Sub-convective wall pressure sources in high Reynolds number incompressible turbulent boundary layers

Wall-pressure (p_w) fluctuations in turbulent boundary layers, spanning a wide range of scales, are critical in high Reynolds number (Re) flow applications. Among these, the sub-convective motions, located between the acoustic and convective regions in the wavenumber-frequency spectrum, are of particular significance due to their strong coupling with structural vibration modes and their contribution to in-cabin noise in aircraft. Turbulent motions in the outer region of the boundary layer influence the near-wall dynamics through mechanisms such as superposition and amplitude modulation, suggesting they may also contribute to sub-convective p_w content. As Re increases, these outer-layer structures become more energetic, and their potential impact on the sub-convective motions is expected to grow. Identifying the spatio-temporal characteristics of these outer-layer motions is therefore essential for improving predictive models aimed at mitigating noise and vibration in engineering systems. Prior investigations exploring these nonlinear sources have been limited to friction Reynolds numbers (Re_τ) up to 6000. The present study extends the Re_τ​ range through a collaborative experimental effort by Stanford, Princeton, and the University of Melbourne. The experiments include synchronized p_w​-array measurements with two-component, large-field-of-view particle image velocimetry, spanning 4.5δ × δ (where δ is the boundary layer thickness), to capture the relevant outer layer dynamics.