A numerical simulation is performed to predict the multiphase flow structure of a slug of liquid salt water as it is accelerated through a launch tube by high-pressure, high-temperature gas. This effort is performed to support the design of recoilless launch systems, in which the momentum produced by the launch of a solid projectile is balanced by the ejection of a liquid countermass in the opposite direction. Careful design of the countermass can reduce the net recoil of the launch system to nearly zero, thus allowing higher launch energies to be obtained from relatively light platforms. Simulating the behavior of the liquid countermass during the launch cycle is important for predicting net recoil, as well as for calculating the overall performance of the launch system.

The current work builds on previous efforts to analyze the liquid slug ejection by considering the fluid system in three dimensions and by recognizing the importance of surface tension and mesh refinement at the gas-liquid interface. Effects of shear stress at the slug-wall interface are considered, as are Rayleigh-Taylor instabilities that arise due to the high acceleration and large density differences of the gas and liquid phases.

A transient 3D simulation of the launch event is performed using the VOF (volume of fluid) method to track the gas-liquid interface, while a realizable k-ε model is used to calculate turbulent viscosity in both the gas and liquid phases. The initial conditions of the simulation are that the liquid slug is initially at rest, and that a relatively flat and uniform gas-liquid interface discretely separates the two phases. The properties of the driving gas are calculated a priori using a separate combustion model. The driving gas enters the computational domain of the fluid model as an inlet boundary condition, specified by an entrance pressure as a function of time. The simulation is allowed to run until the bulk of the liquid slug has exited the tube into the atmosphere.

The predicted flow properties, including liquid volume fraction, pressure, velocity, and temperature fields as functions of time are reported. The predicted structure and velocity of the exiting slug are compared to measurements taken using high-speed video during empirical testing of a representative launch system. Gas pressures inside the tube during the launch cycle are compared to electronic pressure data collected from the same test. There is excellent agreement between the predicted and empirical exit velocity of the liquid slug, though the video data shows a higher degree of atomization of the liquid than is predicted by the model.

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