An experimental investigation of vortex pair interaction with a clean or contaminated free surface.

Abstract

The interaction between a pair of trailing vortices and a free surface was studied. The vortices were produced by a submerged delta wing at a negative angle of attack. In order to isolate and study the interaction, an apparatus was built to generate reproducible vortex pairs with the same Reynolds number and Froude number as the trailing vortices. The apparatus consisted of a pair of counter rotating flaps which were moved by a computer controlled stepping motor. The vortex pairs generated with the flaps were laminar and remained laminar during the time that significant interaction with the free surface was observed. The Froude number for the vortices was low and the resulting surface deformations were small. Particle image velocimetry along with flow visualization (laser induced fluorescence and shadowgraph cinematography) was used to study the flow field. Striations, first observed by Sarpkaya and Henderson (1985) during the interaction of trailing vortices with the free surface, were also observed during the interaction of the vortex pair with the free surface. Striations are surface deformations which first occur at the center and intensify and spread to each side so that in a short time they are predominantly oriented in the cross-stream direction. The striations were found to be caused by cross-stream vortices which are stretched by the strain field between the trailing vortices (or the vortex pair) and their images. Contaminants on the free surface were found to have a large influence on both the free surface deformations and the flow field of the vortices. Also observed on the free surface was a Reynolds ridge on each side of the wake. The Reynolds ridge marks the leading edge of a boundary layer formed as a result of the shear stress produced at the free surface by surfactants. Vorticity of opposite sign is produced at a contaminated surface as a result of the vortex interaction. This vorticity can roll-up and form a pair of secondary vortices which will then alter the trajectory of the original vortex pair, causing them to rebound from the free surface. The effect of surface contamination was studied by adding known amounts of an insoluble surfactant to the free surface. Surface depressions, termed scars, were observed above the primary vortices as well as the secondary vortices, although in the case of the secondary vortices the depression was much greater.