Effect of propeller immersion depth on the flow around a marine propeller

Abstract

The effects of the free surface on the flow around a rotating propeller were investigated experimentally in a circulating water channel by varying the propeller immersion depth. Instantaneous velocity fields were measured using a two-frame particle image velocimetry (PIV) technique at four blade phases. These fields were ensemble averaged to obtain the phase-averaged flow structure of both the inflow and the wake. The flow around the propeller was influenced by the propeller rotation speed, the loading on the blades, and the proximity of the propeller to the free surface. The boundary layers developed on the blade surfaces led to the formation of viscous wake at all immersion depths. As the propeller was moved closer to the free surface, the axial velocity within the slipstream decreased. Interaction between the tip vortices and the slipstream seemed to cause the tip vortices to follow an oscillating trajectory. The presence of the free surface greatly affected the wake structure, especially for the propeller immersion depth of 0.6 D. At small immersion depths, the free surface modified the tip and trailing vortices and the slipstream flow structure downstream of X/D = 0.3 in the propeller wake. The present study shows that wake modeling should consider the free surface effects to accurately predict propeller performance or cavitation at small propeller immersion depths.