On the action of viscosity in increasing the spacing ratio of a vortex street

1—von Kármán, by considering two parallel rows (of indefinite extent) of isolated, equal, point-vortices existing in a non-viscous fluid, has shown that the only stable vortex arrangement is the asymmetrical staggered one; and then only provided that the geometry of the system is such that h / a = 0·281, where h = width between the rows, and a = distance between consecutive vortices in one row. Since von Kármán’s investigation was published, writers on the subject have attempted to connect up the street with an obstacle producing it; and to investigate the effect of channel walls upon the stability and spacing ratio of the ideal street. At the same time efforts have been made to verify von Kármán’s spacing prediction by experiment, and to check the theoretical conclusions concerning the effect of parallel walls ;§ but the results have been far from satisfactory.