On the interdependence between acoustic and turbulent fluctuating motions in a moving fluid

Abstract

In a moving, turbulent, compressible fluid there is not necessarily a clear distinction between acoustic motion and turbulent motion, nor have such components of the motion been unambiguously identified in previous work. It has been found that, without approximation, it is possible to make unambiguous and conceptually satisfactory definitions of acoustic and turbulent components, respectively, of the fluctuating motion of a fluid. These definitions lead naturally to a formulation of the problem of “aerodynamic noise generation” in which it is possible to clearly identify the “sources” of aerodynamic noise and to distinguish between noise generation and noise scattering. The formulation reveals an essential interdependence between the acoustic and turbulent components of the motion. It provides a logical, self-consistent basis for the theoretical discussion of such problems as the generation, propagation and radiation of acoustic disturbances in a turbulent fluid having a non-uniform mean flow velocity, and the energy density balance and energy flow in such a fluid.