声悬浮条件下黏性液滴的扇谐振荡规律研究

采用单轴式声悬浮方法研究了黏度μ =0.94-75.65 mPa·s的甘油-水溶液液滴的扇谐振荡规律. 发现一定阶数的振荡模式存在一定的临界黏度μ_c, 只有当μ < μ_c时, 该阶扇谐振荡才能被激发. 实验测定了声场调制幅度η = 0.23 时, l =2-9 阶扇谐振荡的临界黏度, 发现ln μ_c与l近似呈线性递减关系. 采用参数共振理论分析了黏性液滴的扇谐振荡过程, 发现激发扇谐振荡的液滴赤道半径扰动阈值h_c正比于液滴黏度μ, 并随l增大而增大, 因此扇谐振荡难以在高黏度和高阶模式下发生. 实验还发现, 各阶扇谐振荡的振幅和共振频率宽度随液滴黏度增大而减小, 黏度对液滴本征频率无明显影响.     

基于振荡气泡法测定声悬浮液滴的表面张力

通过调节阵列式声悬浮装置的悬浮声压,使悬浮液滴转变为气泡.利用振荡气泡法的原理,结合声悬浮气泡在声场中处于缓慢变形的非平衡状态特点,对原始气泡表面膨胀系数的公式进行了修正.实验过程中采用高速摄像机记录气泡形成及振荡过程的图像,并对图像进行处理,测定悬浮气泡的表面膨胀模量,得到液体的动态表面张力.以锅炉用油为样品,利用声悬浮气泡的膨胀模量,对液滴动态表面张力进行非接触式测量.     

基于亚波长管道增强的漩涡声场悬浮操控微粒和液滴的实验研究

声悬浮技术可以在无接触的情况下操控微粒和液滴,因此已被广泛应用于化学分析、液滴动力学和生物反应器等领域.目前声悬浮技术的主要工作是在开放环境中进行悬浮等操控.本文提出了亚波长管道增强型空气声镊的概念,利用亚波长声波导管进行声场操控及微粒和液滴悬浮.通过4个小型换能器激发有限长度亚波长圆波导管的单一低阶声学模态,可以在有限长度的波导管内产生漩涡声场.实验发现由于亚波长结构对声场的增强作用,亚波长管道增强的漩涡声场在径向和轴向悬浮力大小上均有较大提升,因此可对发泡聚苯乙烯颗粒和水滴实施悬浮和自转等操控.这项工作将亚波长声波导管的概念引入声场操控中,有望加深对声场和物质相互作用的物理理解,开发新型小型化悬浮微粒和液滴的声学操纵器件.     

双声源驱动热声系统声场调制研究

本文在回热器边界声场调制理论的基础上,考虑了两端换热器和谐振管变径的影响,推导出热声系统(谐振管和回热器)内声场与双声源复声压相关的声场调制关系式。理论和实验分析了双声源复声压对谐振管中的行波比率以及回热器中的声阻抗的调制,并给出通过复声压调制后的系统内声场分布图。对比结果表明该声场调制关系式的适用性,通过双声源复声压的调节,能调制出谐振管及回热器所期望的工作声场。     

水下低频振荡涡流场声散射调制机理与特性研究

水下涡流场对声波的散射问题是声波在复杂流场中传播的基本问题,在水下目标探测和流场声成像领域具有重要意义.针对水下低频振荡涡流场声散射调制问题建立了理论分析模型与数值计算方法,探究了其声散射调制声场的产生机理与时空频特性.首先,基于运动介质的波动方程,通过引入势函数将波动方程分解为流声耦合项和非耦合项,并对流声耦合项进行频域分析处理,揭示了水下振荡涡流场的声散射调制机理;其次,采用间断伽辽金数值方法对水下低频振荡涡流场中声传播过程进行了数值模拟,分析了低马赫数条件下,不同入射声波频率、涡流场的振荡频率和涡核尺度对涡流场声散射调制声场时空频特性的影响规律,并结合理论分析模型对其特性进行了解释.研究表明:低马赫数下,振荡涡流场对声波的散射可产生包含涡流场振荡频率双边带调制谐波的散射调制声场,且随着入射声波频率、涡核尺度的增大,散射调制声场强度增强,总散射声场空间分布具有对称性和明显主瓣,且主瓣方位角趋近于入射波传播方向;在频率比远大于1条件下,涡流场振荡频率对散射调制声场强度影响较小.     

莱顿弗罗斯特水滴振荡模式的影响因素及机理探究

探讨了莱顿弗罗斯特效应下水滴振荡的二维边界条件,在此基础上可求解液滴的本征频率.将振荡归结为由表面波叠加形成的驻波,并引入参量共振描述振荡的物理机制.探究了一定温度下振荡模式的演化规律,给出了振荡模式随半径的分布及振荡频率随时间的变化.验证了热对流为液滴与表面间主要的传热方式,并对真实液滴的几何形状做了定性探讨.     

声悬浮过程的激光全息干涉研究

利用二次曝光全息干涉术实现了对单轴式声悬浮声压场的研究.分别拍摄了悬浮不同物体和 不同输出功率情况下声悬浮场的多幅全息图，并进行了对比分析.结果表明，实验中获得的 声压分布图样与由声波动方程获得的理论声压分布基本一致，其相应中轴线的强度分布也具 有很好的一致性.与以往的声场测量方法相比，二次曝光法非接触、无干扰及全场测量的优 势在声悬浮场测量中得以充分体现，该方法的引入不但简化了声悬浮场测量的实际操作，而 且可以更直观地获得全场信息，为优化声悬浮系统提供了实验依据. 关键词： 全息干涉术 二次曝光法 声悬浮 谐振     

时均流诱导声振荡的数值计算研究

时均流诱导的声振荡可以为热声制冷提供驱动源或驱动发电机和换能器发电,为风能利用提供了新思路,是热声领域的最新研究方向之一。本文基于计算流体动力学(CFD)方法,建立了正十字型时均流激声发动机的三维数学模型,采用大涡模拟湍流模型计算。计算结果验证了时均流诱导声振荡效应,揭示出谐振管内声场分布和谐振管内部压力与开口处涡的关系,为后续的实验研究奠定了理论基础。     

声悬浮高温小球轴向稳定性研究

针对驻波声悬浮高温小球的轴向稳定性,建立了声-热-流-重力场耦合的物理模型,采用有限元方法计算了高温小球周围空间的不均匀温度场和声场,分析了驻波声场中直径2 mm的氮化硅小球在300～2000 K温度区间的轴向悬浮稳定性,并通过常温下声悬浮实验验证了仿真模型的准确性。结果表明在初始悬浮间距满足谐振并保持不变的条件下,随着悬浮小球的温度升高,小球的平衡位置降低,并且存在能保持稳定悬浮状态的温度最大值。在小球升温的过程中,通过反馈调节发射端-反射端间距和发射端激励电压,可以在一定程度上保持高温小球的轴向悬浮稳定性。     

声悬浮条件下环己烷液滴的蒸发凝固

采用单轴式声悬浮方法研究了环己烷液滴的蒸发过程,发现环己烷液滴的蒸发可以使自身温度降至熔点以下并发生凝固.高速摄像实时观测表明,环己烷晶核开始形成于液滴赤道附近,并以枝晶方式长大,平均生长速度为12.5-160.4 mm/s.进一步研究发现,声悬浮条件下平均Sherwood数与平均Nusselt数的比值Sh/Nu是在自然对流条件下的1.3倍,这表明声流边界层有效提高了环己烷液滴的蒸发速率而对传热的促进作用相对较小,因而可以使液滴降至更低温度,进而发生凝固.据此,提出了挥发性液体在声悬浮条件下发生蒸发凝固的必要条件. 关键词： 声悬浮 声流 环己烷 蒸发凝固     

Parametric resonance in acoustically levitated water drops

Liquid drops can be suspended in air with acoustic levitation method. When the sound pressure is periodically modulated, the levitated drop is usually forced into an axisymmetric oscillation. However, a transition from axisymmetric oscillation into sectorial oscillation occurs when the modulation frequency approaches some specific values. The frequency of the sectorial oscillation is almost exactly half of the modulation frequency. It is demonstrated that this transition is induced by the parametric resonance of levitated drop. The natural frequency of sectorial oscillation is found to decrease with the increase of drop distortion extent.     

Non-Axisymmetric Oscillation of Acoustically Levitated Water Drops at Specific Frequencies

A category of non-axisymmetric oscillations of acoustically levitated water drops was observed. These oscillations can be qualitatively described by superposing a sectorial oscillating term upon the initial oblate shape resulting from the effect of acoustic radiation pressure. The oscillation frequencies are around 25 Hz for the 2-lobed mode and exactly 50 Hz for the 3- and 4-1obed modes. These oscillations were excited by the disturbance from the power supply. For the same water drop, higher mode oscillations were observed with more oblate initial shape, indicating that the eigenfrequencies of these non-axisymmetric oscillations decrease with increasing initial distortion. The maximum velocity and acceleration within the oscillating drop can attain 0.3 m·s^-1 and 98.7 m·s^-2 respectively, resulting in strong fluid convection and enhanced heat and mass transfer.     

Internal flow of acoustically levitated drops undergoing sectorial oscillations

We present the experimental observation and theoretical analysis of fluid flow in acoustically levitated water drop undergoing sectorial oscillations. The fluid always flows between the extended sections and the compressed sections. The magnitude of fluid velocity decreases from the equatorial fringe to the centre of levitated drop. The maximum fluid velocity is 60-160 mm/s and the Reynolds number of the oscillations is estimated to be 137-367. The internal flow of the drop is analyzed as potential flow, and the fluid velocity is found to be horizontal. In the equatorial plane, the calculated stream lines and velocity profiles agree well with the experimental observations.     

倾斜均质表面上等径水滴聚合过程的可视化实验

对倾斜均匀表面上等直径水滴的聚合过程及特性进行了可视化实验研究,获得了水滴直径和表面倾角等参数对液滴聚合过程中液滴液桥半径、接触角和接触线变化特性的影响,分析了水滴聚合对其运动的影响.实验结果表明:表面倾角越大,下滑的临界半径越小;液滴的直径越大,液滴聚合后越容易下滑;液滴聚合可以加快液滴的运动,使下滑临界半径减小.     

水平均质表面上液滴聚合过程的可视化实验研究

对水平均匀表面上液滴的聚合过程及特性进行了可视化实验研究,获得了液滴半径和液滴物性等参数对液滴聚合过程中液滴液桥半径和接触角变化特性的影响规律。实验结果表明:液滴聚合中液桥半径和接触角都呈衰减振荡变化; 聚合前液滴半径越小,液桥半径振荡频率越大,振幅越小,振荡时间越短;液滴的粘度越大,液桥半径的振荡频率越小, 振幅越小,振荡时间越短;液滴聚合前的接触角明显大于聚合液滴静止后的接触角,其差值与固体界面状况和气、固、液物性相关。     

倾斜均质表面上非等径液滴的聚合特性

对倾斜均匀表面上非等径液滴的聚合过程及特性进行了可视化实验研究,获得了液滴半径和表面倾角等参数对液滴聚合过程中液滴液桥半径、接触角和接触线变化特性的影响规律,进一步说明了倾斜表面上液滴聚合可以加快液滴的运动.     

Experimental Evidence of Parametric Instabilities in an Unmagnetized Plasma Subjected to a Strong H.F. Electric Field

Experimental evidence of parametric excitation, by an intense external H.F. field, of an electron surface mode and an ion wave is presented. The pumping electromagnetic energy density is equal to or slightly larger than the thermal energy density of the electrons. The value of f_pc/f₀ (electron plasma frequency/external field frequency) is that for an electron surface wave. Depending on the pressure and field intensity, this decay instability can lead to three types of low frequency oscillations, with frequencies close to the ion plasma frequency. Two of these are described by Aliev and Silin's intense field theory: one is the volume ion plasma oscillation and the other a surface ion plasma oscillation. The third corresponds to no known ion eigenmode. Several other features of the theory by Aliev and co-workers are also confirmed experimentally, such as the harmonic excitation of the instability (nf₀ ≈ f_pe/√2, where n is an integer), the instability amplitude as a function of f_pe/f₀ (above threshold conditions), the value of the mismatch parameter as a function of field strength and ion mass, and the existence of a fine structure corresponding to the symmetric and antisymmetric electron surface oscillations. Even at high pump field strengths, the decay products are nearly monochromatic i.e. the plasma does not become turbulent.     

Star-drops formed by periodic excitation and on an air cushion – A short review

When simply put on a solid, a liquid drop usually adopts the shape of a spherical cap or a puddle depending on its volume and on the wetting conditions. However, when the drop is subjected to a periodic field, a parametric excitation can induce a transition of shape and can break the drop’s initial axial symmetry, provided that the pinning forces at the contact-line are weak enough. Therefore, a standing wave appears at the drop interface and induces a periodic motion, with a frequency that equals half the excitation frequency. In the first part, we review the different situations where star drops can be generated from various types of periodic excitations. In the second part, we show that similar star drops can occur in a much less intuitive fashion when the drop is put on an air cushion, where no periodic motion is imposed a priori. Preliminary experiments as well as theoretical clues for a hydrodynamic interpretation, suggest that the periodic vibration is due to an inertial instability in the air layer below the drop.     

Zero frequency shift of an oscillating-rotating liquid droplet

Free-decay oscillations and rotations of a levitated liquid droplet are simulated numerically, and the frequency shift of drop-shape oscillations is studied. It is shown for an oscillating-rotating liquid droplet that the oscillation frequency decreases as the amplitude of drop-shape oscillations increases, while it increases as the rotation rate increases. The pressure difference between the equator and the pole of the droplet is found to correspond to the frequency shift. It is also found that the relation between the amplitude and the rotation rate is linear both for zero frequency shift and for zero pressure difference.