The interaction of modulated vortex pairs with a free surface

Spatially modulated vortex pairs were generated below a free surface by two counter-rotating flaps whose edges approximate a sinusoid. The surface interactions of the vertically approaching vortex pairs were visualized by the shadowgraph technique. For certain initial conditions each modulated vortex core was seen to form a line of circular surface depressions. Subsequent digital particle image velocimetry (DPIV) measurements of several vortex pair cross-sections and the subsurface plane confirmed the existence of vortex core terminations at the surface--the modulated vortex tube was broken into a line of U-vortices. A prerequisite for reconnection of the vortex with the surface is that the flow's kinematics force the vortex core, that is regions of high vorticity, toward the surface. The ensuing locally concentrated viscous flux of surface-parallel vorticity through the surface is balanced by a local surface deceleration. Surface-normal vorticity appears on either side of the decelerated region whose gradually increasing circulation is directly balanced by the loss of circulation of the surface-parallel vortex. The circulation transfer for the ;This work makes extensive use of digital particle image velocimetry (DPIV) in obtaining instantaneous flow field information from two-dimensional spatial cuts through the flow domain. A large number of laser-sheet illuminated particles are imaged by a solid state sensor yielding image sequences of single-exposed particles. A local interrogation of a pair of consecutive images by means of a local two-dimensional cross-correlation extracts the average displacement of the sample particle images whose velocity is then inferred from the magnification factor and time difference between the particle illumination. The acquisition of image sequences gives insight to the temporal evolution of the images flow field. Although the image acquisition rate is limited to the 30Hz video standard, asynchronous particle illumination in conjunction with a specially configured camera yield particle image pairs whose time difference is on the order of 2 milliseconds.