Coherent organizational states in turbulent pipe flow

Turbulent pipe flow is still an essentially open area of research, boosted

in the last two decades by considerable progress achieved both on the experi-

mental and numerical frontiers, mainly centered around the identification and

characterization of coherent structures - hypothesized to be the building blocks

of turbulent dynamics. It has been a challenging task, however, to detect and

visualize these coherent states. We address, by means of stereoscopic particle

image velocimetry, that issue with the help of a large diameter (6 inches) pipe

loop, which allowed us explore coherent states at various moderate Reynolds

numbers (5300 < Re < 29000) of single-phase Newtonian flow. Although these

states have been observed at flow regimes around laminar-turbulent transition

(Re ≈ 2300) and also at high Reynolds number pipe flow (Re ≈ 35000), at mod-

erate Reynolds numbers their existence has remained largely unproven until this

study. By conditionally averaging the flow fields with respect to their dominant

azimuthal wavenumber of streamwise velocity streaks, we have been able to

uncover the existence of ten distinct coherent flow patterns. In this work, we

present their statistical weight contributions, streamwise organization, and tran-

sition dynamics. Interestingly, the weight contribution and transition dynamics

of these wavenumber states remain relativly consistent across all Reynolds num-

bers. This indicates that the Reynolds number effect—where turbulent statistics

depend on the Reynolds number in conditions of moderate turbulence—dos not

seem to be reflected in the dynamics of these COS. Moreover, their occurrence

probabilities are noted to be reasonably well described by a Poisson distribu-

tion, which suggests that the streaks are created as a Poisson process on the

pipe circular geometry.