A remote sensing study of a surface ship wake

Abstract

The turbulent wake of the USNS Hayes, a twin hulled ship, was imaged simultaneously by a thermal infrared scanner, an X-band microwave radar and a 35 mm strip camera mounted in an NRL RP-3A aircraft. Thermal surface effects and centimeter-scale surface roughness characteristics were determined for both natural ship wakes and those treated with oleyl alcohol, an organic material which produced a monomolecular film on the surface of the turbulent wake. The turbulent motions and the presence of the monomolecular film at the wake surface strongly influenced the centimeter-scale surface roughness. This influence appeared as a significant reduction in the power of the reflected microwave signal from the wake surface compared to the surrounding ambient surface. The persistence of this reduction appeared to increase when the film was present. Various computer codes were employed to analyze the digitized IR video data. They generated temperature contour plots and temperature profiles across the wake at various locations behind the ship. These computer plots along with the original 70 mm photographic representation of the data and the corresponding water temperature data indicated that all the wakes were significantly cooler than the surrounding ambient surface water and slightly cooler than the water at keel depth. The thermal signatures of the wakes treated with the surface film were more persistent than the natural wakes, and the cool surface was maintained over a broader cross section of the treated wakes. These observations can be explained on the basis of changes in emissivity and related surface properties, the ship wake hydrodynamics, wind stress considerations and surface film physics.