Ship Wakes and their Surface Signatures
Ermakov, Stanislav; Kapustin, Ivan; Kalimulin, Rashid; Ermoshkin, Alexey
Institute of Applied Physics RAS, RUSSIAN FEDERATION
Spatial/temporal evolution of turbulence generated by surface ships and the effect of the wake on short wind waves has been studied on the Black Sea and on the Gorky Water Reservoir. Measurements of currents in ship wakes were conducted using an Acoustic Doppler Current Profiler deployed from a motor boat. It has been obtained that the temporal/spatial evolution of the wake width could be described approximately by a 0.4-power dependence, and the wake depth remained nearly constant at its initial stages. A simple one-dimensional model of ship wake evolution and of the wake widening has been developed using a semi-empirical theory of turbulence.
The wake surface signatures which were studied using photographs and radar images, including SAR images, are characterized by "flattening" of surface roughness due to turbulence at short distances (small "wake ages"), by formation of rough bands at the wake edges at small to intermediate stages, and finally by slick bands ("rail trace") at large ages. A physical explanation of these signatures has been done based on experimental data. Namely, mean circulating currents near the boundaries of the wake were detected in experiment. These currents result in the wind wave intensification due to wave straining at the edges of the wake. The rough bands appeared to be strongly asymmetric when the wind direction is nearly perpendicular to the wake. It was shown that slick bands appeared at the later stage of the wake evolution. Film samples were collected from the slick bands and outside the bands when crossing the wakes and the enhanced surfactant concentration was detected in the slicks as a result of the surfactants transport to the water surface by air bubbles in the wake and compression of surface films by the mean circulating currents.
The work was supported by RFBR (projects 12-05-31237, 11-05-00295, 12-05-31336), the Program RAN Radiophysics, and by the Russian Government (Grants No. 11.G34.31.0048 and 11.G34.31.0078).