Coherent structures incl. large-scale and very large-scale motions at high Reynolds numbers and the definition of their sizes in fully developed pipe flow are not fully understood. Recent studies with intrusive measurement techniques show that the streamwise extension of these structures is highly dependent on the Re-number. They are represented in wave length (λ) or wave number (k). The large-scale and very-large-scale structures are quite important with respect to the turbulent kinetic energy and Reynolds stresses. According to Kim & Adrian (1999), Guala et al. (2006) and Vallikivi (2014) some open questions remain unsettled for identifying accurate sizes of the large-scale motions (LSM) and very large-scale motions (VLSM). The theory of Kim & Adrian (1999) claims sizes of LSM (λ_LSM=2R-3R) and VLSM (λ_VLSM=8R-16R), but that does not estimate precisely magnitudes of the larger streamwise structures in turbulent pipe flow. The reason is that their experimental studies had been carried out for lower Re-numbers. The so called CoLa-Pipe (Cottbus Large Pipe) [König et al. (2014)] is a high Reynolds number test facility (60x10³ < Re_b < 10⁶) for various purposes ranging from basic science to applied researches. It is a closed-return facility with the suction side made of high precision smooth acrylic glass, having an inner pipe diameter of 190±0.23 mm and total length of 148 pipe diameter, i.e. L/D"148. The facility has a return pipe section also made of smooth acrylic glass with an inner diameter of 342±0.32 mm, leading to L/D"78. The CoLa-Pipe is providing an opportunity to approach higher Re-numbers with better resolution.

It can be observed that the sizes of these coherent structures can reach even couple of meters along the pipe axis. This phenomenon is investigated with Particle Image Velocimetry (PIV) as a non-intrusive measurement technique in CoLa-Pipe (in collaboration with LaVision GmbH) to validate and compare the results of Hot-Wire Anemometry (HWA) which have been obtained at LAS in previous studies.

As being a high Reynolds number pipe facility, for CoLa-Pipe slightly different PIV setups are necessary in comparison the other pipe facilities. Considering the relationship between the Re-number and the length of structures, large laser planes are expected in axial streamwise direction. This step is mandatory for capturing turbulent properties at high velocity ranges. Calibration of the PIV and as well as optimization of the measurement parameters such as spatial resolution, time separation between images, and number of samples should also be done.

Obtaining turbulent structures at high Reynolds numbers optically with PIV requires long laser plane setups in axial direction. First of all a 2D laser plane is applied in streamwise direction to enable the identification of axial extensions of turbulent structures. In this step spatial auto-correlation is performed using streamwise velocity. The results show that after traveling a certain distance the turbulent structures show a specific correlation in terms of lengths.

Their periodic properties can be observed at higher wall-normal distances and they show a similar behavior to previous HWA-results in CoLa-Pipe as mentioned in Öngüner et al. (2016).

Bibliography

1. Adrian, R.J., 1991, Particle imaging techniques for experimental fluid mechanics, Ann. Rev. Fluid Mech. 23, 261-304
2. Guala, M., Hommena, S.E., Adrian, R.J.; 2006, Large-scale and very large-scale motions in turbulent pipe flow, J. Fluid Mech., 554, 521-542
3. König, F., Zanoun, E.-S., Öngüner, E., Egbers, Ch., 2014, The CoLaPipe - The New Cottbus large pipe test facility at Brandenburg University of Technology Cottbus-Senftenberg, Review of Scientific Intstruments 85, 075115
4. Vallikivi, M., 2014, Wall-bounded turbulence at high Reynolds numbers, PhD thesis, Princeton University