单气泡上浮运动的层析PIV实验研究

Tomographic PIV Experimental Study on Motion of Rising Single Bubble

关键词:  单气泡上浮  层析PIV  三维尾流结构  李雅普诺夫指数  流体输运

Key words:  Single bubble rising  Tomographic PIV  Three-dimensional wake structure  Lyapunov exponent  Fluid transport

摘要:

为了揭示上浮过程中的尾流涡结构及其与气泡之间的相互作用，分别采用阴影图像法和层析PIV技术，对单个气泡在静止水中自由Z字型上浮的过程进行了实验研究，得到气泡的形状、运动和三维的尾流速度场。采用λci涡判据和有限时间李雅普诺夫指数（Finite-time Lyapunov exponent，FTLE），从速度场识别出三维的尾流涡结构和二维的拉格朗日拟序结构。结果表明，Z字型上浮过程中，气泡周围环绕有涡环，涡环会沿运动路径脱落交替的、方向相反的发卡涡；单个发卡涡脱落过程中，气泡底部的FTLE脊线会形成一个位置不对称的流量窗口。由此可得到结论：发卡涡的周期性脱落，使涡环相对于气泡的对称性被周期性交替破坏，导致气泡形成Z字型周期运动；单个发卡涡脱落过程中，涡环两侧各自对发卡涡的流体输运存在差异。

Abstract:

In order to uncover the wake vortex structures and their interaction with the bubble， the shadow image method and the tomographic PIV were used respectively to conduct experiments on the process of a single bubble with a zigzag path rising freely in quiescent water. Therefore， the bubble shape， motion and three-dimensional wake velocity field were obtained. Using the λci vortex criterion and the finite-time Lyapunov exponent （FTLE）， the three-dimensional wake vortex structures and the two-dimensional Lagrangian coherent structures（LCS） were identified from the velocity field. The results show that during the zigzag rising process， there is a vortex ring around the bubble， and oppositely-oriented hairpin vortices shed alternately from the vortex ring along the bubble path. During the shedding of a single hairpin vortex， the FTLE ridges at the bottom of the bubble will form an asymmetrical flux window. The conclusions can be drawn from these results. First， the periodic shedding of the hairpin vortices periodically and alternately breaks the symmetry of the vortex ring relative to the bubble， resulting in the periodic zigzag motion of the bubble. Second， during the shedding of a single hairpin vortex， the fluid transport from one side of the vortex ring to the hairpin vortex is different from the other side’s.