，颗粒与气体的大规模湍流混合是能量消耗的主要原因。得到了流化层厚度、流化层流速分布和主体阻力的计算公式。这些公式可以解释实际火山碎屑流的行为。本文还从理论和实验两方面探讨了水流沉积的机理。从而使云仙火山火山碎屑流的计算机模拟成为可能。模拟结果很好地解释了实际情况。

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.