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One of the key observations in the Princeton Superpipe was the late start ofthe logarithmic mean velocity overlap layer at a wall distance of the order of$10^3$ inner units. Between $y^+\approx 150$, the start of the overlap layer inzero pressure gradient turbulent boundary layers, and $y^+\approx 10^3$, theSuperpipe profile is modeled equally well by a power law or a log-law with alarger slope than in the overlap layer. In this paper it is shown that the meanvelocity profile in turbulent plane channel flow exhibits analogouscharacteristics, namely a sudden decrease of logarithmic slope (increase of$\kappa$) at a $y^+_{\mathrm{break}}\approx 600$, which marks the start of theactual overlap layer. This demonstration results from the first construction ofthe complete inner and outer asymptotic expansions up to order$\mathcal{O}(\mathrm{Re}_\tau)^{-1}$ from mean velocity profiles of directnumerical simulations (DNS) at moderate Reynolds numbers. A preliminaryanalysis of a Couette flow DNS, on the other hand, yields an increase oflogarithmic slope (decrease of $\kappa$) at a $y^+_{\mathrm{break}}\approx400$. The correlation between the sign of the slope change and the flowsymmetry motivates the hypothesis that the breakpoint between the possiblyuniversal short inner logarithmic region and the actual overlap log-lawcorresponds to the penetration depth of large-scale turbulent structuresoriginating from the opposite wall.

The late start of the mean velocity overlap layer at $y^+=\mathcal{O}(10^3)$ -- A generic feature of ducted turbulent flows