Reaearch On The Flow Characteristics Of Cloud Cavitation Around S-shaped Hydrofoil

S-shaped hydrofoil is widely used in the design of reversible fluid machinery.Rotating machinery designed from symmetrical S-shaped hydrofoil has the same performance in positive and negative direction.Considering the labor cost and operation difficulty,bi-directional axial flow pump is the most convenient way to achieve the integration of pumping station.In the cavitation test of axial pump,it is found that the cavitation performance of bi-directional axial pump is obviously different from that of unidirectional axial pump,and the use of S-shaped hydrofoil in the design of high Angle of attack is easy to induce the generation of cloud cavitation in advance.Therefore,the study of cloud cavitation flow phenomenon based on S-shaped hydrofoil can lay a foundation for the further study of cavitation in impeller of bi-directional axial pump and the reduction of influence of cloud cavitation on bi-directional pump station in the future.In this paper,the cavitation flow around S-shaped hydrofoil was studied by cavitation water tunnel test combined with numerical calculation method.The instantaneous flow characteristics of cloud cavitation in different dynamic forms of S-shaped hydrofoil were found by high-speed photography test.The correlation between cloud cavitation movement and main frequency change of lift and drag were analyzed by six axis force and torque sensors.The FBDCM turbulence model was used to obtain the flow field under cloud cavitation of S-shaped hydrofoil,which was in good agreement with the experimental results.The internal motion mechanism causing cavitation shedding of different shapes was revealed,and the special pressure distribution of S-shaped hydrofoil and the dominant frequency variation caused by the surface pressure change were discussed.Based on different vortex identification criteria and vorticity transport equations,the vortex evolution mechanism caused by cloud cavitation under different working conditions is mastered.Combined with the modified one-dimensional flow theory,the source of unstable pressure pulsation on the surface of S-shaped hydrofoil induced by cloud cavitation is clarified.The main conclusions are as follows:1.The cavitation flow and lift-drag characteristics of S-shaped hydrofoil under different cavitation numbers and different angles of attack were tested.Through highspeed photography test,it is found that the cloud cavitation evolution characteristics of Sshaped hydrofoil are different from other hydrofoils.With the decrease of inlet cavitation number,when the angle of attack α>0,the development of cavitation stage of S-shaped hydrofoil is similar to that of conventional hydrofoil,which has experienced the development process from bubble birth to sheet cavitation and then to cloud cavitation;When the angle of attack α<0,the cloud cavitation phenomenon will gradually transforms to sheet cavitation with the decrease of cavitation number,and the sheet cavitation stage and cloud cavitation stage will appear alternately for many times with the decrease of cavitation number,which indicates that the special shape of S-shaped hydrofoil will induce the phenomenon different from the conventional hydrofoil in the process of cavitation development.2.The lift-drag of S-shaped hydrofoil is collected by a six-dimensional force and torque sensor.It is found that the change of Reynolds number is linearly correlated with the Angle of attack at stall occurrence under the condition of no cavitation.The lifting resistance fluctuation is small in the stage of no cavitation,and the stable sheet cavitation morphology can restrain the lift-drag fluctuation.Before the transition oscillation cavitation occurs,the main frequency of the lift and drag are about twice that of the cloud cavitation.After the transition oscillation cavitation occurs,the main frequency of the lift and drag is consistent with the main frequency of the cloud cavitation,which is not affected by the angle of attack and cavitation number.However,the unique alternating shedding phenomenon of S-shaped hydrofoil will lead to the generation of different harmonic frequencies of components.Under the negative Angle of attack,the amplitude lift and drag reaches the maximum at the first cloud cavitation stage,and then decreases and tends to be stable as the decrease of cavitation number.3.Based on the FBDCM turbulence model and ZGB cavitation model,the numerical simulation of cloud cavitation with different dynamic morphology was carried out,and the reliability of the numerical calculation was verified by the test results of the lift-drag characteristics and the evolution of cloud cavitation.Combined with the re-entrant jets movement and strength distribution in the process of cloud cavitation,it is explained that the partial cloud cavitation phenomenon at low angle of attack and designed angle of attack is caused by the significant increase in the thickness of the lower part of the cavity caused by the special shape of the S-shaped hydrofoil,but the thickness of the first part is not enough to provide conditions for the re-entrant jets movement.The special shape of Sshaped hydrofoil will cause the maximum strength of the re-entrant jet at the position of 0.5C,resulting in the shedding at the position where the first cavitation length develops to 0.5C.Meanwhile,it also disrupts the velocity field at the tail of the cavity,reduces the strength of the return jet at the second development to 0.5C,and ensures the smooth development of the cavitation length to 1.0C.The result is multiple cloud cavitation with different degrees of alternating shedding.The main reasons for the continuous rise of lift coefficient are explained by the time-average surface pressure coefficient of S-shaped hydrofoil,and the main reasons for the change of main frequency of lift-drag under different degrees of cloud cavitation are explained by the change of pressure coefficient induced by cloud cavitation movement at different times.4.The vortex around s-shaped hydrofoil caused by cloud cavitation was identified by Q-criterion and Q-criterion,and the correlation between cloud cavitation evolution and vortex motion was explored.U-shaped vortex structure existed in s-shaped hydrofoil flow field at design angle of attack stage and small scale shedding cloud with higher angle of attack(0.25C<l<0.5C)was found.U-shaped vortex comes from the collapse movement stage of cloud cavitation,which is caused by the uneven distribution of re-entrant jets and side-entrant jet in the spanwise direction.When the falling cavitation cloud completely dissipates,the vortex volume in the flow field will develop to the maximum,and the alternating of cavitation and vortex will easily cause the blockage and performance decline of hydraulic machinery.Combined with the vorticity transport equation,it is found that the vorticity distribution caused by cloud cavitation mainly comes from the stretching effect of cloud cavity,while the vortex dilatation term and baroclinic term respectively provide vorticity evolution in cavitation and vorticity transfer at the vapour-liquid interface.5.Considering the effect of vapour-liquid alternating cover,the proposed modified function of one-dimensional theory can improve the prediction accuracy of pressure pulsation on suction surface under different cloud cavitation conditions of S-shaped hydrofoil.Dissipation process in the cloud cavitation caused the strong pressure on the suction side of the S-shaped foil,the main factors of small scale cloud cavity fall off the high pressure intensity coefficient of concentration of cloud cavity fracture and dispersed location,and large scale cloud cavitation loss caused by high stress intensity factor is mainly come from the cloud cavitation at the trailing edge.