Numerical simulations of pyroclastic surge in eruptions of Usu Volcano

Pyroclastic surge developed during explosive volcanic eruption can cause hazardous damage as serious as other phenomena such as blast waves or eruptive fragments. A numerical code is developed for simulating the pyroclastic surge. The code is an extension of a shock capturing code that treats shock wave propagation and other complex wave interactions (Saito, 2002). It is applied for simulating imaginary eruptions of Usu volcano, one of the most active volcano in Japan. The eruption is modeled as a high-speed jet ejected from a vent opened at the ground into a stationary atomosphere. The governing equations of mass, momentum and energy are solved by finite volume methods known as WAF(Weighed Averaged Flux) method with the HLLC approximate Riemann solver(Toro, 1997). The numerical grids of the terrain are generated from a digital elevation map. Preliminary numerical tests are carried out and followings are found: a) At the beginning of eruption, a blast wave is generated and propagates rather quickly(supersonic propagation). b) After the blast wave, a column of of high density erupting as developes. c) In about 100 sec from the beginning of the eruption, it is observed that high density gas flow spreads out along the ground as pyroclastic surge. The correlation between the gas ejection speed and the speed of the pyroclastic surge is also investigated.

Saito, T. (2002), Numerical analysis of dusty-gas flows, J. Comput. Phys., 176, 129-144. Toro, E. F. (1997), Riemann solvers and numerical methods for fluid dynamics, Springer-Verlag, Heidelberg.