运动物体与两相界面相互作用的动力学研究

发布时间：2018-08-23 14:01

【摘要】：运动物体与两相界面相互作用的问题是日常生活、自然界和工业应用中十分常见的流动现象。由于该问题的流动复杂性,虽然已有前人大量的研究工作,然而仍有一些流动机理尚未明确,如物体入水形成空腔的流动机理以及物体出水的完整物理图像等。本文结合数值模拟和实验方法研究了两类典型的运动物体与界面相互作用的流动问题:物体入水和物体出水,重点分析了其中出现的流动现象及内在的流动机理。主要工作及研究成果如下:(1)采用数值模拟和实验方法研究了运动物体入水问题,揭示物体表面润湿性、流体惯性、物体几何形态和接触线钉扎位置对空腔形成的影响及其内在的流动机理和动力学演化。以圆柱和圆球这两种几何形态的物体作为研究对象,我们发现接触线的钉扎与固体侧壁面附近发生的流动分离有关,钉扎位置一般位于圆球的赤道上方和圆柱下端边缘附近。通过对钉轧接触线附近的准静态流场进行力平衡分析,定量地得到了接触线钉扎位置与固壁表面浸润性、流体惯性和物体几何形态等因素的标度率关系,该理论分析结果与数值和实验结果相符;基于势流理论的Rayleigh-Besant方程求解了空腔发展形态,发现无粘流主导了空腔的径向伸展;数值发现了空腔内壁毛细波传播的色散关系与空心圆柱射流上扰动传播的色散关系相一致,而且与入水物体的几何形态无关。此外,我们还研究了物体几何形态和液相粘性对空腔形成的影响,并发现流线型物体入水更易形成空腔;通过考虑粘性耗散能,修改了无粘势流的伯努利方程,从而得到了液相粘性对接触线钉轧位置的标度率关系,该关系与数值结果一致且预测并解释了液相粘性越小越易形成空腔的原因。(2)采用数值模拟和理论分析的方法,研究了初始浸没物体出水问题。以圆柱作为研究对象,探讨圆柱出水过程中的界面演化和接触线运动的动力学行为及其内在的流动机理,给出了圆柱出水整个出水阶段的物理图像,并根据是否形成接触线定义了出水问题中的两种典型流动模态:液膜破碎和液膜包裹。参数研究表明液体粘性是决定物体是否能被包裹的重要因素,数值结果表明液膜包裹的临界条件仅由Oh数决定,与圆柱运动速度无关。液膜包裹模态中包裹圆柱表面的液膜形态最终会达到准静态,且圆柱侧壁面液膜厚度仅依赖于Re数,基于功能关系的简单模型,提出相关标度率关系。我们还发现圆柱尾端液柱的断裂时间仅依赖于Ca数。液膜破碎模态中发现运动接触线相对于圆柱的运动速度是由Oh数和表面润湿性决定。接触线的运动随着Oh数的增大逐渐从惯性模式发展到粘性模式,而且在惯性模式下接触线运动的相对速度与圆柱自身运动速度相关,而在粘性模式下则与圆柱自身运动速度无关。物体几何形态对出水问题的影响主要表现在对流动模态和阻力做功,研究发现不同几何形状的物体入水会导致生成接触线位置的不同。由于接触线更容易在长椭球的北极点处生成,因此具有流线型特征的长椭球出水所承受的阻力较其他形态物体小得多,这为一般水生生物拥有流线型形态提供了合理解释。
[Abstract]:The interaction between a moving object and a two-phase interface is a very common phenomenon in daily life, nature and industrial applications. Due to the complexity of the problem, although a lot of previous studies have been done, there are still some unspecified flow mechanisms, such as the flow mechanism of the cavity formed by the object entering water and the effluent of the object. In this paper, the interaction between two typical moving objects and the interface is studied by means of numerical simulation and experimental methods. The flow phenomena and the internal flow mechanism are analyzed in detail. The main work and research results are as follows: (1) Numerical simulation and experimental methods are used. In this paper, the problem of moving objects entering water is studied, and the effects of surface wettability, fluid inertia, geometric shape of objects and pinning position of contact lines on cavity formation are revealed. The pinning position is generally located above the equator of the sphere and near the edge of the lower end of the cylinder. Through the force balance analysis of the quasi-static flow field near the pinning-rolling contact line, the scaling rates of the pinning position of the contact line, the surface wettability of the solid wall, the fluid inertia and the geometric shape of the object are obtained quantitatively. Based on the potential flow theory, the Rayleigh-Besant equation is solved and it is found that the inviscid flow dominates the radial expansion of the cavity. The numerical results show that the dispersion relation of capillary wave propagation in the cavity wall is consistent with the dispersion relation of perturbation propagation on the hollow cylindrical jet. In addition, the influence of geometry and liquid viscosity on cavity formation is also studied, and it is found that streamlined objects are more likely to form cavities when entering water. By considering viscous dissipation energy, the Bernoulli equation of inviscid potential flow is modified, and the scale of liquid viscosity on the rolling position of contact wire pins is obtained. (2) Numerical simulation and theoretical analysis were used to study the problem of initial submerged body effluent. Cylinder was taken as the object of study to investigate the interfacial evolution and the dynamic behavior of contact line motion in the process of effluent. The physical images of the whole stage of the cylinder effluent are given, and two typical flow modes of the effluent problem are defined according to whether the contact line is formed: the breakage of the liquid film and the encapsulation of the liquid film. The critical condition of wrapping is determined only by the Oh number and is independent of the velocity of the cylinder. The liquid film morphology on the surface of the cylinder in the liquid film wrapping mode will eventually reach quasi-static state, and the thickness of the liquid film on the side wall of the cylinder only depends on the Re number. It is found that the velocity of the contact line relative to the cylinder is determined by Oh number and surface wettability in the liquid film breaking mode. In the viscous mode, it is independent of the velocity of the cylinder itself. The influence of the geometric shape of the object on the water outflow is mainly manifested in the work done on the flow mode and resistance. It is found that the position of the contact line is different when the object with different geometric shape enters the water. The resistance of streamlined ellipsoidal effluent is much less than that of other bodies, which provides a reasonable explanation for the streamlined morphology of aquatic organisms.
【学位授予单位】：中国科学技术大学
【学位级别】：博士
【学位授予年份】：2017
【分类号】：O35

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