Linear and nonlinear approaches and digital signal processing in aeroacoustics

The linear and nonlinear approaches to the calculation of small acoustic disturbance propagation and evolution in nonuniform flows are compared. In the conventional linear approach it is the linearized equations of time-dependent, ideal (inviscid and non-heat-conducting) or viscous gas flow that are integrated. In the nonlinear approach the original nonlinear equations governing the same time-dependent flow (Euler equations for an ideal gas) are integrated; these are the same equations that, together with time relaxation procedure, are used in the linear approach for calculating the stationary background. It is shown that the application of digital signal processing, widely used in acoustic experiments, makes it possible to isolate the harmonic acoustic waves from the results of integration of the nonlinear equations, though their intensity is smaller than that of the noise due to computational errors, including inadequate attainment of the stationary background.