ON SECONDARY VORTICES IN THE CYLINDER WAKE

The wake of a circular cylinder is investigated for Reynolds numbers b etween 160 and 500 by means of particle image velocimetry (PIV). For t he first time cross-stream velocity fields are determined for two clas ses of secondary vortices (A-mode and B-mode). The circulation of the A-mode secondary vortices in this plane is approximately twice the cir culation of the B-mode secondary vortices. The spanwise wavelength of the secondary vortices is four to five cylinder diameters for the A-mo de and one diameter for the B-mode. The spatio-temporal development of the wake is analyzed by acquiring a time sequence of PTV images cover ing several Karman periods. On the basis of the vorticity field, the A - and B-modes can be identified as topologically different vortex stru ctures. Two vortex models are developed to explain the differences bet ween these modes. (C) 1996 American Institute of Physics.