Acoustic Radiation of a Turbulent Boundary Layer Over a Flat Smooth Boundary

A consistent theory of sound generation in a turbulent boundary layer developing over a flat smooth boundary at low Mach numbers is presented. The main source of sound and the long-wavelength part of pressure fluctuations on the boundary are incoming shear (viscous) waves generated by Lighthill quadrupoles in the near-wall region of the turbulent boundary layer. It is shown that with an increase in the Reynolds number (decrease in viscosity), the role of viscosity in sound generation does not decrease, but instead increases. Quantitative estimates of the spectrum of the sound power density generated in a turbulent boundary layer are given.