A numerical study on the interaction between forward and aft propellers of hybrid CRP pod propulsion systems

This paper presents a numerical study on the interaction between the forward and aft propellers of hybrid CRP pod propulsion systems (CRP-POD) in open water with a focus on both global quantities, that is, thrust and torque, and flow details, such as vortical structure, slipstream evolution, and lateral load on the pod unit. Improved delayed detached eddy simulation (IDDES) method is adopted and combined with the gamma - Re-theta transition model. The open-water characteristics of CRP-POD obtained by CFD are in satisfactory agreement with model test data. Two sets of CRP-PODs with different propeller geometries are considered. It is found independently of propeller geometry that (i) in terms of thrust and torque, the forward propeller is slightly affected by the aft propeller, while it has a large effect on the aft propeller; (ii) the wake of the forward propeller is accelerated in the axial direction and weakened in the tangential direction by the aft propeller; and (iii) the time-averaged lateral force on the pod unit is reduced remarkably by the propeller-propeller interaction. However, the vortex interaction between the forward and aft propellers depends significantly on the propeller geometry. In the case when the tip vortices shed by the forward and aft propellers can directly collide with each other, the vortical structure in the wake becomes unstable and rapidly breaks down.