A Laboratory Study of Strong Modulation of Radar Signals due to Long Waves on the Water Surface Covered with a Surfactant Film

Modulation of radar backscatter due to long wind waves in the presence of surfactant films is studied for the first time under wave-tank conditions. It is found that the modulation coefficient can increase significantly with increasing surfactant concentration. We show that the existing theoretical models of modulation of free surface waves underestimate the observed modulation levels. It is demonstrated that the experimental radar Doppler shifts cannot be explained by only the scattering due to free gravity-capillary waves (GCW) and are also determined by coupled waves, which are nonlinear harmonics of the longer wind wave components. Estimates of the relative intensity of coupled waves in the wind-wave spectrum at the Bragg wavelength are presented. It is shown that the contribution of coupled waves is considerable and increases with increasing surfactant concentration, thus confirming the hypothesis for a cascade mechanism of strong backscatter modulation in the long-wave field.     

Two-polarization Ku-band radar imagery of sea surface in presence of atmospheric boundary layer motions

Abstract

The influence of atmospheric boundary layer stability on radar imagery at two polarizations is considered. A strong dependence of ripple manifestations on the relative directions of sounding and the wind is noted in vertically polarized images. The main mechanisms for modulation of resonant scattering ripples by long surface waves are examined.     

Large-eddy simulation of formation of three-dimensional aeolian sand ripples in a turbulent field

With the method of large-eddy simulation, the equation of spherule motion and the method of immersed boundary condition, numerical simulations of three-dimensional turbulent aeolian motion and the formation of sand ripples under three-dimensional turbulent wind and the mutual actions of saltation and creeping motion were carried out. The resulting sand ripples have the form that is flat on the upwind side and steep on the leeward, which is identical to the sand ripples in nature. We also realized the self-restoration process of three-dimensional sand ripples, which shows the correctness of the method of numerical simulation and the models of saltation and creeping. Finally, We analyzed the influence of sand ripples on the three-dimensional turbulent wind field, and found that due to the appearance and development of sand ripples, in the normal direction of ground there exists stronger energy exchange, and moreover, there is close correspondence between the forms of sand ripples and the vorticity close to the ground surface. Supported by the Key Project of National Natural Science Foundation of China (Grant No. 10532040)     

Study of the mechanism for broadening of the spectrum of a low-frequency reverberation signal for sound scattering by near-surface inhomogeneities under conditions of intense wind waves

The paper considers the problem of backscattering of sound waves by near-surface volumetric inhomogeneities under conditions of intense wind waves. We calculate the expected share of the scattered signal spectrum based on the given wind-wave intensity and the depth distribution of volumetric inhomogeneities. For deep ocean conditions in the frequency range of 500–1000 Hz for a pulse duration of 10 s, we measure the levels and shape of the reverberation spectrum for time delays from 20 to 100 s. Comparison of the measured and calculated reverberation spectra has shown their good coincidence.     

Modelling of Internal and Surface Waves of the Real Ocean in the Large Thermostratified Experimental Tank at the Institute of Applied Physics of the Russian Academy of Sciences

We discuss the results of studies of surface-wave transformation by nonuniform flows, performed in the tank of the Institute of Applied Physics of the Russian Academy of Sciences (IAP RAS), and the results of modelling of the influence of iceberg motion on regular background internal waves in the subsurface pycnocline.Transformation of surface waves in the flow field past an immersed sphere is studied both experimentally and theoretically. It is shown that even fairly weak nonuniform flows can cause noticeable changes in the surface-wave field. The sizes of the spatial region in which the characteristics of the surface waves are changed exceed considerably the sizes of the nonuniform-flow region. It is found that the nonlinearity of surface waves leads to an increase in the variability of the surface-wave amplitude in a broad frequency range. The proposed theoretical model describes well the main experimentally observed features of the transformation of nonlinear surface waves in the nonuniform-flow field.It is proved experimentally that background internal waves with frequencies close to those of internal waves in an iceberg wake lead to a considerable transformation of the field of lee waves. The parameters of the resulting wave system are independent of characteristic horizontal sizes of the iceberg model and the length of the internal wave. The total wave system is stationary in the entire velocity range of the model in the case of counterpropagation of background waves. In the case of copropagation of background waves, the nature of the wave system depends on the ratio between the towing velocity and the phase velocity of background waves. In particular, the wave system in the wake can have both a pronounced nonstationary nature and a typical stationary phase pattern.     

飞秒脉冲激光1维强度调制的传输特性

实验研究了飞秒激光干涉条纹在CS2介质中传输的增长率与条纹间隔以及增长率与中心峰值强度的关系。结果发现,在非线性介质中传输的1维干涉调制特别是当其调制频率位于中心光强对应的最快增长频率附近时,随着传输距离的增加调制场迅速获得增长,将高斯光束分裂成多根峰值强度很高的带状细丝。相应的数值模拟与实验结果具有较好的一致性。     

Model for modulated and chaotic waves in zero-Prandtl-number rotating convection

The effects of time-periodic forcing in a few-mode model for zero-Prandtl-number convection with rigid body rotation is investigated. The time-periodic modulation of the rotation rate about the vertical axis and gravity modulation are considered separately. In the presence of periodic variation of the rotation rate, the model shows modulated waves with a band of frequencies. The increase in the external forcing amplitude widens the frequency band of the modulated waves, which ultimately leads to temporally chaotic waves. The gravity modulation, on the other hand, with small frequencies, destroys the quasiperiodic waves at the onset and leads to chaos through intermittency. The spectral power density shows more power to a band of frequencies in the case of periodic modulation of the rotation rate. In the case of externally imposed vertical vibration, the spectral density has more power at lower frequencies. The two types of forcing show different routes to chaos.      

Wavy liquid film in the presence of co- or counter-current turbulent gas flow

The wavy downflow of a viscous liquid film in the presence of the turbulent gas flow was analyzed theoretically. Two-dimensional stationary running waves are calculated in a wide range of Reynolds numbers of liquid and gas. Hydrodynamics of liquid is calculated based on complete Navier-Stokes equations. The wave interface surface is considered as a small perturbation and equations in gas are linearized near the main turbulent flow. Different optimal downflow regimes are determined, and the main wave characteristics are compared in detail with and without the co- and counter-current gas flows. It is shown that at high velocities of the co-current gas flow, the calculated waves correspond to ripples observed in experiments.     

Analysis of optical re-modulation by multistage modeling of RSOA

The time-resolved multistage reservoir model well-known for semiconductor optical amplifier (SOA) is extended to analyze the behavior of a bulk homogeneous InP-InGaAsP buried heterostructure reflective semiconductor optical amplifier (RSOA). Parameters for simulation have been extracted from the experimental RSOA characteristics. We have employed the model to explain the steady-state and re-modulation dynamics in the RSOA. Electrical modulation bandwidth and intermodulation distortion in the RSOA have been derived from the model and close agreement is obtained with the reported data. It is found out that the ripples in the upstream output from the RSOA for incomplete modulation erase of downstream modulated data follow Gaussian distribution, which simplifies the calculation of upstream SNR and bit error rate. It is explained in detail that amplitude ripples in the upstream data can be reduced by judicious choice of optical and electrical parameters of the RSOA. In particular, for an average low downstream power level (<−20 dBm) a good downstream modulation erase factor about 89% and 23 dB extinction ratio in the upstream modulated signal can be achieved.     

The short-wave model and waves in two directions

In the region where a sinusoidal wave in the cochlea reaches its maximum amplitude, the long-wave (or one-dimensional) model of the cochlea is deficient. In this region a short-wave model is more appropriate. However, in its current form, the short-wave model supports only waves in one direction. Therefore, it cannot cope with reflection effects associated with, e.g., inhomogeneities. Theoretical explorations of creation and internal reflection of otoacoustic emissions have almost exclusively been based on the long-wave model. In this article the road is paved for future explorations on a generalized form of the short-wave model, one that supports forward as well as backward waves, and thus can include internal reflections.     

Plasma Maser Theory of Whistler Waves in the Presence of Ion Cyclotron Turbulence

In this paper we consider the plasma maser theory of whistler waves in the presence of ion cyclotron waves in a magnetized plasma. In a plasma with low frequency ion cyclotron turbulence and a high frequency test whistler wave, growth of the whistler wave takes place because of the turbulent bremsstrahlung interaction between the resonant electrons and the modulated electric fields. The growth rate of the whistler wave is calculated and the results discussed.     

Temporal intermittency of chaos in parametrically excited capillary ripples. I

The results of an experimental study of disordered capillary ripples parametrically excited on a liquid surface in various types of resonators are presented. It is found that, in addition to spatio-temporal modulation of capillary ripples, excitation of large-scale motions (gravity waves) and intermittency of chaos, when regular and chaotic patterns alternate in time, are possible in certain supercriticality and liquid-depth ranges.Institute of Applied Physics, Russian Academy of Sciences. Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Radiofizika, Vol. 36, No. 5, pp. 404–411, May, 1993.     

Controlling the radiation spectrum of quantum-well heterostructure lasers by ultrasonic strain

The radiation of a semiconductor laser has been modulated in frequency by variable strain. The strain is excited by injecting bulk or surface ultrasonic waves. Dynamic and static analyses of the variations in the spectral characteristics of the radiation in the presence of sound are performed. A model is suggested and the data obtained are analyzed theoretically. The radiation frequency modulation in InGaAsP/InP heterostructures produced by surface waves is shown to be determined mainly by the band gap modulation of the active region.     

Under which conditions the Benjamin-Feir instability may spawn an extreme wave event: A fully nonlinear approach

Within the framework of the fully nonlinear water waves equations, we consider a Stokes wavetrain modulated by the Benjamin-Feir instability in the presence of both viscous dissipation and forcing due to wind. The wind model corresponds to the Miles’ theory. By introducing wind effect on the waves, the present paper extends the previous works of [6] and [7] who neglected wind input. It is also a continuation of the study developed by [9] who considered a similar problem within the framework of the NLS equation. The marginal stability curve derived from the fully nonlinear numerical simulations coincides with the curve obtained by [9] from a linear stability analysis. Furthermore, it is found that wind input goes in the subharmonic mode of the modulation whereas dissipation damps the fundamental mode of the initial Stokes wavetrain.     

Effect of relativistic elliptical beam modulation on excitation of surface plasma waves in a magnetized dusty plasma column with elliptical cross section

Abstract

The excitation of surface plasma waves due to the interaction of an elliptical relativistic density modulated electron beam with the magnetized dusty plasma column with elliptical cross-section has been studied. The dispersion relation of surface plasma waves has been retrieved from the derived dispersion relation by considering that the beam is absent and there is no dust in the plasma elliptical cylinder. It is shown that the Cherenkov and fast cyclotron interactions appear between the beam and eigen-modes of plasma column. The growth rate of the instability increases with the beam density and modulation index as one-third power of the beam density in Cherenkov interaction and is proportional to the square root of beam density in fast cyclotron interaction. The numerical results and graphs are presented, too.     

Generation of strong Langmuir fields during optical breakdown of a dense gas

Results are presented from theoretical studies and computer simulations of the resonant excitation of Langmuir waves during the ionization of a homogeneous gas by high-intensity laser radiation. Two mechanisms for the formation of nonuniform resonant structures in the discharge are examined: plasma-resonance ionization instability, resulting in the density modulation along the electric field vector, and gas breakdown in the field of a transversely inhomogeneous laser beam (a Bessel beam produced by an axicon lens). In both cases, the transition of the plasma density through the critical value is accompanied by the generation of intense Langmuir waves, the formation of fast ionization fronts, and the appearance of long-lived quasi-turbulent states.     

Doppler spectra of laboratory wind waves at low grazing angle

Eight radar frequencies over a range from 3 GHz to 94 GHz have been used to measure Doppler spectra of wind waves in a laboratory wind-wave tank, at low grazing angle. The VV spectra show relatively little change in shape at different frequencies. The HH spectra show significant trends in the lower radar frequencies, some of which are probably artefacts of the experiment. A new model, the 'fish model' is developed empirically from measurements of the peak Doppler spectra for both vertical and horizontal polarization. This model indicates that the H-pol, not just the V-pol, peak Doppler shift is dependent on the radar frequency.     

Parametrische Anregung von elektrostatischen Oberflächenwellen in einer inhomogenen Plasmaschicht durch ein amplituden-moduliertes UHF-Pumpfeld

Parametric resonance phenomena are investigated in a plasma layer with thickness d and thin inhomogeneous boundary regions. The modulated UHF electric field is parallel to the plasma layer. We consider both strong and low modulation of the field amplitude and suppose, that the carrier frequency ω₀ of the pump wave is much larger than the Langmuir frequency ω_Le. We find the region for the modulation frequency ω, in which the parametric growing of the asymmetric and symmetric surface waves occurs. The maximum growth rates of these waves, the direction of their propagation and the threshold value of the modulation depth α of the UHF pump field are calculation.     

Doppler spectra of laboratory wind waves at low grazing angle

Abstract

Eight radar frequencies over a range from 3 GHz to 94 GHz have been used to measure Doppler spectra of wind waves in a laboratory wind-wave tank, at low grazing angle. The VV spectra show relatively little change in shape at different frequencies. The HH spectra show significant trends in the lower radar frequencies, some of which are probably artefacts of the experiment. A new model, the ‘fish model’ is developed empirically from measurements of the peak Doppler spectra for both vertical and horizontal polarization. This model indicates that the H-pol, not just the V-pol, peak Doppler shift is dependent on the radar frequency.