A particle image velocimetry measurement of flow over a highly confined circular cylinder at 60% blockage ratio

In this study, a Particle Image Velocimetry (PIV) measurement is carried out to investigate the flow past a highly confined circular cylinder at a fixed blockage ratio of 60%. The Reynolds number (Re) in terms of the cylinder diameter and the mean incoming flow velocity is varied from 318 to 1431. It is observed that the characteristic frequency of the separated shear layers is amplified through a so-called frequency-filtering process. The instability of the separated shear layers causes two different vortex shedding modes including alternating and symmetric shedding modes, which are observed at Re = 927-1242. An intermittent switch between the two shedding modes is also observed. The theory of mixing layers is applied with modification to predict the characteristics of the separated shear layers, and the prediction shows a good agreement with the experimental data. It is found that the vortex roll-up and merging locations can be estimated by calculating the energy contents of individual frequency modes.

Automated summary

In this study, Particle Image Velocimetry (PIV) measurement was used to investigate the flow past a highly confined circular cylinder, revealing amplified characteristic frequency of separated shear layers and transition between different shedding modes. The theory of mixing layers was applied to predict the characteristics of separated shear layers with good agreement with experimental data, and vortex roll-up and merging locations were estimated based on energy contents of individual frequency modes.