Acoustic-Vortex Decomposition Method for CFD-CAA Study of Blade Machine Noise

Abstract

This chapter presents a method for calculating the generation of pressure pulsations and noise emission by blade machines based on the decomposition of the compressible medium velocity field into vortex and acoustic modes. This method of acoustic-vortex decomposition of the basic equations of motion of a compressible medium leads to an inhomogeneous wave equation with respect to enthalpy pulsations, which includes pseudo-sonic oscillations in the source vortex region and acoustic oscillations in the near-field and far-field. The source function in the wave equation is determined from the independent solution of the vortex mode equations. The boundary conditions for the wave equation are formulated using complex specific acoustic impedance and pseudo-sonic oscillations. This method allows for the consideration of the influence of inhomogeneity and turbulence of the flow, rotor interference, sound diffraction on the elements of the flow part, and impedance characteristics of the machine surfaces, while ensuring the accuracy and speed of calculations. The acoustic-vortex method represents the noise source as a function of the vortex mode velocity field. This approach eliminates the arbitrariness and conventionality of the aeroacoustic analogy, defining the source, pressure pulsations, and noise propagation in the near-field as a direct result of numerical modelling.