Hydrodynamic structure of a vortex ring

Abstract

Theoretical models of vortex rings obtained in the framework of the ideal fluid theory apparently give only a qualitative idea of a structure and characteristics of vortex rings that form and exist in a real viscous fluid. Until recently, direct experimental studies of the hydrodynamic structure of real vortex rings have been lacking, and thus it has not been clear how realistically theoretical models of vortex rings reflect the actual phenomenon. The lack of reliable experimental data on the structure of a vortex ring and on the structure of concentrated vortices in general, with the latter being one of the basic types of fluid and gas motion in natural conditions and in technical devices, is mostly explained by complexity and unsteady character of vortex motion in a fluid, and by imperfection of suitable scientific instrumentation. There are only experimental investigations by Timme (1957) and Kirde (1962), who studied the structure and evolution of one of the types of concentrated vortices forming in a crossflow past a cylindrical body and representing the so-called Karman vortex street (Karman and Rubach 1912; Schlichting 1969). In the experiments these vortices appeared on the water surface during the horizontal motion of a vertically set cylinder which was partly extended above the surface. A flow field of these vortices was studied by observing and registering tracks of small particles on the free surface of the liquid. However, it is clear that each of the vortices constituting the Karman street is not a completely isolated vortex structure in an unbounded medium, because it experiences the influence of the closely located body that is flowed past and the whole chain of the Karman vortices. These investigations give only an indirect idea of the structure of single concentrated vortices. The difficulties in performing direct measurements of a velocity field of vortices developing in actual conditions, such as whirlwinds, tornados, and hurricanes, are quite obvious.