ABSTRACT
Much has been learned about vortices by means of flow visualization. Particularly successful have been the vortex-tagging techniques whereby smoke is released in air or dye or fine bubbles are released in water from vorticity-generating solid surfaces. This chapter emphasizes the visualization of two-dimensional vortices; three-dimensional vortex patterns. It describes examples of the various vortex interactions in time sequence. Vortex tagging appears most useful in unsteady flows at modest Reynolds numbers where vortices can be visualized near bodies. Developing vortices have spiral shapes but the simplest vortex is circular. Such a structure is often attained by a tendency of noncircular vortices to become rounded in the absence of strong external deformation. Another basic vortex interaction is the marching of a pair of vortices rotating in opposite directions on a straight or curved path, depending on their relative strengths.
