Alexei Moussatov

Cone-like bubble formation in ultrasonic cavitation field.

A new phenomenon in ultrasonic cavitation field is reported. Cavitation bubbles are observed to self-arrange in a cone-like macrostructure in the vicinity of transducer radiating surface. The cone-like macrostructure is stable while its branch-like pattern microstructure changes rapidly. The structure is constituted by moving bubbles which undergo attractive and repulsive Bjerknes forces caused by high acoustic pressure gradients and strongly nonlinear oscillations of cavitation bubbles. The cone-like bubble structure is a chemically active formation. Its remarkably high activity is confirmed by chemiluminescence experiments.

Porous material characterization – ultrasonic method for estimation of tortuosity and characteristic length using a barometric chamber

An ultrasonic method of acoustic parameter evaluation for porous materials saturated by air (or any other gas) is discussed. The method is based on the evolution of speed of sound and the attenuation inside the material when the static pressure of the gas saturating the material is changed. Asymptotic development of the equivalent fluid model of Johnson-Allard is used for analytical description. The method allows an estimation of three essential parameters of the model: the tortuosity, and the viscous and thermal characteristic lengths. Both characteristic lengths are estimated individually by assuming a given ratio between them. Tests are performed with industrial plastic foams and granular substances (glass beads, sea sand) over a gas pressure range from 0.2 to 6 bars at the frequencies 30-600 kHz. The present technique has a number of distinct advantages over the conventional ultrasonic approach: operation at a single frequency, improved signal-to-noise ratio, possibility of saturation the porous media by different gases. In the case when scattering phenomena occur, the present method permits a separate analysis of scattering losses and viscothermal losses. An analytical description of the method is followed by a presentation of the set-up and the measurement procedure. Experimental results and perspectives are discussed.

Hysteresis in response of nonlinear bistable interface to continuously varying acoustic loading.

In many experimental situations it is an equation of the forced relaxator and not of the forced oscillator that describes a variation in the acoustic field of the interface width (i.e. of a characteristic distance between the surfaces composing the interface). The developed theory predicts that some types of the nonlinear relaxators (depending on the structure of the nonlinear interaction force between the surfaces) exhibit hysteresis in their response to continuous acoustic loading of first increasing and then decreasing amplitude. Nonlinear (unharmonic) variation of the interface width starts at threshold amplitude of the incident sinusoidal acoustic wave, which is higher than threshold amplitude for returning to sinusoidal motion. This dynamic hysteresis (and accompanying it bistability) are possible, in particular, if the dependence of the effective interaction force on the interface width admits two quasi-equilibrium positions of the interface (bistable interface) or if the force itself is hysteretic (hysteretic interface). These theoretical predictions are relevant to some recent experimental observations on the interaction of powerful ultrasonic fields with cracks.

Frequency up-conversion and frequency down-conversion of acoustic waves in damaged materials

Experimental results have been obtained on thermally damaged thick glass plates. A study of nonlinear phenomena with different amounts of damages from the virgin (undamaged) case to a strongly cracked configuration has been performed. The cascade process of harmonic excitation, interaction of a high-frequency wave with a low-frequency one (nonlinear parametric receiving antenna) and demodulation of the amplitude modulated high-frequency wave (nonlinear parametric emitting antenna) have been implemented. One observes a dramatic increase in generation of the second harmonic as well as the demodulation signals versus the pump wave amplitude. A clear correlation exists between these nonlinear signatures and the amount of damages. Preliminary determinations of the coefficient of quadratic nonlinearity have been achieved. The results in the configuration of parametric emitting antenna are believed to be the very first on cracked materials.

Ultrasonic characterization of plastic foams via measurements with static pressure variations

A method for ultrasonic characterization of plastic foams by changing the static pressure of air that saturates the foam has been proposed. The method is based on high frequency asymptotic expressions of the standard Johnson–Allard equivalent fluid model. It is shown, both experimentally and theoretically, that the real part of squared acoustical refractive index and logarithm of the transmission coefficient depend linearly on the inverse of the square root of applied static pressure. These linear relations provide a simple and convenient way to determine experimentally the constitutive parameters. The method is illustrated with industrial open-cell foams. Advantages, limitations, and perspectives are discussed.

Observation of nonlinear interaction of acoustic waves in granular materials: demodulation process

Abstract Experimental study of the demodulation effect of ultrasonic waves in glass beads are reported. Both impulsive (burst) and continuous regime have been studied for amplitude modulated signals with its carrier frequency of 100 kHz. It is shown that two low frequency acoustical modes corresponding to the modulation frequency (in the range of few kHz) are excited due to the nonlinear processes in the granular medium. The excitation of a first mode propagating mainly through the beads (solid-based mode) and the excitation of a second mode propagating mainly through the air saturating the granular material (fluid-based mode) are observed. Possible physical mechanisms of generation and regimes of propagation in the granular assemblages are discussed.

Transport parameters and sound propagation in an air-saturated sand

Measurements at ultrasonic frequencies of the transmission coefficient and the sound speed in layers of quarry sand saturated by air and helium are performed at different pressures. The measurement of surface impedance at audible frequencies is also performed for air-saturated layers. Evaluation of transport parameters can be obtained from these measurements. Close sets of parameters can be obtained at high and low frequencies with the model by Pride et al. [Phys. Rev. B 47, 4964 (1993)].

Experimental study of the dispersion of ultrasonic waves sandy sediments

Abstract The present note discusses the dispersion that occurs in sediments saturated by water at medium ultrasonic frequencies (in the 0.2–1.2 MHz range). A standard algorithm based on the phase spectrum is used to calculate the phase velocity versus frequency. Some measurements have been performed on a granular material (a sand sample provided by IFREMER). It is shown that the wavespeed decreases slightly versus frequency. The implications of such measurements are discussed in relation with theoretical models and some recently published results.

Insitu measurements of the absorption coefficient in porous materials performed with parametric arrays and with standard audio instrumentation

Metrology applications in the field of the characterization of sound absorbing porous materials were considered quite early by using ‘‘parametric arrays’’ of nonlinear acoustics. Some experimental work was performed in our lab with various devices having a central frequency varying from 40 kHz to 200 kHz [Castagnede et al., Ultrasonics, 44 (2006)]. Reflection and transmission coefficients are available versus frequency, as well as dispersion curves. Experimental data are compared to numerical predictions in the frame of the standard poroelastic models for different porous materials. Some dedicated instrumentation has also been designed for industrial applications, e.g. for ‘‘on‐line’’ and ‘‘in‐situ’’ measurements of the LF coefficient of absorption over the 100 Hz–6 kHz bandwidth [Castagnede et al., Applied Acoustics, 68 (2007)]. More recently, absorption coefficient measurements have been performed with the same type of configurations with standard audio instrumentation, including a simple loudspeaker an...

Physical analysis of a stable cavitation bubble structure at high acoustic intensity

A cavitation bubble structure stable at high acoustic intensity (from 1.8 to more than 8.2 W/cm2) has been experimentally observed [A. Moussatov et al., Ultrason. Sonochem. 10, (2003)]. At the vincinity of an axisymmetrical radiating surface, big streamers of bubbles get ejected from the surface and build up a bubble structure of conical shape denoted CBS. In this paper, results on the observation and analysis of the CBS are reported for 20‐kHz horn‐type transducers with different sonotrode diameters. It is found that: (i) the CBS is always a zone of high chemical activity, even when the bubble structure is not observed due to high speed streaming; (ii) the geometry of the CBS is determined by nonlinear acoustic wave propagation. These results are supported by various experimental data: chemiluminescence measurements, high speed movies (2250 frames/s) under CW scattered light or LED flashes synchronized with driving signal and measurement of the acoustic pressure and of the time‐averaged acoustic pressure...