Source locations and convection velocities of outer-scaled wall-pressure fluctuations in canonical turbulent boundary layers

This study identifies that the large-scale ($δ$-scaled) sources of wall-pressure fluctuations ($p_w$) beneath a zero-pressure-gradient turbulent boundary layer extend from the near-wall region up to the top of the logarithmic region (approximately 0.15$δ$, where $δ$ is the boundary layer thickness). Even though contributions from the wake region are present, they are found to be statistically less dominant than those from the logarithmic region. The findings are based on bespoke measurements using an array of 63 microphones spanning 5$δ$ in the streamwise direction, which synchronously captures space-time $p_w$ data alongside streamwise velocity fluctuations ($u$) from a single hotwire probe at the array's downstream end. The array is designed to spatially filter $p_w$ signals to uncover outer-scale contributions, by accurately resolving the large-scale portion of the frequency-wavenumber $p_w$ spectrum while avoiding aliasing from small-scale energy. This design is validated against previously published low-Reynolds-number simulation datasets of turbulent boundary layer flow. Present experiments span a friction Reynolds number range of $1400 \lesssim Re_τ \lesssim 5200$, over which the large-scale energy in the boundary layer grows significantly. This growth is reflected in both the frequency-wavenumber $p_w$ spectrum and the space-time $p_w$ correlations, both of which show scaling trends reflective of the large-scale pressure field convecting at an outer-scaled velocity of $0.75U_\infty$, where $U_\infty$ is the freestream velocity. The Reynolds-number-dependent amplification of large-scale $p_w$ sources is directly quantified through reconstructed space-time $p_w$--$u$ correlations, which show increasing magnitudes across the inner region with rising $Re_τ$.