Numerical investigation of driving forces in a geyser event using a dynamic multi-phase Navier–Stokes model

A geyser is an explosive flow of air–water mixture shooting out of a manhole. It has been demonstrated experimentally that the releasing motion of confined air pockets in a dropshaft is the key mechanism to trigger a geyser. Other release events, unassociated with air–water mixtures, can occur, but the intensity is significantly smaller than the air–water geysers. Existing numerical models that simulate vertical air movement in mixed-phase flows typically solve a series of lumped-mass continuity, momentum and energy equations, greatly simplifying the interactions between the water and air phases. Hence, existing models are unsatisfactory in capturing the complex dynamics of a geyser because of the violent interactions between the water and air phases. In this work, a two-phase numerical model solving the Navier–Stokes Equations was applied to investigate the driving forces in an air–water geyser formation in storm sewer system. The simulated dynamics include buoyancy, air compressibility, momentum and pressure. The numerical model revealed the key factor that triggers an air–water geyser, which involves compressed air pockets that are pushed into the dropshaft by pressure surges from the main pipe. The numerical model also captured the two distinctive features of an air–water geyser, which are a violent mixture of water–air and a high-speed jet. This study also revealed how a pressure head in the main pipe, which is much lower than the ground elevation, could lift the water to the ground and push it out of the manhole