Research On Supression Of Unsteady Cloud Cavitation Based On Hydrofoil With Bionic Fin Structure

Cavitation is the process of localized bubble generation caused by local pressure drop in the flow field.The negative impact of cvitation cannot be ignored in spite of many examples of using cavitation in industry.In the field of hydrodynamics,the study of cavitation mainly focuses on the negative effects of cavitation.Whether it is a pump,a turbine or a propeller,stable sheet cavitation usually does not bring serious effects.Once the cavitation is unstable,a series of problems such as vibration,noise and cavitation will occur.Among various cavitation types,cloud cavitation has strong unsteady characteristics and its damage is also the greatest.Therefore,it is important to study cloud cavitation control.This paper introduces the cavitation mechanism and cavitation control method.It mainly studies the unsteady process of cavitation in the current process of cavitation mechanism research.Cavitation control methods can be subdivided into active control methods and passive control methods.In order to understand the mechanism of cavitation and its control methods,the hydrofoil model is selected to study in this paper.In this paper,methods to improve or control hydrofoil cloud cavitation are explored.It provides a reference for anti-cavitation design of hydraulic machinery based on airfoil and cascade lift theory.This paper takes NACA 0015 hydrofoil as the research object.The unsteady cloud cavitation process of 3D hydrofoil is simulated.By studying the swimming characteristics of fish,it was found that the fish propelled by the tuna model had good hydrodynamic performance.Typical characteristics of tuna-propelled fishes are that their tails provide power,and the tail fins control the flow by changing the angle of attack with the current.After data extraction,it was found that the tail fin profile of swordfish was symmetrical airfoil,and there was an inclined spin structure on the surface of the tail fin.Based on the research of hydrofoil cloud cavitation and the idea of bionic fin-fin structure,the bionic structure is arranged on the hydrofoil suction surface to explore its control effection.The impact of the position and height of the bionic structure is studied on the cloud cavitation process.Based on previous research of cavitation at home and abroad,the research is mainly carried out by numerical simulation.The SST k-? turbulence model and the ZGB cavitation model are used in the numerical simulation.At the same time,in order to solve the problem that the viscosity coefficient ?t of the kinetic energy of the two-phase region is too high,the reliability of the numerical algorithm is verified by introducing the density function.The numerical simulation results are in good agreement with the experimental results,which verify the reliability of the numerical algorithm.In this paper,the research on 3D NACA 0015 hydrofoil cloud cavitation flow and cavitation control based on bionics mainly includes the following contents:(1)When the angle of attack is 8 degrees and the number of cavitations is 0.6,0.8 and 1.0,the three-dimensional NACA 0015 hydrofoil cavitation flow is studied.The variation of the hydrostatic lift coefficient,draft coefficient and the suction surface pressure under different cavitation numbers is studied.It was found that there are two distinct peaks in pressure fluctuations in a complete cloud cavitation cycle.It indicates that the cavitation vacuolization process is not directly collapsed.There is a process that the bubble is stretched to compression and then stretched and collapsed.The causes of the U-shaped shedding structure during cloud cavitation and the secondary compression process in the process of vacuolar shedding were analyzed.By analyzing the pressure distribution and the velocity vector of the cross section of the hydrofoil,it was found that the jet intensity in the chord direction and the span direction are different.The unevenness of the spread pressure distribution causes the "U" type when the cloud vacuole falls off.(2)Based on the bionics idea,the discontinuous tilting obstacles of the fishtail fin fin structure are arranged on the suction surface of NACA 0015.The research,numerical simulation of unsteady cloud cavitation process with chord wise positions of 0.2c,0.3c and 0.5c,was analyzed.It is found that the cloud cavitation shedding period becomes longer,and the cavitation volume is reduced by nearly 60% compared with the previous one when the bionic structure is arranged at the chord of 0.3c.The control effect is the best.In order to clarify its control mechanism further,the turbulent kinetic energy of the hydrofoil cloud cavitation process was analyzed.It was found that the surface turbulent energy was effectively reduced and the flow field structure was optimized when the bionic structure was arranged.By the vortex dynamics analysis of cloud cavitation flow,it was found that the vortex stretching term can be effectively increased after the bionic structure is arranged.(3)Based on the research on the position parameters of bionic structures,the height parameters of bionic structures are studied.The unsteady cloud cavitation flows were studied at the heights of 1% c,1.5% c,2% c,2.5% c and 3% c.The hydrofoil can obtain the maximum lift-drag ratio,which is higher than that without bionic structure.The volume of the bubble is reduced by nearly 60%.After the bionic structure is arranged,the force performance of the hydrofoil in the cavitation state is optimized.By the analysis of the volume change of the bubble,it is found that when the height of the bionic structure is 2.5%c.The volume of the bubble is reduced by nearly 62%.