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

TL;DR

This paper investigates how ships generate long-lasting surface currents through Langmuir-type circulations caused by the Craik-Leibovich vortex force, influencing surface-active substance distribution and surface wake structure.

Contribution

It demonstrates the formation and persistence of Langmuir-type circulations in ship wakes through numerical simulations and links them to observed radar imagery.

Findings

Surface currents persist due to large-scale circulations.

Circulations depend on ship heading relative to wave direction.

Simulations agree with at-sea radar observations.

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 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 current 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 Langmuir-type circulations. Since 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.