Characteristics And Mechanism Of Tip Leakage Cavitating Flow And Its Noise

Noise radiated by tip leakage vortex(TLV)cavitating flow is a common yet serious problem in marine propulsion and many kinds of hydraulic machineries.It not only serves as a major issue in negatively affecting the stealth capabilities of malitary ships or submarines,but also contributes to detrimental effects on marine life and mental health of ship crew members.In order to obtain an in-depth understanding of this phenomenon and to establish a theoretical basis for the solution of the problem,it is necessary to investigate characteristics and the underlying mechanisms of TLV cavitating flow noise under different conditions.In the present Ph D thsis,aiming at understanding the mechanisms involved in the generation of noise by both TLV cavitation inception and developed TLV cavitation,Large Eddy Simulation(LES)coupled with Schnerr–Sauer cavitation model is used to solve the flow field and a porous Ffowcs Williams–Hawkings(FW-H)acoustic analogy is used to obtain far field sound signals.By carrying out vortex identification and parameter extraction of flow in the vortex core,the flow characteristics at the TLV center,cavitation inception,fully developed cavitation and their noise under conditions of variable angles of attack and clearance sizes are systematically studied.The main contents and innovative achievements of this thesis are as follows:(1)A new model is proposed for predicting TLV cavitating flow and its noise considering the growth and collapse of nuclei.By calculating the kinematic and dynamic equations,various forces exerted on nuclei are included,such as drag force,pressure gradient force and force induced by volume change of the nuclei.Based on the Rayleigh-Plesset equation,the growth and collapse behaviors of the nuclei are solved under the encountered pressure.Combined with the noise of a single sphere bubble and the far-field solution of FW-H equations,the prediction of TLV cavitation inception noise is finally realized.(2)Based on the analysis of the applicability of different types of vortex identification methods in TLV flow,an extraction method of TLV center trajectory at different locations and the parameters at the TLV center is proposed.the spatiotemporal evolution characteristics and formation mechanism of TLV center flow are discussed in detail.The main specific results are as follows: through the comparative application of various types of vortex identification methods and vortex center location methods,it is found that ? method and Liutex method have a better ability in filtering out the strong shear layers around the hydrofoil and the gap wall,which can obtain clearer vortex structure profiles.As vector variable with clear physical meaning,the maximum value of Liutex method is used to identify the TLV center.Meanwhile,by assuming that the geometric center of the cavitation structure is the center of cavitating TLV,we can finally obtain the TLV center line along the main flow direction and extract parameters at TLV center in both fully wetted and cavitating flow.By comparing the distribution of TLV structure and its intensity variation in the flow direction under 16 working conditions,it is found that the TLV intensity increases monotonously with the increase of incidence angles.However,the change of clearance sizes can significantly change the vortex structure types in the gap region,which leads to different variation patterns of TLV intensity with gap sizes at different angles of attack.The time-averaged pressure distribution on the center line of TLV is also significantly affected by the change of incidence angles and clearance.The minimum pressure at the TLV center under different gaps is closely related to tangential velocity distribution within and around TLV core.Under the condition of small gap size,the vortex core region can be divided into rigid body rotation region,turbulent viscous mixing regiona and leakage flow dominant region.As the tip clearance size increases,tip separation vortex(TSV)will be rolled up around TLV,and the TLV core region can be mainly divided into rigid body rotation region,turbulent viscous mixing region,TSV roll-up region and leakage flow dominant region.The existence of TSV and tip leakage flow is responsible for the suppression and promotion of TLV development,respectively.In addition,intense distribution of the fluctuating pressure in the vortex center indicates that there is a strong pressure fluctuation in the TLV center.By analyzing the distribution of different Reynolds stresses terms,it is clarified that the intense fluctuation at the TLV center mainly comes from the turbulent structures which are rolled up around TLV,including the induced vortices(IVs),TSV,wake vortices(WVs)and shear layer near the gap wall.According to the x-component of time-averaged velocity distribution on the TLV center line,axial flow in the TLV core can be classified as jet-like and wake-like states.Both strong excess and deficit of the axial velocity can be found in the present investigation.During the formation of TLV above the foil,a positive pressure gradient in the axial direction develops in the TLV core,which leads to the acceleration of axial flow,whose maximum value can reach about twice the mainstream velocity.Deceleration of the TLV core fluid is mainly affected by mixing of TLV core flow with flows characterized by low axial velocity,such as TSV and WVs under low incidence angle conditions,as well as IVs and the boundary layer of the gap wall uhder high incidence angle-small clearance size conditions.As a result,the wake-like axial flow is formed,and the minimum value can be as low as about 0.5 times of the mainstream velocity.(3)The noise characteristics and sound generation mechanism of TLV cavitation inception are analyzed and revealed.The main research works and findings are as follows: The far-field noise of TLV cavitation inception is dominated by thickness noise and presents characteristics of a monopole source.The change of angles of attack and gap sizes has a significant effect on the sound intensity,which increases monotonously with the increase of the angle of attack.And it first increases and then decreases with the expansion of the gap.The maximum value of cavitation volume has the same variation pattern as the far field noise.By looking into the distribution of time-averaged pressure and fluctuating pressure in the vortex center,it is found that the variation of gap size shifts the position of the lowest pressure point at the vortex center,so does the location of cavitation inception.As the lowest time-averaged pressure point superimposes on the location with strong pressure fluactuation,the low-pressure region at the TLV center is broadened,which greatly increases the cavitation inception volume,under the conditions with dimensionless clearance of 1.0 and 1.5.Based on the theoretical formula of bubble noise during its collapse derived from the potential energy stored in the expanded bubble,it is found that the bubble formed by the growth of a nucleus will release a strong pulse during collapse,and its intensity is mainly related to the maximum radius of the bubble.(4)Based on the numerical results of the flow field and sound signals of sheet cavitation and TLV cavitation in fully developed TLV cavitating flow,evolution characteristics and collapse mechanisms of sheet cavitation and TLV cavitation are expounded respectively.Moreover,acoustic characteristics and mechanisms of sound generation by sheet and TLV cavitating flow are analyzed and clarified.Through the analysis of cavitation volume variation and evolution process,it is found that the maximum volume and the evolution period decrease with the increase of gap sizes.During its evolution,the TLV cavity firstly brokes in the radial direction into multiple sub-TLVCs,and then shrinks or completely collapses.The collapses of cloud and TLV cavities are triggered by the rise of pressure inside and outside the cavities.Outward transport of vorticity inside the cavity by the dilatation term acts as an important role to continue weakening the vortex together with its ability in maintaining low pressure for cavitation.High pressure distribution and jets are observed at the ends of Sub-TLVCs,which may further accelerate the collapse of sub-TLVCs and induce intense noise pulses consequently.Though the proportion of TLV cavity volume is very small as compared with that of sheet cavity,the radiated noise induced by TLV cavitating flow is found to be comparable to that of sheet cavitating flow,which both play important roles in the overall sound field and worth to be investigated separately.There is a great difference in the spectral characteristics between sheet cavitation noise and TLV cavitation noise.The sheet cavitation noise also shows clear periodicity in its variation as that of sheet cavity volume,and there is an obvious dominant frequency in the low frequency range of the spectrum.On the other hand,TLV cavitation noise has no dominant frequency and has a strong distribution of sound at the middle and low frequencies.Both of sheet cavitation noise and TLV cavitation noise have strong broadband components in the high frequency band.The contributions of thickness source term and loading source term in the FW-H equations to far field acoustic results are analyzed.It is found that cavitation evolution mainly enhances the middle and low frequency noise through the second derivative of cavity volume to time.On the other hand,the hydrofoil loading mainly enhances the high-frequency far field noise through the first derivative of lift force to time.