The coupling between acoustic waves and fluid flow motion is basically nonlinear, with the result that flow and sound modify themselves reciprocally with respect to generation and propagation properties. As a result this problem is investigated by many different communities, such as applied mathematics, acoustics and fluid mechanics. This book is the result of an international school which was held to discuss the foundation of sound--flow interactions, to share expertise and methodologies, and to promote cross-fertilization between the different disciplines involved. It consists essentially of a set of pedagogical lectures and is meant to serve not only as a compact source of reference for the experienced researcher but also as an advanced textbook for postgraduate students, and nonspecialists wishing to familiarize themselves in depth, at a research level, with this fascinating subject.

A brief review of some fundamental aspects of sound arising from turbulent boundary layers, noise due to flow past a single discontinuity (trailing edge), noise from airfoils operating in turbulent flow, and noise due to rigid flow discontinuities (spoilers) immersed in rigid ducts is presented. Emphasis is on dipole-like sound fields associated with turbulent flow past a trailing edge, rigid bodies in turbulence and in-duct spoilers. Representative available data are reviewed and evaluated in terms of theoretical considerations and, where possible, empirical prediction techniques are given in terms of convenient aerodynamic and geometric parameters. Limitations on current knowledge are discussed.

The scattering of sound by the nonlinear interaction of two sound beams in the presence of turbulence is used to experimentally measure the turbulent velocities generated by a submerged water jet. When two sound beams of primary frequencies f01 and f02 intersect in a region of turbulent flow, the nonlinear scattering generates sum and difference frequency components (f0+ = f01 f02 and f0- = f01 - f02) which radiate outside the interaction region. In the absence of turbulence, the crossed beams do not produce radiated sum and difference frequencies. In this experiment, two transducers emit continuous wave focused sound beams of frequencies f01 = 2.0 MHz and f02 = 2.1 MHz, respectively. A receiving transducer, located outside the interaction region, detects the scattered sum frequency (f0 = 4.1 MHz.) Scattering results measured at 80 degree angles for each of 11 scan positions across the jet are used to map out the velocity correlation coefficients of the turbulence. Results are then compared with earlier published experiments that use conventional hotwire probes to measure a similar turbulent jet flow in air. nonlinear acoustics; sound- scattering; sound-waves; ultrasonic-waves.

Modern Methods in Analytical Acoustics considers topics fundamental to the understanding of noise, vibration and fluid mechanisms. The series of lectures on which this material is based began by some twenty five years ago and has been developed and expanded ever since. Acknowledged experts in the field have given this course many times in Europe and the USA. Although the scope of the course has widened considerably, the primary aim of teaching analytical techniques of acoustics alongside specific areas of wave motion and unsteady fluid mechanisms remains. The distinguished authors of this volume are drawn from Departments of Acoustics, Engineering of Applied Mathematics in Berlin, Cambridge and London. Their intention is to reach a wider audience of all those concerned with acoustic analysis than has been able to attend the course.