Turbulent Boundary Layer Superstructures Near the Wall and Their Relationship to Wall-Pressure Fluctuations
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Abstract
Turbulent boundary layer superstructures are characterized as regions of organized coherent motions that are on the order of several boundary layer thickness long in the streamwise direction and meander in the spanwise direction as they move and evolve within the boundary layer. They are presumed to exist within the outer region of the boundary layer, but studies have shown their presence in the lower part of the logarithmic layer as well. They are hypothesized to induce pressure fluctuations upon their interaction with rigid surfaces of various geometrical shapes, surface roughness and other aerodynamic features. Due to their large sizes, they are attributed to the low-wavenumber pressure fluctuations on aerodynamic surfaces that radiate far-field sound with minimal attenuation in energy, as well as internal noise within vehicles.
This study provides experimental evidence of the existence and statistical nature of the superstructures throughout the various regions of a turbulent boundary layer. They are shown to increase in their streamwise length under favorable pressure gradients, while their spanwise meandering gets pronounced in adverse pressure gradient flows. Results from Particle Image Velocimetry (PIV) have shown that they evolve over time and space, by merging, diverging and branching during their life, making it important to characterize their unique statistics in space and time, separately. Moreover, it is shown that in the lower part of the log-layer (
Synchronous measurements of the velocity fluctuations with alias-free surface pressure fluctuations reveal a strong relationship between the two. Specifically, at