超疏水小球低速入水空泡研究

物体入水问题是一类复杂的流固耦合问题,具有广泛的工程应用背景.物体在跨越自由液面入水的过程中,在一定的条件下,会向水中卷入空气形成空泡,空泡的运动还可能形成指向物体的射流,从而对物体的受力及其运动过程产生影响.超疏水表面能够在物体入水过程中形成多尺度流固耦合作用,进而影响物体的运动和宏观流动现象.而对于小尺度的小球低速入水问题,表面和界面力往往起主导作用.为了在更广的参数空间获得超疏水小球入水空泡类型和小球的运动特性,采用高速摄影实验方法,研究了半径0.175～10 mm的超疏水小球低速入水及空泡动力学行为,获得了小球漂浮振荡、准静态空泡、浅闭合空泡、深闭合空泡和表面闭合空泡5种类型的动力学行为,探讨了这些运动行为与韦伯数We和邦德数Bo之间的关系,并推导了小球漂浮振荡与下沉现象的无量纲关系.研究结果表明:超疏水小球的入水及空泡动力学行为主要与韦伯数We和邦德数Bo有关.在邦德数Bo -1)范围内,表面张力对流动的影响显著,随着韦伯数We的增大,小球入水及空泡动力学行为依次经历漂浮振荡、准静态闭合、浅闭合、深闭合和表面闭合;在邦德数O(10^-1) O(1)后,浅闭合现象也不再发生;小球漂浮振荡与下沉现象的临界关系可以用相似律关系描述.

STUDY ON LOW-SPEED WATER ENTRY OF SUPER-HYDROPHOBIC SMALL SPHERES

Water-entry ofobject is a type of fluid-structure interaction problem withcomplex physics, which has a wide range of engineeringapplications. During the water-entry process, as the object passesthe water-air interface, the possible air entrapment may formcavity under specific conditions and the dynamic movement ofcavity can generate a jet pointing towards the object. Both thecavity and jet can greatly influence the force on the object andtherefore influence its movement. Moreover, the super-hydrophobicsurface can form multi-scale fluid-solid coupling phenomena in theprocess of water-entry of object, which in turn affects the motionof the object and the macroscopic fluid flow. For small scalewater-entry problem, the surface or interfacial force rather thanbody force is usually dominant. The objective of present study isto obtain the cavity behaviors and motion of characteristics forthe problem of water-entry of small super-hydrophobic spheres in awider parameter space. Based on such purposes, the high-speedphotography experiment method is used to study the low-speedwater-entry and cavity dynamics of super-hydrophobic spheres witha radius of 0.175$\sim $10 mm. Five dynamic modes are obtainedincluding sphere floating oscillation, quasi-static impact cavity,shallow seal impact cavity, deep seal impact cavity and surfaceseal impact cavity. The relationship between these dynamic modeswith the Weber number and Bond number is discussed, and thedimensionless relationship between the floating oscillation andsinking behaviors of the sphere is derived. The results show thatthe cavity behaviors of water-entry of super-hydrophobic spheresare mainly related to the Weber number and the Bond number. In therange of $BoO(1)$, the shallow seal state also does not happen. Thecritical relationship between the floating oscillation and thesinking of sphere can be described by the scaling laws. A formulais developed to categorize the floating oscillation andquasi-static impact cavity phenomena.