In the course of development of controlled impinging jets, a phenomenon was discovered that may seem to be rather strange at first sight. It is a change of topology - in fact, in the central region, the very reversal of topological character - of the surface flows occurring when oscillation is superimposed on the supply now into an annular nozzle generating the impinging jet. The change, apart from its interesting hydrodynamic aspects, is of importance for heat and mass transfer applications as it promises a considerable increase of total transfer rate as well as an increase of the area covered by the impingement. (C) 2008 Elsevier B.V. All rights reserved.