绕弹性水翼空化流激振动特性实验研究

Experimental investigations of cavitating flow induced vibration around a flexible hydrofoil

弹性结构在空化载荷作用下易发生振动,从而威胁结构安全,研究绕弹性水翼的空化流激振动特性显得尤为重要。采用高速摄像、测力装置和激光多普勒同步测量系统,分别获取了不同空化阶段的空泡形态、水动力、结构变形及相应诱导的结构振动特性。研究结果表明,弹性水翼的水动力系数在无空化阶段几乎保持不变,随着空化产生,升力、阻力和扭转力矩系数都随着空化数减小呈现先增大后减小的趋势。水翼在云状空化阶段下振动最为剧烈,大尺度空泡的周期性脱落导致结构振动速度呈现周期性变化。初生空化、片状空化和云状空化阶段的振动速度、概率密度分布均服从高斯分布,其中云状空化阶段的高斯分布曲线均方根最大,这是由于云状空泡团的脱落和溃灭导致的速度波动较大。在云状空化阶段,较大空化数下呈现回射流非定常脱落机制,较小空化数下呈现激波非定常脱落机制。回射流机制下,空穴演化呈现附着空穴发展-回射流发展-云状空穴脱落和溃灭的非定常过程。激波机制下空穴演化具有附着空穴发展-回射流发展-云状空穴脱落和溃灭-残余空穴溃灭的非定常特征。不同脱落机制下结构振动特性存在显著差异,回射流机制呈现小幅、高频的振动特性。由于大尺度空泡团的溃灭激波及其引起的附着空泡的二次溃灭,导致结构振动加剧,激波机制呈现大幅、低频的振动特性。

Flexible structures are prone to vibrate under cavitation loads,threatening the safety of structures.Therefore,it is important to analyse the cavitating flow induced vibration around flexible hydrofoils.A highspeed camera,a Laser Doppler Vibrometer(LDV),and a hydrodynamic load cell are exploited to observe the cavitation structures,measure the hydrofoil vibrations,and measure the lift and drag static forces,respectively.The load coefficients of the flexible hydrofoil keep constant in the sub-cavitation stage,while they first increase then decrease with the decay of the cavitation number in the cavitating flow.The probability density functions of vibration velocities in the incipient cavitation,sheet cavitation,and cloud cavitation stages are all Gaussian.The vibration velocity of the hydrofoil has the largest amplitude and root-mean-square in the cloud cavitation stage and varies periodically due to the periodic shedding and collapse of the large-scale cloud cavitation.The re-entrant jet and shockwave shedding are the two main shedding mechanisms in the cloud cavitation stage.Similarly,the evolution of the cloud cavity induced by the shockwave mechanism can be divided into four stages:1)the attached cavity growth;2)the re-entrant jet development;3)the shedding and collapse of the cloud cavity;4)residual cavity collapse.The vibration induced by the re-entrant jet mechanism has small amplitude and high frequency,while the opposite is true for that induced by the shockwave mechanism due to the collapse of the large-scale cavity and the subsequent secondary collapse of the attached cavity.