Showing papers on "Swell published in 1983"

Wave set-up, percolation and undertow in the surf zone

Abstract: Waves approaching a sloping beach induce a tilt in the mean water level within the surf zone. The existence of this ‘set-up’ is here demonstrated by observing the mean flow in a straight tube laid parallel to the incoming waves; also by showing that the waves induce a siphon in a U-tube laid on the sloping bottom. It is argued theoretically, and confirmed by experiment, that the set-up should help to drive an offshore bottom current (the undertow) between the shoreline and the breaker line. Seawards from the breaker line the bottom current is reversed. The consequent convergence of the bottom currents may contribute to building up the ‘breaker bar’. Further experiments show that the mean onshore pressure gradient drives a circulation of water within a porous beach. The associated pattern of streamlines also extends into the land, inshore from the run-up line. Theoretically, the injection of dye at the sediment-water interface might be used to probe the porosity of the beach material.

Measurements of wind velocity and pressure with a wave follower during Marsen

Abstract: Air pressure data are used in determining the rate of momentum transfer from wind to waves. On the basis of the wind velocity measurements, the wave-induced airflow and its coherence with waves are obtained for various wind velocities and phase speeds of the ocean waves. The pressure results suggest that momentum transfer to waves can be specified by a certain relation, which is given. The wind-velocity results suggest that the wave-induced airflow is much smaller than the mean wind speed. An empirical equation is proposed for the modulation of atmospheric transfer to short waves caused by the orbital velocity of long waves.

A wave prediction system for real-time sea state forecasting

Abstract: The considerable increase in requirements for sea state forecasts in recent years has led to development of a numerical wave forecasting system in the Meteorological Office. This is based on a wave prediction model which combines the advantages of the parametric technique in predicting a growing wind-sea with those of a discrete spectral model in the swell regime. This is done using a discrete model by parametrizing the nonlinear interactions term in the energy balance equation in a way that reproduces the behaviour of the parametric model. The main difficulty with the method is in the separation of wind-sea and swell required to do this. Propagation of wave energy is performed using an accurate form of the Lax-Wendroff integration scheme. A two-term representation of wave growth is used whilst dissipation is modelled by an explicit whitecapping mechanism. Shallow water effects are included by representations of shoaling, refraction and bottom friction. The operational numerical atmospheric model at the Meteorological Office provides the wind input. An extensive program of evaluation has shown that the results provide high quality guidance with 24-hour forecasts of wave height having a r.m.s. error ranging from 0·6 m in the southern North Sea to 1·0 m east of the Shetlands.

A mechanism for the formation of ice edge bands

Abstract: We propose a mechanism for the formation of multiple bands at an ice edge that is subjected to an off-ice wind. The wind moves the outermost floes seaward faster than the interior ice, dilating the marginal ice zone and opening up random polynyas. In the largest of the polynyas, the off-ice wind generates a significant fetch-limited wave spectrum, which produces a wave radiation pressure on floes at the downwind end of the polynya, increasing their velocity. They are herded against their downwind neighbors, creating a band of high ice concentration. The band is stabilized by an internal compacting stress due to the exponential decay of the wind-wave spectrum across the band and to a slower decay of swell incident from seaward. The band's growth terminates when it encounters a second downwind polynya of sufficiently large size. We show how an array of ‘immature’ bands generated in this way ‘matures’ by selective mergers into larger, regularly spaced bands of approximately equal width.

Near-Surface Measurements of Quasi-Lagrangian Velocities in Open Water

Abstract: Near-surface water velocities have been measured in the coastal zone of Lake Huron and Cape Cod Bay by tracking drifters and drogues using acoustic travel time and compass sighting techniques. The near-surface current, defined as the velocity of near-surface drifters and drogues relative to a drogue set at 1.8 m, varied on the depth scale on the order of 1 m, and was directed nearly parallel to the wind and to predominant wave propagation velocity. Velocity profiles were logarithmic with depth to order 1 m depth, and realistic values of stress were calculated using a law of the wall formula and a Karman's constant of 0.4. Inferred roughness lengths were of the order of 30 cm. Anomalously high values of wind stress were inferred from velocity profiles observed during conditions of light wind and steady swell. These may be due to the similarity of Stokes drift distribution to turbulent shear flow profiles.

Nonlinear focusing of surface waves by a lens-theory and experiment

Abstract: One of the recent developments in harnessing wave energy is to construct a line of submerged structures parallel to the incident swell crests in order to transform the straight crests to circular crests converging to a focus. To understand the neighbourhood of the focus, we have carried out theoretical studies by accounting for diffraction and nonlinearity, both separately and jointly. Experiments have also been conducted in a large outdoor basin and are compared with the theories. These comparisons tend to favour the approximate nonlinear theory, but the efficiency of the focusing device as an energy concentrator does not appear to be significantly impaired by nonlinear effects.

Imaging radar observations of directional properties of ocean waves

Abstract: SEASAT-A synthetic aperture radar (SAR) and side-looking airborne radar (SLAR) images of ocean waves are examined in the form of normalized directional distributions of backscatter variance at series of frequencies. This method provides a more detailed description of radar results than have contoured two-dimensional wave number spectra and reduces some of the uncertainties in relating radar measurements to the waves. The range of aspects of the radar distribution that parallel those of ocean waves is defined. Within this restriction, not only can dominant wave frequencies and directions be determined accurately, but also the shape of a directional peak at a frequency, its directional width, and the background level can be determined approximately. Some of these aspects are examined with SLAR images obtained near reference wave measurements. Through its superior directional resolution, the radar appears to have distinguished two wave trains at a single frequency only 20° different in direction. The SEASAT-A satellite SAR provided an unusual opportunity to examine directional properties of waves in the hostile environment about Hurricane Fico. A swell highly dispersed in frequency and direction at a distance from the center of 450 km had a minimum observed directional width of 11°. Wave directions, their changes with frequency, and directional widths were in accord with those expected from the hurricane winds. Thematic maps of the direction and width of the swell energy as it spread across the ocean surface show smooth changes in these properties over distance, with relatively small scatter of individual values. These patterns also are in accord with those from a simple hurricane wave emission concept, but details of the distributions show distinct departures that must represent unrecognized smaller-scale fluctuations of the process.

Southern ocean mean monthly waves and surface winds for winter 1978 by Seasat radar altimeter

Abstract: Data acquired by the SEASAT radar altimeter during the 3 month satellite lifetime are analyzed in a study of the sea state of the southern hemisphere oceans. The lifetime of the SEASAT satellite, July 7 to October 10, 1978, corresponds to the Antarctic winter. Mean monthly maps of wind speed, significant wave height, and swell have been generated from the altimeter measurements along the satellite tracks. These maps delineate spatial and temporal differences of these parameters in the Atlantic, Indian, and Pacific oceans. Several features of the Southern Ocean wind and wave fields agree with conventional descriptions. For example, the principle zonal wind regimes established by the Southeast Trades and Westerlies are clearly evident in the monthly averages. Significant wave height and swell also exhibit minima near the Doldrums at low latitudes with steady increases southward to the latitudes of the Westerlies. However, superimposed on these general patterns is significant variability with horizontal scales as small as 1000 km. The maps also document a gradual migration of the region of absolute maximum wind and wave from the Atlantic eastward to the Indian Ocean and finally into the Pacific.

Measurement of tsunamis in the open ocean

Abstract: The spectrum of long waves, which are a background to tsunamis, is analyzed on the basis of records of near‐bottom pressure sensors obtained in the Northwest Pacific during the first and second USA‐USSR expeditions on the investigation of tsunamis in the open ocean (1975 and 1978). Instrumental trends, tidal oscillations, and quasistationary longwave noise were contained in the records. Special filters were used to pick out small waves generated by the seismicity of the ocean bottom. A decrease of noise level from 102 cm (including tides) to 10°1 cm could be reached. The level of long‐wave noise is found to depend on the instrument's location. Minimal disturbances in the records were observed at stations situated on the edge of the continental slope. The influence of cyclones passing over the instrument's site is deduced. It shows an increase in noise level on Nyquist frequency (0.5 min°1), which possibly is caused by the action of swell generated by the cyclone. Seismicity of the region under in...

The response of Antarctic icebergs to ocean waves

Abstract: The heave, tilt, and strain responses of three Antarctic tabular icebergs to ocean waves were measured during a 1980–1981 cruise of HMS Endurance to the South Atlantic. The three icebergs, located near the South Sandwich and South Orkney islands, were instrumented with accelerometers, tiltmeters, and wire strainmeters, while a Waverider buoy was used to record the ocean wave field. The thickness of the icebergs was surveyed by a helicopter-borne radio echo sounder. The heave response occurred mainly at the swell period but with outbreaks of bobbing which lasted for a few cycles at a resonant period (about 40 s), which agreed well with the predictions of a numerical finite element model. The roll response occurred mainly at a long resonant period (40–50 s), which again agreed well with the model, but there was also a significant response at ocean wave periods (5–20 s), which exceeded predictions. The strain response had a component at very long periods, which is unexplained by theory, while the surface strain at ocean wave periods agreed with the simple analytical model of Goodman et al. (1980). Using this model it is possible to predict a wave height and period that will cause breakup of the icebergs, and we conclude that swell-induced breakup is likely to occur during major storms in the open southern ocean.

Short-Period Atmospheric Gravity Waves: A Study of Their Statistical Properties and Source Mechanisms

Abstract: Gravity waves for the one year period beginning 19 October 1976 around Palisades, New York, are investigated to determine their statistical properties and sources The waves have typical periods of 10 min, pressure amplitudes of 3 Pa and velocities of 30 m/s In general, the largest, amplitude waves occur during late fall and early winter when the upper tropospheric winds directly overhead are fastest and the static stability of the lower troposphere is greatest Mean wave amplitudes correlate highly with the product of the mean maximum wind speed and the mean low level stratification directly aloft A distinct diurnal variation of wave amplitudes with the largest waves occurring in the pre-dawn hours is also observed as a result of the increased static stability then The majority of waves are generated by shear instability; however, a number of waves are generated by distant sources such as nuclear detonations or large thunderstorms The waves with distant sources can be distinguished on the basis of their generally much higher coherency across the grid and velocities that depart markedly from the wind velocity at any point in the sounding

Solitary Wave Evolution Over a Gradual Slope with Turbulent Friction

Abstract: A solitary wave of amplitude a evolving in water of depth d over a bottom of gradual slope δ and turbulent friction coefficient Cf is found to have the asymptotic (as d ↓0) relative amplitude a/d = α1 = 15α/4Cf α1<αb where αb, is the relative amplitude above which breaking occurs. It is argued that an initially sinusoidal wave of sufficiently small amplitude evolves over a shoaling bottom into a periodic sequence of solitary waves with relative amplitude α1. This prediction is supported by observations (Wells, 1978) of the evolution of swell over mud flats.

Apparatus for measuring the vertical acceleration due to the swell

Abstract: A buoy has accelerometers and magnetometers for measuring characteristics of an ocean swell. The main plane of the buoy floats on and follows the motion of the free surface of the ocean water. A first accelerometer and a magnetometer are mounted on the buoy and oriented along the axis which is perpendicular to the main plane of the buoy (i.e. perpendicular to the surface of the ocean swell). A pair of accelerometers and magnetometers are mounted in the main plane of the buoy and are oriented along mutually perpendicular axes of that plane. The swell caused acceleration vector is perpendicular to the main plane (i.e. the free surface of the water). The accelerometer oriented perpendicular to the main plane measures the sum of the swell caused acceleration vector and the projection of the gravity vector along the axis of the buoy. The mutually perpendicular pair of accelerometers mounted in the main plane measure the projection of the gravity vector in the main plane of the buoy. The projection of the gravity vector along the axis perpendicular of the buoy are derived from the values of the gravity vector and of the gravity vector components in the main plane of the buoy. The value of the projection of the gravity vector is deducted from the value measured by the first accelerometer for obtaining the acceleration vector due to the swell.

Laminated type piezoelectric body

Abstract: PURPOSE:To obtain a laminating type piezoelectric body having the stable quantity of displacement and excellent durability by limiting the degree of surface roughness and degree of swell of a piezoelectric element used for a laminate. CONSTITUTION:The degree of swell of a piezoelectric element 1 for a laminate A is decided to be less than 0.07%. When the degree of swell exceeds 0.07%, the quantity of displacement of the laminate A reaches 45 micron or less, and the laminate cannot be used when it is employed as a driving source for an injector. Partial stress by swells is generated in each piezoelectric element 1 under the state in which high load is applied in the axial direction of the piezoelectric body A. When the laminate is used as the driving source for the injector, the degree of surface roughness is decided to be less than five micron. That is, the quantity of displacement is made smaller than 45 micron and the laminate cannot be employed substantially on five micro or more, and fine cracks are generated in the piezoelectric element and durability is deteriorated in the same manner as in the case of the degree of swell of 0.07% or more.

Transition of mechanically generated regular waves to wind waves under the action of wind

Abstract: The transition process of mechanically generated regular waves into wind waves is investigated in a wind-wave tunnel. Modulations of characteristic quantities of the waves are examined in both space and time using wave-gauge arrays. The transition begins with the occurrence of wave breaking, and it is associated with the following two processes: (i) the irregularization, i.e. the generation and amplification of a random modulation whose wave height and period are in phase; and (ii) the frequency shift to the lower side by the mutual coalescence of waves through the amplification of the modulation, the coalescence occurring at the troughs of the period modulation. The modulational properties under the effect of wind with wave breaking are thus in part different from the theoretical prediction of modulational instability of a freely travelling wavetrain. It is concluded that the mechanism of the transition is the modulational instability coupled with the wind effects.

Wave evolution over a gradual slope with turbulent friction

Abstract: The evolution of a weakly nonlinear, weakly dispersive gravity wave in water of depth d over a bottom of gradual slope δ and Chezy friction coefficient Cf is studied. It is found that an initially sinusoidal wave evolves into a periodic sequence of solitary waves with relative amplitude a/d = α1 = 15δ/4Cf if α1 < αb, where αb is the relative amplitude above which breaking occurs. This prediction is supported by observations (Wells 1978) of the evolution of swell over mudflats.

Multimodal Properties of the Surface-Wave Field Observed with Pitch-Roll Buoys During GATE

Abstract: A sophisticated analysis technique is applied to a subset of pitch-roll buoy data collected by the research vessels Gilliss and Quadra during the GARP Tropical Atlantic Experiment (GATE) in September 1974. The procedure enables the examination of directional properties of the wave field at a level of detail not previously along 44°N latitude. BY comparing properties of the observed spectra with the predictions of a simple schematic model of the storm, we conclude that swell reaching the GATE area was emitted during the first half of the storm's lifetime; swell subsequently radiated from the storm was heavily attenuated, either by sheltering of the site by the Cape Verde Islands or because of radically lower emission levels from the storm itself. This work illustrates the power, as well as the limitations of the pitch-roll buoy when used in conjunction with a fully effective analysis technique.

Influence of long chain branching on extrudate swell of low‐density polyethylenes

Abstract: An experimental study of extrudate swell has been carried out, involving five low-density polyethylene (LDPE) samples of approximately the same molecular size but of different frequencies of long-chain branching (LCB). The results show that the samples with higher frequencies of LCB exhibit a tendency to swell more than samples of lower frequency. This tendency is more pronounced for short L/D dies and high shear rates. It seems possible that LDPE samples of varying LCB frequencies can be differentiated by determining the swelling ratio (d/D) with orifice dies (L/D ⋍ 0) at high throughput rates.

Problems in the Analysis of Data for the Assessment of Longshore Sediment Transport: An Example from North Queensland

Abstract: The calculation of longshore transport rates using the well established CERC and Bijker methods is discussed for sites where the local wave climate exhibits pronounced bi-modal spectral characteristics and longshore currents are influenced by the presence of wind-drift currents. A simple procedure for spectral decomposition is presented by means of which the complex wave climate is represented by separate sea and swell influences each with an identifiable height, period and direction. The results of transport calculations based on local sea conditions are discussed and compared with longshore transport differentials inferred from beach volume changes based on repetitive beach surveys over the last decade. The Bijker method is considered to be preferable for areas where nearshore wave and current behaviour is complex provided that high quality field measurements are used as a basis for the input data. In North Queensland, this will usually mean that spectral analysis of wave recordings will be necessary. The paper concludes that further research into longshore transport rates under the influence of more than one wavetrain is required.

Low Reynolds number oscillatory flow past a slowly rotating sphere

Abstract: In this paper the flow of a Newtonian viscous fluid, oscillating with angular velocityλ and amplitudeV∞, past a rigid sphere of radiusa which is slowly rotated with angular velocity ω is studied. Denoting the kinematic viscosity of the fluid byv, three parameters are involved in the analysis, namely: the Reynolds numberR =V∞ α/v and the Taylor numbersτ=a2ω/v andσ=a2gl/v. Asymptotic approximation is employed assuming thatR andτ are small whileσ is arbitrary. In the leading approximation, the swell velocity represents a double roll system while the oscillatory radial and polar velocities are reminiscent of Stokes (1851) celebrated solutions. In the terms of orderτ2/R approximation, the swell velocity is suppressed while the remaining two velocities are purely steady and independent ofσ. When the approximation is of orderR, the swell velocity is now purely oscillatory while the radial and polar velocities exhibit steady streaming components on which are superimposed oscillatory components. A quantitative study reveals that the steady streaming velocities are larger whenσ is small than when it is large.

Short review and recent results on microseisms

Abstract: This paper endeavours, mostly in a bibliographical manner, to show the place of meteorological microseims among the whole of geophysics. Relations with swell are mentioned with their theoretical and practical consequences for the properties of both phenomena, and the work of members of the International Commissions on Microseisms is summarized, covering velocity and bearing of the waves, spectral analysis, relation with atmospheric circulation, and world-wide extension of these researches since the I. G. Y. In a recent project, undecennial variation of yearly means of microseismic disturbance, the maximum of which often occurs 2–3 years after sunspot maxima, is investigated using data from several stations. A similar variation is therefore inferred for atmospheric cyclones, and the brightness of other planets seems to vary in the same way. Changes in the solar constant are suggested as the physical influence responsible, and new recording devices are proposed to follow on an absolute scale the future behaviour of such an index of atmospheric disturbances.

Extrudate swell for a round jet with large surface tension

Abstract: The problem of extrudate swell of a viscoelastic fluid from a round pipe is studied by the method of domain perturbations. The perturbation problems are solved by a finite-element method through second-order in the flow rate parameter ∈ for small flow rates. The analysis extends the work of Sturges on swelling in two-dimensional channels to round capillary tubes. In perturbation studies for small ∈, the rheology of the fluid may be expressed by three parameters, the viscosity and the two constants α 1 and α 2 appearing at order two in the expansion of the extra stress around zero shear. Surface tension has an important influence on the shape of the jet at low speeds. The shape of the surface on a round jet depends on α 1 and α 2 , in the plane jet only on α 1 . The analysis predicts that no matter what the constitutive equation may be, the jet will first contract if the radius of the pipe is sufficiently small. The contraction takes place in a length less than 1 10 the diameter of the jet and is followed by a swell. The contraction is usually small and may be hard to observe. There are five different contributions to the jet shape at second-order but only the viscoelastic ones persist as the pipe radius goes to zero.

Analysis of disastrous structural damage of a bulk carrier

Abstract: A bulk carrier in the fully laden condition suffered disastrous structural damage in rough seas of significant wave height 8 m and lost the forebody; she finally sank forty days later. The process of the disaster is analysed from the viewpoint of hydro-elasticity on the basis by buckling of the upper deck due to slamming impact in an unhappy superposition of a regular swell and an irregular swell.

A technique to estimate the elastic shear compliance from swell measurements

Abstract: Viscoelastic solutions were ejected vertically downwards into air and various Newtonian fluids. The measured swell increased significantly when ejected into a liquid rather than air. The observed increase is considered a result of both bouyancy and drag forces on the solution. The following dimensions expression relating the ratio of the swell diameter in liquid and air D L / D A to the elastic shear compliance of the ejected solution J e was experimentally observed. ( D L / D A ) 6 -1=30(Δϱ/ϱ s ) − 1 2 ([ g 2 η 2 N ϱ s ] 1 3 J e ) 3 5 , where Δϱ is the density difference between the extruded and Newtonian fluid, ϱ s is the solution density, g is the gravitational constant, and η N is the Newtonian fluid viscosity. Thus with this expression a simple extrudate swell technique exists to estimate the elastic shear compliance of a viscoelastic solution.

Directional Wave Spectra From Heave/Pitch/Roll Data Buoys

Abstract: Directional wave data measured by the NORWAVE heave/pitch/roll buoys moored on the Norwegian Continental Shelf has been used to consider in more detail the shape and spreading of the directional wave distribution of developing seas and long distance swell trains. The traditional cosine-2s distribution is rejected in favour of a family of wrapped stable distributions. By the use of a simple sufficient test multimodal distributions are found to occur, as expected, most frequently at lower wave frequencies. Simultaneous current measurements have been carried out near to the buoy locations and the effect of local currents on the measured dispersion relation has been studied. Although certain agreement with theory is seen, the presence of other effects shrouds the details making absolute agreement difficult.