Concluding remarks

A brief overview of the studies made in the field of boundary layer noise in the last 60 years, has been reported, with particular emphasis on the interior noise problem and the mechanisms underlying the generation of the wall pressure fluctuations responsible for the panel vibrations and the transmission of noise.

The problem of the acoustic radiation due to the interaction of a turbu­lent boundary layer with a solid surface, has been treated only qualitatively. The prediction of the far field noise can be achieved by integral formulations and the main feature outlined in the present notes consisted in an order of magnitude estimation of the terms representing the far field pressure solu­tion. The practical consequences of those results have been discussed in the framework of the airframe noise problem.

More emphasis has been given on the description of the wall pressure statistics mainly in terms of their spectral content estimated in the Fourier domain. The scaling parameters of the frequency spectra have been dis­cussed in connection with the properties of the near wall and the outer – layer regions of the turbulent boundary layer. The main properties of the wavenumber-frequency spectra have been also reviewed and discussed along with the main statistical models proposed in literature to predict the auto – and cross-spectra behaviors.

More practical aspects have been treated by considering the case of sep­arated flows and the complex behavior arising by the interaction of the boundary layer with shockwaves.

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