An experimental study of the parallel and oblique vortex shedding from circular cylinders

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Abstract
An experimental study of the origin of oblique vortex shedding in the laminar wake of circular cylinders was conducted in the range of Reynolds numbers from 40 to 160. Two transverse circular cylinders were positioned upstream of the main shedding cylinder to control the angle of shedding from the main cylinder. The respective distances between each transverse cylinder and the main cylinder were used to induce oblique shedding of different angles, curved shedding, as well as parallel shedding. Measurements of the mean static pressure distribution in the base region of the cylinder and of the mean spanwise component of the velocity in the wake were taken. These measurements revealed that a non-symmetric pressure distribution, which induced a spanwise flow in the base region of the cylinder, was responsible for the oblique shedding. By using a simple model based on the ratio of the streamwise to the spanwise vorticity components, the angle of shedding was predicted within 2° of the value measured from flow visualization. The vorticity was simply evaluated from the spanwise and streamwise velocity profiles of oblique vortex streets obtained with the LDV measurement technique. Parallel vortex shedding showed a symmetric pressure distribution with zero spanwise component of the velocity and zero cross-shear in the cylinder base. It was shown that parallel vortex shedding results in a continuous Strouhal–Reynolds number curve.