English

Derivation of Pitot corrections for the Zagarola & Smits Superpipe data and their composite fit

Fluid Dynamics 2019-02-12 v1

Abstract

The original turbulent pipe flow experiments in the Princeton "Superpipe" by Zagarola & Smits (1997, 1998) at unprecedented laboratory Reynolds numbers have started an ongoing vigorous debate on the logarithmic law in the mean velocity profile U+(y+)U^+(y^+) and the intimately related question of Pitot probe corrections for mean shear, viscous effects and turbulence level. Considering that the Pitot probe diameter d+d^+ exceeded 7000 wall units at the highest Reynolds number, the various \textcolor{black}{traditional Pitot} corrections had to be extended into uncharted territory \textcolor{black}{where they may no longer be additive}. In this note, the inverse approach is adopted, where the net result of all the corrections is assumed to be compatible with the model for U+U^+ developed by \cite{Monk17}. The latter has an inner part which is, up to higher order corrections, identical to the zero pressure gradient turbulent boundary layer profile and switches around ybreak+400y^+_{\mathrm{break}} \approx 400 to a logarithmic overlap layer with a K\'arm\'an "parameter" κ\kappa \textcolor{black}{that depends on pressure gradient and possibly on other flow parameters}. The simplicity of the resulting global Pitot correction proportional to (d+)0.9(R+)0.4(d^+)^{0.9}(R^+)^{-0.4}, with only two fitting parameters, indirectly supports this model. \textcolor{black}{Based on the required equality of the overlap and centerline κ\kappa's, it is furthermore shown that ybreak+y^+_{\mathrm{break}} must be a constant.} \textcolor{black}{Finally,} the outer "wake" part of the profile is argued to be asymptotically linear between the wall and about half the pipe radius. This gives rise to a linear higher order tail y+/R+\propto y^+/R^+ in the logarithmic overlap layer, which has been \textcolor{black}{the subject} of asymptotic analysis over the last decades.

Keywords

Cite

@article{arxiv.1902.03829,
  title  = {Derivation of Pitot corrections for the Zagarola & Smits Superpipe data and their composite fit},
  author = {Peter A. Monkewitz},
  journal= {arXiv preprint arXiv:1902.03829},
  year   = {2019}
}

Comments

11 pages, 6 figures