In this paper, the fluid transport in the interaction of two co-axial co-rotating vortex rings are investigated. Vortex rings are generated using the piston-cylinder apparatus, and the resulting velocity fields are measured using digital particle image velocimetry. The interaction process is analysed by means of vorticity contour, as well by investigation of the Lagrangian coherent structures (LCSs) defined by the ridges of the finite-time Lyapunov exponent (FTLE). Experimental results demonstrate that two types of vortex interaction are identified, namely strong and weak interactions, respectively. For the strong interaction, the Lagrangian boundaries of the two vortex rings are merged together and form a flux window for fluid transport. For weak interaction, only the Lagrangian drift induced by the motion of the front vortex ring is observed and affects the Lagrangian boundary of the rear vortex ring. Moreover, the fluids transported in the strong interaction carry considerable momentum but no circulation. By contrast, there are nearly no fluxes occurring in the weak interaction. By tracking the variations of circulation and impulse occupied by the separated regions distinguished by the LCSs, it is found that the circulation nearly has no change, but the impulse occupied by vortex core region has significant change. In the strong interaction, the impulse of rear vortex ring decreases but the impulse of the front vortex ring increases. Based on the impulse law, it is speculated that the fluid force generated by the formation of the rear vortex rings can be enhanced. Therefore, the strong interaction between wake vortices can actually improve the propulsive efficiency of the biological systems by operating the formation of large-scale vortices.

本文研究了在两个同轴同向旋转涡流环相互作用中的流体传输。使用活塞缸设备产生涡流环，并使用数字粒子图像测速仪测量所得的速度场。相互作用过程通过涡度轮廓进行分析，还通过研究由有限时间Lyapunov指数（FTLE）的脊定义的拉格朗日相干结构（LCS）进行分析。实验结果表明，确定了两种类型的涡旋相互作用，即强相互作用和弱相互作用。为了实现强相互作用，两个涡流环的拉格朗日边界合并在一起，形成流体传输的通量窗口。对于弱互动，仅观察到了由前涡流环的运动引起的拉格朗日漂移，并影响了后涡流环的拉格朗日边界。而且，在强相互作用下输送的流体带有相当大的动量，但没有循环。相反，在弱相互作用中几乎没有通量发生。通过跟踪由LCSs区分的分离区域所占据的环流和冲量的变化，发现环流几乎没有变化，但涡旋核心区所占据的冲量却发生了显着变化。在强相互作用中，后涡流环的脉冲减小，而前涡流环的脉冲增大。基于脉冲定律，推测可以增强由于形成后涡流环而产生的流体力。因此，