Development of numerical models on explosive volcanic eruptions

建立了火山喷发云和管道流的数值模型，以便将火山喷发动力学与火山碎屑沉积中的野外观测联系起来。得到以下3个结果。(1)喷发云三维数值编码的建立。三维湍流对喷发云与周围空气的混合起着至关重要的作用，从而影响喷发云的动力学。结果表明，在采用三维坐标和高空间分辨率的三阶精度的情况下，我们的数值代码可以正确地再现这种湍流混合。(2)建立了气泡成核生长与管道动力学的耦合模型。建立了微观气泡成核生长与宏观管道动力学的耦合模型，采用细胞模型（一种将气泡流动近似为具有不可压缩粘性或粘弹性材料外壳的气泡聚集的模型）。该模型使我们能够预测黏度对岩浆破碎的依赖，以及岩浆中气泡成核和气泡大小分布的时间。(3)实际爆发喷发详细喷发序列重建。在详细的喷发物地质调查和岩石学调查的基础上，结合文献资料分析，重建了浅间1783年火山喷发的喷发序列。结果表明，浅间1783火山喷发具有爆炸性喷发云和碎屑性熔岩流同时产生的特征，这种喷发方式可以解释为此次喷发期间岩浆破碎程度较低（喷出物粒度较粗）。

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.