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 mou nted rigid cylinder oscillating transverse to a fluid flow, using DPIV meas urements. It is shown that there are large qualitative changes in these vel ocity fields, depending on the mode of cylinder oscillation. In particular, the characteristic "recirculation bubble", usually seen in the mean veloci ty 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 count er-rotating vortices of opposite sign to what is expected, causing a downst ream-oriented jet-type flow close to the cylinder, which in turn results in a 'double-wake' type velocity profile. Measurements of both the total Reyn olds stresses, and the periodic stresses evaluated using phase-averaged vel ocity data, show that more than 90% of the total stresses are due to the re peatable large-scale coherent structures in the wake, when the body is vibr ating. Periodic stresses make up only about 60% of the total stresses, in t he case of the stationary body. Interestingly, for the fixed body, the peri odic stresses remain relatively unchanged between our experiments (Re = 390 0) and those of Cantwell & Coles, at Re = 140 000, although the total stres ses are significantly increased at the larger Re. Our experimental evaluati on of Reynolds stress is stimulated by the need for such data in developing turbulence modelling of these flows, as well as to enable detailed compari son with direct numerical simulations. (C) 2001 Academic Press.