Friction scaling and the onset of large scale motions in pipe flow*

Fully turbulent flow is stable in pipes once the bulk Reynolds number

exceeds a value of 3,000. By carrying out laboratory experiments and

highly resolved direct numerical simulations we show that starting from

this point and for one and a half decades in Re, the friction scaling

precisely matches the Blasius power law, while it deviates considerably

from the logarithmic Prandtl-vonKarman prediction.  Surprisingly we find

that with increasing Reynolds number a distinct transition occurs where

velocity fluctuations in the  log layer become correlated with wall

shear stress fluctuations. More specifically we observe that the mutual

information between log-layer large-scale motions and wall shear stress

fluctuations sharply increases once the Reynolds number exceeds a value

of 62,500. This point coincides with the break down of the Blasius power

law and the approach of the friction factor to the logarithmic

Prandtl-vonKarman relation. Further changes of the near wall flow

structure that are encountered at this transition will be discussed.