HIGH-VELOCITY ACOUSTICS

The noises emitted by unsteady mixing layers and boundary layers are attributed to the drop in density along the axis of an eddy as it grows and increases its circulation by entrainment of the surrounding external fluid. This concept is both derived from and supported by observations of the traces of eddies on a free surface, and also by other available observations of turbulent flows. If one considers each large eddy discretely, defines its axis and assigns it an appropriate strength, one can then propose a model that describes very roughly how the eddy varies. In turn, this variation can be regarded as a monopole source strength for the associated sound power radiated into the outer surrounding fluid at rest. This very simple model provides only the prediction that the sound power will be proportional to the eighth power of the mean flow velocity for a mixing layer, or the sixth power for a boundary layer. It is not detailed enough to predict the coefficients of these proportionalities. Nonetheless, the model provides physical insight into the source models of Lighthill and Ribner. In the light of the concepts basic to this flow modelling, some discussion is presented of the mechanisms governing various sources of flow noise. © 1992.