Formation and interaction of two parallel vortex streets

The instability of two pairs of infinitely long parallel vortex sheets for initial disturbances is investigated by a linear analysis in an inviscid incompressible fluid. It is shown paradoxically that two pairs of parallel vortex sheets cannot exist at a gap distance between two pairs less than the distance between vortex sheets of each pair. By calculating the nonlinear growth in time of periodic disturbances in the vortex sheets, the essential features of the formation and interaction of two parallel vortex streets are analyzed. It is demonstrated that the initially antisymmetric disturbance is more apt to cause the interaction between two vortex streets in their developing stage than the symmetric one. These theoretical results are compared with the experimental ones which were observed in a flow past two circular cylinders spaced in the direction perpendicular to a uniform flow.