| The pump-jet propulsor is a kind of propeller that equipped on the underwater vehicle.Compared with the traditional heavy inclination propeller,the pump-jet propulsor has the advantages of reducing radiation noise and increasing navigational speed when operating in low noise condition.However,there is a tiny clearance between the blade tip and the chamber while in this flow region the TLV(Tip Leakage Vortex)and TSV(Tip Separation Vortex)are found to be easy to form.Moreover,due to the effect of the vortex of TLV and TSV,the cavitation phenomenon can be found occurring in the blade tip clearance,inducing radiation noises and decreasing the thrust efficiency of the pump-jet propulsor together with the risk of exposing underwater vehicle.Therefore,it is very important to study on the dynamic characteristics of the flow instabilities involved in the blade tip clearance of the pump-jet propulsor.Meanwhile,the relevant research results can be taken as a good reference or instructions for designing high-performance pump-jet propulsors with improved noise characteristics,providing a significant value from the point view of theory and engineering application.In present work,firstly,the steady numerical simulation of the NACA0009hydrofoil was carried out based on the SST k-ωturbulence model,and the pressure and velocity distribution in the tip clearance area was analyzed with the evolved mechanisms of TLV and TSV obtained.Secondly,the steady and unsteady flow in the pump-jet propulsor with different blade tip clearance size(TCS)which are 0.5mm,1mm,2mm and 3mm,respectively,have been numerically calculated under non-cavitation and cavitation conditions.Moreover,the pressure fluctuation in the TLV cores was studied by applying the method of the 3Dscatter diagram in order to better describe the TLV shape.Furthermore,the turbulent kinetic energy transport equation and entropy theory was also firstly applied to analyze the energy generation and losses in the tip clearance field.Finally,the mechanism of the TLV morphological change and energy losses as well as the interaction between cavitation and the TLV vortex were additionally studied.The main work and conclusions are as follows:Firstly,the domestic and abroad research background for the pump-jet propulsor and the flow instabilities in the blade tip clearance were systematically introduced and summarized.Meanwhile,the numerical theory and methods including the RANS equations,main turbulence models,the Rayleigh-Plesset equation and Zwart-Gerber-Belamri cavitation model as well as vorticity transport equation and entropy theory,were also briefly reviewed to provide the theory basis for the further research work in present study.Secondly,the steady numerical simulation of the NACA0009 hydrofoil with tip clearance size equal to 10mm was carried out.Compared with the experimental data,the accuracy of the numerical simulation method applied in present work was verified.Meanwhile,the evolved mechanisms of TLV and TSV was obtained by analyzing the pressure and velocity distribution in the tip clearance field,the vorticity transportation and turbulent kinetic energy.The main conclusions are as follows:(1)The generation of the TLV and TSV vortex are mainly caused by the interaction of the shear stress near the wall together with the pressure and velocity gradient where the clockwise vortex occurs.Moreover,the turbulent kinetic energy of TSV was found much higher than that in other areas.(2)The TLV core identified by the Q criterion descends gradually with the main flow and go towards the water tunnel wall along the spanwise direction.(3)The TLV is distorted along the positive x direction and negative y direction,and the strength of TLV is weakened by the TSV.Thirdly,the steady and unsteady flow inside the pump-jet propulsor with different blade tip clearances which are 0.5mm,1mm,2mm and 3mm,respectively,were numerically calculated under the design and off-design conditions.Especially,the SST k-ωand the DES(Detached Eddy Simulation)turbulence models were selected for the steady and unsteady simulation,respectively.Moreover,the monitor points of pressure fluctuation were set and located near the vortex core.In addition,the vorticity of TLV and its effect on the energy losses were analyzed based on the vorticity transport equation and entropy theory.The main conclusions are as follows:(1)The steady inner flow of the E779A propeller was numerically simulated when operating under different advancing speed coefficients,and the corresponding results agree well with the relevant experimental data,which verifies the accuracy of the numerical method applied in present work and can be trusted as a good reference for the further numerical research work on the pump-jet propulsor.(2)At the initial status of the TLV,it locates at the region ranged from Z=11mm to Z=7mm;as it grows to the fully-developed status,it locates in the range between Z=7mm and Z=-1mm;the final debilitating status locates at the locations between Z=-1mm and Z=-9mm.(3)The thrust efficiency of the pump-jet propulsor with the TCS equal to 1mm is still higher in comparison with other sizes under high advancing speed coefficients,which illustrates the optimal tip clearance size for the pump-jet researched in this paper is 1mm.(4)It can be obtained that 9 peaks and 9 valleys were found among the pressure fluctuation of the TLV through the time domain analysis,and the dominant frequency is 187.5Hz(9f_z).When the blade tip clearance(TCS)is equal to 0.5mm and 1mm,the increased advancing speed coefficient makes the TLV vortex form in advance,which is also the main reason for the energy losses.(5)The entropy production rate in the pump-jet propulsor with the TCS equal to0.5mm and 1mm is about 12.5%higher than those when the TCS is 2mm and 3mm.Meanwhile,the entropy production rate caused by the oscillation velocity occupies a large proportion of the total entropy production rate.(6)The portion of the entropy production rate among the rotor total entropy production rate in the tip clearance area is more than 80%along the spanwise direction of the rotor,and 47%along axial direction,respectively.Therefore,the TLV is the main cause of energy losses.Finally,the cavitating flow in the pump-jet propulsor was numerically simulated based on the steady by adding the Z-G-B cavitation model with the effect of cavitation on the TLV and propulsion performance of the pump-jet discussed.The main conclusions are as follows:(1)When the cavitation numberσis equal to 2.32,the thrust efficiency of the pump-jet with TCS equal to 0.5mm,1mm,2mm,3mm,decreases 25.6%,4.3%,20.5%,17.9%,respectively,which illustrates the optimal tip clearance size is 1mm after cavitation occurs.(2)Whenσis equal to 2.73,the cavities are found on the rotor blades with TCS equal to 0.5mm and 1mm near the leading edge,and there also exist much cavities in the tip clearance field while little for the other tip clearance sizes.(3)Whenσis equal to 2.73,the amplitude of the dominant frequency dropped substantially for the monitors locating at the cavitation area while there is little change for the dominant frequency,which illustrates that the development strength of TLV can be suppressed by cavitation,and the energy loss caused by cavitation is equivalent with TLV. |
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