Showing papers on "Swell published in 1974"

Wave breaking in the presence of wind drift and swell

Abstract: Wind, blowing over a water surface, induces a thin layer of high vorticity in which the wind stress is supported by molecular viscosity; the magnitude of the surface drift, the velocity difference across the layer, being of the order of 3% of the wind speed. When long waves move across the surface, there is a nonlinear augmentation of the surface drift near the long-wave crests so that short waves, super-imposed on the longer ones, experience an augmented drift in these regions. This is shown to reduce the maximum amplitude that the short waves can attain when they are at the point of incipient breaking. Theoretical estimates of the reduction are compared with measurements in wind-wave tanks by the authors and by Mitsuvasu (1966) in which long mechanically generated waves are superimposed on short wind-generated waves. The reductions measured in the energy density of the short waves by increasing the slope of the longer ones at constant wind speed are generally consistent with the predictions of the theory in a variety of cases. (Author)

Whistler waves observed upstream from collisionless shocks

Abstract: Waves in the frequency range 0.5 - 4 Hz were studied in the region upstream of the earth's bow shock using data from the fluxgate magnetic field experiment on IMP-6. Analysis of 150 examples of these waves during a three month interval indicates that amplitudes are generally less than 1 or 2 gammas and propagation directions generally make angles of between 20 and 40 degrees with the field direction. The waves as measured in the spacecraft frame of reference are either left or right hand polarized with respect to the average field direction. It is concluded that the observed waves are right handed waves in the plasma frame of reference with wavelengths of approximately 100 km propagating upstream in the whistler mode. Doppler shifting reduces the observed frequencies in the spacecraft frame and reverses the observed polarization for those waves propagating more directly upstream. Similar waves are seen ahead of most interplanetary shocks.

Spatial variability of underwater ambient noise at the Arctic ice‐water boundary

Abstract: Quasisynoptic measurements were made to determine the spatial variability of ambient noise at the Arctic ice‐water boundary over a frequency range from 100 to 1000 Hz. The results of this investigation show that the ice‐water boundary acts as a spatially well‐defined source of ambient noise with measured noise levels near a compact edge about 12 dB higher than open water levels and about 20 dB higher than levels far in the ice field. Measured noise levels near a diffuse ice edge were about 4 dB higher than open water levels, and about 10 dB higher than levels far in the ice field. The relatively high noise levels at the ice edge are probably generated by wave and swell interactions with individual ice floes.

Waves and irregularities in the solar wind

Abstract: This review is meant for the nonspecialist who wants a simple physical picture. Some simple calculations are given to show why waves and irregularities in the solar wind are of importance in obtaining a physical understanding of the dynamical processes occurring in the interplanetary medium. The basic physics of various phenomena observed in the heliosphere, resulting because of the presence of waves in the solar wind, is spelled out in some detail. The most fundamental point made in this review is that a considerable body of phenomena occurring in the solar wind are either caused directly or modified significantly by the turbulent waves that are always present in the solar wind.

Water particle velocities measured under ocean waves

Abstract: Simultaneous measurements of waves and two orthogonal water particle velocities were made at six elevations in 19 m of water by using a penetrating wave staff and an electromagnetic flowmeter. Moderate swell and low-wind conditions prevailed during the experiment. The measured wave-induced velocities were 2–4% greater than those calculated by using linear wave theory. Coherence of the wave height and wave-induced velocities in the significant energy-density range was computed to be over 0.85, indicating that the motion was almost totally wave induced. At higher frequencies it was apparent that the motion was primarily turbulence. Phase spectra computed for the measured wave heights and orbital velocities compared very well with linear theory. Measured frequency distributions were compared with both Gaussian and Gram-Charlier distributions by using the chi-squared goodness-of-fit test. Qualitatively, the Gram-Charlier distribution gave the better fit to the data suggesting a weakly nonlinear system.

Mode and period of sand transport in the surf zone

Abstract: Three almometers-water opacity measuring devices-emplaced perpendicular to the beach, measure instantaneously and continuously the sediment concentration across the surf zone. Most of the variance of the sand movement is centered in frequencies of less than 0.25 Hz and between 1.15 and 1.25 Hz. Modes and frequency of sand transport differ within each of the dynamic zones of the surf. The motion of sediment in the inner and outer surf zones is small and virtually independent of the deep water wave periods. Outside the breaker zone, bed load movement is somewhat coincident with the prevailing swell period. Lighter concentrations move predominantly with a 0.8-0.9 second periodicity. In the breaker zone, sand moves along the bottom with frequencies equal to that of both the swell and sea, but most of the power is in lower frequencies. In the breaker zone sand is rarely thrown into suspension. In the transition zone, sediment motion is largely by suspension with a period a little longer than the swell.

On the Planimetric Shape of Wreck Bay, Vancouver Is.

Abstract: The planimetric shape of Wreck Bay has been studied by fitting various segments of the bay to theoretical logarithmic spiral curves, and by using the characteristics of incident swell to estimate longshore sediment transport along the beach. We have found that the cliff base in the northwest half of the bay closely follows log-spiral curvature, whereas the southeast half reflects only poor correspondence. This situation is due largely to distortion of the incident swell by two islands in the bay, Florencia Is., and Seal Rock. Quasi-permanent cliff sections supporting large trees occur on the landward side of the theoretical log-spiral curves, and those sections exposing unconsolidated glaciofluvial outwash are generally seaward of the curves. Present day erosion, due mainly to wind action on the exposed parts of the cliff, indicates that gradual mutation of the planimetric shape, particularly for the southeast half of the bay, is tending towards an improved fit. Longshore current distribution along the beach accounts for the direction of spit-bar growth at the mouth of Lost Shoe Creek, and also for the presence of Sand Point in the middle of the bay. It has been found useful when comparing the goodness of fit of beaches of different lengths to introduce a normalized root mean squared error. Furthermore, it has been shown that if a headland remains part of a receding spiral beach, then the spiral center must gradually migrate away from the headland as erosion proceeds.

Equilibrium range in wave spectra observed at an open‐ocean tower

Abstract: The equilibrium range constant α in S(ω) = αg2ω−5 estimated for typical sequences of North Atlantic open-ocean wave spectra ranged from 7 to 12×10−3, the overall mean value being 8.8×10−3, and regression lines fitted to mean spectra yielded exponents that differed little from −5. Frequency bands over which these equilibrium values of S(ω) held were narrow. A sequence of wave spectra associated with nonstationary winds illustrated some of the difficulties in dealing with open-ocean wave data. Observations of turbulent wind velocity spectra under these conditions illustrated sea-to-air momentum transfer associated with advecting swell.

The interaction of wind waves and internal waves: experimental measurements

Abstract: Internal waves were generated by a ship using the dead water effect in areas where the water contains a strong near-surface density gradient. The effects of these internal waves on the wind waves were examined. The principal measurements included height and slope statistics of the wind waves and horizontal currents in the internal waves. The effects on the wind waves were always observable from an aircraft; however, in measurements made only along the ship's track, the effects of the internal waves were not always readily distinguishable from other factors that influence the wind wave field. By using statistical techniques, relationships have been established between the wind waves, internal waves and the wind velocity. The principal finding is that the wind wave field is relatively more sensitive to internal wave currents at low wind speeds than in high winds. Numerical values are given.

Estimation of Boundary Conditions for Coastal Models

Abstract: In this study, frequency response and transfer function techniques are used together with cross-spectral and fast Fourier transform methods to determine the proper boundary values for computing the flow field of a coastal sea. Tide data containing considerable perturbations from swell and meteorological disturbances are analyzed. In computing the frequency response estimates, the effect of noise in the input is treated by a cancelling technique and by the choice of a reference station to evaluate the interdependencies among the other stations at the boundary. The usefulness of the network frequency response function is threefold: (1) future conditions can be simulated using observed water levels at any single location, (2) boundary information for models of different grid size can be obtained by interpolation, and (3) missing data at a given location can be estimated optimally using data at neighboring stations and the network response function. The paper discusses an example of such an application, the determination of a boundary of a two-dimensional model of Jamaica Bay, New York City, U.S.A.

Calculation of the Fluctuations of Propeller Load Induced by Ship Motions in Oblique Wave (Part 1)

Abstract: The fluctuations of the propeller load of a high speed container carrier oscillating on the swells have begun to come into question as it's speed becomes higher and higher. The measured results of model experiments in the self-propulsion test in oblique regular waves have shown the fluctuations that amount to 50% of mean value, at the wave height 1/40 of model length. These phenomena of remarkably great vibrating bearing propeller forces have already been pointed out by Hoshino et. al., Schwanecke, Vedeler and Lipis. In the present work, the author attempted to estimate the fluctuations of propeller load applying the unsteady aerofoil theory and the theoretical calculations of ship motions based on the New Strip Method in oblique regular waves. The fluctuations of propeller load were shown to consist of three harmonics for one blade, namely encounter circular frequency, omega sub iota; the frequency compounded by circular frequency of propeller revolution, omega Sub iota, plus or minus omega. For the total blades only one harmonic, omega Sub iota was shown to act. fluctuations have made clear that wave orbital motion contributes exceedingly, and then in the order of surging and pitching motions of a vessel. Theoretical calculations by the present method have been compared with the above mentioned model experiments, and shown to be in good agreement with them, except the region of short wave length where the fluctuations of fluid velocity are affected by the deformation of waves due to the presence of the vessel. The thrust fluctuations of loads acting on one blade and that for the total blades as a whole propeller were simulated by the computer, the results of which are similar to some experimental results measured by Takaishi and others.

Basic systems of wind wave field

Abstract: The study of wind waves is usually carried out in the following manner. At the first moment a homogenous wind field with the constant speed directed from the shore to the basin is occurred over the water surface restricted by a straight shore line. It is required to calculate statistic wave characteristics as functions of time and distance from the shore. When solving the problem in such a way the explorers [.1-6 ~] usually came to a conclusion of the system development of gravitational waves with a main energy maximum the amplitude and period of which rise in process developing from small magnitudes to limiting values. Some explorers noted that the two -or three-wave systems under the conditions of constant wind are available. The first results of this theory were obtained by L.Ph. Tytov. In studies of stereophotographs of sea waves he noted and described quantitatively two types of waves: "prevailing" and "large", f7l I* is possible to show that the first type of waves has a phase speed that is less than wind speed, and the second one is equal to wind speed. At a later time G.Neumann [6J generalizing results of ocean observations has come to the conclusion that under the action of constant wind three "specific" wave systems which have phase speeds less, equal and 1.2 more than the speed of wind are developed. However, Tytov's and Neumann's results didn't receive a progress, and later on they were substituted by the conception of continuous wave spectrum with one energy maximum £3-6J • Nevertheless, the opinions of availability of two-or three-wave systems as a typical feature of wind rough sea [8,9] were published in the press.