Comparison of the wall pressure fluctuations in artificially generated turbulent spots, natural transition and turbulent boundary layers

Knowledge of the wall pressure fluctuations beneath transitional and turbulent flows is required in order to understand and reduce aerodynamically and hydrodynamically generated noise. Measurements inside and outside turbulent boundary layers indicate that the wall pressure fluctuations generated by turbulence may well be the dominant mechanism in the generation of near field noise (self-noise) and, if the wall pressure fluctuations are coupled with the vibratory modes of the structure, there will be a significant increase in the sound level radiated into the far field. Wall pressure fluctuations were measured in the plane of symmetry of turbulent spots convecting in a laminar boundary layer having zero, favorable and adverse pressure gradients. The results show that the magnitude of the rms(root mean square) wall pressure and the spectra of the turbulent spot phase of boundary layer transition are strongly influenced by the local mean flow pressure gradient. The data also verifies that boundary layer transition in the presence of an adverse pressure gradient produces rms wall pressure fluctuations which are 1.5 to 2.5 times larger than those found in either a zero or favorable pressure gradient.