Theoretical Design And Performance Investigation About Hybrid CRP Podded Propulsor System

As a new kind of energy saving propulsor,the hybrid CRP podded propulsor provides an additional choice for building ―Green Ship‖.The combined propulsor has better maneuverability,higher propulsive efficiency and lower installed power compared with traditional propulsor.Two sets of independent propulsion systems raise the redundancy without stern thruster and maneuvering rudder.Two sets of propulsion systems can improve the fluctuating pressure imposed on the hull surface by separating the hydrodynamic load and lower the air pollution by improving the load of prime mover.As people have higher and higher requirement for ship propulsor,the fluctuating pressure and noise performance of ship propulsor become the focus of attention.This paper establishes the design model of the hybrid CRP podded propulsor using the propeller vortex theory and surface panel method,and has hydrodynamic performance and flow field calculations about the designed CRP podded propulsor through CFD technique.In order to have better investigation about the performances of hybrid CRP podded propulsor,the prediction models of the fluctuating pressure field and noise filed of the hybrid CRP podded propulsor are established using theoretical methods.The FORTRAN program written in this paper is used to predict the unsteady hydrodynamic performance of DTRC4118 propeller,and the calculation result agrees well with that of literature.A single propeller is designed in the ship wake for some containership,the effective wake field of full scale vessel needed in design is obtained through ship model experiments and existing theoretical empirical formulas.The open water and unsteady hydrodynamic performances of the designed single propeller are calculated,and the pressure distributions on the propeller surface over one revolution in the wake field are given.The results show that the pressure distributes on the propeller surface evenly.The hybrid CRP podded propulsor is designed comprehensively based on the propeller vortex theory and surface panel method,and the containership used in the single propeller design is studied again.The design process mainly involves the main propeller design and the pod propeller designs,and the pod unit is given first.The hydrodynamic interactions among different elements are considered through induced velocities,and the unsteady design is carried out until the requirement is met.The streamlined pod unit is chosen according to the hull form,and the flow field performance of the streamlined pod unit is significantly improved compared with conventional pod unit.The prediction program written in this paper is used to calculate the hydrodynamic performance and analyze the wake field of a set of contra-rotating propeller,and the results show good agreement with test data.The unsteady hydrodynamic performance calculation results show that the thrust generated by the main propeller is obviously larger than that generated by the pod propeller,and that the fluctuating amplitude of the main propeller thrust is close to that of the designed single propeller and the fluctuating amplitude of the pod propeller thrust is obviously smaller than those of the main propeller and the designed single propeller.Under the design condition,the propulsive efficiency of the designed hybrid CRP podded propulsor increases by 8.533% compared with the single propeller.In order to analyze the effect of viscous force on the hydrodynamic performance of the hybrid CRP podded propulsor,the CFD technique is used to calculate the hydrodynamic performance of the system.The ICEM is used for geometric modeling and grid partition,and the FLUENT is used to predict the hydrodynamic performance of the system.The pod unit,front and rear propellers and the inner computational domain adopt unstructured grids,and the outer computational domain structured grids.The axial wake is only considered,and the calculation results show good agreement the results of the surface panel method.The pressure distributions of the front and rear propellers and the pod unit and the changing trend of flow field are given.The fluctuating pressure calculation model of propeller is established,and the pressure signal is analyzed in the frequency domain through Fourier Transform.Two given containership propellers’ fluctuating pressures at specific field points are calculated,and the results are in good agreement with the experiment data.The method given in this paper is used to calculate the fluctuating pressure fields of the hybrid CRP podded propulsor and corresponding single propeller.The results show that the fluctuating pressure of the hybrid CRP podded propulsor is obviously smaller than that of the single propeller,and that the hybrid CRP podded propulsor can improve the stern vibration effectively.The potential-based panel method coupled with acoustic analogy is used to calculate the low frequency discrete noise directivity of DTRC4119.The result shows that the thickness noise has significant effect on the overall noise at the rotation plane and that the effect on the overall noise can be ignored in the downstream and upstream.The low frequency discrete noise performance of the system without considering the acoustic scattering effect of pod unit is calculated,the sound pressure level superposition is used to consider the acoustic interaction of two propellers.The noise of the hybrid CRP podded propulsor considering the acoustic scattering effect of pod unit is calculated and the noise directivity at the first interaction frequency as well as the three-dimensional noise directivity are given.The results show that the pod unit has significant effect on the noise directivity,and the noise field in the upstream of the combined propulsor is slightly reinforced and that in the downstream is significantly weakened.