Experimental Characterization of Turbulent Motions Using Wall-Pressure Measurements in Low Reynolds Number Turbulent Boundary Layers

The wall-pressure fluctuations induced by low Reynolds number turbulent boundary layers are experimentally studied using flush-mounted microphones. The spatial coherence of the energy is characterized using traditional time-averaged statistical descriptors. A novel analysis is developed, based on the wavelet transform, to study the organization of coherent turbulent events, and their corresponding wall-pressure signatures. This analysis identified that induced irrotational motions/entrained fluid, between neighbouring packets, have wall-pressure signatures below 100 Hz, and packets of hairpin vortices contribute to the wall-pressure energy between 100 Hz and 250 Hz. The packets contain a hierarchy of organized, well-defined events, which contribute to the wall-pressure fluctuations at frequencies above 250 Hz. It is estimated that wall-pressure signatures from packets can be retained for up to seven boundary layer thicknesses in the streamwise direction. The composition of events within hairpin packets depends on Reynolds number, showing a shift towards higher-frequency events, with increasing Reynolds number.