Radar scattering on gravity-capillary waves: Laboratory investigation

The mechanism of the resonance (Bragg) scattering of microwaves by gravity-capillary waves (GCWs) is checked experimentally. Resonant regularities of backscattered signal intensity are obtained as functions of frequency and GCW propagation direction. It is shown that the width of the resonance curves is determined by the width of the directivity pattern of the scatterometer’s antenna. The excitation of the GCW second harmonic and the spatial structure of the wave field at the GCW doubled frequency are investigated. The ratio of the amplitudes of the free wave and forced harmonic that originated during excitation of the primary wave is determined. The resonance curve is obtained for the second-order scattering of radio waves (on forced harmonics and free GCWs). The correction to the backscattering cross section is investigated in the second order of smallness relative to the Bragg term.     

Dynamics of gravity-capillary waves on the surface of a nonuniformly heated fluid

The method of slowly varying amplitudes is used to determine the reflection of surface gravity-capillary waves from regions of regular surface convection in the approximation of an ideal and homogeneous viscous fluid. A model of a quasi-steady distribution of temperature is employed. The conditions of both weak (nonresonant) interaction, when the wave passes almost completely through the convective zone, and strong resonant interaction are revealed. When interaction is strong, standing waves form in front of the convective region and the spectral composition of the wave packet changes. Viscosity and the accompanying boundary layers lead to the broadening of forbidden zones.     

Effects of three-wave interactions in the gravity-capillary range of wind waves

The formation of the spectrum of short wind waves from the gravity-capillary and capillary ranges under the effect of three-wave interactions is considered. In order to determine the spectrum, the kinetic equation for wave packets is integrated to the point where the solution is established. Three-wave interactions are described by a collision integral without introducing any additional assumptions simplifying the problem. This calculation procedure reproduces the Zakharov-Filonenko theoretical spectra, which correspond to the cases of energy equipartition and the inertial range. It is shown that the main role of three-wave interactions lies in the energy transfer from the range of short gravity waves to waves with shorter wavelengths. This transfer is accomplished both locally in the Fourier space and as a result of interactions between short and long waves. Its characteristic features are the formation of a dip on the curvature spectrum in the region of a minimum phase velocity of waves and the formation of a secondary peak in the capillary range. The dip is filled and disappears as the wind speed increases. Taking into account the interaction between short and long waves increases the spectrum in the capillary range several times, and the balance between energy input from long waves and viscous dissipation is established in the capillary range. The energy sink caused by three-wave interactions, viscous dissipation, and wind forcing cannot give the stability of the spectrum of short gravity waves.     

Modeling the generation and gravity-capillary waves using an underwater sound source

This is an experimental study of the acoustic method of surface-wave excitation using an underwater source of high-frequency (950 kHz) sound. The surface waves are excited at the sound-beam modulation frequency (3–55 Hz). For a normal fall onto the free surface, the modulated sound beam efficiently generates waves in the gravity-capillary range. This provides flexible electronic control of the main wave parameters (frequency and amplitude) in the packet and continuous modes. The amplitude-frequency characteristics of the process of surface-wave generation were obtained by numerical calculations (based on equations for the rate of acoustic flux and propagation of gravity-capillary surface waves) and by experiments (based on surface wave measurements by optical and contact methods). Both values are very consistent: on the background of a similar monotonic attenuation with frequency, they have a local dip near the minimum of the phase velocity and oscillation in the frequency range above 20 Hz. The experiments on the excitation of wave packets by single acoustic messages with varying lengths and powers, as well as by falling water drops, indicated that, in all cases, the phase characteristics are satisfactorily consistent with one another and the time needed for the signal to arrive at the measurement point is determined by the group velocity.     

Experimental investigation of fluctuations of radar signals caused by surface waves

On the oceanographic platform of the Marine Hydrophysical Institute of the Ukrainian National Academy of Sciences, we performedin situ investigations of surface waves based on the analysis of the fluctuations of radar signals at a wavelength of 1.2 cm. We analyzed both the traditional modulation transfer function used to describe variations of the scattering cross section for the scales of surface waves and the correlation of mean variations of the level of radar signals with the group structure of waving. This is of interest for applications in medium-resolution radar systems (−200–500m) recording the group structure of waving in the process of its smoothing. The experimental estimations of the amplitude of variations of radar signals caused by the group structure of waving are in good agreement with numerical results obtained with the help of a two-scale model of scattering of radar signals by the sea surface. Translated by Peter V. Malyshev and Dmitry V. Malyshev     

Laboratory study of the dynamics of a layer of turbulence excited by standing surface waves

The evolution of a turbulent layer excited by standing surface parametric waves is studied experimentally. The time dependence of the depth of the turbulized region is obtained. It is found that the thikness of the layer is described by a nonlinear dependence as distinguished from the previously obtained theoretical results. The dynamics of turbulent-layer embedding is described on the basis of a semiempirical theory and the assumption that the outer scale of turbulence is constant. Satisfactory agreement with experimental data is obtained. In the context of the proposed model, a relationship is found between the quantities characterizing the turbulence and surface waves. The dependence of the steepness of surface waves on the acceleration of the bottom is obtained experimentally. The data obtained in this study can be used for estimation of turbulence generation during a submarine earthquake.     

A speckle noise suppression method based on surface waves investigation and monitoring data

The internal energy distribution of waves can be described using ocean-wave spectra. In many ways, obtaining wave spectra on a global scale is critical. Surface waves investigation and monitoring onboard the Chinese-French oceanography satellite is the first space-borne instrument for detecting wave spectra specially, which was launched on October 29, 2018. It can avoid the shortage of synthetic aperture radar detection results while still having some problems, especially with the effects of spe...     

Study of nonlinear currents induced by coincident surface waves in liquid

This is an experimental study of the mixing induced by coincident surface waves in a liquid. The main mechanism leading to the emergence of mixing was shown to be the middle currents generated by coincident waves. The regime of these currents strongly depends on the amplitude of surface waves. For waves of near-critical amplitudes, an intense turbulization of middle currents is observed. Patterns of the velocity field were obtained using the Particle Imaging Velocimetry (PIV) technique for different amplitudes of surface waves. The results obtained can be used to estimate mixing in the near-surface oceanic layer.     

光滑表面内孤立波光学遥感图像的实验研究

基于光学遥感图像的内孤立波参数反演是一项重要的工作。本文在实验室条件下提出一种新方法用于模拟光滑表面情况下内孤立波的光学遥感成像。基于二维内波水槽、LED平板面光源和CCD相机搭建仿真光学遥感系统探测内孤立波。水平表面的光学遥感图像和垂向内孤立波传播图像被同时探测,旨在探讨在光滑表面下,光学遥感与内孤立波的响应。结果表明,内孤立波传播经过时,CCD1相机获得暗纹,暗纹的特性随光源入射角的变化而变化。光学遥感特征参数和垂向波要素相对应。实验还显示光学遥感图像的暗纹宽度与内孤立波的半波宽度在不同内孤立波振幅下呈现正相关关系。该系统有着现象清晰,重复性高的优点,为定量研究光学遥感成像机理提供科学依据。     

The high frequency waves produced on the surface of a liquid by an oscillating translating pressure distribution

In a previous paper (Magnuson, 1977) expressions were derived for the two-dimensional surface elevation resulting from oscillatory translating pressure distributions. Wave elevations were computed for the subcritical frequency range for the uniform pressure distribution by using linearized water wave theory. This paper extends the work in Magnuson (1977) to the supercritical frequency range. Wave profiles are presented for frequencies and speeds covering the operating range of air-cushion vehicles. In addition, the power radiated into the far field is computed as a function of frequency for several speeds.     

Damping of gravity-capillary waves in the presence of oil slicks according to data from laboratory and numerical experiments

Laboratory study results on the damping of gravity-capillary waves (GCWs) on water covered with oil slicks are presented. The nonmonotonic behavior (the presence of a local maximum) of the dependence of the damp coefficient on the slick thickness has been ascertained. The dispersion relation for GCWs in the presence of a viscoelastic slick of arbitrary thickness has been numerically calculated; this confirmed the presence of a damp coefficient maximum as a function of slick thickness. The parameters of oil slicks (interfacial elasticity and viscosity) have been estimated in a wide range of their thickness from a comparison of the calculation results with the data from laboratory measurements of wave parameters on water covered with an oil slick.     

Experimental investigation of the lift component of roll damping

The lift force and turning moment acting on a model towed obliquely to the direction of motion have been measured. Two models were used; one of them was tested fitted with and without a rudder. These measurements were used to determine the magnitude of the lift coefficient and the point of application of the transverse force acting on the model. The data were then used to determine the lift component of the roll damping moment. It has been found that the equivalent linear damping coefficient due to lift is a nonlinear function of the forward speed of the ship.     

Laboratory investigation on the compressibility of Singapore marine clays

The compressibility characteristics of Singapore marine clay in reconstituted and undisturbed states were studied using oedometer, constant rate of strain, Rowe cell, and isotropic consolidation tests. The intrinsic compression curve of the reconstituted clay was found to be similar to that proposed earlier with some minor deviations at low vertical stresses of less than 100?kPa. The field and laboratory compression behaviors were found to be similar, hence the laboratory curve could be used as a reference for interpreting the field behavior. Factors affecting the measurements of compression index and yield stress were discussed. As the coefficient of lateral earth pressure at the top upper clay was close to 1, the compression curves of vertically and horizontally trimmed samples were almost similar. The yield stress was mainly controlled by the strain rate; i.e., the higher strain rate resulted in the higher yield stress. Constrained modulus as derived from cone penetration tests and flat dilatometer tests were also examined and compared with laboratory test results. The in situ tests showed the decrease in constrained modulus with depth and generally the dilatometer test was found to register a higher modulus value.     

Laboratory study of deep-water breaking waves

In an attempt to elucidate the mechanics of deep-water wave breaking, a variety of breaking waves, including spilling and plunging waves, of different length scales and geometries was studied. The waves were generated through wave-wave interactions using wave packets with constant-steepness components, constant-amplitude components, and also components following the Pierson-Moskowitz distribution. Wave steepening prior to breaking were found to cause an increase in the high frequency spectral slope of the wave spectrum. The slopes were correlated to the type of breaking and the intensity of the breaking. The energy loss through breaking varied with the spectral characteristics of the wave packet. On the other hand, it was also noted that, irrespective of the wave packet, the losses were from the higher frequency end of the first harmonics.     

台阶型海脊俘获波的实验研究

基于物理模型实验研究瞬态冲击波在台阶地形上传播过程,揭示由于反射而在海脊上出现的波浪俘获现象。结果表明:在瞬态波产生区域附近,海脊上所测到的先导波即为最大波,其由泄漏至海脊外的深水波绕射至海脊所致。随着传播距离的增加,由于频散效应的影响,先导波逐渐减小,沿台阶近似直线传播的海脊俘获波和在台阶上曲折传播的海脊俘获波逐渐显现。在距离波浪产生较远区域所测的最大波晚于先导波出现,且这些由俘获波所叠加而成的最大波随着传播距离的增加而呈现出更加复杂的波面过程。     

Laboratory measurements of instantaneous sediment concentrationunder waves

An optical instrument which detects the light attenuation in sediment-laden flows has been developed to measure the instantaneous and time-average suspended concentration of cohesive sediments. In a flume test with natural muds, the device showed that the instantaneous sediment concentration oscillated with the waves but that the amplitude decreased rapidly with the distance above the mud bed. Strong sediment motion was found near the benthic boundary layer. This indicates a weak vertical turbulent diffusion in the upper portion of the water column in a pure wave erosion process. It is concluded that the probe design can be easily modified and the apparatus can be deployed for both laboratory and field measurements     

Laboratory investigations on impulsive waves caused by underwater landslide

Laboratory investigations have been performed on the submarine landslide generated waves by performing 120 laboratory tests. Both rigid and deforming-slide masses are considered. The effects of bed slope angle, initial submergence, slide geometry, shape and deformation on impulse wave characteristics have been inspected. Impulse wave amplitude, period, energy and nonlinearity are studied in this work. The effects of bed slope angle on energy conversion from slide into wave are also investigated. Laboratory-based prediction equations are presented for impulse wave amplitude and period in near and far-field and are successfully verified using the available data in previous laboratory and numerical works.     

The energy cycle of wind waves on the sea surface

The problems of wind-induced waves on the sea surface are considered. To this end, the empirical fetch laws that determine variations in the basic periods and heights of waves in relation to their fetch are used. The relation between the fetch and the physical time is found, as are the dependences of the basic characteristics of waves on the time of wind forcing. It is found that about 5% of wind energy dissipated in the near-water air layer contributes to the growth of wave heights, i.e. wave energy, although this quantity depends on the age of waves and the exponent in the fetch laws. With consideration for estimates of the probability distribution functions for the wind over the world ocean [11], it is found that the rate of wind-energy dissipation in the near-water air layer is on the order of 1 W/m². The calculations of wind waves [19] for the world ocean for 2007 have made it possible to assess the mean characteristics of the cycle of wave development and their seasonal variations. An analysis of these calculations [19] shows that about 20% of wind energy is transferred to the water surface. The remaining amount (80%) of wind energy is spent on the generation of turbulence in the near-water air layer. About 2%, i.e., one tenth of the energy transferred to water, is spent on turbulence generation due to the instability of the vertical velocity profile of the Stokes drift current and on energy dissipation in the surf zones. Of the remaining 18%, 5% is spent directly on wave growth and 13% is spent on the generation of turbulence during wave breaking and on a small-scale spectral region. These annually and globally mean estimates have a seasonal cycle with an amplitude on the order of 20% in absolute values but with a smaller amplitude in relative values. According to [19] and to the results of this study, the annually mean height of waves is estimated as 2.7 m and their age is estimated as 1.17.     

Laboratory simulation of the Kolmogorov flow on a spherical surface

The first results of a laboratory simulation of the Kolmogorov flow on a spherical surface are described. The primary laminar regime was found to be a system of zonal laminar jets of alternating directions. When the first critical value is passed, the primary regime loses its stability, and on its background a secondary vortex quasi-periodic regime with low frequency is formed. With a further increase in the Reynolds number and when the second critical value is passed, this vortex regime becomes unstable and self-excited oscillations emerge in the flow. Specifically, it was found that, if the spherical layer radius is chosen as a length scale, the wavelengths of perturbations in the vortex regime fall in the range of maximum intensity in the spectrum of the horizontal component of wind speed at the tropopause level. We explain the maximum peak shift in the wind spectrum on synoptic time scales when the observational height increases from 3000 km in the surface layer up to 8000?C10000 km in the upper troposphere and lower stratosphere.