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.
Two limiting cases were investigated in detail: the interaction
with a surfactant-rich
(contaminated) surface and with a surfactant-poor (‘clean’)
surface. In the latter
case shadowgraph images showed that the underlying vortex core formed a
line of
circular surface depressions. Subsequent measurements of the temporally
evolving
velocity fields using digital particle image velocimetry (DPIV) of the
vortex pair
cross-sections and the subsurface plane confirmed the connection process
of the main
vortex core with the surface. As a result of the connection the initially
modulated
vortex tube was broken into a line of U-vortices. In the presence of surfactants
this connection could not be observed; rather a Reynolds ridge (or stagnation
line)
was formed and a very weak connection of the secondary separation vortex
could
be seen in the shadowgraphs as well as measured with the time-resolved
DPIV
technique.
A prerequisite for connection of the vortex with the
surface is that the flow's
kinematics force the vortex core, that is, regions of concentrated 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 each side of the decelerated region whose gradually
increasing circulation is directly balanced by the loss of
circulation of the surface-parallel
vortex. However, the shear forces caused by small amounts of surface
contamination and its associated subsurface boundary layer inhibit
the connection process by
preventing the essential viscous flux of parallel vorticity through the
surface.
Instead, the subsurface boundary layer is associated with
a flux of parallel vorticity
into the surface which then concentrates into the observable secondary
separation
vortex.