MEASUREMENTS OF THE UNSTEADY WALL SHEAR STRESS VECTOR IN THE TURBULENT PIPE FLOW UP TO REτ = 28 000

Kurzfassung

Three-dimensional particle tracking velocimetry (PTV) was conducted in the turbulent boundary layer of the CICLoPE long pipe test facility using a multi-aperture 3d 3c micro PTV system. The wall distance of imaged particle is encoded in the size of particle imaged triplets that are formed by a triple aperture placed in the optical path, following the “defocusing” concept originally described by Willert & Gharib 1992. In contrast to previously published multi-aperture microscopic particle tracking in a canonical TBL using a back-scattering configuration (Klinner & Willert, 2025), the present setup illuminates the particles from the side which achieves a higher scattering efficiency and mitigates light scatter at the air-glass interface. Thereby, the number density of successfully reconstructed particle triplets is roughly doubled which improves spatial resolution and statistical convergence. Extensive image data of the turbulent pipe BL was recorded for center line velocities of 5.2 ≤ Ucl ≤ 30.1m/s with corresponding shear Reynolds numbers of 5700 ≤ Re_tau ≤ 28100. The recovered PTV data sets provide velocity statistics up to the fourth moment up to wall distances of 600µm and capture the near-wall turbulence peak. In addition, both the stream- and spanwise fluctuations of the wall shear stress vector and accelerations within the viscous sub-layer are estimated. Probability distributions of the WSS vector exhibit no particular Reynolds number trend.