WAKEX 86: A Ship Wake/Films Exploratory Experiment

摘要

In an exploratory experiment on Chesapeake Bay we examined several aspects of ship wakes and their interaction with natural and artificial surface films. We used aerial photography to record these interactions, the geometries of white water wakes and the motions of drifters distributed in the wakes of ships. We also measured the surface tension across ship wakes in natural waters. Our velocity measurements using the drifters are consistent with the theory (Garrett and Smith, 1984; Swanson, 1986) that a displacement hull sheds a pair of vortices which moves surface water away from the center of the wake. (Our observations of the artificial slick/wake interaction corroborate this conclusion.) On either side of the wake in natural waters, slicked bands were visually observed and measurements showed their surface tension to be several dynes/cm lower than the ambient; this indicates these slicked bands to be compacted surfactant material which damp the capillary and capillary-gravity waves. This pattern of slicked bands is very similar to those highly persistent ones photographed from space (Scully-Powers, 1986) and is consistent with the wake vortex theory. Our photographs also suggest that surfactant material is being deposited in these bands by rising bubbles which scour surfactants from the water column; these bubbles are generated by the bow wave. The white water wake observations are consistent with Peltzer 1984a but also show the importance of the bow wave in foam generation. Keywords: Ship wakes; Remote sensing; Surface tension; Ocean films; Ship waves; Hull flows; Ship hydrodynamics; Vortices. (EDC)