A Large-Scale Experiment for the Visualization of Near-Wall Structures in Turbulent Pipe Flow Using Stereoscopic PIV

Turbulent pipe flow is still an essentially open area of research, boosted in the last two decades by considerable progress achieved both on the experimental and numerical frontiers, mainly related to the identification and characterization of near-wall structures and their possible roles in the statistical features of turbulent fluctuations.However, it has been a challenging task in particle image velocimetry due to various experimental constraints to visualize these structures.To address this issue, this paper presents the setup of a large-scale pipe experiment with a stereoscopic PIV system for flow field characterizations, which allows us to go deep in visualizing the turbulent boundary layer, down to the viscous sublayer, in conditions of fully developed turbulence.Flow measurements of Re = 4298 and Re = 24414 (based on the diameter) are performed.The measured streamwise velocity profiles show excellent agreement with data from the literature.We further detect a number of interesting and still barely understood structures of turbulent pipe flows, namely streamwise vortices and very large-scale motions (VSLMs) formed by long, meandering structures of high and low streamwise momentum, as well as coherent symmetry patterns of streamwise vortices alternating with sweeps and ejections.