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Gaseous jet injected into liquid typically occurs in underwater propulsion. The flow is unsteady and turbulent; the high density ratio between water and gas can induce many complicated phenomena. To investigate the underwater propulsion performance, the analytical gas bubble model which has been widely used to predict the thrust of underwater engine is summarized. In this model, water is assumed to form a conservative system, so that the work done on the water by the gas is equal to the sum of the kinetic and potential energies stored in water. To assess the validity of this approximate model, the detailed Navier-Stokes flow computations were made to elucidate flow structures of the gasliquid interactions and to evaluate their effect on the propulsion system. Specifically, multiphase unsteady flows were simulated using the Volume of Fluid model; fluid compressibility, viscosity, and energy transfer were taken into account. The results show that phenomena including expansion, necking/breaking, and back-attack were highlighted in the jet process, which cannot be described by the analytical model. Our analysis indicates that the back pressure in underwater propulsion is much different from the ambient pressure and can significantly affect the propulsion performance. When gas is fully expanded, the back pressure can be lower than the ambient pressure, which results in a decrease in total thrust. However, when the necking/breaking appears, the back pressure will jump to a high value, which affects the mass flow rate through the nozzle, the velocity and the pressure at the nozzle exit. Thus, the total thrust in underwater propulsion is not only determined by the nozzle geometry but also by the flow structures and associated pressure distributions.

Flow Structures of Gaseous Jet Injected into Liquid for Underwater Propulsion