  Two direct numerical simulation (DNS) databases are investigated to
understand the effect of the outer-layer turbulence on the inner layer's
structures and energy transfer mechanisms. The first DNS database is the
non-equilibrium adverse-pressure-gradient (APG) turbulence boundary layer (TBL)
of Gungor et al. (2022). Its Reynolds number and the inner-layer pressure
gradient parameter reach above 8000 and 10, respectively. The shape factor
spans between 1.4 and 3.3, which indicates the flow has various velocity defect
situations. The second database is the same flow as the first one but the outer
layer turbulence is artificially eliminated in this flow. Turbulence is removed
above 0.15 local boundary layer thickness. For the analysis, we chose four
streamwise positions with small, moderate, large, and very-large velocity
defect. We compare the wall-normal distribution of Reynolds stresses, two-point
correlations and spectral distributions of energy, production and pressure
strain. The results show that the inner layer turbulence can sustain itself
when the outer-layer turbulence does not exist regardless of the velocity
defect or the pressure gradient. The two-point correlations of both cases show
that outer large-scale structures affect the inner layer structures
significantly. The streamwise extent of the correlation contours scales with
pressure-viscous units. This shows the importance of the pressure gradient's
effect on the inner-layer structures. The spectral distributions demonstrate
that the energy transfer mechanisms are probably the same in the inner layer
regardless of the velocity defect, which suggests the near-wall cycle may exist
even in very-large defect APG TBLs where the mean shear in the inner layer is
considerably lower than small-defect APG TBLs.
