Skresanova I. Correlation functions and ultrasonic Doppler spectra in weakly inhomogeneous isotropic condensed matter

The thesis is devoted to the study of ultrasound Doppler response in weakly inhomogeneous isotropic condensed matter. Analytical expressions for correlation functions and power spectra of Doppler response for the cases of stationary motion of matter are obtained. An expression for determination of fluid flow parameters on the basis of data on spectral characteristic is derived in the case of arbitrary relationship between parameters of the probing pulses and geometry of the flow. The influence of the degree of correlation among ultrasound scatterers and chaotization of their motion during the diffusion process on the Doppler spectra are examined. It is shown that increasing of correlation radius and coefficient of diffusion results in the broadening of the Doppler spectra. The influence of field parameters on the spectra is analyzed in the case of non-stationary motion of condensed mater. An expression for partial power of the Doppler spectrum is obtained in the case of harmonically oscillated elastic matter. The dependence of the partial power on the displacement amplitude for the different values of threshold frequency is analyzed. The factors that influence on the formation of the Doppler spectra are established for the cases of non-stationary accelerated movement of fluids. The influence of correlated and uncorrelated motion of discrete ultrasound scatterers on the Doppler spectra are examined. For such a system dynamic motion model that takes into account the presence of distributed internal active centers and frictional forces is developed. The frequency dependence of absorption coefficient and second viscosity in the media with the set of internal degrees of freedom and relaxation processes is studied.