Numerical Investigation Of Openwater Performance Of Hybrid CRP Podded Propulsor

Nowadays,all types of ships all over the world,expecially container ships,are developped to become not only larger and larger but also faster and faster,conventional single screw propeller is faced with many unavoidable problems,such as lower propulsion efficiency,severer cavitation and noise performance,meanwhile,a large amount of exhaust gas is discharged into the air leading to serious pollution problem.Under this background,hybrid CRP poded propulsor becomes a more promising new propulsion way,because it inherits advantages both from contra-rotating propellers and poded propulsor,such as better propulsion and maneauveribility performance,effective energy saving performance,lower level of greenhouse gas emission.In practical application,unsteady forces and energy saving performance of this hybrid propulsor catch much more attention,therefore it is very crucial to accurately predict the influence of different design parameters on the hydrodynamic performance of the hybrid propulsor.With the continuous improvement of algorithms and computer computing power,advantages of CFD(Computational Fluid Dynamics)become more and more evident.CFD can help us not only obtain accurate prediction of ship resistance performance,maneuverability and seakeeping performance,but also investigate physical mechanisem.A single phase solver,pimpleDyMFoam from OpenFOAM is used to solve RANS equations and SST k-ω turbulence model,where coupling of pressure and velocity is solved by algorithm named PIMPLE who is a hybrid version of SIMPLE and PISO,PIMPLE algorithm can keep computing stability even with a relatively larger timestep.In this thesis,a standard propeller model from ITTC,named DTMB4119,is firstly buit by CATIA based on propeller projection theory,then its hydrodynamics at different advance coefficients are computed and compared with experimental data to exam confidence of propeller modeling method and computational method used in this thesis.Secondly hydrodynamics of contra-rotating propellers are computed and validated,furthermore,affects of blade ratio,clearance between front and rear propellers and revolution ratio on hydrodynamic performance of CRPs are investigated in depth.Based on computing results,several conclusions can be drawn as follows.Firstly,CRPs whose blade number ratio is 4:5 is proved to be more stable than CRPs whose blade number ratio is 4:4.Secondly,best energy saving performance CRPs can be achieved by CRPs when its revolution ratio is around 1.Thirdly,clearance between front and rear propellers has obvious influence on the amplitude of unsteady forces,but have little affect on energy saving performance.Based on comparison of tangential velocity distribution in slipstream between CRPs and single propeller,it has been proved that rotational energy can be recovered drastically.Finally,model of hybrid CRP podded propulsor is built by adding a streamlined pod to CRPs.The interactions between front propeller and rear poded unit,affect of clearance and revolution ratio are investigated in detail.Based on computing data,the following conclusions can be drawn.Firstly,the rear podded propulsor has little influence on front propeller,however,the front will couse larger fluctuating amplitude of the rear propeller and reduce propulsion efficiency of the rear poded propeller.Secondly,in a certain length range,clearance is proved to have little influence on fluctuating amplitude and energy saving performance of hybrid propulsor.Thirdly,in a certain revolution ratio range,it can be found that revolution ratio has little influence on fluctuating amplitude and propulsion efficiency,it maybe because revolution ratios studied in this thesis are too close to one.From view of recoving rotational energy in slipstream,CRPs will obtain best energy saving performance when revolution ratio is around1.In the last,pressure distribution on the pod surface is investigated,it can be found that higher pressure is centered on incident flow surface of struct,then large drag force may be induced,therefore,pod shape in this region should be optimized to reduce blockage effect of struct.