NONLINEAR AND THERMOVISCOUS PHENOMENA IN ACOUSTICS .3.

The present paper is intended as a review (over 300 Refs.) of some bas ic concepts and modern investigations of nonlinear and thermoviscous p henomena in acoustic fields in fluids. The focus is on theoretical mod els of nonlinear acoustics for acoustic time-averages, sound beams, an d nonlinear standing waves. The well-known topics of general use are c onsidered and the facts which are essential to implementation are also given. Special attention is paid to the combined analysis of the effe cts of nonlinearity and absorption, to secondary processes in intense sound fields, and to numerical simulation of nonlinear waves. Section 1: Equations of state. Fluid state. Section 2: Energy and momentum tra nsfer in the sound field. Section 3: Non-linear wave equation. Section 4: Evolution equations and their solutions. Section 5: Standing waves . Part III of the paper includes section 5. In this section nonlinear standing waves are reviewed. The method of Krylov, Bogoljubov, and Mit ropolski is described as a general calculation tool for treating weak nonlinear wave equations. This method can be divided into the asymptot ic method and the averaging method. The asymptotic method and its exte nsion are applied to problems without internal resonances. For example , nonlinear Helmholtz resonators are investigated. The averaging metho d is used for studying problems with internal resonances and is applie d to standing waves in closed tubes, in thermoacoustic systems, and in rods with dispersion. Finally, the combination of the averaging metho d with the Green's function approach and an integral equation, which i s suited especially for numerical calculations, are presented.