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Faraday waves in small cylinders and the sidewall non-ideality.

机译:小圆柱体中的法拉第波和侧壁不理想。

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This work is the result of a scientific inquiry into the current understanding of experimental single-mode Faraday waves, from the perspective of the linear stability theory. Given an electromechanical shaker capable of imposing vibrations of several centimeters at frequencies of up to 15 Hz, experiments were directed toward laterally "small" systems in which the cell modes are discretized and the excited wavelength was of the order of the lateral dimension. In this regime, the theoretically tractable boundary condition for the sidewalls is a stress free condition, which is a challenge to produce experimentally. In reality, the no-slip behavior of the fluid along the sidewalls and interfacial contact line effects such as capillary hysteresis introduce sidewall stresses. This marks the first attempt to match the single mode Faraday experiment to a linear theory that rigorously treats viscosity.;In these experiments it was found that the liquids FC70 and silicone oil, which, other than a slight meniscus, produced a flat interface that moved with little effort once the container was tilted. This behavior stands in contrast to that of a water-air or an immiscible silicone oil-water interface in a glass beaker, which suffer from pinning to the sidewall. Closer inspection of the FC70-oil interface at the sidewalls shows formation of a tiny oil film in a tilted cell where bulk FC70 had displaced bulk silicone oil. Wave decay experiments confirm the dominance of the bulk viscous contribution to the sidewall damping effects when measuring the overall system damping.;In interpreting the experiments, first the viscous linear stability theory of Kumar & Tuckerman[52] is presented, and modified with the stress free boundary condition to account for mode discretization. The linear theory is capable of predicting the threshold amplitude, above which the flat interface is unstable and deflection occurs. In horizontally infinite systems the well-known result is that the instability is subharmonically excited---with a frequency half that of the forcing frequency. The main implication of mode discretization is the continuum of available modes is discretized, and each available mode can be excited inside of its own frequency band. The corresponding threshold amplitudes for each band descend to a minimum amplitude near the natural frequency of the mode, and the points at which the thresholds of neighboring modes intersect are co-dimension 2 points, conditions where two modes are neutrally stable.;The theoretical concepts of the critical threshold and frequency bands at which modes appear are studied with the FC70 and silicone oil system. In a system oscillating below the instability threshold, tiny flow perturbations are seen due to emission of waves from the oscillating meniscus, an unavoidable non-ideality. Imposing vibrations above the threshold amplitude results in the gradual (or sudden) deflection and growth of the interface to some new state. Experimental thresholds are therefore marked by performing a series of trials at different amplitudes, and the lowest amplitude at which the instability is observed is marked as the threshold. Data sets are built by measuring the threshold for the experimental frequency band, and bounded with the co-dimension 2 points.;Two complete data sets of several subharmonic, harmonic, and superharmonic modes are presented for FC70 and 1.5 cSt silicone oil systems of different layer heights, and are compared to the predictions of the linear theory. While slightly higher than predicted thresholds are observed near the mode natural frequencies, the agreement is quite good as one moves toward the co-dimension 2 points. The deviation near the natural frequencies appears to be greater for modes with greater number of azimuthal nodes, suggesting an associated increase in wall damping. Considerable deviation is seen in the lower-than-predicted thresholds for the harmonic modes, most noticeably for the mode showing the same azimuthal uniformity as the sidewall meniscus, suggesting an interaction between the harmonic modes with the meniscus waves, which are also harmonic with the cell motion. Similar deviation is seen for the other harmonic modes, albeit less due to the spatial mismatch of the instability with the meniscus wave.;The power of the linear theory comes in being able to predict the mode of instability and the critical threshold at which it appears, whereas growth beyond the infinitesimal state a nonlinear theory is required. A nonlinear theory was not completed in this work and therefore the experiment was used primarily for comparison to the linear theory. Qualitative nonlinear observations, however, were numerous, and many are presented in the interest of both giving a broader sense of the experiment and insight into the parameter spaces a weakly nonlinear theory might show agreement.;The final result presented is for when the instability is excited with two frequencies, a novelty for single mode experiments. (Abstract shortened by UMI.).
机译:这项工作是从线性稳定性理论的角度对当前对实验性单模法拉第波的理解进行科学探索的结果。给定一种能够在高达15 Hz的频率下施加几厘米振动的机电振动器,将实验针对横向“小型”系统,在该系统中,单元模式被离散化,激发波长约为横向尺寸。在这种情况下,侧壁的理论上易处理的边界条件是无应力条件,这是通过实验产生的挑战。实际上,流体沿着侧壁的无滑动行为和界面接触线效应(例如毛细管滞后)会引入侧壁应力。这标志着首次尝试将单模法拉第实验与严格处理粘度的线性理论相匹配;在这些实验中,发现液体FC70和硅油(除了轻微的弯液面外)产生了可移动的平坦界面一旦容器倾斜,只需很少的力气。该行为与玻璃烧杯中的水-空气或不溶混的硅油-水界面的行为相反,后者会钉扎到侧壁上。仔细检查侧壁上的FC70-油界面,可以发现在倾斜的单元中形成了微小的油膜,在该单元中,散装的FC70取代了散装的硅油。波动衰减实验证实了在测量整体系统阻尼时主体粘性对侧壁阻尼效应的贡献。在解释实验时,首先提出了Kumar&Tuckerman [52]的粘性线性稳定性理论,并根据应力对其进行了修正。自由边界条件以解决模式离散问题。线性理论能够预测阈值振幅,在该阈值振幅以上,平坦界面不稳定并且会发生挠曲。在水平无限系统中,众所周知的结果是,不稳定是次谐波激发的,其频率是强迫频率的一半。模式离散化的主要含义是将可用模式的连续性离散化,并且每个可用模式都可以在其自己的频带内被激发。每个频带的相应阈值幅度下降到模态固有频率附近的最小幅度,并且相邻模态阈值相交的点为共维2点,这是两个模态保持中立的条件。使用FC70和硅油系统研究临界阈值和出现模式的频带。在不稳定性阈值以下振荡的系统中,由于振荡的弯月面发出了波,这是不可避免的非理想情况,因此会看到微小的流量扰动。施加高于阈值幅度的振动会导致界面逐渐(或突然)偏转和增长到某种新状态。因此,通过在不同幅度下执行一系列试验来标记实验阈值,并将观察到不稳定的最低幅度标记为阈值。通过测量实验频段的阈值建立数据集,并以共维2点为界。;针对不同的FC70和1.5 cSt硅油系统,给出了两个亚谐波,谐波和超谐波模式的两个完整数据集层高度,并与线性理论的预测值进行比较。尽管在模态固有频率附近观察到的阈值略高于预期阈值,但是当人们朝维数2点移动时,一致性非常好。对于具有更多方位角节点的模式,固有频率附近的偏差似乎更大,这表明壁阻尼也随之增加。在谐波模式的低于阈值的阈值中可以看到相当大的偏差,最显着的是与侧壁弯月面具有相同方位角均匀度的模式,这表明谐波模式与弯月形波之间存在相互作用,而弯月形波也与侧壁弯月形产生谐波。细胞运动。对于其他谐波模式也可以看到类似的偏差,尽管由于不稳定性与弯月面波的空间失配而导致的偏差较小。线性理论的强大之处在于能够预测不稳定性的模式以及出现它的临界阈值,而要超越无穷小状态增长,则需要一个非线性理论。非线性理论尚未完成,因此该实验主要用于与线性理论进行比较。但是,定性非线性观测的数量众多,并且提出许多目的是为了使实验具有更广泛的意义,并且洞察弱非线性理论可能显示出一致的参数空间。给出的最终结果是针对不稳定性为用两个频率激发,是单模实验的新颖之处。 (摘要由UMI缩短。)。

著录项

  • 作者

    Batson, William R., III.;

  • 作者单位

    University of Florida.;

  • 授予单位 University of Florida.;
  • 学科 Engineering Chemical.
  • 学位 Ph.D.
  • 年度 2013
  • 页码 144 p.
  • 总页数 144
  • 原文格式 PDF
  • 正文语种 eng
  • 中图分类
  • 关键词

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