Noise Transmission from Boundary Layer Pressure Fluctuations

Abstract

A theoretical study is made of noise spectra radiated by the vibration of thin, stiff, flat plates under the action of turbulent boundary layer pressure fluctuations. The transmitted radiation investigated arises from the streamwise convection of quasi-static pressure fluctuations by the steady flow and is critically influenced by the dispersive character of transverse wave propagation in the plates. At moderate subsonic Mach numbers and typical parameter values the total transmitted power varies approximately as the fifth power of Mach number, for sufficiently thin boundary layers. At lower Mach numbers the dependence may be flatter and near certain transonic velocities should be steeper. The dominant transmitted frequencies increase with Mach number, varying, under certain conditions, as Mach number squared. Plate parameters which importantly influence sound transmission included mass/unit area, size of independent plate sections, stiffness, and damping factors for the various vibrational modes. For sufficiently thin boundary layers, the transmitted spectrum shape is essentially independent of boundary layer thickness, but the total transmitted power varies approximately as the fourth power of the thickness. For thicker boundary layers, the dependence is flatter and the Much number dependence is also flattened. Several possible procedures for reducing the noise transmission are mentioned briefly.