Ing. Jaroslav Plocek, CSc.

The possibilities of measuring sound pressure in liquids without obvious influence of the measured fields are discussed. A new type of sensor is presented that allows point measurements, practically does not affect the measured acoustic fields and at the same time allows to easily distinguish useful signals from background disturbances.

A newly developed method for indication of the bubble state in classical single-bubble cavitation experiments is introduced. The method is based on processing the signal from a sensor, positioned on the flask from outside. The technical means of the method are further explored to improve the position stability of the bubble.

Near field and far field of intensively radiating zeroth order acoustic sources are studied theoretically. The acoustic source is modelled by a spherical gas bubble freely oscillating in water. The frontier between the near fieldan far field is determined from the acoustic velocity of the radiated waves. The distance of this frontier from the source center is calculated for different intensities of the source oscillations.

In the paper thermal behavior of spark generated bubbles is studied experimentally using an optic sensor (a photodiode) and an acoustic sensor (a hydrophone). It is shown that there is plasma in the bubble interior during the whole first bubble oscillation. A simple method is used, based on several assumptions, which allows determining an estimate of the radiated optic energy and from it the surface temperature of the bubble when the thermal radiation reaches its maximum. This temperature can be then exploited when computing a number of other quantities. An example of the energy partition in the first bubble growth phase and in the first compression and the following expansion phases is given. The estimate of the surface plasma temperature when the bubble is compressed to its first minimum volume is also given and it has been found to be approximately 5900 K in a selected experiment. It is also shown that spark generated bubbles do not behave adiabatically and that the plasma in the bubble interior represents a highly non-homogeneous medium far from an equilibrium state, for which the equation of the state of an ideal gas cannot be used.

The paper deals with description of nonlinear standing waves in acoustic resonators that are coupled mechanically by means of an elastically mounted wall which is implemented between the resonators. The coupling represents a linear oscillators. For the purpose of the behavior description of the nonlinear acoustic fields, the system of three model equations were derived. Two of them are the modified inhomogeneous Burgers equations and the third model equation is the oscillator's equation of motion. The investigated resonant system is excited by the harmonically vibrating pistons. The system of model equations was solved numerically in the frequency domain. The whole system obtains many parameters which can be changed. With help of these parameters we can adjust various configurations of the resonant system. The configurations, which offer interesting results, were studied. One of the configurations ensures that the resonant system behaves as a frequency convertor. Other selected configuration causes suppression of higher harmonic components in the one of the resonators.

This paper is concerned with study of low-frequency sound beams generated as difference-frequency secondary field in parametric array. As model equations for theoretical investigation, KZK equation and Higher-order parabolic equation (HOPE) [Kamakura, T., Masahiko, A., Kenicii, A, Acoust. Sci. & Tech. 25, 2, (2004)] were used. Efficient numerical algorithm capable of massive parallelization was proposed for numerical integration of the model equations. Numerical results obtained using KZK equation and HOPE show that the KZK equation overestimates amplitude of the difference-frequency secondary wave in the near-field of the primary wave at the axis of symmetry. Both the equations provide the same results in the far-field and at the off-axis for both lowand high-frequency secondary fields.

Low-voltage high-energy spark discharges in water and associated bubble oscillations are studied experimentally. Pressure waves emitted during spark discharges and during following bubble oscillations have been recorded and analyzed to determine the radiated acoustic energy and potential energy of the bubbles.These energies are then used to determine the efficiency of energy conversion.Due to specific features of the low-voltage apparatus used, bubbles of different sizes have been generated and these bubbles were found to oscillate with different intensities.To explain this great diversity in the generated bubbles, the electrical circuit of the apparatus and the early stages of the spark discharge are analyzed. While it is confirmed that the bubble size grows with the energy used for discharge, the relation between the intensity of bubble oscillations and the circuit parameters is more complex.To explain this relation,two hypotheses,partly supported by the data available,are presented.

The conditions for measurements focused to single-bubble cavitation and single-bubble sonoluminiscence were prepared. Cell for these experiments was chosen on the base of eigenfrequencies determination. The obtained values were compared to values figured out on the base of mathematical model. The optimum frequency for single-bubble cavitation and single-bubble sonoluminiscence measurements was assessed. The swept frequency acoustic interferometry method was prepared for the determination of characteristics of used liquid. Liquid characteristics like sound speed, attenuation, and density can be determined. These characteristics will be used for mathematical model of single-babble cavitaion.

The ultrasonic research at the CTU in Prague Faculty of Electrical Engineering, Department of Physics is described.

Measurements of the acoustic emissions from the single bubble cavitation utilizing different type of hydrophones have been made. The experiment arrangement is introduced and important hydrophones properties and measurements results are discussed. Some recommendations for improvement of the operating apparatus are also introduced.

Singe-bubble cavitation experiment is described.The basic arrangement and difficulties,arising at its realisation are discussed.Some practical recomendations for successful realisation of the operating apparatus are also introduced.

The paper is concerned to processing of broad band signal from laser interferometer, which is arisen due to high acoustic pressure and displacements measurement.

This paper shortly reviews several recently checked methods for optimal interferometer frequency searching in liquid markin flowmeters and describes their problems.It introduces some other searching criteria and their testing in view to their use in these flowmeters.

The paper deals with new method of the measurement of the absorption zones on roots.

Contemporary ultrasonic liquid flowmeters based on usual methods are unable to measure small flowrates (1 ÷ 100 ml/s) in narrow (>>1 cm in diameter) opaque pipes. Under such conditions, methods based on measuring drift velocity of a defined part of the liquid with artificially changed ultrasonic parameters (sound velocity and/or attenuation) may be used. A change is induced in one place of the pipe, and this change is detected in another place downstream. One of alternatives is a flowmeter based on checking temperature changes of ultrasonic velocity in the flowing liquid.

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