The appearance of P+S modes in the wake of a freely vibrating, highly flexible cylinder

We present visual evidence of a stable P+S (vortex pair and singlet) wake mode in the free vibration of a highly flexible, cantilevered cylinder. Our experiments cover a reduced velocity range of 17–91, and a Reynolds number range of 70–363, over which the flow remains laminar. The P+S wake appears at certain spanwise locations, coexisting with a 2S (2 singlet) wake at other locations. The presence of a co-existing P+S mode sometimes occurs with high amplitude transverse oscillations of the cylinder, but the magnitude of oscillations is not necessarily predictive of wake modes. Hysteresis in the transverse amplitudes occurs with increasing or decreasing reduced velocity. The hysteresis region indicates the lowest range of reduced velocity over which P+S wakes are found, though the P+S wake modes occur with the lower amplitude bound of the hysteresis region, rather than the larger amplitude bound.