Structure and dynamics of the wake of bubbles and its relevance for bubble interaction
Structure and dynamics of the wake of bubbles and its relevance for bubble interaction
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DOI：
10.1063/1.870043
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发表时间：
1999-06
影响因子：
4.6
通讯作者：
C. Brücker
中科院分区：
文献类型：
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作者：
C. Brücker
采用数字粒子图像测速仪结合高速记录技术，对单个和两个相互作用的气泡在水中自由升起后的流动进行了实验研究。实验对象为直径约0.4-0.8 cm的椭圆形气泡，它们在水中上升时表现出螺旋形、之字形和摇摆运动，并在水中加入示踪剂小颗粒用于流动显示。在垂直通道的逆流条件下，气泡被保留在观察区的中心，这使得可以在几个连续的周期内观察到尾迹振荡和气泡相互作用。除了光片外，通过同时漫射照明，我们能够记录气泡的路径和形状振荡，以及水平和垂直截面上的尾迹结构。结果表明，曲折运动与交替的相反方向的发夹式涡旋结构的产生和释放是有规律的耦合。伴随着尾迹振荡，气泡在曲折路径反转点的赤道平面上经历了强烈的非对称形变。Z字形运动叠加在气泡的一个小的横向漂移上，这意味着净升力的存在。这是由于在锯齿形路径和锯齿形路径上观测到的发卡涡强度不同所致；在最近的球状尾迹流数值结果中发现了一个看似熟悉的现象。对于螺旋状气泡，尾迹与观察者随气泡一起移动的尾迹大致相同。它由一对缠绕在螺旋路径上的流向涡丝组成，并以不对称的位置附着在气泡底座上。气泡的短轴在切向平面和径向平面向螺旋中心倾斜。由于非对称附着的尾迹所产生的压力场，存在两种升力分量，一种是引起横向运动的升力，另一种是使气泡保持在圆形路径上的向心力。提出了一种机制来解释单个气泡螺旋或曲折的原因。用两个同时释放的气泡进行的实验表明，气泡相互作用是由尾流动力学强烈触发的。一旦一个气泡在摇晃的气泡之后被捕获，它就会通过连续的跳跃加速并上升，直到它们相撞。跳跃的原因是发夹涡的环状头部从前导气泡中脱落的向上诱导效应。最后的碰撞和排斥使尾迹在短时间内突然增大，这被认为是泡状流中湍流产生放大的主要原因之一。
The flow in the wake of single and two interacting air bubbles freely rising in water is studied experimentally using digital-particle-image-velocimetry in combination with high-speed recording. The experiments focus on ellipsoidal bubbles of diameter of about 0.4–0.8 cm which show spiraling, zigzagging, and rocking motion during their rise in water, which was seeded with small tracer particles for flow visualization. Under counterflow conditions in the vertical channel, the bubbles are retained in the center of the observation region, which allows the wake oscillations and bubble interaction to be observed over several successive periods. By simultaneous diffuse illumination in addition to the light sheet, we were able to record both the path and shape oscillations of the bubble, as well as the wake structure in a horizontal and vertical cross section. The results show that the zigzagging motion is coupled to a regular generation and discharge of alternate oppositely oriented hairpin-like vortex structures. Associated with the wake oscillation, the bubble experiences a strong asymmetric deformation in the equatorial plane at the inversion points of the zigzag path. The zigzag motion is superimposed on a small lateral drift of the bubble, which implies the existence of a net lift force. This is explained by the observed different strength of the hairpin vortices in the zig and zag path; a seemingly familiar phenomenon was found in recent numerical results of the sphere wake flow. For spiraling bubbles the wake is approximately steady to an observer moving with the bubble. It consists of a twisted pair of streamwise vortex filaments which are wound in a helical path and are attached to the bubble base at an asymmetrical position. The minor axis of the bubble is tilted in the tangential plane as well as in the radial plane toward the spiral center. Due to the pressure field induced by the asymmetrically attached wake two components of the lift force exist, one that causes the lateral motion and the other a centripetal force that keeps the bubble on a circular path. A mechanism is proposed to explain the reason for one bubble to spiral or to zigzag. Experiments with two simultaneous released bubbles show that bubble interaction is strongly triggered by the wake dynamics. Once a bubble is captured in the wake of a rocking bubble, it accelerates and rises via successive jumps until they collide. The jumps are explained by the upwards induction effect of the ring-like heads of the hairpin vortices being shed from the leading bubble. The final collision and repulsion thereafter abruptly enlarges the wake for a short moment, which is suggested to be one major contribution to the amplification of turbulence production in bubbly flows.