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Published April 26, 1996 | Published
Journal Article Open

Experimental studies of vortex disconnection and connection at a free surface


An experimental study is presented that examines the interaction of a vortex ring with a free surface. The main objective of this study is to identify the physical mechanisms that are responsible for the self-disconnection of vortex filaments in the near-surface region and the subsequent connection of disconnected vortex elements to the free surface. The understanding of those mechanisms is essential for the identification and estimation of the appropriate spatial and temporal scales of the disconnection and connection process. In this regard, the velocity and vorticity fields of an obliquely approaching laminar vortex ring with a Reynolds number of 1150 were mapped by using Digital Particle Image Velocimetry (DPIV). The evolution of the near-surface vorticity field indicates that the connection process starts in the side regions of the approaching vortex ring where surface-normal vorticity already exists in the bulk. A local strain rate analysis was conducted to support this conclusion. Disconnection in the near-surface tip region of the vortex ring occurs because of the removal of surfaceparallel vorticity by the viscous flux of vorticity through the surface. Temporal and spatial mapping of the vorticity field at the surface and in the perpendicular plane of symmetry shows that the viscous flux is balanced by a local deceleration of the flow at the surface. It is found that the observed timescales of the disconnection and connection process scale with the near-surface vorticity gradient rather than with the core diameter of the vortex ring.

Additional Information

© 1996 Cambridge University Press. Received 14 August 1995 and in revised form 22 March 1996. Published online: 26 April 2006. This work has been supported by the Office of Naval Research, ONR-URI grant N00014-92-J-1610. The authors would like to thank Dr Willert for his contributions during the initial processing of the DPIV data. The valuable discussions with Dr Rood and Dr Ashurst on the subject of vortex kinematics and vorticity flux have been essential to our presentations in the paper.

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August 20, 2023
August 20, 2023