A PIV study of blockage ratio effects on flow over a confined circular cylinder at low Reynolds numbers

The present study examines the flow over a circular cylinder confined between two parallel walls in the lateral direction using a Particle Image Velocimetry (PIV) method. The experiment covers five blockage ratios ( β ) of 1/6, 1/5, 1/3, 1/2 and 3/5, respectively, at Reynolds numbers ( Re ) up to 1300. It is found that two vortex shedding modes including symmetric and alternating shedding modes appear at β ≥ 1/3 for Re > 600. The symmetric shedding mode appears intermittently and only lasts for short time periods, whereas the alternating shedding mode dominates wake dynamics. The frequencies corresponding to the two vortex shedding modes ( f VK for alternating shedding mode and f SL for symmetric shedding mode) are clearly detected in separated shear layers. The occurrence of both vortex shedding modes at blockage ratios greater than 1/3 has not been reported previously in the literature, and only the alternating shedding mode was observed. The present study reveals that the f VK -based Strouhal number increases exponentially with β , which also depends on the Reynolds numbers. It is further revealed that the correlation between the f SL -based Strouhal number and Re follows a 1/2 power law at relatively large blockage ratios (e.g. β = 1/3 and 1/2).