Study On Design Methods For Contra-Rotating Propellers Of High Speed Underwater Vehicles

As a kind of high-efficiency propulsor,contra-rotating propellers(CRPs)have many advantages.CRP propulsion consumes less power,which contributes to increasing the cruising range of various autonomous underwater vehicles;furthermore,as torque cancellation between forward and aft propellers effectively reduces the rolling moment acting on the carrier,therefore,CRPs can improve the directional course stability of underwater vehicles,and are the preferred propulsion method for torpedoes.Compared to single propellers,CRPs divide their thrust onto two sets of blades,therefore,the lower loading on blade delays the cavitation inception,which reduces vibration and hydrodynamic noise,enhance the concealment performance of underwater weapons,and also improve the sensitivity of underwater detection equipments.In view of the above merits,more and more attention has been paid to the application of CRPs,so the study on design method of CRPs is of great significance.In the past two decades,rapid development of viscous flow CFD technology has not only provided various numerical tools for the study of propusion,but also raised people's understanding on propusion to a new level.Unfortunately,study on the theoretical design method of CRPs has not been promoted in time,and still remains at the level of last century as a whole,which is unable to satisfy the increased requirements for design quality nowadays.In this context,aiming at high-speed underwater vehicles(hereinafter referred to as vehicles or carriers)and combining the viscous flow CFD method and the potential flow lifting-surface method,design methods for CRPs are proposed and developed in this dissertation,which includes simulation of resistance and self-propulsion of the vehicle,prediction of the effective wake distribution,and lifting surface design of CRPs with prescribed and optimum circulation distributions.A complete procedure of CRP design and numerical verification has been established as outcome of the present research.A summary of the work of this dissertaion is given as follows,(1)Based on the solution of the RANS(Reynolds Averaged Navier Stokes)equations,a method is established for simulating,at full scale,the self-propulsion of high speed underwater vehicles propelled by CRPs,which provides a direct way to evaluate the propulsive performance of CRPs by avoiding errors due to the Reynolds scale effects.To establish an accurate and reliable simulation method for selfpropulsion,firstly,grid dependency and turbulence model influences are investigated for the computation of vehicle resistance and CRP open-water performance at model scale.Compared with experimental data,the error in predicted resistance is less than3%,and those in thrust and torque are less than 2% and 4%,respectively.Then,vehicle resistance and CRP open-water performance are simulated at full scale,and the scale effects on CRP hydrodynamic performance are investigated.Finally,the simulation of self-propulsion is carried out for a vehicle using quasi-steady and unsteady models,respectively,and the self-propulsion factors are analyzed.The analysis results indicate that the self-propulsion factors yielded from the two simulation models are both within a reasonable range,and the differences between the self-propulsion factors yielded from the two simulation models are less than 2%,which suggests that the quasi-steady model can be adopted to save time.(2)The accuracy of wake distribution is cruicial for propeller design,which affects the hydrodynamic,cavitation and noise performance of the designed propeller.Therefore,for CRPs,a numerical method is proposed to predict the full-scale effective wake distributions including axial,circumferential,and radial components.The action of CRP blades on the flow field is simulated with body force distributions.Flow simulations for the body forces working behind the vehicle and in the open water are conducted respectively,and the effective wake distributions are obtained by processing and analyzing flows yielded from the two simulatons.To verify the accuracy of the method,the hydrodynamic performance of CRPs is computed using a potential flow method,taking as input the effective wake distributions predicted by the present method.Compared with the results obtained via RANS simulation of self-propulsion using real blades,the errors in thrust and torque obtained via the potential flow method are within 3.6% and 2.8%,respectively.Furthermore,the influences of different wake components on blade forces are investigated via potential flow computations.It is found that,for the aft propeller,the circumferential wake component cannot be ignored when designing CRPs.Finally,the scale effects on effective wake distributions are investigated via RANS simulations at different Reynolds numbers.(3)A design method is established for wake-adapted CRPs with prescribed radial distributions of circulation.The task of lifting-surface design is to determine appropriately pitch distributions and camber-line shapes according to the inflow conditions,so that the loading is distributed more uniformly over the blades,which can improve the cavitation performance.Based on the vortex lattice lifting-surface model(VLM)and by solving nonlinear equations with Newton-Raphson's iterative scheme,two design procedures of CRPs are established.In one of them,the maximum camber and pitch of blade sections are designed by using prescribed camber line shapes.By controlling circulation at the leading edge,the chordwise distribution of blade loading is made as close to that of NACA a=0.8 as possible.In the other procedure,the camber line shape and pitch of blade sections are designed according to prescribed chordwise circulation distributions.Among the two procedures,the former one is highly efficient computationally,but the chordwise distribution of loading is generally not uniform enough;the latter one is more expensive computationally,but can accurately meet the design requirements for chordwise loading distribution.Therefore,the pressure distributions on blade surfaces yielded from the latter procedure is more uniform as compared with those from the former one.For an underwater vehicle,CRPs are designed using the two procedures,and the results are numerically validated via RANS simulations of self-propulsion.(4)Although propeller design is generally performed with prescribed circulation distributions,to make the hydrodynamic efficiency of designed CRPs as high as possible,the design often starts from optimum circulation distributions.Based on the VLM,a method is established for determining optimum circulation distributions of CRPs.With prescribed camber line shape,a relation between propeller efficiency and pitch angle distributions is established by assuming that the local inflow is tangent to the camber line at the leading edge,and the optimum hydrodynamic efficiency is achieved by iteratively modifying pitch angles.Firstly,the pitch angle distributions which make blade loadings become zero need to be found.Then,the increment of pitch angle in each iteration step is determined according to the local gradient of efficiency against pitch angle.Taking the preset thrust as design target,the pitch angle distributions corresponding to optimum circulation distributions are determined via an iterative process.Taking an underwater vehicle as the carrier,the present method is applied to the design of optimum CRPs,and the results are numerically validated via RANS simulations of self-propulsion.