In the present study, we experimentally investigated the freely rising of bubble-in-chain and the evolution of bubble-induced flow for a quiescent fluid. Quantitative data on bubble morphology, kinematics, and bubble-induced flow dynamics were obtained by combining the methods of shadowgraphy and laser-induced-fluorescence particle image velocimetry (PIV/LIF). Under the current experimental condition, after detached from the orifice, the rising of the bubble can be divided into three stages: rectilinear rising, transition regime and zigzagging. In the rectilinear rising stage (0 < y/D_n < 12), the bubble-induced flow is dominated by a pair of counter-rotating vortices, which rise together with the bubble moving, i.e., the standing eddies. Each bubble experiences an acceleration along the orifice centerline, and gradually becomes flattened with its aspect ratio increasing. At y/D_n ≈ 12, the bubble aspect ratio reaches the maximum (χ ≈ 4) and the induced wake vortices are significantly strengthened. Coupled with the destabilizing effect of disturbance from previous leading bubbles, the wake flow becomes unstable leading to the onset of wake vortex shedding that triggers the bubble path instability. As a result, the bubble deviates from the rectilinear path and goes into the transition regime. As rising to y/D_n ≈ 30, the bubble is strongly affected by the wake from previous bubbles since the vorticity has not been accumulated to trigger the next vortex shedding. Thus, the bubble enters in the zigzagging stage with its trajectory exhibiting significant differences among different bubbles. In particular, the bubble-induced flow in the zigzagging stage has the strong entrainment ability, which might be helpful for applications using bubbles for enhancing the mixing and transport of liquids.

在本研究中，我们通过实验研究了链中气泡的自由上升和静态流体的气泡诱导流动的演变。通过结合阴影成像和激光诱导荧光粒子图像测速 (PIV/LIF) 的方法，获得了关于气泡形态、运动学和气泡诱导流动动力学的定量数据。在目前的实验条件下，气泡脱离孔口后的上升可分为直线上升、过渡状态和锯齿形三个阶段。在直线上升阶段（0 < y/D _n< 12)，气泡引起的流动由一对反向旋转的涡流控制，它们随着气泡的移动而上升，即立涡。每个气泡沿着孔口中心线经历一个加速度，并随着其纵横比的增加而逐渐变平。在y/D _n ≈ 12 时，气泡纵横比达到最大值 ( χ ≈ 4)，诱导的尾流涡流显着增强。再加上先前引导气泡扰动的不稳定效应，尾流变得不稳定，导致尾涡脱落，引发气泡路径不稳定。结果，气泡偏离直线路径并进入过渡状态。随着上升到y/D _n≈ 30，气泡受到前一个气泡尾流的强烈影响，因为涡度尚未累积以触发下一个涡旋脱落。因此，气泡进入锯齿形阶段，其轨迹在不同气泡之间表现出显着差异。特别是曲折阶段的气泡诱导流动具有很强的夹带能力，这可能有助于利用气泡增强液体的混合和输送的应用。