Structure and Persistence of Ship Wakes and the Role of Langmuir-Type Circulations

Abstract

Surface ships operating in ocean waves are shown to generate transverse surface currents which persist long after the initial ship-induced currents have decayed. These surface currents are due to the formation of large-scale Langmuir-type circulations (LTC) and are suggested to be the mechanism for the concentration of surface-active substance into streaks or bands, which are regularly seen in synthetic aperture radar imagery of the ocean surface. These circulations are generated by the Craik-Leibovich vortex force, which results from interaction of ship-induced currents with ambient surface waves. Once generated, the circulations persist due to their large-scale nearly inviscid nature. Numerical simulations of the wake behind a surface ship in calm, head, and following seas are presented and show good agreement with radar imagery from at-sea experiments. It is demonstrated that surface currents do not persist in the absence of surface waves, but can persist for tens of kilometers through the formation of LTC. Because the circulations are driven by the cross product of the Stokes drift and the wake vorticity, the structure of the persistent wake is a function of relative heading of the ship with respect to the direction of surface-wave propagation. 1. Introduction The study of ship wakes presents a field that has had great theoretical and applied interest for over a century, starting with the description of the Kelvin wake (Thomson 1887). The theoretical interest is still significant because not all mechanisms related to ship wakes are understood, particularly the long persistence of ship-induced perturbations. There is a high degree of applied interest in the matter, as ship wakes can be observed in synthetic aperture radar (SAR) imagery (Munk et al. 1987; Reed & Milgram 2002; Brusch et al. 2011) and optical imagery (Munk et al. 1987). There are several distinct processes generated by a self-propelled ship: surface waves generated by the ship hull (Kelvin wake); propeller and hull-induced near-surface flow and turbulence; and bubbles produced by the propellers and breaking bow waves (Peltzer 1984). These features are well documented in field (Peltzer 1993) and towing-tank (Swean 1987; Peltzer et al. 1992) experiments. Most of these features decay several ship lengths behind the vessel (Swean 1987), which is also shown by simulations presented in this article. Nevertheless, it is well known that some features of ship wakes can be observed well after the passage of the ship, in some cases tens of kilometers behind the vessel. This article addresses the physical mechanism for the formation and resulting structure of such long-lasting ship wakes, and describes their qualitative and some quantitative features.