Universal wake structures of Karman vortex streets in two-dimensional flows

Formation of Karman vortex streets by circular rods is studied in a flowing soap film channel. The two-dimensional fluid flow in the film allows stable vortex streets to be generated and studied over a broad range of Reynolds numbers, 10 < Re < 2000. There have been many studies of the Karman vortex streets. The current study focuses on the universal feature of the near-wake structures generated by circular cylinders in a uniform two-dimensional (2D) flow. The new approach in the investigation is to decompose the street into a series of vortex couples, which defines the outer envelope of the wake, and to study the expansion rate of these couples. Our experiment shows that motion of vortex couples are powered by bursts of transverse jets, and the jet intensity determines the asymptotic width and the expansion rate of the wake. A simple model is constructed that not only allows us to extract the kinetic energy encapsulated in the vortices but also explains the universal appearance of the 2D laminar wakes generated by large and small rods. To the best of our knowledge, this is the first time that the efficiency of energy injected into vortices is estimated and this efficiency turns out to be similar to 5%. (c) 2005 American Institute of Physics.