Study On The Hydrodynamic Performance And Design Method Of Tunnel Thrusters

The tunnel thruster is not only used to improve the ship's maneuverability,but also a necessary part of the dynamic positioning system.As human beings devote more and more efforts on the exploration of deep sea resources,the tunnel thruster will play an increasingly important role in the maritime field.However,the research work with regeard to the tunnel thruster has been mostly experimental,numerical study is insufficient and far from mature.In this context,the hydrodynamic performance,excitation force and radiated noise of the tunnel thruster have been studied in this dissertaion by means of viscous flow CFD simulations as well as physical experiments,and a design method for the impeller has been proposed based on the lifting line theory and the concepts of equivalent inflow and effective wake.A summary of the present research work is given as follows,(1)Numerical simulation methods have been developed for the quasi-steady and unsteady hydrodynamic performance and flow of tunnel thrusters by solving the RANS equations and the k-? SST turbulence transport equations.While the quasisteady method is meant for efficient evaluation of the hydyodynamic performance at reasonable accuracy,the unsteady one is mainly used for the prediction of impeller-excited fluctuating forces acting on the thruster.The flow passages between adjacent blades are discretized with prismatic cells so that the boundary layer flow is resolved down to the viscous sub-layer.Due to the complexity of flow near the blades,the grid density on blade surfaces and the number of grid layers in the clearances between blade tips and the tunnel wall are determined via a grid-dependency analysis.Unsteady simulations are performed by using the sliding mesh model,and the results are compared with those of the quasi-steady model to investigate the influences of the quasi-steady approximation on predicted loads.For three impellers covering a range of extended area ratio and pitch ratio the hydrodynamic performances predicted by the quasi-steady model agree well with experimental data.Through analysis of the flow field,the reason why the hub of impeller contributes a small amount of thrust,and how the impeller induces an axial force on the hull,which can amount to 40~60% of the impeller thrust,are clarified.Besides,several flow features of interest are investigated based on the simulation results.(2)Aiming at predicting the hull thrust at the preliminary design stage of a tunnel thruster,a RANS simulation method incorporating the body force model for the impeller is proposed.According to the characteristic loading distribution of the impeller blades mentioned above,a body force model is established.To account for the blockage effect of the impeller,a flow rate correction approach is proposed,and the empirical formulas of the flow rate correction coefficient are given for the cases of with and without the gearbox.Numerical examples indicate that the present method is capable of predicting the hull thrust,and hence the impeller thrust given the total thrust required,at reasonable accuracy.Besides,the body-force based simulation method can also be applied to simulating the ventilation phenomenon of tunnel thrusters.(3)Based on the analysis of the flow field characteristics of the tunnel thruster,it is found that,due to the existence of the flow induced by the tunnel and the ship hull,the impeller does not necessarily experience heavy loading.By introducing an equivalent advance speed and the concept of effective wake,a design method for the impeller has been developed by utilizing the lifting line model for open propellers.By comparison of the RANS-simulated velocity fields due to identical body-force distributions placed in the tunnel and in the open water respectively,the equivalent advance speed and the effective wake of the gearbox are determined.A tip-unloaded impeller has been designed by means of the present design method,and the hydrodynamic performance of the deisgned impeller is found to be reasonably accurate according to RANS simulation results.(4)Cavitation occurs more easily on tunnel thruster blades due to the small tip clearance and heavy loads as compared with an open propeller,which makes the problems of structure vibration and radiated noise more serious.Unfortunately,there are few public sources of relevant experimental study.In this context,a number of tests have been conducted for a tunnel thruster model using three different impellers in the cavitation tunnel of Shanghai Jiao Tong University.The influences of flow speed,impeller rotation speed,and the cavitation number upon the fluctuating pressures on the tunnel wall and the noise at a distance from the thruster are investigated for the case that the gearbox is located upstream of the impeller.The experimental results indicate that the influences of the flow and impeller rotation speed are negligible.However,the amplitude of pressure fluctuation and the sound pressure level are both higher in cavitating conditions than in non-cavitating ones.The pressure fluctuation at the impeller disk becomes stronger as the cavitation number decreases,whereas the noise does not.In addition,the experimental results show that,when their thrusts are equal,the impeller with unloaded tip can reduce the fluctuating pressures and sound pressure level relative to the one with ‘flat plate blade',but the effect of non-linear rake is not obvious,which needs to be studied further.(5)In order to further study the effects of unloaded tip and nonlinear rake,numerical simulations based on the sliding mesh model are carried out for the three impellers used in the experiment.The numerical simulation results also show that,compared with the impeller with ‘flat plate blade',the impeller with unloaded tip has lower fluctuating pressures,and its hydrodynamic performance is also improved.However,the numerical simulation results indicate that the nonlinear rake can further reduce the fluctuating pressures,which is not obvious in the physical experiments.Numerical simulations are also carried out for the three impellers at negative pitches and for two fixed-pitch impellers at forward and reverse conditions.The results show that the unloaded tip will reduce the hydrodynamic performance and increase the fluctuating pressures at negative pitch,which is just the opposite at positive pitch.In the case of fixed-pitch impellers,however,the unloaded tip is effective to reduce the fluctuating pressures and improve the hydrodynamic performance regardless of the impeller's rotating direction.The blade geometry mainly affects the mean values of axial and lateral forces of the tunnel thruster,and has little influence on the amplitudes of the impeller's excitation forces.