The CoLaPipe is a novel test facility at the Department of Aerodynamics and Fluid Mechanics, Brandenburg University of Technology Cottbus-Senftenberg (BTU Cottbus-Senftenberg), set up to investigate fully developed pipe flow at high Reynolds numbers (Re(m) ⩽ 1.5 × 10(6)). The design of the CoLaPipe is closed-return with two available test sections providing a length-to-diameter ratio of L/D = 148 and L/D = 79. Within this work, we introduce the CoLaPipe and describe the various components in detail, i.e., the settling chamber, the inlet contraction, the blower, bends, and diffusers as well as the cooling system. A special feature is the numerically optimized contraction design. The applications of different measuring techniques such as hot-wire anemometry and static pressure measurements to quantitatively evaluate the mean flow characteristics and turbulence statistics are discussed as well. In addition, capabilities and limitations of available and new pipe flow facilities are presented and reconsidered based on their length-to-diameter ratio, the achieved Reynolds numbers, and the resulting spatial resolution. Here, the focus is on the facility design, the presentation of some basic characteristics, and its contribution to a reviewed list of specific questions still arising, e.g., scaling and structural behavior of turbulent pipe flow as well as the influence of the development length on turbulence investigations.

Experiments have been conducted to assess the sizes and energy fractions of structure in fully developed turbulent pipe flow regime in two pipe facilities, Co-laPipe at BTU Cottbus-Senftenberg, and CICLoPE at University of Bologna, for shear Reynolds number in the range 2.5 · 10 3 ≤ Re τ ≤ 3.7 · 10 4 , utilizing a single hot-wire probe. Considerations are given to the spectra of the streamwise velocity fluctuations, and to large scale motions and their energy contents from the pipe nearwall to centerline. The analysis of the velocity fluctuations revealed a Reynoldsnumber dependent inner peak at a fixed wall normal location, however, an outer peak seems not to appear that might be attributed either to low Reynolds number effect or not high enough spatial resolution of hot-wire probe, motivating further study utilizing nanoscale probes. Sizes of the large scale, and very large scale structures were estimated to have wavelengths of 3R, and 20R at high Reynolds number, srespectively. The fractional energy contents in wavelengths associated with the large scale motions at various wall normal locations showed maximum contribution to the turbulent kinetic energy near the outer limit of the logarithmic layer.

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The present work focuses on spectral analysis of the streamwise velocity fluctuations obtained, utilizing the Cottbus Large Pipe (CoLaPipe) test facility for R + ≤ 3500, where R + is the Reynolds number based on the wall friction velocity, uτ , and the pipe radius, R. Measurements of the streamwise spectra have been conducted using a single hot-wire probe. Few runs have been also performed using Particle Image Velocimetry (PIV) as a structure visualization evidence and for spatial correlation purposes. The spectral analysis is being carried out to reveal few insights into pipe flow structure, allowing to follow the foot prints of such structures as well as providing estimates of their energy contents. The cumulative energy is examined as a function of the streamwise wavelength, describing the most energetic motions found in spectral data at various wall normal locations. For the current Reynolds number range, the Very Large Scale Motions (VLSM) and the Large Scale Motions (LSM) were evident as localized peaks in pre-multiplied spectra, having mean wavelengths approximately of 12R, and 3R, respectively.

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