Efficiency of sound field focusing at a long distance in an oceanic waveguide with background internal waves

The efficiency of focusing a reversed wave and the possibility of scanning with the focal spot at long distances in a shallow sea in the presence of an anisotropic field of background internal waves are theoretically investigated. The localized fields are controlled by varying the transmission frequency without a change in the distribution of the reversed field over the array aperture. The effect of periodically repeated focal spots is analyzed. Numerical calculations are performed for the longitudinal and transverse orientations of acoustic path with respect to the propagation direction of internal waves. The effect of perturbation on the stability and efficiency of focusing is discussed. A comparative analysis of the data obtained for long and short distances is performed.     

Focusing of acoustic fields in a randomly inhomogeneous waveguide by using the time reversal method

We perform theoretical analysis of the method of field focusing in a randomly inhomogeneous waveguide using reradiation of the received signal with time reversal. The simplest case where point sources and receivers are used for emission and reception is considered. As an example, the waveguide is chosen which simulates an underwater sound channel with refractive-index fluctuations caused by random internal waves. In underwater acoustics, the considered method of field focusing is usually applied at relatively short distances that are shorter than or about 10 km. This work deals with much longer paths, along which sound waves propagate under conditions of well-developed ray and wave chaos. Main attention is given to studying the width of the focal spot and the field amplitude at its center. It is shown that the amplitude distribution in the vertical section of the focal spot and the peak amplitude value at its center can be estimated analytically using the stochastic ray theory.     

Robust time reversal focusing in the ocean

Recent time-reversal experiments with high-frequency transmissions (3.5 kHz) show that stable focusing is severely limited by the time-dependent ocean environments. The vertical focal structure displays dynamic variations associated with focal splitting and remerging resulting in large changes in focal intensity. Numerical simulations verify that the intensity variation is linked to the focal shift induced by phase changes in acoustic waves resulting from sound speed fluctuations due to internal waves. A relationship between focal range shift, frequency shift, or channel depth changes is illustrated using waveguide-invariant theory. Based on the analysis of experimental data and numerical simulations, methods for robust time-reversal focusing are developed to extend the period of stable focusing.     

“Hot spots” in the field of internal waves in the ocean

A review of observations of high-amplitude internal waves is presented. These waves play an important role in the global scenario of water mixing in the world ocean and also cause a considerable variability of oceanographic fields, including the sound velocity field. The main sources of intense internal waves and the regions where such waves arise are considered. It is shown that, in addition to the much-investigated trains of soliton-like internal waves, which are formed near large-scale inhomogeneities of the bottom relief because of the nonlinear evolution of internal tides, internal waves encountered in the open ocean also possess considerable amplitudes and nonlinearity.     

复频聚焦超声换能器水中焦区商场特性的研究

本文对新研制的一种复频聚焦超声换能器的辐射声场进行了实验研究,测定了辐射压在轴向上的分布曲线,进而确定了焦区位置,这与理论所得结果符合较好,测定了辐射声压在焦平面上的分布曲线,并对焦区声场进行了定位,最后对换能器在水中焦区辐射声的频谱进行了研究,不仅观测到了两个源波,而且还观测到了和频波,差频波以及倍频波,证实了声散射声效应的存在。     

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.     

The almost perfect lens and focusing of evanescent waves

We propose a general method for evaluating the field in the focal region of nearly perfect one-dimensional lenses, and study how evanescent waves contribute to the electric field behind subwavelength apertures. The approach presented here may give a better insight into how to shape and focus evanescent waves.     

浅海内波环境下声场时间相关特性

浅海内波是导致声场时间相关半径减小的一个重要原因.利用2015年南中国海声传播起伏实验,对比分析了线性内波以及孤立子内波环境下声场时间相关半径的统计特性.实验数据(175～225 Hz)表明,大振幅孤立子内波的存在极大地降低了声场的时间相关半径,声场时间相关半径从线性内波环境下的1～3 h,降低为孤立子内波环境下的小于...     

圆极化波反射聚焦超表面

基于圆极化波入射条件下的高效同极化反射超表面实现了对圆极化反射波相位的自由调控, 设计了一维圆极化波反射聚焦超表面. 在中心频率f=16 GHz附近, 右旋圆极化平面波入射时, 反射波聚焦于焦距L=200 mm的实焦点; 左旋圆极化波入射时, 反射波近似聚焦于焦距L=-200 mm的虚焦点. 仿真计算得到聚焦波束的波束宽度、焦深. 结果表明, 这种圆极化反射聚焦超表面具有很好的聚焦效果, 同时具有长焦深和宽带特性.     

Sound scattering by spatially localized inhomogeneities in a shallow-water waveguide in the presence of internal waves

A method is developed for solving the problem of sound scattering by concentrated inhomogeneities in a waveguide of small depth in the presence of internal waves (IW) typical of an oceanic shelf. The sound field fluctuations related to the motion of a model scatterer (a soft spheroid) and to the propagation of the IW are calculated and analyzed. It is shown that the field of internal waves considerably affects the scattered sound field even when the source-receiver and source-scatterer distances are relatively small (about several kilometers). This effect depends not only on the amplitude of the IW, but on their propagation direction as well.     

Effect of shallow water internal waves on ocean acoustic striation patterns

Contour plots of underwater acoustic intensity, mapped in range and frequency, often exhibit striations. It has been claimed that a scalar parameter 'beta', defined in terms of the slope of the striations, is invariant to the details of the acoustic waveguide. In shallow water, the canonical value is β=1. In the present paper, the waveguide invariant is modelled as a distribution rather than a scalar. The effects of shallow water internal waves on the distribution are studied by numerical simulation. Realizations of time-evolving shallow water internal wave fields are synthesized and acoustic propagation simulated using the parabolic equation method. The waveguide invariant distribution is tracked as the internal wave field evolves in time. Both random background internal waves and more event-like solitary internal waves are considered.     

Effect of shallow water internal waves on ocean acoustic striation patterns

Abstract

Contour plots of underwater acoustic intensity, mapped in range and frequency, often exhibit striations. It has been claimed that a scalar parameter ‘beta’, defined in terms of the slope of the striations, is invariant to the details of the acoustic waveguide. In shallow water, the canonical value is β=1. In the present paper, the waveguide invariant is modelled as a distribution rather than a scalar. The effects of shallow water internal waves on the distribution are studied by numerical simulation. Realizations of time-evolving shallow water internal wave fields are synthesized and acoustic propagation simulated using the parabolic equation method. The waveguide invariant distribution is tracked as the internal wave field evolves in time. Both random background internal waves and more event-like solitary internal waves are considered.     

Focusing of sound pulses using the time reversal technique on 100-km paths in a deep sea

Numerical and analytical studies are performed on how unstable fluctuations of the parameters of the medium in a deep sea affect the focusing of sound pulses using the time reversal method. The simplest situation, when point sources and receivers are used for emission and reception, is considered. Pulse propagation in the direct and backward directions is numerically simulated by the parabolic equation method. Calculations are performed for sound signals with frequencies of several tens of hertz. It is shown that, in the presence of sound velocity fluctuations caused by random internal waves, noticeable attenuation of the field amplitude at the center of the focal spot can be observed beginning from distances of 200 to 400 km. As the central frequency of the pulsed signal increases, the effect of nonstationarity of the perturbation on the focusing is amplified. This phenomenon is explained qualitatively and quantitatively in the geometrical optics approximation.     

Effect of internal waves on intrusions in a thermocline and on upwelling

Results of the investigation into the interaction of internal waves and flows and the elaboration of a relevant mathematical model are reported. It is found in the field experiments that internal waves develop against the background of upwelling, which results in formation of jets causing intrusion velocity fluctuations. The mechanism and mathematical model of this process are proposed. A mathematical model of the suspended matter transport by jets with a wave-modulated trajectory is developed and verified.     

Far fields of internal gravity waves in stratified media of variable depth

The present paper is devoted to presenting the solution of the problem on the far field of internal gravity waves in a stratified medium whose depth linearly depends on the space variable. Uniform asymptotic representations of solutions are constructed; these representations enable us to describe the far fields of internal gravity waves excited by a moving source and characterize the main specific features of the wave fields near caustics and wave fronts.     

Application of time-reversed wave field focusing to reconstructing the frequency spectrum of background internal waves

A numerical experiment is used to analyze the possibility of focusing the time-reversed wave field for reconstructing the frequency spectrum of the vertical displacements of water layers by measuring the frequency shifts of the sound field maximum at the focal spot. The focusing of the field is controlled by varying the transmitted frequency at a fixed distribution of the reversed field, which is formed in the unperturbed waveguide, over the array aperture. The data of computations are compared with those obtained without focusing.     

Detection of defects in cylindrical structures using a time reverse method and a finite-difference approach

The detection and characterization of defects in structures is an important issue in non-destructive testing. To avoid the scanning of large samples, guided elastic waves, which propagate along the structure, are excited. These waves interact with a defect, which results in a scattered wave field. In an experiment, the displacements of these scattered waves are recorded over time for a fixed axial coordinate at a number of circumferential positions of a circular cylindrical tube. Since in complex structures it is difficult to determine the axial and particularly circumferential position of the defect directly from the time signals, a time reversed numerical simulation is performed. There the measured displacement histories are reversed in time and used as displacement excitations in a simulation of the tested structure. A three-dimensional code in cylindrical coordinates, based on a velocity-stress finite-difference method, is used to simulate the wave propagation. As long as the geometric and material parameters are chosen equivalent to the performed experiment, the scattered waves travel back through the simulated structure and interfere, even if no defect is present in the numerical model. The result is an increase of the amplitudes of the stress and displacement components at the location where the defect was in the tested sample.     

The possibility of reconstruction of two-dimensional random inhomogeneities in a shallow sea by frequency shifts of the spatial interference structure of the sound field

Fluctuations of frequency shifts of the spatial interference structure of the sound field in the oceanic waveguide, caused by a two-dimensional field of a random perturbation, are described. Various schemes of observation point spacing are considered. The possibility is shown to reconstruct the spatial spectrum of waveguide perturbations by measuring the spatial spectrum of the frequency shift of the interference pattern. The results of the theoretical treatment are illustrated by the examples of background internal waves and bottom roughness. The sensitivity of monitoring based on measurements of frequency shifts of the interference structure of the sound field is estimated. For medium perturbations by background internal waves, the fluctuations of liquid layer vibrations, sound speed, and temperature, which are minimum detectable by frequency shifts of the interference pattern, were estimated.     

Exact analytic self-similar solution of a wave attractor field

Stratified and rotating fluids support obliquely propagating internal waves. A symmetry-breaking shape of the fluid domain focuses them on a wave attractor. For a trapezoidal basin, it is here shown how to determine the internal wave field analytically. This requires solving the wave equation on a closed domain-an ill-posed Cauchy problem-whose solution exhibits a remarkable self-similar spatial structure. These results are relevant for mixing and mean flow generation in oceans, atmospheres and stars whose symmetry is generally broken and where internal waves are tidally forced.