Alternative interpretation of the Superpipe data and motivation for CICLoPE: The effect of a decreasing viscous length scale

Pressurization and cryogenic conditions have been used in some experiments to change the kinematic viscosity ν of the flowing gas by many orders of magnitude in order to achieve high Reynolds number conditions in facilities of limited size. This leads to a substantial reduction of the viscous length scale ℓ∗=ν/uτ, as in the so-called Princeton “Superpipe” experiments. We demonstrate that the limited dimensions of the facilities and probes can lead to inaccuracies in the near-wall measurements for increasing Reynolds number. Specifically, a lack of accurate wall-normal probe positioning is simulated using three different datasets of wall-bounded turbulent flows. Relatively large errors in the overlap region parameters are observed for position errors of small physical magnitude that become greatly amplified in wall units as ℓ∗ is reduced. This offers an alternative interpretation to some of the key findings reported by the Superpipe team, such as the increasing lower limit of the logarithmic region ylog,min+, the existence of a power law region between the wall and the logarithmic layer, and the “mixing transition” phenomenon in wall-bounded turbulence.