Long time scale fluctuation in the irregularity of vortex shedding (spectral analysis of the local rms value and circulation)

Irregularity of vortex shedding from a circular cylinder is studied experimentally. The study is based on a spectral analysis of local rms value of velocity fluctuation behind the circular cylinder. Spectral analysis of circulation of vortices obtained by both PIV and image processing of the streak line visualized by hydrogen bubbles extending from the location near the separation point of shear layer is made in order to investigate the relationship between velocity fluctuations and circulation fluctuations. From this study, long time scale periodic fluctuation, approximately 20 times the vortex shedding periodicity, is clarified. Also a good correlation between the time variation of the local rms value and that of the circulation was obtained.