Development of numerical models on explosive volcanic eruptions

为了将火山碎屑沉积中爆炸喷发的动力学与现场观测联系起来，建立了喷发云和管道流动的数值模型。结果表明：(1)喷发云团三维湍流数值模拟的发展对喷发云团与周围空气的混合以及喷发云团的动力学起着至关重要的作用。(2)建立了气泡成核-长大与管道动力学的耦合模型，建立了微观气泡成核-长大与宏观管道动力学的耦合模型。(3)重建了实际喷发的详细喷发序列。在详细地质调查和岩石学研究的基础上，结合对古文献的分析，重建了阿萨玛1783年喷发的喷发序列。结果表明，ASAMA 1783年喷发具有同时产生爆发性喷发云和碎屑状熔岩流的特点，这种喷发方式是由于喷发过程中岩浆破碎程度较低(喷出物的粗粒度)所致。

Numerical models of eruption cloud and conduit flow were developed in order to link the dynamics of explosive eruption and field observations in the pyroclastic deposits. The following 3 results were obtained.(1)Development of a 3-D numerical code of eruption cloud3-dimensional turbulence plays an essential role in mixing between eruption cloud and surrounding air, and hence, the dynamics of eruption cloud. It has been shown that such turbulent mixing can be correctly reproduced by our numerical code where 3-D coordinate and the third order accuracy with high spatial resolution are employed.(2)Development of a coupled model of bubble nucleation-growth and conduit dynamicsA coupled model of microscopic bubble nucleation-growth and macroscopic conduit dynamics has been developed, using the cell model (a model where bubbly flow is approximated as an aggregate of bubbles with shell of incompressible viscous or viscoelasitc materials). The model enables us to predict the dependence of viscosity on magma fragmentation and timing of bubble nucleation and bubble size-distribution in magma during explosive eruptions.(3)Reconstruction of detailed eruption sequence for actual explosive eruptions.Eruption sequence of the Asama 1783 eruption was reconstructed on the basis of detailed geological survey and petrological investigation for the ejecta as well as analyses of old documents. The results show that the Asama 1783 eruption is characterized by simultaneous generation of explosive eruption cloud and clastogenic lava flow, and that such an eruption style is explained by relatively low degree of magma fragmentation (coarse grain-size of the ejecta) during this eruption.