Laser Slope Gauge Research Articles

Measurements of Waves in a Wind-wave Tank Under Steady and Time-varying Wind Forcing.

This manuscript describes an experimental procedure that allows obtaining diverse quantitative information on temporal and spatial evolution of water waves excited by time-dependent and steady wind forcing. Capacitance-type wave gauge and Laser Slope Gauge (LSG) are used to measure instantaneous water surface elevation and two components of the instantaneous surface slope at a number of locations along the test section of a wind-wave facility. The computer-controlled blower provides airflow over the water in the tank whose rate can vary in time. In the present experiments, the wind speed in the test section initially increases quickly from rest to the set value. It is then kept constant for the prescribed duration; finally, the airflow is shut down. At the beginning of each experimental run, the water surface is calm and there is no wind. Operation of the blower is initiated simultaneously with the acquisition of data provided by all sensors by a computer; data acquisition continues until the waves in the tank fully decay. Multiple independent runs performed under identical forcing conditions allow determining statistically reliable ensemble-averaged characteristic parameters that quantitatively describe wind-waves' variation in time for the initial development stage as a function of fetch. The procedure also allows characterizing the spatial evolution of the wave field under steady wind forcing, as well as decay of waves in time, once the wind is shut down, as a function of fetch.

Investigation of statistical parameters of the evolving wind wave field using a laser slope gauge

Statistical parameters of water waves generated by wind in a small scale facility are studied using extensively a Laser Slope Gauge (LSG), in addition to conventional measuring instruments such as a wave gauge and Pitot tube. The LSG enables direct measurements of two components of the instantaneous surface slope. Long sampling duration in a relatively small experimental facility allowed accumulating records of the measured parameters containing a large number of waves. Data were accumulated for a range of wind velocities at multiple fetches. Frequency spectra of the surface elevation and of the instantaneous local slope variation measured under identical conditions are compared. Higher moments of the surface slope are presented. Information on the waves’ asymmetry is retrieved from the computed skewness of the surface slope components.

On the two-dimensional structure of short gravity waves in a wind wave tank

Experiments on wind-waves in a laboratory tank are carried out for a variety of operational conditions. In addition to using a conventional wave gauge to determine the temporal variation of the surface elevation at the sensor location, independent measuring techniques such as 3D reconstruction of stereo video images and 2D laser slope gauge are applied in order to gain information on the three-dimensional structure of the wind-wave field. The consistency of results obtained by different methods under identical forcing conditions is examined to establish the limits of applicability and accuracy of each method. The accumulated results on the spatial and temporal coherence of the surface elevation variation, on the directional wave spectra, and on the probability distribution of the instantaneous surface slope directions demonstrate that the wind-wave field is essentially three-dimensional and short-crested.

Laboratory study of the fine structure of short surface waves due to breaking: Two‐directional wave propagation

This study was stimulated by the need to identify the influence of breaking on the evolution of the short surface wave field responsible for microwave scattering. Laboratory measurements of the fine space‐time structure of short gravity‐capillary waves and Ku band scattering at grazing and moderate incidence from spilling and plunging breaking waves in a laboratory wave channel are presented. Unsteady breaking waves are generated by focusing wave groups in space‐time domains. A scanning laser slope gauge was used for measuring capillary‐gravity waves with wavelengths of 2 mm to 10 cm and frequency ranges of up to 150 Hz. A dual polarized (VV, HH) coherent pulsed Ku band scatterometer with a 3 ns temporal resolution was used to simultaneously obtain Doppler spectra of the scattered signals from the breaking area for grazing angles from 6° to 25°. Two‐dimensional filtering in the wave number‐frequency plane as well as plus/minus Doppler spectra were used to separate the direction of propagation of the surface waves within the breaking region. It was found that the breaking splash is the main source of the surface wave generation. The short surface wave slope field produced by breaking could be separated into short (4–8 mm wavelength), fast waves and free gravity‐capillary waves. Both types of waves were found to be co‐ and counterpropagate relative to the dominant wave propagation direction. The Doppler spectrograms reveal the presence of high‐frequency waves at the moment of breaking that can be associated with short, fast waves. The low‐frequency part of the Doppler spectrogram is consistent with the appearance of free waves.

Directional spectrum of wind waves: Part II—comparison and confirmation

A model on the directional frequency spectrum of wind waves for deep water is introduced. The comparisons of the proposed model with other existing models show that the proposed model is very close to the JONSWAP model and DHH model for describing the developing waves under the normal spectral bandwidth, and has a better description for the transition of the unidirectional spectrum from ω−4 to ω−5 at a position around ωp, i. e., three time the peak frequency. Comparisons also show that the proposed model describes closely both field data measured by a four-frequency radar and a laser-optical sensor, and laboratory data measured by a laser slope gauge and an imaging optical method. The comparisons further demonstrate that the inverse spectral bandwidth as a new wave parameter is robust for describing the spectral, steepness. Finally, the formula on the local spectral-peak angular frequency is confirmed using the observed two-dimensional spectra.

Observation of hydrodynamic modulation of gravity‐capillary waves by dominant gravity waves

Accurate evaluation of the hydrodynamic modulation of short wind waves by dominant gravity waves is needed for various applications of microwave radar remote sensing of the ocean surface. Such knowledge is also essential for the studies of wind wave coupling and air–sea momentum/energy transfer. Here, we present direct field observations of the hydrodynamic modulation transfer function (MTF) during two field programs (High‐Resolution Remote Sensing Experiment (High‐Res) off Cape Hatteras, June 1993, and Coastal Ocean Processes (CoOP) experiment off the California coast, April and May 1995). Gravity‐capillary wave spectra were obtained using a scanning laser slope gauge (SLSG) for wave numbers between 25 and 800 rad m−1. An array of capacitive wave wires and a motion detection package were used for the measurement of surface gravity waves. These instruments were mounted on a research catamaran, which was towed to the side of a research vessel. The observations were mostly made under low wind conditions. The observed coherence is mostly below 0.1, suggesting that the modulation of short wind waves is not strongly correlated with dominant gravity waves. The coherence is slightly higher for wave numbers 50–100 rad m−1 than for wave numbers above 200 rad m−1. When wind and dominant waves are aligned and the water surface is clean, the magnitude of the hydrodynamic MTF is around 2–4 and its phase is close to 0. When surfactants are present and short wind wave spectra are reduced, the MTF magnitudes become significantly larger for wave numbers above 200 rad m−1. When wind and dominant waves are not aligned, the coherence becomes lower as expected. The results from the two field experiments are roughly consistent with each other. The existing relaxation model may predict the hydrodynamic MTF values that are roughly consistent with the observations if the effect of modulated wind stress over gravity waves is included.

Laboratory observations of gravity‐capillary waves under transient wind forcing

Laboratory experiments were conducted to investigate the response of short wind waves (wave numbers between 50 and 800 rad m−1) under transient wind forcing. Three‐dimensional wave number frequency spectra of gravity‐capillary waves were measured using a scanning laser slope gauge (SLSG). When wind is started rapidly, gravity‐capillary waves of a particular wave number (initial wavelets) first appear, and they are immediately followed by waves over a wider wave number range (both in magnitude and in direction). These waves all appear to grow exponentially, with each wave number growing at its own growth rate. The observed energy input rates from wind to waves are always maximum for along‐wind waves and are smaller for waves in directions oblique to the wind. When wind speed is modulated periodically in time, the modulation of the wind stress lags behind that of the wind speed, and the short wind wave spectra lag further behind the wind stress. Assuming that waves are modulated according to existing relaxation theory and that the relaxation rate is identical to the wave growth rate, very short wind waves appear to respond to the instantaneous wind speed while longer wind waves appear to respond to the instantaneous wind stress.

Relationship between air‐sea gas transfer and short wind waves

Laboratory studies have been conducted in two circular wind wave flumes to investigate the relationship between air‐sea transfer velocities of weakly soluble, nonreactive gases and wind‐generated surface waves over clean water surfaces and in the presence of surface films. Detailed surface wave measurements have been made using a scanning laser slope gauge. In the circular tanks, longer gravity waves (wavenumber below 12 rad/m) are hardly affected by surfactant, while shorter waves (above 100 rad/m) are significantly reduced. With higher surfactant concentrations, waves above 200–300 rad/m may be completely eliminated. Because of the absence of narrow‐banded fetch‐limited gravity waves, the wave fields in the circular tanks are significantly different from those in linear wind wave flumes. At a given wind friction velocity, the transfer velocity may decrease by as much as 60% because of surface films. Regardless of the surfactant concentrations, the transfer velocity shows a reasonable correlation with the total mean square slope and with the mean square slope of shorter wind waves (wavenumber above 200 rad/m). However, it shows a poor correlation with the mean square slope of longer wind waves (wavenumber below 50 rad/m). These observations suggest that short wind waves play an important role in air‐sea gas exchange.

Free and bound capillary waves as microwave scatterers: laboratory studies

Laboratory measurements of the fine space-time structure of short gravity-capillary waves, as well as Ku-band scattering at grazing and moderate incidence from wind waves in the large Delft Hydraulics Laboratory wind-wave channel are presented. This study was stimulated by the need to verify the processes that significantly contribute to scattering at grazing and moderate incidence. A scanning laser slope gauge was used for measuring capillary waves from 2-mm to 2-cm wavelengths and frequencies ranging up to 100 Hz. A dual-polarized (vertical, VV, and horizontal, HH), coherent, pulsed Ku-band scatterometer with good temporal resolution (3 ns) was used to obtain simultaneous Doppler spectra and the absolute cross section of scattered signals for grazing angles 6 and 25/spl deg/ and for winds in the range 2.5-12.5 m/s. Two-dimensional (2D) filtering and bispectral analyses were used to separate and study the influence of free and bound surface waves. The results of this study demonstrate that the frequency-wavenumber spectra of capillary waves consist of two parts. The first corresponds to free capillary waves, which satisfy the dispersion relationship. The second corresponds to bound parasitic capillary waves, which are located near the crests of steep wind waves. The phase velocity of these capillary waves is approximately equal to the phase velocity of the steep waves. Measurements of the Doppler frequency of the scattered signals show that the Doppler spectra also have a bimodal structure. While the first low-frequency part of the spectrum corresponds to the Bragg scattering from the free capillary waves, the highfrequency part is associated with Bragg scattering from the bound capillary waves on the crests of the steep waves. This type of scattering is predominant for the upwind direction of illumination (especially for HH-polarization).

An Experimental Study of Rain Effects on Fine Structures of Wind Waves

Abstract Previous experiments suggested that rain had dual effects on wind waves: damping in the gravity range and enhancement in the capillary–gravity/capillary range. These results in the frequency domain obtained from fixed-point measurements, however, might be contaminated by the Doppler effect. In the present study, wave slopes were spatially mapped with a scanning laser slope gauge under wind velocities of 3, 5, and 7 m s−1 and rain intensities of 42, 68, and 115 mm h−1. Synthesized spectral densities, which are not affected by the Doppler effect, were obtained by processing the data in both “fixed point” and “spatial scan” modes. The existence of dual effects is clarified with the present results.

Frequency‐wavenumber spectrum of wind‐generated gravity‐capillary waves

Spatial‐temporal information on wind‐generated gravity‐capillary waves is obtained using a scanning laser slope gauge in a wind wave flume. The results show that a significant portion of short wind wave energy may be contained in wave modes that propagate with the dominant waves rather than at their phase speeds predicted by the dispersion relation. Double‐peaked directional spreading of the spectrum is observed in a certain range of wavenumber and wind stress. A close examination of such results suggests that nonlinear interactions may take place between short waves and higher harmonics of dominant waves or among short waves and enhance the growth of short waves in oblique directions.

Optical Measurements of Capillary-Gravity Wave Spectra Using a Scanning Laser Slope Gauge

Abstract A description of a new scanning laser slope gauge (SLSG) is given and the results obtained from both laboratory wind-wave tank and field measurements are presented. The device relies on the measurements of two components of surface slope to compute spatial and temporal lags that are used to estimate the full three-dimensional slope spectrum. The device is capable of resolving frequencies up to 34.7 Hz and wavelengths in the range between 7.9 × 10−3 and 3.08 × 10−1 m. The technique makes use of a two-dimensional laser scanner that samples the perimeter of a circle of 0.154-m diameter (an unfilled aperture). Both laboratory and field results indicate the device is well suited to measure the full three-dimensional spectra of capillary-gravity waves and is capable of providing ground-truthing measurements for the verification of remotely sensed ocean surface features.

Surface strain modulation of insoluble surface film properties

The complex dispersion relation of capillary waves on water has been measured in a wavelength regime (λ∼1–2 cm) of interest to radar imaging. Two noninvasive sensors: a capacitive wave height antenna and a one-dimensional scanning laser slope gauge, are used to study the alteration of surface tension and surface elastic modulus in the presence of monomolecular surface films. On a static water surface, these sensors are compared to accepted standard techniques. The surface tension results are compared to a Wilhelmy plate while the elastic modulus results are compared to equations of state obtained by Langmuir trough techniques. The sensors are then employed to measure the response of surface film properties to surface areal strains for film pressures and strain rates, which are comparable to natural films in the presence of ship-generated surface currents. In the experiments reported in this paper, only insoluble surfactants are used. The monolayer surface films considered in this study are found to exhibit higher elastic modulus (∼8 mN/m) at small surface pressure (π≤1 mN/m) than expected from previous work reported in the literature. The response of these films to surface strain is consistent with our static surface measurements. The modulation of film parameters is found to be in phase with internal wave-generated oscillatory surface strain with periods on the order of 9 s. This implies that the film conforms to the strained water surface with negligible slippage in this regime. These measurements demonstrate the capability of this approach to continuously monitor surface films in situ with detector system response time of the order of 1 s. The technique is suited to large or deep containers and is applicable to sheltered ocean environments.

In situ measurements of capillary‐gravity wave spectra using a scanning laser slope gauge and microwave radars

Capillary‐gravity wave spectra are measured using a scanning laser slope gauge (SLSG), and simultaneously by X and K band Doppler radars off the Chemotaxis Dock at the Quissett campus of the Woods Hole Oceanographic Institution at Woods Hole, Massachusetts. Wave spectral densities estimated from the radar measurements using the Bragg theory agree with those measured using the SLSG at the Bragg wavenumber to within a few decibels, suggesting that Bragg scattering theory is valid for the conditions of this experiment. The observed degree of saturation of capillary‐gravity waves is in reasonable agreement with measurements by Jähne and Riemer (1990) obtained from measurements in a large wind‐wave flume at intermediate wind speeds, but our data indicate a higher degree of saturation at very low wind speeds. The rate at which the slope‐frequency spectrum falls off, however, is much lower in the field than in laboratories, even at moderate winds, suggesting long waves are responsible for a large Doppler shift of capillary‐gravity waves. Close examination of combined wavenumber‐frequency slope spectra also reveals significant smearing of the spectra in the frequency domain due to long waves. These observations confirm that spatial measurements (wavenumber spectra measurements) are essential for characterizing short capillary‐gravity waves, since this strong Doppler shift will dramatically change apparent frequency spectra.

Two-dimensional scanning laser slope gauge: measurements of ocean-ripple structures

A compact, two-dimensional scanning laser slope gauge has been developed for measuring ocean ripples, leading to the determination of their directional slope spectrum in the wavelength range of 4.5-81 mm. A telecentric scanning mechanism is used to enhance the dynamic range of slope measurements and to eliminate constraints caused by water-surface undulations associated with long waves. Successful laboratory tests and field trials have been performed; sample results are provided.

On ripple dynamics: V. Linear propagation of cylindrical waves on liquids with and without a surface dilatational viscoelastic response

The propagation of continuous-wave cylindrical ripples as observed in the laboratory is the subject of this paper. The first explicit formulation of the linearized Navier-Stokes in cylindrical coordinates for waves in the presence of a surface dilatational viscoelastic response is derived. Observations of wave slope as measured by a laboratory laser slope gauge are reported; they are described accurately by the theoretically predicted Hankel functions of the second kind. The study uses n-decane as the liquid on which the waves propagate. Three surfaces are tested: they are pure n-decane, n-decane with a surface film of polydimethylsiloxane (PDMS) and a film of a fluorinated surfactant (FC740). The observed dispersion relation is shown to be in excellent agreement with theory and indicates values of surface dilatational elasticity equal to −0.048 ± .091 mN/m, 0.477 ± 0.012 mN/m, and 1.72 ± 0.300 mN/m for the cases of pure n-decane, n-decane with PDMS, and n-decane with FC740, respectively.

Modulation of radar backscatter from the ocean by a variable surface current

Calculations of surface wave and radar cross section modulation induced by a spatially varying surface current have been compared with field measurements made with synthetic aperture radar (X and L bands), real aperture radar ( X band), laser slope gauge, and charge‐coupled device video camera during the Joint Canada‐U.S. Ocean Wave Investigation Project (JOWIP), which took place in August 1983. The comparison reveals that (1) many existing wind relaxation models underpredict the hydrodynamic effect of the current, (2) the simple Bragg scattering model underpredicts radar backscatter modulation for higher‐frequency radars (e.g., X band) but appears acceptable at lower radar frequencies (e.g., L band), and (3) the discrepancy between measurements and calculations at high radar frequency is reduced, but not eliminated, when the effects of long surface waves have been accounted for.

Laboratory measurements of polarization radios of wind wave surfaces

Polarization ratios ( <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">\sigma_{vv}/\sigma_{HH}</tex> ) of wind-generated rough water surfaces are studied experimentally by means of radar backscatter power measurements. The measurements were made at 9.23 GHz with incidence angles between <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">45\deg</tex> and <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">55\deg</tex> for wind speeds between 3 m/s to 10 m/s. Scattering surface statistics at all wind speeds were also measured by means of a wave height gauge and a laser slope gauge. The polarization ratio data are presented against parameters pertinent to scattering theory, which are obtained from the measured surface statistics. The results are compared to "composite surface" theory calculations for an assumed wave-height spectral density of the form <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">F(k) \propto k^{-m}</tex> , where <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">m</tex> is an adjustable parameter.