The present paper is intended as a review (over 300 Refs.) of some basic concepts and modern investigations of nonlinear and thermoviscous phenomena in acoustic fields in fluids. The focus is on theoretical models of nonlinear acoustics for acoustic time-averages, sound beams, and nonlinear standing waves. The well-known topics of general use are considered and the facts which are essential to implementation are also given. Special attention is paid to the combined analysis of the effects of nonlinearity and absorption, to secondary processes in intense sound fields, and to numerical simulation of nonlinear waves. The content includes: Section 1: Equations of state. Fluid state. Section 2: Energy and momentum transfer in the sound field. Section 3: Non-linear wave equation. Section 4: Evolution equations and their solutions. Section 5: Standing waves. Part II of the paper includes sections 3 and 4. In section 3, we present a derivation of the nonlinear wave equation in a thermoviscous fluid, its connection with Lighthill's equation, and outline the effect of acoustic streaming and heating on the nonlinear wave propagation. In section 4, frequently used model equations of nonlinear acoustics are reviewed starting with the familiar simple wave equation. Computer codes and some experimental results are discussed for the Khokhlov-Zabolotskaya-Kuznetsov (KZK) equation and for more general problem statements including the nonlinear wave equation.
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