The acoustic properties of real liquids are largely related to the phase inclusions contained in them, of which gas bubbles are the most common. The aim of the work was to find the relationship between the nonlinear acoustic parameter and the cavitation strength of the liquid with the distribution of bubbles in the liquid, which has so far been poorly studied. The theoretical studies of the parameter of acoustic nonlinearity and the cavitation strength of a liquid with bubbles were carried out within the framework of the homogeneous approximation of a micro-homogeneous liquid; the relationship of these parameters with the bubble distribution function was established, and the typical values of these parameters for different concentrations of bubbles were calculated. Experimental measurements of the parameter of acoustic nonlinearity and the cavitation strength in the upper layer of seawater were carried out; these measurements were consistent with the theoretical estimates. A connection was established between the thresholds of acoustic and optical cavitation—the optical breakdown of a liquid by laser radiation. The results obtained can find practical application in the measurement of the cavitation strength of seawater at great depths in the sea, and the use of an optoacoustic method associated with the use of optical cavitation is proposed.

The problem of propagation of low-frequency sound in a shallow waveguide with random hydrological inhomogeneity caused by background internal waves is considered. A new approach to statistical modeling of acoustic fields, based on the application of the random matrix theory and previously successfully used for deep-water acoustic waveguides, is used to the case of shallow-water waveguides. In this approach, sound scattering on random inhomogeneity is described using an ensemble of random propagator matrices which describe the transformation of the acoustic field in the space of normal waveguide modes. A study of the effect of sound “escaping” from a waveguide was carried out. The term “escaping” here means energy transfer to modes with stronger attenuation due to scattering on internal waves. A model of an underwater sound channel with an axis at a depth of about 45 meters is considered. It is shown that the first few modes propagating inside the water column are very little subject to losses due to the “escaping”. The strongest impact of the leakage scattering is experienced by the middle group of modes capable of reaching the sea surface. It is revealed as significant increasing of losses as compared to a horizontally homogeneous waveguide. On the other hand, the existence of linear mode combinations for which loss enhancement is practically absent has been revealed. These linear combinations correspond to the eigenfunctions of an inhomogeneous waveguide. Statistical analysis of propagator eigenfunctions indicates on qualitative differences of mechanisms of scattering for frequencies of 100 and 500 Hz.

Practically important properties of real liquids, including seawater, are their nonlinear properties, which include a nonlinear acoustic parameter, as well as cavitation strength – a rupture of the continuity of a liquid at high intensities in an acoustic wave. The manifestation of nonlinear effects is greatly facilitated by the presence of various nuclei in the liquid – gas bubbles, foreign particles, and other inclusions of various origins. For practical applications, it is important to study the properties of real liquids with inclusions in them. Due to the difficulties of measuring cavitation strength by acoustic methods, other methods are being sought. It is proposed to use an optoacoustic method associated with the use of laser radiation that causes optical breakdown - optical cavitation, that accompanied by a strong sound generation effect. The connection between optical breakdown thresholds and acoustic cavitation thresholds has been established, which can later be used to identify cavitation thresholds at high static pressures (at great depths in the sea), where the use of acoustic emitters is extremely difficult. The paper studies the bubble size distribution function in seawater, combined with studies of the acoustic nonlinearity parameter and cavitation strength at various depths. The interrelation of these characteristics for seawater is shown.

The spatial structure of a far-field acoustic wavefield created by a sparse horizontal array of nondirectional emitters is considered. It is shown that the array can selectively excite certain modes of the acoustic wavefield. The number of an excited mode depends on the angle with respect to the array axis. The results of numerical simulation are presented for two models of a waveguide and for an array mounted at the ocean bottom. It is shown that the efficiency of single mode excitation grows with an increase in the modal number. The angular dependence of the excited modal spectrum is studied. It is shown that this dependence consists of several branches corresponding to the most excited modes.

In this paper, we show the possibility of using nonstationary and nonlinear sound scattering for diagnosing bubbles in a liquid. Nonstationary sound scattering arises due to transient processes of bubble swinging under the action of acoustic pulses. It was previously used for bubble spectroscopy in seawater using parametric emitters. Transient processes cause the coherent swinging of bubble eigenoscillations by high-frequency phase-shift keyed pulses, which are realized at certain ratios between values of high- and low-frequency resonance for the bubble and are refined by phase manipulation in the pulse. The nonlinear scattering of high-frequency pulses at difference frequencies for a monotonic bubble size distribution function g ( R ) is shown to be mainly determined by bubble resonance at the pumping frequency rather than at the difference frequency, regardless of the pulse duration. The use of nonstationary nonlinear scattering is justified in the case of a nonmonotonic size distribution of bubbles. Nonstationary nonlinear spectroscopy of bubbles is if that the pumping frequency corresponds to bubbles on the descending branch of g ( R ) for small sizes, while the difference frequency corresponds to large bubbles on the other side of the maximum of the g ( R ) function. We demonstrate that the use of nonlinear nonstationary scattering by colliding beams will allow remote spectroscopy of bubbles in a liquid that are formed in natural and technological processes and to correctly estimate the gas content in bubble structures.

The results of studying the spectra of optical breakdown plasma under two-pulse and single-pulse excitation in an ultrasound field are reported. It is shown that the efficiency of using ultrasound for recording the trace concentrations of chemical elements in a liquid under one-pulse excitation of the liquid is comparable to the efficiency under a two-pulse optical breakdown.

Представлены результаты исследования спектров плазмы оптического пробоя при двухимпульсном и одноимпульсном возбуждении в поле ультразвука. Показана эффективность использования ультразвука для регистрации следовых концентраций химических элементов в жидкости при одноимпульсном возбуждении жидкости, сравнимая с эффективностью при двухимпульсном оптическом пробое. Ключевые слова: оптический пробой, лазерно-искровая спектроскопия, спектральные линии, оптоакустические эффекты, ультразвук.

Studies of the opto-acoustic effects accompanying laser breakdown in water generated by focused laser and ultrasonic radiation are performed. For the first time, the experimental results were obtained showing sharply increasing effects of acoustic emission from the breakdown zone under the combined influence of laser and ultrasonic irradiation. Experiments were performed using the nanosecond pulses of a Nd:YAG laser operating at a wavelength of 532 nm. Acoustic radiation was generated by acoustic focusing systems at various frequencies. It is shown that recording of acoustic emission from the breakdown zone makes it possible to study the thresholds and dynamics of the laser breakdown, which coincide with the high-speed optical methods. The results obtained make it possible to find applications of the acoustic method of diagnostics of laser breakdown and cavitation in opaque environments for which optical methods are not applicable. The effect of the increase in the resolution of the spectral lines of chemical elements in seawater under the action of an ultrasonic field is established. This effect indicates the possibility of development of a combined method of laser–spark spectroscopy using ultrasound, which will make it possible to approach the sensitivity of the precision chemical analysis, and which should be more practical and advanced because it yields a high efficiency and detailed measurements.

Новые объекты в океане, подводные газовые факелы (ГФ), образованные газовыми пузырьками, выходящими из дна моря, повсеместно встречаются в районах выброса газов как из толщи донных осадков в различных районах океана, так и в районах выгрузки газа при таянии вечной мерзлоты в арктических морях, и к ним проявляется все больше внимания. Стандартное применение рассеяния звука позволяет обнаружить наличие ГФ в море, но не позволяет в полной мере корректно оценить функцию распределения пузырьков по размерам в факеле и поэтому возникают неопределенности с оценкой мощности выброса газов из моря. Обсуждаются возможности использования метода нестационарного и нелинейного рассеяния звука для получения информации о структуре и динамике подводных газовых факелов, образованных выходом газа из морского дна. Нелинейное рассеяние звука обусловлено высокой нелинейностью пузырьковых структур в воде. Нестационарное рассеяние звука возникает вследствие переходных процессов раскачки пузырьков под действием акустических импульсов, и оно ранее использовалось для изучения распределения пузырьков в приповерхностных слоях морской воды. В работе показано, что применение нелинейного нестационарного рассеяния на встречных пучках позволит проводить дистанционную спектроскопию пузырьков в газовых факелах и проводить корректные оценки газосодержания в факелах. New objects in the ocean, underwater gas flares (GF) formed by gas bubbles emerging from the sea floor, are ubiquitous in areas where gases are released from the bottom sediments in various areas of the ocean, and in areas where gas is discharged during permafrost melting in the Arctic seas, and they are receiving increasing attention. The standard application of sound scattering allows detecting the presence of GF in the sea, but does not allow us to fully correctly estimate the bubble size distribution function in the flare, and therefore there are uncertainties with the estimation of the power of gas emission from the sea. The possibilities of using the method of non-stationary and nonlinear sound scattering to obtain information about the structure and dynamics of underwater gas flares formed by gas escaping from the sea floor are discussed. Nonlinear sound scattering is caused by the high nonlinearity of bubble structures in water. Non-stationary sound scattering occurs due to transient processes of bubble swinging under the action of acoustic pulses, and it was previously used to study the distribution of bubbles in near-surface layers of seawater. It is shown that the use of nonlinear non-stationary scattering on colliding beams will allow remote spectroscopy of bubbles in gas flares and correct estimates of the gas content in the flares.

We have used a nanosecond pulsed laser to study the dynamics of laser- induced breakdown with traditional optical detection on both nanosecond time scale and at later stages. Experiments on the induction of optical breakdown in the volume of liquid were performed using an Nd:YAG laser. It is shown that the optical breakdown in the liquid in the ultrasonic field is accompanied by an increase in the intensity of the spectral lines of potassium and oxygen with an increase in the amplitude and frequency of ultrasound. It was found that the effect of ultrasound on the intensity of the lines varies depending on the time of the breakdown evolution. Along with the optical spectra, the acoustic emission accompanying the pulsations of the cavitation bubble formed at the late stages of liquid breakdown source was studied. It was shown that the acoustic emission varies significantly with different ultrasound parameters. It is shown that an excited signal at its own switching frequency has a sufficiently high amplitude for its registration under typical experimental conditions. It is shown that the saturation effect is observed at frequencies above 200 kHz and at high ultrasound power, when the growth of the intensity of spectral lines slows down sharply. This effect indicates the possibility of using relatively small ultrasound powers for the implementation of the identified optoacoustic effects and spectroscopic properties in the laser breakdown in the liquid.

Abstract A strong dependence of the optical breakdown in a liquid in the ultrasonic field on the amplitude and frequency of ultrasound is demonstrated. The increase in the intensity of spectral lines of potassium and oxygen during the optical breakdown is detected by the increase in the amplitude and frequency of ultrasound. At the same time, the effect of saturation is observed at high frequencies (above 200 kHz) and for high ultrasonic powers, when the growth in the intensity of spectral lines slows sharply down. This shows that there is no necessity to use high powers and frequencies of ultrasound to study the opto-acoustic effects during optical breakdown in a liquid.

A strong dependence of the optical breakdown in a liquid in the ultrasonic field on the amplitude and frequency of ultrasound is demonstrated. The increase in the intensity of spectral lines of potassium and oxygen during the optical breakdown is detected by the increase in the amplitude and frequency of ultrasound. At the same time, the effect of saturation is observed at high frequencies (above 200 kHz) and for high ultrasonic powers, when the growth in the intensity of spectral lines slows sharply down. This shows that there is no necessity to use high powers and frequencies of ultrasound to study the opto-acoustic effects during optical breakdown in a liquid.

It is established that the action of ultrasound leads to both sharp enhancement of the acoustic emission and increase in the intensity of spectral lines of laser-excited elements dissolved in aqueous salt solutions (NaCl, NaHCO3, CaCl2), which provides a new method of combined laser–ultrasonic spark spectroscopy. The task of synchronization of the acoustic and optical emission has been solved, which reveals dependence of the intensity of spectral lines on the phase of ultrasonic wave action.

Показано, что при воздействии ультразвука наблюдаются резкое увеличение акустической эмиссии и увеличение интенсивности спектральных линий растворенных элементов в водных растворах NaCl, NaHCO3 и CaCl2, что позволяет говорить о новом комбинированном методе лазерной и ультразвуковой искровой спектроскопии. Решена задача синхронизации акустического и оптического излучения, что позволило выявить зависимость интенсивности спектральных линий от фазы ультразвукового воздействия. DOI: 10.21883/PJTF.2017.16.44933.16650

The acoustic emission from the zone of the optical breakdown in liquid is experimentally studied. The spectral characteristics and energy of the acoustic wave that is generated in liquid due to expansion of the plasma formation initiated by the optical breakdown at a wavelength of 532 nm are analyzed. Two spectral peaks that characterize the acoustic emission and the low-frequency shift of the low-frequency peak owing to an increase in the laser pulse energy are demonstrated. In general, the linear dependence of the acoustic pressure on the laser pulse energy is observed. The acoustic data can be used to reconstruct function R(t) that is in agreement with dependences R(t) resulting from the optical data. This circumstance is important for the study of breakdown in opaque media.

Motion of an ensemble of non-interacting classical particles in a space-periodic potential subjected to a weak external wave-like perturbation is considered. With large values of the wavenumber, the perturbation causes a resonance-induced chaotic layer in a certain area of phase space. Different scenarios of the emergence of chaos are considered. Adiabatic space or time modulation of the phase of the perturbation changes the location of the chaotic layer. A significant number of particles trapped within the chaotic layer is retained inside in course of the adiabatic modulation, revealing autoresonant behavior. We show that this phenomenon can be used to generate a directed ballistic current using a weak perturbation, even if particle’s energies are initially close to the minimal value.

The motion of an ensemble of noninteracting classical particles in a spatially periodic potential field has been considered, assuming that the ensemble initially undergoes ballistic motion. It is shown that an external perturbation in the form of a plane wave with adiabatically modulated frequency can lead to localization of a significant part of the ensemble with a decrease in the average energy of particles. The effect is related to the capture of particles in a chaotic layer in the phase space, which is induced by scattering on a resonance with the perturbation wave.

The hydrocarbon seeps emitting buoyant bubble plumes from seafloor vents—gas flares have been actively investigated in different regions of the World Ocean, in particular, on the Sakhalin slope in the Sea of Okhotsk. The gas flares can be easily detected by regular echo sounders, because the scattering cross section of a gas bubble is large. Within the gas-hydrate stability zone—for high hydrostatic pressures and low temperatures, methane-hydrate ice skins are formed on rising seep bubbles which are typically methane. The objective of the present study was to develop a suitable model describing rheological characteristics of gas-hydrate shell and to analyze acoustic manifestations of such bubbles for the frequency range used in marine field experiments.