基于晶格Boltzmann方法研究接触角的测量和迟滞

发布时间：2018-11-20 19:55

【摘要】：在计算流体力学中,起源于格子气自动机和分子动理论的晶格Boltzmann方法,发展成为一种可靠的新途径,其高效性、准确性和鲁棒性也被普遍证实。它不再依赖于宏观连续介质的Navier-Stokes方程,而从微观模型出发模拟复杂流体的宏观行为,在研究颗粒流、热流动、微流体等复杂流体运动中得到成功的应用。特别是在多相流研究中,由于晶格Boltzmann方法不需要边界积分,避免了传统计算流体力学方法在跟踪大量分散相界面时遇到的困难。基于热力学原理,课题组提出了具有热力学一致性和伽利略不变性的多相流模型,为研究表面润湿现象打下了基础。表面润湿是一种常见的自然现象;作为体现表面润湿性的特征量,接触角是气体、液体和固体表面三相相互作用的结果,也是研究和应用中常用的物理量。虽然晶格Boltzmann方法模拟研究表面润湿已经取得了很大成功,但是还没有一种可靠的算法计算动态的接触角;即使是静态接触角也只能采用理想的球冠模型计算或使用外部工具测量。首先采用化学势边界条件,将液滴直接落在基板上,不再需要过渡区,接触角可以直接从三项接触点开始测量。然后通过分析接触角的微观形态,使用气液界面和第二排格线的交点确定切线,提出了在模拟过程中实时地计算动态接触角的方法。在忽略重力的条件下,对该方法计算的接触角和理论计算结果做了系统的比较。在温度为0.7和0.8时,在30至160度范围内符合的非常好,说明该方法的计算结果是准确的。在相同的固体界面上,液滴半径从20至200个格子,计算所得的接触角保持不变,说明该方法的计算结果是稳定的。在包含重力的情况下,计算了大小从0.1cm到0.5cm的不同的平面上液滴和悬垂液滴。结果显示,随着尺寸的变大,液滴的形变程度越来越大,但是当前方法计算得到的液滴接触角保持不变。这与理论预期是一致的,也真实地反映了接触角的微观本质。进一步将液滴放置在亲疏水交替排布的固体表面,在表面倾斜程度不太大的时候,液滴在重力的作用下会发生倾斜,从而产生接触角迟滞现象。使用三种不同亲疏水配置的表面计算接触角迟滞,结果显示,尽管疏水的模式表面比亲水表面具有更大的接触角,而且液滴更不容易失稳滑动,但是接触角迟滞随着表面的倾斜增大的趋势却是基本一致的。当失稳滑动以后,液滴会连续的滑过亲疏水条带,前进角会逐渐增加然后突然下降,后退角会逐渐减小然后突然增加,二者的变化一般不同步,导致接触角迟滞呈现动态的波动状态。接触角迟滞的动态波形与前进和后退角的相位有关,而且包含了由于液滴的抖动说产生的高阶波动。这些关于接触角和迟滞的研究促进了深入理解并实际应用润湿现象。颗粒在管道流中的跨流层侧向迁移是奇妙的自然现象,近几年已发展成为微流控领域的一个研究热点。利用多弛豫晶格Boltzmann方法,我们研究了椭圆颗粒的惯性聚集现象。与经典的Segre-Silberberg效应类似,椭圆颗粒也表现出侧向迁移和平衡。但是因为其非圆几何形状,椭圆颗粒的运动中还包括着非匀速扭转和周期性振动。我们分别研究了雷诺数、阻塞比和纵横比对椭圆运动轨迹的影响,发现在雷诺数改变对椭圆平衡位置的影响很小,雷诺数从3变到300时,平衡位置的变化率只有3%;阻塞比越大的颗粒,其平衡位置越接近靠近管道的中心线,并且颗粒对流场的影响越强烈;随着纵横比的增加,椭圆颗粒的旋转周期变短,而且平衡位置呈现马鞍形变化,在纵横比接近0.5时达到最低点。这项工作对于研究具有卵形或椭圆形红细胞的鸟类的血液流有积极的促进作用。
[Abstract]:In the computational fluid dynamics, the lattice Boltzmann method, which is derived from the lattice gas and the molecular dynamic theory, has been developed into a reliable new way, and its efficiency, accuracy and robustness are also widely confirmed. It is no longer dependent on the Navier-Stokes equations of the macro continuum, and the macroscopic behavior of complex fluid is simulated from the micro-model, and the successful application in the complex fluid movement such as particle flow, thermal flow, micro-fluid and the like is studied. In particular, in the multi-phase flow research, since the lattice Boltzmann method does not need the boundary integration, the difficulties encountered by the traditional CFD method in tracking a large number of dispersed phase interfaces are avoided. Based on the principle of thermodynamics, the research group put forward a multi-phase flow model with thermodynamic consistency and Galileo invariance, which laid the foundation for the study of surface wetting. Surface wetting is a common natural phenomenon; as a feature of surface wettability, the contact angle is the result of three-phase interaction of gas, liquid and solid surface, and also is a common physical quantity in research and application. Although the lattice Boltzmann method has made great success in the simulation of the surface wetting, there is no reliable algorithm to calculate the dynamic contact angle; even the static contact angle can only be measured using an ideal spherical cap model or using an external tool. First, the chemical potential boundary condition is used to drop the liquid drop directly on the substrate, no transition zone is needed, and the contact angle can be measured directly from the three contact points. The method of calculating the dynamic contact angle in real time is presented by analyzing the micro-shape of the contact angle, using the intersection point of the gas-liquid interface and the second-row lattice line to determine the tangent. The contact angle calculated by the method and the theoretical calculation result are compared with the system under the condition of ignoring the gravity. When the temperature is 0. 7 and 0. 8, the results of the method are very good in the range of 30 to 160 degrees, and the calculation result of the method is accurate. On the same solid interface, the droplet radius is from 20 to 200 grids, and the calculated contact angle remains unchanged, indicating that the calculation result of the method is stable. In the case of gravity, the droplets and the suspended droplets in a different plane from 0. 1 cm to 0. 5 cm were calculated. The results show that, with the change of the size, the deformation of the liquid drops is more and more large, but the contact angle of the liquid drop calculated by the current method remains unchanged. This is consistent with the theoretical expectation, and the micro essence of the contact angle is also truly reflected. the liquid drops are further placed on the solid surface which is arranged alternately in the water-friendly water, and when the surface inclination is not too large, the liquid drops can be inclined under the action of gravity, so that the contact angle hysteresis phenomenon is generated. The contact angle hysteresis is calculated using the surface of three different lipophilic water configurations, and the results show that, although the hydrophobic mode surface has a greater contact angle than the hydrophilic surface, and the droplets are less prone to instability, the tendency of the contact angle hysteresis to increase with the inclination of the surface is substantially uniform. After the unstable sliding, the liquid drops can slide through the hydrophobic strip continuously, the advancing angle gradually increases and then suddenly decreases, the back angle gradually decreases and then suddenly increases, the variation of the two is generally not synchronized, resulting in a dynamic fluctuation state of the contact angle hysteresis. The dynamic waveform of the contact angle hysteresis is related to the phase of the forward and backward angles, and contains high-order fluctuations due to the shaking of the droplets. These studies on contact angle and hysteresis have contributed to an in-depth understanding and practical application of the wetting phenomenon. The lateral migration of the cross-flow layer in the pipe flow is a wonderful natural phenomenon, and has developed into a hot spot in the field of micro-flow control in recent years. By using the multi-relaxed lattice Boltzmann method, we have studied the phenomenon of the inertia of the elliptic particles. Similar to the classical Sere-Silberberg effect, elliptical particles also exhibit lateral migration and balance. but because of its non-circular geometry, the motion of the elliptical particles also includes non-uniform torsional and periodic vibrations. The influence of the Reynolds number, the blocking ratio and the aspect ratio on the trajectory of the elliptic motion is studied respectively. It is found that the influence of the change of the Reynolds number on the equilibrium position of the ellipse is very small, the change rate of the equilibrium position is only 3% when the Reynolds number changes from 3 to 300, and the larger the blocking ratio is, The closer the equilibrium position is to the center line of the pipe, the more the effect of the particle convection field is; with the increase of the aspect ratio, the rotation period of the elliptical particles becomes shorter, and the equilibrium position exhibits a saddle-shaped change, reaching the lowest point when the aspect ratio is close to 0.5. This work has a positive effect on the study of the blood flow of birds with ovoid or elliptical red blood cells.
【学位授予单位】：广西师范大学
【学位级别】：硕士
【学位授予年份】：2017
【分类号】：O35

【相似文献】