Showing papers on "Swell published in 1977"

Measurement of skin friction distribution along the surface of wind waves

Abstract: Measurements of local values of the skin friction have been made at many points along the surface of representative wind wave crests in a wind wave tunnel, by use of the distortion of hydrogen-bubble lines. The results obtained at 2.85-m fetch under 6.2 m s−1 mean wind speed show that the intensity of the skin friction varies greatly along the surface of wind waves as a function of the phase angle. It increases rather continuously at the windward surface toward the crest, attains a value of about 12 dyn cm−2 near the crest, decreases suddenly just past the crest, and the value at the lee surface is substantially zero Values of the skin friction thus determined along the representative wind waves give an average value of 3.6 dyn cm−2, rather exceeding the overall stress value of 3.0 dyn cm−2, which has been estimated from the wind profile. The results are interpreted as that the skin friction bears most of the shearing stress of wind, and that it exerts most intensively around the representative wave crests at their windward faces.

Wind Stress and Roughness Length over Breaking Waves

Abstract: The effects of surface wave breaking on the adjacent atmospheric boundary layer are examined. It is argued that the transition from aerodynamically smooth to rough flow in a neutral atmosphere corresponds to the onset of extensive small-scale wave breaking. The association of wave breaking with the generation of turbulence in the boundary layer above leads to the result that the friction velocity is approximately equal to the phase velocity of the breaking waves. It is argued that this approximate relationship holds even when the small-scale breaking waves are riding on a swell. The existence of a minimum phase velocity for surface waves then requires that there be a minimum friction velocity, in the neighborhood of 23 cm s−1 below which rough flow cannot occur. A result of Phillips and Banner (1974) which describes the limiting amplitude of small gravity waves under the action of wind drift and swell is used to derive a relationship between the roughness length and friction velocity which is a g...

Measurement of the High-Frequency Spectrum of Ocean Surface Waves

Abstract: High-frequency spectra of wind-generated ocean waves were measured at an ocean research tower of Kyushu University using a fast-response wave recorder and an electronic differentiating circuit. Wind waves generated by a northeast wind (speed U10.5=8 m s−1, fetch F=2 km) were superimposed on the swell from the north and in a stationary state. The equilibrium range of the wave spectrum, where the spectral form is given bywas clearly observed in a frequency range fm 5 Hz) was given approximately by>where σ is the surface tension, ρw the den...

Laser probe for measuring 2-D wave slope spectra of ocean capillary waves.

Abstract: A laser-optical instrument for use in determining the two-dimensional wave-slope spectrum of ocean capillary waves is described. The instrument measures up to a 35-deg tip angle of the surface normal by measuring the position of a refracted laser beam directed vertically upward through a water surface. A telescope, a continuous two-dimensional Schottky barrier photodiode, and a pair of analog dividers render the signals independent of water height and insensitive to laser-beam intensity fluctuations. Calibration is performed entirely in the laboratory before field use. Sample records and wave-slope spectra are shown for one-dimensional wave-tank tests and for two-dimensional ocean tests. These are presented along with comparison spectra for calm and choppy water conditions. A mechanical wave follower was used to adjust the instrument position in the presence of large ocean swell and tides.

Effects of long waves on wind boundary layer and on ripple slope statistics

Abstract: Simultaneous measurements of the wind boundary layer and the wind-generated ripples were conducted in a wind-wave tank with and without preexisting long regular surface waves. These long waves were found to have little effect on the roughness length of the disturbed water surface and to suppress the growth of short waves.

Evaluation of Laboratory Suction Tests for Prediction of Heave in Foundation Soils.

Abstract: : Commonly used methods for determining swell potential and predicting volume change and in situ heave are evaluated from the standpoint of simplicity, economy, reliability of test data, and simulation of field conditions. Swell potential is often defined as the percent swell from in situ water content to saturation under a surcharge pressure of 1 psi, whereas in situ heave is often defined as the percent swell from changes in the initial to assumed equilibrium moisture and confining conditions in the field. Increasing degrees of expansion or swell potentials usually correlate with increasing liquid limits and plastically indices.

Swell waves at Saint Helena related to distant storms

Abstract: This paper discusses the relationship between a frequency-time plot of energy density of swell, from records taken at St Helena in December 1970 and January 1971, and meteorological records of distant storms. A similar exercise in 1969 had suggested that some of the swells originated from the southern hemisphere, even possibly from the Pacific Ocean, but wave amplitudes were all very low. Higher waves were recorded in the present exercise, but all the major events in the energy spectra were attributable to northern hemisphere storms, a few minor events only to the southern hemisphere, and none to the Pacific Ocean. It is concluded that large northwesterly storms in the area southeast of Newfoundland are probably responsible for the spectacular ‘rollers’ which are frequently reported from St Helena.

Barus effect in polymer flows through cylindrical and flat dies

Abstract: The determination of the swell factor of a polymer jet extruded from a cylindrical or flat die is examined. Relations are obtained for the dependence of the swell factor on shear rate and shear stress, expressed in terms of the constants of the rheological model.

Soft mooring systems at exposed terminals

Abstract: This paper describes a soft mooring system developed for an exposed tanker terminal proposed on the Bay of Fundy in Eastern Canada. The response of moored ships to both waves and wave groups and low energy portions of wave spectra is discussed. Natural periods of ship motions for various mooring configurations are compared to the periods of the waves, swell and wave groups. Analytic and model test results showing the decrease in mooring forces with the increase in system flexibility are presented.

Kelvin-Helmholtz waves in the ocean?

Abstract: Large amplitude short waves confined near the crests of a swell have been observed when a stiff breeze was blowing against the swell. This would seem to imply the existence of both a wavelength-selective generating mechanism and a trapping mechanism, neither of which is to be expected of surface gravity waves of the observed length. It is suggested that there are significant changes in the dynamics of such waves if allowance is made for the dynamic coupling between wind and waves. For a Kelvin-Helmholtz model it is shown that energy transfer rates from the turbulent pressure fluctuations are greatly enhanced for subcritical conditions by the inclusion of the dynamic coupling. The group velocity of subcritical waves is profoundly affected, becoming infinite at the stability boundary. Thus subcritical waves could be trapped on a swell. An examination of the effects of wind shear suggest that Kelvin-Helmholtz type instability could still be present, although for stronger winds, particularly for rather longer waves.

Method for collecting energy from swell and plant for carrying out said method

Abstract: A method and apparatus for collecting energy from a swell in a fluid medium using a mobile float which is displaceable in response to movements of the swell. The float is submergible in the fluid medium at a predetermined depth selected such that, in the absence of movement of the fluid medium, the upwardly and downwardly directed forces acting on the float counterbalance each other so that the float is in equilibrium at such predetermined depth. The buoyancy of the float is adjusted to maintain it in equilibrium.

On the hysteresis of the sea surface and its applicability to wave height predictions

Abstract: Because of the low dissipation rate of wave energy on the ocean's surface, the wave height at some location and time must be dependent upon wind fields in existence there at previous times and upon swell propagated there from other regions. To study these relationships, significant wave height (SWH) measurements from the Geos-3 radar altimeter are used in conjunction with anemometer windspeed measurements from weather ships, L, C, and R. During the passage of large cyclonic disturbances near the fixed locations of these vessels in the North Atlantic in February 1976, distinct hysteresis profiles that characterize the sea's memory during generation and dissipation conditions are observed. Examples are given that demonstrate the influences of cyclone intensity, movement, velocity, and shape on the configuration of these profiles.

The Oceanographic/Meteorological Environment West of St. Croix,

Abstract: : A review of oceanographic and meteorological data was undertaken for the underwater tracking range west of St. Croix in order to assemble a scenario of the physical environment. Recent current meter data from three current meter arrays moored during February 1976 indicated that the most significant contributions to the time dependent flow are rotary motions that have maximum amplitudes coinciding with the semidiurnal tidal period (12.42 hours). Maximum horizontal current shear (1.2 cm/sec/m) occurs at about 100. Convective mixing appears to be the principal process of layer depth variations, which varied from a minimum of 50 during August to a maximum of about 120 during March. Surface winds are out of the east during all months, with speeds averaging from 10.4 knots in October to 14.0 knots in July. Maximum average wave heights (sea and swell) most frequently occur with periods between 10-11 seconds. Seasonal variation in sound velocity amounts to about 3 m/sec in the first 100 m, 2 between 100 and 300 and less than 2 m/sec from 300 to 900.