On secondary vortices in the cylinder wake

The wake of a circular cylinder is investigated for Reynolds numbers between 160 and 500 by means of particle image velocimetry (PIV). For the first time cross-stream velocity fields are determined for two classes of secondary vortices (A-mode and B-mode). The circulation of the A-mode secondary vortices in this plane is approximately twice the circulation of the B-mode secondary vortices. The spanwise wavelength of the secondary vortices is four to five cylinder diameters for the A-mode and one diameter for the B-mode. The spatio-temporal development of the wake is analyzed by acquiring a time sequence of PIV images covering several Kármán periods. On the basis of the vorticity field, the A- and B-modes can be identified as topologically different vortex structures. Two vortex models are developed to explain the differences between these modes.