Microstructure Design Of Hydrofoil Surface And Its Influence On Cavitation Flow Field

Cavitation is an important reason for the decline of hydraulic mechanical performance,vibration and noise of the system and cavitation erosion on the blade surface.Due to the complicated structure and high rotating speed of hydraulic machinery,it is not easy to study the mechanism of the occurrence and evolution of cavitation in numerically and experimentally.Therefore,study of cavitation around the hydrofoil can help to better understand the cavitation problem in hydraulic machinery with its results can be of great help for exploring ways to suppress cloud cavitation and further improving hydraulic machinery performance.In this paper,we study the impact of hydrofoil surface water injection on cavitation suppression around the NACA0066 hydrofoil.The accuracy of modified RNG k-? turbulence model combined with Singhal et al.full cavitation model is validated on the basis of comparison and optimization analyses.The cavity shedding frequency and the periodic evolution of cavitation are all predicted accurately.Then,the spectrum characteristics of cavitation is studied by using the selected model.It is found that when the angle of attack is 6°,the cavity oscillation frequency decreases with the increase of cavitation number.In the cloud cavitation stage,the power spectral density,which characterizing the cavity instability,is relatively high.In the transition period of cloud cavitation and sheet cavitation,there is a sudden jump in power spectral density.In the sheet cavitation stage,the power spectral density is gradually reduced.While it is very different when the angle of attack is 8° and 10°.The cavity oscillation frequency does not change with the cavitation number and its strouhal number(St)is stable in the vicinity of 0.21 in cloud cavitation stage.In the transition period of cloud cavitation and sheet cavitation(near ?/2? =3.5),there is also a sudden jump in power spectral density.On the basis of this analysis,the paper focuses on the research of the interaction law between the water injection position,the jet exit angle and the cavitation suppression effect and the hydrodynamic performance of the 2D hydrofoil.Study of water injection at 6° angle of attack and 8° angle of attack under different cavitation conditions shows that the optimal jet position is only related to the angle of attack of hydrofoil,and this position is located at the vertex position on the upper surface of hydrofoil at the corresponding angle of attack.The angle of jet is another important factor affecting the cavitation suppression effect.Study found that when jet direction is opposite to flow direction and having angle of 155°,the cavitation suppression effect is significant with the lift-drag ratio increased to some extent.The possible mechanism of cavitation suppression has preliminary studied as well.Water injection in an appropriate position and with a proper jet angle is equivalent to inject energy into the boundary layer,which changes the pressure distribution on the suction side of hydrofoil and reduces the reverse pressure gradient near the trailing edge of hydrofoil.The reduction of adverse pressure gradient near the trailing edge weakens the power of re-entrant jet formation,thus cloud cavitation is effectively suppressed.Based on the simulation results of 2D hydrofoil,the effect of spanwise jet hole porosity(?)on cavitation suppression is studied.The influence of the introduction of jet on the cavitating flow field is revealed from the periodic evolution of cavitation,the motion characteristics of the re-entrant jet,and the dynamic characteristics of the hydrofoil.In the optimal jet position(x=0.25c),the single row spanwise jet hole porosity should not exceed 58.92%,for excessive jet hole porosity will result in a significant decrease in hydrofoil dynamic performance.