Computational aeroacoustics of phonation, part I: Computational methods and sound generation mechanisms

The aerodynamic generation of sound during phonation was studied using direct numerical simulations of the airflow and the sound field in a rigid pipe with a modulated orifice. Forced oscillations with an imposed wall motion were considered, neglecting fluid-structure interactions. The compressible, two-dimensional, axisymmetric form of the Navier-Stokes equations were numerically integrated using highly accurate finite difference methods. A moving grid was used to model the effects of the moving walls. The geometry and flow conditions were selected to approximate the flow within an idealized human glottis and vocal tract during phonation. Direct simulations of the flow and farfield sound were performed for several wall motion programs, and flow conditions. An acoustic analogy based on the Ffowcs Williams-Hawkings equation was then used to decompose the acoustic source into its monopole, dipole, and quadrupole contributions for analysis. The predictions of the farfield acoustic pressure using the acoustic analogy were in excellent agreement with results from the direct numerical simulations. It was found that the dominant sound production mechanism was a dipole induced by the net force exerted by the surfaces of the glottis walls on the fluid along the direction of sound wave propagation. A monopole mechanism, specifically sound from the volume of fluid displaced by the wall motion, was found to be comparatively weak at the frequency considered (125 Hz). The orifice geometry was found to have only a weak influence on the amplitude of the radiated sound.