Surface‐active films are found in all parts of the ocean. They are easily detected visually on the ocean surface when the wind speed is less than 5 m/s (10 knot). They are generally of biological origin and consist mainly of fulvic and humic acids, carbohydrates, proteins, and lipids. Their molecules have hydrophilic (partially water soluble) heads and hydrophobic (insoluble carbon chain) tails. Surface‐active films that have been concentrated at the edges of the centerline wake generated by the passage of a surface ship strongly influence the propagation of the short gravity and capillary waves which interact with electromagnetic waves at both radar and visible wavelengths. An accurate method for detecting the presence of surface films and measuring the fine‐scale surface film pressure and surface elasticity distributions on a water surface was developed at the Naval Research Laboratory (NRL). These measurements, together with model‐scale flow visualization experiments, have allowed us to develop a better understanding of the role these ship‐generated surface‐active film distributions play in the formation of the dark centerline and “railroad track” or dark lines which are aligned at some narrow angle to the ship's track in synthetic aperture radar (SAR) images of ship wakes.
Measurements of short wave energy, surface tension distributions, and radar imagery of centerllne ship wakes are presented. A direct association between reduced radar return and reducedscattering wave energy is demonstrated. The dominant influences of ship-generated turbulence and surface film distributions upon reduced short wave energy in ship wakes are shown. The effect of turbulence is emphasized by the very slow regrowth of attenuated wave energy in the especially turbulent wake of a towed barge. The effect of surface film distributions is emphasized by the suppression of short wave growth during a wind puff in a wake that is about 1 hour old.
The present study examines the vortex street wake behavior of a flexible, helically wound, high aspect ratio marine cable in a linear shear flow. Particular attention is paid to the lock-on phenomena associated with uniform and sheared flow past the cable when it is forced to vibrate in the first mode, normal to the flow. An analysis is given of the effects on the vortex shedding and synchronization phenomena that are generated by placing distributions of spherical bluff body shapes along the span of the cable in uniform and sheared flow. The latter geometry is representative of a number of cable system deployments and has special consequencies for strumming in a shear flow. The effectiveness of these attached spheres as strumming-suppression devices is evaluated. Synchronized vibration and/or the presence of the bluff bodies significantly affected the spanwise character of the near wake cellular vortex shedding structure. The spanwise extent of the resonant, vortex-excited oscillations was significantly extended by the presence of the spheres along the cable span. This finding was particularly significant because it meant that the undesirable effects that accompanied synchronization would be extended over a longer portion of the cable span.
A mathematical model based on the spectral energy balance equation for suppression of shortwave energy in centerline ship wakes is developed. At distances behind a ship of more than a few ship lengths, the dominant influences on short wave energy are the growing effects of wind and nonlinear energy transfer from longer waves, and the attenuating effects of turbulence and viscous attenuation which can be greatly enhanced by surface film distributions which are modified by passage of the ship. These influences are modeled as "source terms" in the energy balance equation. The model is exercised using turbulence data from scale model experiments and surface film distributions measured in an actual ship wake. Agreement between the resulting distribution of Bragg-scattering wave energy and radar image intensity in the far wake is excellent. The influences of variations in the source term model parameters are explored numerically. 96(C6):High-resolution measurements of surface-active film redistribution in ship wakes, J. Geoph•s. Res., 97(C-4):5231-5252, 1992. Phillips O., The scattering of gravity waves by turbulence, Journal of Fluid Mechanics, 5(12):177-192, 1958. Plant W.J., A relationship between wind stress and wave slope, J. Geoph.•ls. Res., 87:1961-1967, 1982. Skop R.A., O.M. Griffin, K.R. Nicolas, and T.F. Swean, Jr., Modification of wave spectra by currents in the wake of a surface ship, Nrl memorandum report 6336, U.S. Naval Research Laboratory,
Experiments were performed to exploit the dispersive properties of unsteady surface waves and to induce breaking by using a modified chirp pulse technique to focus the wave energy at a specific location in the Naval Research Laboratory deep water wave channel. The experiments have resulted in a highly resolved archive of breaking events ranging from wave steepening and incipient breaking to spilling and to plunging. The potential energy density, the crest front steepness, the horizontal asymmetry, and other geometric properties of an incipient breaker vary only within a moderate band about their mean values over the extent of these experiments. Thus the properties of an incipient unsteady breaker are well defined. The application of the phase-time or Hilbert transform method to the data set provides new insights into the local properties of the unsteady wave breaking. Recently, spectral and piecewise-linear algoritbans for two-dimensional potential flow were developed and used by Schultz et al. [1994] to compare the onset of breaking for several methods of energy input to the unsteady wave system. The computations show that steep plunging waves occur when energy input rates are large. The various energy input methods exhibit similar breaking trends in the limit as the energy input rate becomes small in that incipient spilling breakers form when the potential energy is approximately 52 to 54% of the energy for the most energetic Stokes wave, with the formation of a singularity immediately before the crest. 16,515 16,516 GRIFFIN ET AL.: EVOLLrHON OF DEEP WATER BREAKING WAVES Background The fundamental experiments for studying twodimensional wave breaking (apart from wind-generated waves which are discussed briefly later) fall into three main categories: (1) the focusing of essentially two-dimensional waves in the lateral direction [Van Dom and Pazan, 1975; Ramberg et al., 1985; Ramberg and Griffin, 1987]; (2)the towing of a submerged object, such as a hydrofoil to produce steady breakers [Duncan, 1981, 1983]; and (3)the focusing of variable-length waves from a modulated or chirped wave maker to produce unsteady breakers [Dornmermuth et al., 1988; Duncan et al., 1987; Rapp and Melville, 1990; Peltzer et al., 1993; Duncan et al., 1994; Sletten and Savtchenko, 1996] or the overturning of an irregular wave train [Ochi and Tsai, 1983; Bonrnarin, 1989] to produce unsteady breakers. One of the most extensive laboratory studies of unsteady deep water breaking thus far is the work of Rapp and Melville [1990]. The dispersive character of the deep water waves was used to focus a wave packet and to generate a single unsteady breaking event at a controlled location in the wave channel. Losses of excess momentum and energy flux during breaking within a wave group ranged from 10% for spilling breakers to 25% for plunging breakers. Rapp and Melville found that the growth rate of the waves prior to breaking was an important factor in predicting breaking. This had been found previously by Van Dom and Pazan [1975] and, to a lesser e...
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