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C. H. K. Williamson
Cornell University

Despite the large body of research concerned with the near wake of a circular cylinder, the far wake, which extends beyond about 100 diameters downstream, is relatively unexplored , especially at low Reynolds numbers. We have recently shown that the structure of the far wake is exquisitely sensitive to free-stream noise, and is precisely dependent on the frequency and scale of the near wake; indeed it is shown that the presence of extremely low-amplitude peaks in the free-stream spectrum, over a remarkably wide range of frequencies, are sufficient to trigger an 'oblique wave resonance' in the far wake. We show, in the above photograph, a nonlinear interaction between oblique shedding waves generated fromupstream (to the left) and two-dimensional waves amplified downstream from free-stream disturbances (in the central region). We use the smoke-wire flow visualization technique, with the smoke-wire placed 50 diameters downstream of the cylinder. The wake is viewed in planview and the flow is to the right. This two-wave interaction triggers a thrid wave, namely an 'oblique resonance wave' at a large oblique angle, to grow through nonlinear effects (in the right half of the photograph), in preference to the original two waves. If smoke it introduced 100 diameters downstream, then all that is seen is a set of such large-angle oblique resonance waves.

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