On the flow field induced by two counter-rotating propellers at varying load conditions

The hydrodynamics of the flow between and in the wake of contra-rotating propellers is investigated experimentally via Particle Image Velocimetry (PIV) at different advanced coefficients. Results show that the flow dynamics is deeply influenced by the interplay between the two systems of helical vorticity that develop in the wake. The forward and aft propellers' slipstream structure is analyzed in terms of blade loading and relative phase angle. The onset of tip vortex break-up, reconnection and pairing processes occurring within the wake is discussed with a focus on instability mechanisms. Flow field measurements between propellers highlight that the distribution of the tangential component is balanced more effectively by the aft propeller at specific loading conditions.