绕圆头回转体通气空化流型的实验研究

Experimental research on multiphase flow of ventilated cavity around a hemisphere cylinder

为了深入了解通气空化流动现象,利用高速全流场显示技术,对绕圆头回转体通气空化流型进行实验研究。结果表明,重力效应和通气量对通气空化的多相流流型起主要作用。定义了弗洛德数和通气率两个无量纲数,将绕圆头回转体通气空化分为5种多相流流型,即透明空泡、透明气弹、透明分层、水气混合以及半透明水气混合。流动参数对流型的影响分为2个阶段,即重力起主要作用阶段和重力效应不明显阶段。在重力起主要作用阶段,通气率一定时,随着弗洛德数的增大,附着弹体的空泡倾斜程度变小,弹体上表面的断裂空泡转变为贴着弹体壁面的稳定空泡;弗洛德数一定时,随着通气率的增大弹体上表面断裂空泡的尺度不断增大。在重力效应不明显阶段,通气率一定时,随着弗洛德数的增大,雷诺数变大,流场的湍流强度增大,空泡尾流区域水气交换的程度加剧;弗洛德数一定时,随着通气率的增大,通气空化数减小,绕弹体的云雾状空泡逐渐转变为透明空泡。最后,进一步分析了重力影响下透明空泡脱落的非定常过程,以及反向射流作用下云雾状空泡交替脱落的非定常过程。

To understand the characteristics of ventilated cavity,ventilated cavity flow around a hemisphere cylinder is experimentally studied by applying the high speed visualization technolo-gy.The results show that the gravity effect and air entrainment play a major role in forming mul-tiphase flows of ventilated cavity.Froude number and air entrainment coefficient are defined. Five multiphase flows of ventilated cavity around a hemisphere are denoted,which are transpar-ent bubble flow,transparent slug flow,transparent layering flow,air-water mixture flow and semitransparent air-water mixture flow.The influence of flow parameters on fluid structure is analyzed,and there are two stages.One is the stage where the gravity plays a critical role,and the other is the stage where the gravity effect is limited.In the stage where the gravity plays a critical role,when air entrainment coefficient is fixed,fractured cavity changes into steady cavity attaching to the surface of the cylinder as Froude number increases;when Froude number is fixed,the scale of fractured bubble increases as air entrainment coefficient increases.In the stage where gravity effect is limited,when air entrainment coefficient is fixed,Reynolds number and turbulence intensity increase and bubbly wakes created by ventilated cavity become turbulent as Froude number increases;when Froude number is fixed,cavity number based on cavity pressure decreases and cloud cavity around a cylinder changes into transparent cavity as air entrainment co-efficient increases.Finally unsteady bubble shedding is described when fluid structure is domina-ted by gravity and by re-entrant jet.