Spectral analysis of a high-Reynolds-number near-equilibrium adverse-pressure-gradient turbulent boundary layer

The effects of adverse pressure gradients (APG) in high-Reynolds-numbers turbulent boundary layers (TBLs) is thoroughly assessed using both one- and two-dimensional spectra of the Reynolds stresses. The employed database was recently obtained through well-resolved large-eddy simulation (LES) of a near-equilibrium APG TBL with a moderate Rotta-Clauser pressure-gradient parameter β≈1.4, where the momentum-thickness-based and friction Reynolds numbers reach Re_θ=8,700 and Re_τ=1,900, respectively. The effects of the APG will be assessed by comparing the data with a zero-pressure-gradient (ZPG) TBL at similar Re_τ. The increase of energy in the outer region of the TBLs is partly linked with the presence of larger turbulent scales, but there is additional energy due to the rising of energetic smaller scales from the inner region close to the wall, as well as a new peak of production of TKE in the outer region. The two-dimensional spectra show longer spatial and temporal scales in the streamwise and spanwise Reynolds stresses near the wall and in the overlap region.