A numerical bubbly flow investigation of drag reduction for underwater vehicles

This paper discusses the numerical investigation of dispersed bubbly flow within the boundary layer of a fully submerged axisymmetric body in horizontal position. The aim is to analyse the influence of injection position and bubble parameters on the drag reduction behaviour. The numerical study is conducted with the commercial CFD package ANSYS Fluent using the Eulerian-Eulerian modelling approach. Several sets of simulations are carried out with air injection velocities in the rage of 1 m/s to 15 m/s, injection locations between 0 and 0.5 m, and bubble diameters from 0.1 mm to 2 mm. In order to obtain the percentage drag reduction the results are correlated with a model without air injection. The simulations demonstrate a different behaviour between small and large bubble diameters of 0.1 mm and 2 mm respectively. Small bubbles archive drag reduction rates around 10% almost independent from the injection velocity and position, while large bubbles are highly affected by those parameters. The maximum drag reduction of 20.67% is achieved by injecting bubbles of 2 mm diameter with a velocity of 12.5 m/s at the tip of the prow nose. It is presented that the drag reduction increases with increasing injection velocity and bubble diameter. These parameters enable the bubbles to build up a continuous film across large parts of the hull which is required for a sufficient drag reduction.