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The cylinder end boundaries, whether they be end plates or simple free ends, alter the vortex-shedding mechanism near these boundaries. This effect has in the past usually been overlooked. In a region near an end plate or a free end (ranging from 6 to 15 cylinder diameters in length), the shedding frequency f 2 is found to be 10–15% less than the regular Strouhal frequency f s . The latter frequency is observed over the remaining cylinder length. The simultaneous occurrence of two frequencies results in a beat frequency, which is best observed at the junction of the two regions characterized by f s and f 2 respectively. A third frequency f 3 with f s > f 3 > f 2 is observed over the entire cylinder length when the cylinder is bounded by two end plates less than 20 to 30 cylinder diameters apart. Here the critical Reynolds number for the onset of shedding is shifted to about 60 and the laminar Reynolds-number range is extended from about 150 to about 250.
Gerich et al. (Wed,) studied this question.