云状空化是一种以相变为核心，包含多种尺度空泡生长演变行为的强瞬态多相流动现象。该文采用双向耦合欧拉-拉格朗日多尺度空化模型，模拟了空化数σ=1。2时，绕8°攻角三维NACA66水翼空化流动，通过VOF(volume of fluid)界面捕捉法求解欧拉体系下大尺度空穴演化，利用拉格朗日体系下的离散空泡模型(DBM)追踪亚网格尺度离散空泡运动及生长溃灭过程。结果表明:绕水翼云状空化流动，包含附着型片状空泡生长与发展、回射流产生与推进、云状空泡脱落和宏观片空泡再次生长以及压力波推进4个阶段。离散空泡数量及尺度随大尺度空穴周期性演变而变化，主要分布在强湍流脉动区。在各空化发展阶段，离散空泡集中分布在水翼中后部，微空泡直径概率密度函数均符合单峰Gamma分布，最高空泡数密度泡径为50μm。在湍动程度较小的附着型空穴生长阶段，微空泡数密度相对于空泡直径呈-5/3幂率特征;在湍动程度较大的回射流发展阶段、云状空穴脱落溃灭及压力波推进阶段，较小尺度的微空泡数密度相对于空泡直径呈-2/3幂率特征，较大尺度的微空泡数密度相对于空泡直径呈-6幂率特征。
Investigation into the Multiscale Characteristics of Cavitation Flow under Different Cavity Shedding Mechanism
Cloud cavitation is a complex transient multiphase flow phenomenon centered around phase transitions,containing the growth and evolution behavior of bubbles under multiple scales.In this paper,the multiscale cavitating flow on a NACA66 hydrofoil at a cavitation number σ=1.2 and an angle of attack of 8° is studied through a two-way coupling Euler-Lagrange method.The evolution of large-scale cavities under the Euler framework is resolved using the Volume of Fluid(VOF)interface capturing method,while the motion and growth-collapse process of subgrid-scale discrete bubbles are tracked using the Discrete Bubble Model(DBM)in the Lagrangian framework.The results demonstrate that the cloud cavitation flow around the hydrofoil includes four phases:growth and development of attached sheet cavities,formation and evolution upstream of re-entrant jet,detachment and shedding downstream of cloud cavities,and the regrowth of macro-scale bubbles,along with the propulsion of shock waves.The number and size of discrete bubbles change with the periodic evolution of large-scale cavities,primarily distributed in areas of strong turbulent pulsations.During various stages of cavitation development,discrete bubbles are concentrated at the mid-rear part of the hydrofoil.The probability density function of micro-bubble diameters follows a unimodal Gamma distribution,with a dominant size about 50 μm.During the growth stage of attached cavities with lesser turbulence,the micro-bubble number density relative to bubble diameter exhibits a-5/3 power-law characteristic.In stages of higher turbulence,such as the development of re-entrant jets,cloud cavity shedding and collapse,and shock wave propulsion,the micro-bubble number density relative to bubble diameter shows a-2/3 power-law characteristic in smaller size ranges and a-6 power-law characteristic in larger size ranges.
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