Vortex structure for flow over a heaving cylinder with a flexible tail

Hydrogen-bubble visualization technique was applied in the investigation of vortex structure for flow over a heaving cylinder attached with a flexible tail along the wake central-line in a water channel. Wake structures have been mapped in the flexible tail length-frequency (L/D, St) phase space with the flexible tail length of L/D = 2-5 and the oscillation Strouhal number of St = 0-0.34. Four wake modes were identified as: (1) 2S-Kármán vortex mode-a Kármán-vortex-like structure with two single vortices formed per cycle in the near wake; (2) 2S-reverse Kármán vortex mode-a reverse Kármánvortex- like structure with two single vortices per cycle; (3) 2P mode-a bifurcated vortex pair structure with two pairs of vortices per cycle; (4) P + S mode-an unstable vortex structure with three vortices per cycle as a transient mode. Moreover, the typical case of each wake mode was further examined by particle image velocimetry, and the evolutions of vortex structures for the four wake modes were studied in detail.