Mean and fluctuating velocity fields in the wake of a freely-vibrating cylinder

In the present work, we study the wake velocity field of an elastically mounted rigid cylinder oscillating transverse to a fluid flow, using DPIV measurements. It is shown that there are large qualitative changes in these velocity fields, depending on the mode of cylinder oscillation. In particular, the characteristic "recirculation bubble", usually seen in the mean velocity field behind the nonoscillating cylinder, is found to be present in the case of the '2S' wake formation mode, yet is completely absent for the '2P' mode. For the '2P' mode, we find instead the appearance of a pair of counter-rotating vortices of opposite sign to what is expected, causing a downstream-oriented jet-type flow close to the cylinder, which in turn results in a 'double-wake' type velocity profile. Measurements of both the total Reynolds stresses, and the periodic stresses evaluated using phase-averaged velocity data, show that more than 90% of the total stresses are due to the repeatable large-scale coherent structures in the wake, when the body is vibrating. Periodic stresses make up only about 60% of the total stresses, in the case of the stationary body. Interestingly, for the fixed body, the periodic stresses remain relatively unchanged between our experiments (Re = 3900) and those of Cantwell & Coles, at Re = 140 000, although the total stresses are significantly increased at the larger Re. Our experimental evaluation of Reynolds stress is stimulated by the need for such data in developing turbulence modelling of these flows, as well as to enable detailed comparison with direct numerical simulations. (C) 2001 Academic Press.