Sound Scattering Laws for Moving Microinhomogeneous Media

Abstract

Low frequency sound scattering in microinhomogeneneous media, comprising particles moving orderly or chaotically with respect to ambient ideal or viscose fluid or streamlined by fluid is analyzed. It is shown that basic scattering laws are violated in moving media due to acoustic / electromagnetic wave scattering analogy violation related to ambient fluid entrapment by particles (inhomogeneities) playing noticeable role in media sound scattering. Moving inhomogeneous media low frequency sound scattering data observable in experiments is frequently distinguished from predicted by sound scattering theory. That is why scattering laws in moving media are to be generalized and it is main purpose of the paper. Scattering amplitudes and crossections for ideal potential and viscose flows generated by particles moving with respect to media are calculated by means of inhomogeneous wave (Lighthill's) equation. For spherical scatterers in orderly motion Rayleigh law acquires correction in particle Mach number linear approximation even in ideal fluid. However, for chaotically moving particles in ideal fluid, it still holds on the average. Reynolds number of particles motion, angle of scattered wave incidence and flow Mach number -incident wave parameter relationship, defines more complex sound scattering law versions valid in viscous media distinguished from classical Rayleigh law. Linearity of Lighthill's equation (low Mach number requirement) is analysis restriction. PACS numbers: 43.20.Fn, 43.28.Gq, 43.28.Py