旋转圆球入水空泡特性与流场结构的大涡模拟研究

圆球旋转入水过程对于基于先导物投放的新型入水降载方式具有重要研究价值. 采用大涡模拟方法结合均质多相流模型和VOF界面捕捉算法, 对低弗劳德数条件下疏水圆球高速旋转入水的自由运动过程进行了数值模拟, 研究了转速对入水空泡演化、流场结构和水动力特性的影响. 采用动网格与滑移网格技术实现圆球的自由运动, 并基于试验结果对比验证了数值模拟的可靠性与正确性. 旋转运动的升力效应导致圆球入水弹道发生偏转并从水面携带横向楔形射流侵入空泡内部. 采用入水砰击速度与转速进行归一化分析, 结果表明入水转速的增加显著改变了圆球的动力特性: 水平方向的速度和位移以及升力峰值都随入水转速的增加而变大, 但升力峰值受到入水速度的限制; 而垂直方向的速度和加速度以及空泡断裂深度几乎不受转速增加的影响, 并且空泡深闭合发生前圆球转速变化不大. 入水转速的增加也使液面飞溅环和空泡断裂的非对称性增强, 在较低转速时发生空泡表面闭合, 而在较高转速时则发生空泡深闭合. 对于空泡深闭合模式, 入水转速的增加带来更强的横向楔形射流, 并且抑制了空泡断裂产生的高压以及相应涡结构的生成, 致使圆球在入水砰击阶段承受更低的侧向压力. 

NUMERICAL INVESTIGATION ON THE WATER-ENTRY CAVITY FEATURE AND FLOW STRUCTURE OF A SPINNING SPHERE BASED ON LARGE-EDDY SIMULATION1)

The water-entry process of spinning sphere has great significance to the research of the up-to-date load reduction method of water-entry based on pre-launched object. In the present work, large-eddy simulation method is used together with the homogeneous multiphase flow model and VOF algorithm of interface capturing, to simulate the water-entry free motion of a fast-spinning sphere with hydrophobic coating at low Froude number, thus to investigate the water-entry cavity evolution, the flow structure and the hydrodynamic features. The free motion of the sphere is achieved through the dynamic mesh and sliding mesh techniques. The reliability and accuracy of the numerical simulation results are validated by comparison with previously published experimental results with good agreement on the transient cavity shape and the motion of the sphere. The spinning motion induces a lift force on the sphere and the trajectory of the sphere has significant curvature along its descent. A persistent wedge of fluid is emerged across the center of the cavity due to the fluid along the surface dragged by the sphere. The velocity and spin rate were normalized with the impact velocity and spin rate to analyze the numerical results. It shows that the spin rate has significant influence on the cavity evolution and hydrodynamic characteristics. Both of those cavity shapes have asymmetrical splash curtain and collapse asymmetrically. As spin rate increases, the horizontal velocity and the maximum lift force increase, while the maximum lift force is also limited by the impact velocity. The spin rate increase also leads to a stronger wedge of fluid forming. As a result, the pinch-off pressure maximum decreases and less vortex structures are observed. And also, the spin rate increase leads to lower side pressure during the initial impact phase. However, the vertical dynamic characteristics of spheres, like vertical velocity, acceleration and immersion depth of pinch-off, are less affected by the spin rate. Moreover, the sphere spin rate is less affected by the impact spin rate increase before cavity pinch-off.