Spectral features of wall pressure fluctuations beneath turbulent boundary layers

Experimental measurements of the frequency spectra and frequency cross‐spectra of the wall pressure fluctuations beneath a turbulent boundary layer were made in a low‐noise flow facility. The data, taken over a range of flow speeds, clearly display a dimensionless frequency (ωδ/uτ=50) at which the spectra achieve a maximum and a low‐frequency range with an approximately ω2 rolloff. The scaling laws for the low‐, mid‐, and high‐frequency regions of the spectrum are established. The cross‐spectral data, obtained over a range of streamwise separations (0.21≤ξ/δ≤16.4), allow for the computations of the decay Γ(ξ,ω) and convection velocity Uc(ξ,ω) of the wall pressure field. These data show the existence of two distinct wave number groups: a high wave number group that scales on the similarity variable k1ξ=ωξ/Uc(ξ,ω) associated with turbulent sources in the log region of the boundary layer, in which eddies decay in proportion to their size, and a low wave number group that defines the cutoff for the large‐scal...

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