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Ship Wakes

  • Ship wakes often appear in Synthetic Aperture Radar (SAR) imagery. These are due to hydrodynamic processes and their interaction with the radar waves at the surface of the water. The shape of the water surface can be affected by the processes, there can be effects from the surface velocities associated with the processes and the processes can introduce surfactants from below that damp the waves responsible for radar backscatter.
    The hydrodynamic processes can be subdivided into surface gravity waves, internal waves and the turbulent wake. The characteristics of surface gravity waves are affected by the depth of the water; in the open ocean the depth can be considered to be infinite. Internal waves occur when the ocean is stratified in temperature and salinity and their characteristics are affected by the depth and strength of the fluctuations within the layers.
    In addition, the ship wake depends on whether the excitation in the ship frame of reference is stationary or time varying. For example, the Kelvin wake is produced as a ship disturbs the water as it moves forward at constant velocity. In the ship frame, the disturbance in the flow pattern is stationary and, as a direct result, the wake crest pattern is also stationary in this frame. However, there are other possibilities associated with the effect of propeller action, ship motions and the reflection of ambient waves from the hull. A summary of the status of ship wakes was presented at the TEXAS II workshop in Washington, 2008; see J.K.E. Tunaley, Status of Ship Wakes, TEXAS II, Washington, August, 2008. This was also addressed at the TEXAS IV workshop in 2010; see J.K.E. Tunaley, Progress in SAR Ship Detection and Ship Wake Analysis, TEXAS IV, Washington, September 28th-30th, 2010. Alternatively workshop presentations can be found at TEXAS IV 2010, Briefs.
    When the water is stratified, such as when fresh water from the Fraser River overlies salt water in the Strait of Georgia, both steady and unsteady internal wave ship wakes can occur through similar mechanisms as for surface wakes; this includes the reflection of naturally occurring internal waves. Some material on this was presented at the 5th Canadian Conference on Marine Hydrodynamics at St. Johns, Newfoundland in 1999 and a recently updated version of the work is at J.K.E. Tunaley, The Unsteady Wake from a Body Moving Near an Internal Layer", LRDC Technical Report, October, 2010
  • The turbulent wake is produced because water sticks to the ship hull and the Navier-Stokes equation, which describes the flow, is non-linear. Viscosity tends to damp out the fluctuations that are amplified by the non-linear terms but, for practical ships moving at their service speeds, the water will be strongly turbulent within the boundary layer as the stern is approached. When the boundary layer detaches, the ship wake is formed and this comprises flows due to hull surface drag and to form drag as well as flows from the propeller. The last of these includes a swirling component. Superimposed on these is a field of turbulence. The theory of the formation of turbulent wakes has a long history but only a few canonical problems are easy to handle; these have been studied in the laboratory. In the ship case, the Reynolds numbers tend to be much higher than in the laboratory and the distances over which a ship wake occurs can be very long, as much as 30 km or more. Also the ship wake tends to be influenced by both linear flows and swirling flows, which is a situation not covered by the canonical cases.
  • Observations of ship wakes by RADARSAT-2 accompanied by AIS data (in part gathered from an AIS receiver at Kingston, Ontario) were made as part of a research program at the Royal Military College, Kingston. These are described by D.M. Roy and J.K.E. Tunaley, "Visibility of Turbulent Wakes", LRDC and RMC Report, March, 2010. Papers describing the theory behind the zero-angular-momentum wake and a combination of a swirling and linear momentum wake are provided in J.K.E. Tunaley, "The Zero Angular Momentum Turbulent Wake", LRDC Report, December 2010 and J.K.E. Tunaley, "Ship's Turbulent Propeller Wake: Combined Swirling and Linear Momentum Wake", LRDC Report, April, 2010. The latter work relies partly on experimental laboratory observations by other authors and is pertinent to the observation and simulation of ship wakes by synthetic aperture radar. A summary of the theory of the turbulent wake in its far field is provided in J.K.E. Tunaley, Theory of the Turbulent Far-Wake, LRDC Technical Report, January 2011. Another paper J.K.E. Tunaley, "The Narrow-V Wake", LRDC Report,October, 2009 deals with the narrow-V wake. These have applications to Maritime Domain Awareness (MDA).
    In a paper to be published, J.V.Toporkov (of the Naval Research Laboratory) et al., "Surface Velocity Profiles in a Vessel's Turbulent Wake Observed by Dual-Beam Along-Track Interferometric SAR", IEEE Geosci. Rem. Sens. Lett. Vol. 8(4), July 2011, describe observations of the wake of a small craft. The surface flow results are difficult to extract but show axial and radial wake profiles consistent with twin counter-rotating screws as described by the LRDC and RMC observations and theory cited above. In particular the swirling component seems to fall off more rapidly with distance than the axial component. The LRDC position is that hull drag and propeller thrust and swirl should account for the wake and that the twin vortices cited in the paper are speculative. This partial confirmation of the theory strongly suggests that ship wake observations can indeed provide information about a vessel's propulsion system as has been suggested by LRDC and, more recently, the authors.
  • Go-fast boats, sometimes known as "cigarette boats" are often used for smuggling and piracy and it would be useful to be able to detect them easily using radar satellites, such as RADARSAT-2. Theory suggests that these boats usually create a substantial wake of the narrow-V type that would be visible in a SAR image. The characteristics of go-fast boats are summarized in J.K.E. Tunaley, "Smuggler and Pirate Go-Fast Boats", LRDC Report, July, 26th, 2009.
  • When comparing the position of wakes with the positions of ships in a Synthetic Aperture Radar (SAR) image, it is important to correct the apparent ship position in the image for "Velocity Bunching". A simple derivation for this is provided in J.K.E. Tunaley, "SAR Velocity Bunching Relationships", LRDC Report, April, 2009.
    A DRDC (Ottawa) presentation dealing with the extraction of ship velocity (circa 2003) using the Radon transform is at J.K.E. Tunaley, "The Estimation of Ship Velocity, c. 2003.