Numerical simulation of air entrainment by three dimensional pyroclastic surge flow model

Pyroclastic currents, which composed by ash particles and small pyroclasts, is one of destructive ejecta produced during volcanic eruptions. The behavior of this hazardous fluid is still not revealed yet. In particular, dilute fluids called pyroclastic surge can expand and diffuse by entrainment of ambient air and has a possibility of danger. Due to lack of understanding about pyroclastic surge, current disaster prevention measures are inadequate in estimating the run-out distance and range. In recent years, several experiments on the pyroclastic flows have been conducted; however, to reproduce both high temperature and high velocity is quite difficult. Therefore, the numerical calculation is considered as the powerful tool to analyze their flow structures. Here, we applied the numerical model of a pyroclastic surge to the experimental investigation at Smithsonian Institute in order to examine the air entrainment. Our model is based on solving Navier-Stokes equation by finite-difference scheme, CIP-CUP method, and Smagorinsky model applied to turbulent mixing. In this study, pyroclastic surge is treated as a dilute turbulent suspension, and gas and particles are assumed to be well-mixed or have certain settlement velocity. We applied the 2D and 3D model to experiments and investigated the effects of turbulence and settlement. As a result of a series of simulations, we can reproduce the generation of head and wake and it has a strong relationship with mesh size. The large mesh cannot capture the wake at rear of head. Furthermore, the temperature change process by turbulent mixing is confirmed. The experimental data at PELE (the pyroclastic flow Eruption Large-scale Experiment) is also compared and discussed about the change in flow height.