Sub-Convective Pressure Fluctuations in Turbulent Boundary Layer Flows*

Interaction of turbulent boundary layers with surfaces generates vibrations due to coupling of the impinging wall pressure fluctuations with the surface modes. The coupling usually occurs at high phase speeds and sub-convective wavenumbers associated with large spatial scales as big as the surface dimensions. There are challenges in measuring these sub-convective pressure fluctuations as they are orders of magnitude weaker than the pressure fluctuations due to convective turbulence. This study characterizes sub-convective pressure fluctuations using a recently developed measurement technique. The technique uses an array of sub-resonant pressure sensors, comprising multi-neck Helmholtz resonators with microphones designed to selectively filter convective pressure fluctuations. The measured data reveals for the first time a continuous form of the wavenumber frequency spectrum especially at low wavenumbers for different flow conditions including varying pressure gradients. Comparisons of the data with existing wall pressure models reveal striking resemblance to the Chase model (first validation) with lower pressure levels suggesting the need for improved wall pressure models. PIV flow visualization technique performed in sync with the wall pressure measurements help reveal the correlation between large scale structures (velocity sources) and the sub-convective pressure fluctuations. Possible hypotheses on the sub-convective pressure fluctuations origin are presented.