Theory And Experimental Investigation Of Unsteady Cavitation Suppression Around A Hydrofoil

The unsteady cavitation phenomenon involves mass transfer of vapor-liquid two-phase working fluid,large-scale turbulent pulsation,and compressibility of cavitation regions,including cavity inception,development,cavity cut off,shedding and eventual collapse.Cavitation is a complex and unsteady flow phenomenon.Therefore the cavitation research involving the safe and stable operation of nuclear power plants,ship propulsion,high-speed water weapons,hydropower stations and relative other fields have received great attention.How to weaken the development of cavitation and thus inhibit the shedding of cavitation has always been the focus and difficult topic of hydrodynamic research.In order to weaken and suppress cavitation development and cavity shedding,and improve the safety of hydraulic components,this paper proposes a new idea and new structure hydrofoils.One of the new forms use jet injected to mainstream through hydrofoil surface to suppress cavitation,the other use special configuration of hydrofoil surface to weaken cavitation.The purpose is to study the control technology and suppression mechanism of unsteady cavitation,so as to solve the key issue of cavitation suppression that exists in numerous engineering fields.Based on the experimental platform of cavitation water tunnel,the high-speed video recording technology and particle image velocimetry system are used to carry out the series of cavitation flow experiments around hydrofoils which include the original hydrofoil,new hydrofoils with active jet injection and special configuration hydrofoils according to the unsteady cavitation flow characteristics.The study realized the measurement and analysis of unsteady cavitation pattern around the hydrofoil in different cavitation stages and the kinematic parameters of the cavitation flow.The RNG k-? turbulence model based on density correction is established,combined with Schnerr-Sauer cavitation model,to perform a mechanism analysis of the unsteady cavitation flow around the hydrofoil.The experimental study on the unsteady cavitation flow characteristics of the NACA66(MOD)hydrofoil reveals the unsteady periodic evolution process of cloud cavitation.Quantitative evaluation of unsteady cavitation flow by multiple indicators such as dimensionless length,dimensionless thickness,dimensionless area and shedding frequency of cavity is also performed.The results show that in the development stage of cloud cavitation,the re-entrant jet which generated at the fully-developed adherent cavity tail above the hydrofoil suction surface is the main cause of cavity breakage and shedding.Combined with numerical simulation analysis,the interaction law between the re-entrant jet intensity and the cavity is revealed,therefore the dimensionless characteristic number Cre is proposed to evaluate the impact force and vaporization effect between the re-entrant jet and the cavity,further the cavitation pattern is achieved.The structural type of the active jet injection through the hydrofoil surface is proposed,and the experimental study of suppressing cavitation by active jet is taken first.The impact of the hydrofoil surface jet feature parameters on the cavitation flow field characteristics and dynamic characteristics of the hydrofoil is analyzed in detail,and the mechanism of cavitation suppression is revealed.The results show that according to the degree of cavitation in the flow field,the flow control from sheet cavitation to cloud cavitation can be realized by adjusting the jet flow rate,and the optimum jet flow corresponds with the best cavitation suppression effect.In the cloud cavitation stage,when the jet is arranged at the position 0.19C from the leading edge of the hydrofoil,the cavitation suppression effect and hydrodynamic performance can be optimized.The study on the mechanism of cavitation suppression based on active jet method reveals that the jet injects momentum into the flow field,results in reducing the low pressure area of the hydrofoil suction surface and the thickness of the boundary layer near the surface of the hydrofoil behind jet injection.The adverse pressure gradient of the tail of the cavitation zone reduces the intensity and the thickness of the re-entrant jet causing the cavitation development weakened and the frequency of cavitation shedding reduced,finally cavitation flow is controlled.According to the idea of improving of the pressure distribution above the suction surface of the hydrofoil,weaking the strength of the re-entrant jet,and blocking the development of the re-entrant jet travelling to the leading edge of the hydrofoil,the special configuration hydrofoils are proposed to suppress the development and shedding of unsteady cavitation.The results show that the single-row pits on the hydrofoil suction surface can reduce the thickness of boundary layer and low pressure area scope,therefore cavitation flow is weakened.The proposed through-hole jet of the hydrofoil surface shrinks the low-pressure area of flo'w and decreasing the intensity of re-entrant jet,as a result,the cavity development is controlled.