Lagrangian Frame PIV for capturing vortex shedding from a moving cylinder close to a free surface

摘要

We have conducted laboratory experiments on the flow past a circular cylinder moving beneath an air-water interface. The vortex street and the free surface profile are visualized for Reynolds number between 200 and 15000 and for Froude number between 0.01 and 3.2. For the lower Reynolds number (Re = 3000), the vortex shedding from a circular cylinder is suppressed by the free surface at the small gap ratio for Re<1000. As a result, the vortex structure becomes smaller and the frequency of eddy shedding decreases. At the higher Reynolds number (>10000), the surface deformation becomes substantial in the downstream of the cylinder and intermittent bubble injection by wave breaking is observed. In order to investigate the relationship between the frequency of the vortex shedding and the free-surface wave behavior, Lagrangian Frame PIV measurement is conducted. From the results of Strouhal-Reynolds number relationship for gap ratios between 0.25 and 2.0, the period of the vortex shedding becomes long when the gap ratio is small.