Nonlinear acoustics—Coupling between hydrodynamic and acoustic pressure fields

Abstract

Linear or Classical Acoustics considers that sound waves propagate essentially without any medium movement. However, in Aeroacoustics we deal with phenomena where there are the simultaneous generation and propagation of sound in a moving medium. Unless the flow velocity is very low, when in aerodynamics the fluid can be modeled as being incompressible, nonlinearities must be taken into account to better model the flow physics. In this paper, by considering the exact mass and momentum equations in the form of a generalized Ffowcs–Williams and Hawkings equation, the coupling between the hydrodynamic and acoustic pressure fields generated by the motion of a solid body in still air is studied. The extent of the validity of linear hypotheses and decoupling between the two pressure fields is discussed. Strategies to consider the coupling for faster movements are indicated. This coupling appears in computations as stepwise linear iterations between the hydrodynamic and acoustic perturbation pressure fields. The goal of this research is to devise a mathematical and computational model where the formation of normal shock waves in transonic flows appears as a nonlinear interaction process between aerodynamics and aeroacoustics.