不同密度比球体入水空泡流体动力特性研究

基于实验与数值计算相结合的方法,针对不同密度比的疏水性球体开展了垂直入水空泡形态及水下流体动力特性研究。建立了基于高速摄像法的小型航行体入水实验系统,并进行了入水空泡高速录像观察。基于VOF方法和动网格技术建立了考虑表面润湿性的回转体入水数值模拟方法。通过与实验结果对比,验证了数值方法的准确性和有效性。基于对实验与数值结果的分析,总结了疏水性球体的入水空泡及水冠发展随密度比与入水冲击速度的变化规律,对比了不同密度比球体在水下空泡夹断前后的流体动力系数。结果表明:随着入水冲击速度的增加,球体动能加大,入水空泡和水冠尺度增大,并从准静态闭合空泡逐渐发展为深闭合及面闭合空泡,临界速度随着密度比的增加而减小。此外,空泡夹断后会形成上下两股高速射流,射流的进一步运动加速了水面及球体附近空泡的溃灭。在流体动力特性方面,球体带空泡航行阶段的时均流体动力系数随密度比的增加而减小,而随入水冲击速度的变化较小,同时空泡夹断会造成流体动力较大波动。

The Investigation of Cavity Dynamics During Water Entry of the Spheres with Different Dendity

The objective of this paper is to investigate the cavity dynamics during water entry of hydrophobic spheres with different density by combining experimental and numerical methods. A high-speed digital camera system is established for the experiments. The numerical simulations are performed by using VOF (Volume of Fluid) and dynamic mesh methods for the water-entry of revolution body, and surface wettability effect is considered. The effectiveness of the numerical method is validated with experimental results. The effect of the impact velocity and density ratio of the hydrophobic spheres on the splash crown and cavity dynamics during water entry are discussed. Besides, comparison of hydrodynamic characteristics during water entry between the spheres with different density is conducted. Firstly, greater splash crown and air cavity form after impact as the impact velocity increases and different sphere density leads to different cavity regime, especially for the steel sphere which successively experiences the quasi-static impact cavity, deep seal impact cavity and surface seal impact cavity regimes as the impact velocity increases as well as the critical velocity for each cavity regime decreases with increasing density ratio of sphere. Meanwhile, the water jets develop at the point of pinch-off in opposite directions and boost the cavities collapse. Secondly, higher density ratio of sphere leads to a larger time-average hydrodynamic coefficient which is fluctuated by air cavity pinch-off.