Free-surface/vorticity interaction

Abstract

The unsteady flow phenomena resulting from the interaction of wakes and vortices with the free surface are of particular importance in naval hydrodynamics. Ship and submarine wakes produce a three-dimensional complex signature, comprised of a narrow dark band bordered by two bright lines in synthetic-aperture-radar (SAR) images. The dark band signifies the suppression of waves at the Bragg frequency as a consequence of the interaction between the free surface and the imposed vorticity. In the present investigation, the vorticity field is provided by a single tip vortex generated by an airfoil. The results, obtained with an LDV, have shown that the free surface redistributes part or all of the normal turbulent kinetic energy into streamwise and spanwise components. The turbulent kinetic energy first decreases sharply with increasing vertical distance from the vortex and then remains nearly constant within a thin layer below the 'roughened' free surface. The results explain the longevity of the structures and lend further credence to the simulation of near-surface structures via vortex- or contour-dynamics.