Numerical Simulation of Choked Flow in Old Faithful Geyser, Yellowstone

In geologic media, there exist multiphase and multicomponent fluid system, with typically slow fluid flow through the pores or fractures in the subsurface. However, some events such as volcanic and geyser eruptions, as well as artificial CO₂ injection, may cause rapid migration of the fluids. Old Faithful geyser in Yellowstone National Park, has been studied extensively due to its high-speed eruptions. The ejection velocity of the geyser is known to reach the supersonic regime, accompanied by choked flow and the shock wave. These phenomena have been suggested to be causes of the harmonic tremors generation during the geyser eruption; however, the appearance of choked flow and shock wave in Old Faithful Geyser has not been directly confirmed to this date. In order to identify the phenomena behind harmonic tremor generation, we simulated the geyser eruption of water-steam mixture.

The simulation was conducted using the 2D geometry of Old Faithful geyser with two throats and two chambers from the surface to 22 m depth. Eruptions of single-phase and single-component fluid system (water and steam) were simulated, followed by simulations for multiphase fluid system (water-steam mixture). Multiphase fluid system is expected be challenging to predict, as the sound speed of mixture is significantly smaller than that of pure water and steam, while the existence of water interferes with the migration of the steam. Using computational fluid dynamics simulation, thermodynamic properties and the movement of water-steam mixture were considered together, then the generation of choked flow and shock waves in Old Faithful geyser was evaluated. It was observed that the thermodynamic properties and velocity of fluids were dependent on the existence of throats and chambers within the geyser. In addition, generation of shock waves was observed with increase in the steam fraction, as the fluid passed the upper throat of 6.8 m depth.