Flow field and vortex interactions in the near wake of two counter-rotating propellers

The wake hydrodynamics of counter-rotating propellers (CRP) is studied via phase-locked particle image velocimetry for three operating conditions featuring distinct flow dynamics. We focus our work on the flow field and the associated vortex dynamics that take place in the near wake, using a special on-line/off-line phase-locking technique that allows the velocity measurements to be phase-averaged based on both forward and aft propeller angular positions. The effect of propellers relative position on the flow features is investigated with support from high-speed visualizations of the vortex system, which is observed by inducing cavitation in the cores of the vortices. We report the relevant interactions that take place between the forward and aft propellers' tip vorticity, in particular the regular patterns of vortex reconnection and break-up that depend on the advance coefficient, as well as the unique formation of isolated vortex rings and a highly coherent vortex wake.