Orientation and rotation rates of non-axisymmetric fibers in turbulent channel flow

In this work, we investigate the dynamics of long non-axisymmetric fibers in turbulent channel flow. The experimental facility is the TU Wien Turbulent Water Channel, consisting of a closed water channel (aspect ratio 10), and the experiments are performed at the shear Reynolds numbers of 180, 360, and 720. Fibers are neutrally buoyant rods, length-to-diameter ratio of 120, and their asymmetry in shape is induced by to a curvature. We propose, for the first time, a three-dimensional reconstruction and tracking method for non-axisymmetric fibers based on the light intensity distribution, which is obtained with the multiplicative algebraic reconstruction technique (MART). First, we discriminate the fibers, i.e. the voxels corresponding to the location of the fibers are detected. Then the curvature and orientation of the fibers are identified and the fibers are tracked within the measurement volume. We show that the proposed fiber detection methodology allows an accurate and time-resolved reconstruction of both position and orientation of the fibers. We investigate the behaviour of the fibers, from the near-wall region to the channel center, and we produce original statistics on the effect of curvature of the fibers on their orientation and rotation rate. Shape asymmetry shows a significant effect on the distribution, orientation and rotation rates. Our measurements of the orientation and rotation rate of the fibers in the center of the channel, the most homogenous part of the channel, shows that statistics, although influenced by the curvature, bear similarities with those obtained in previous investigations in homogenous isotropic turbulence. Preferential orientation of the fibers in the near wall region shows a noticeable influence by the near wall coherent structures, and shape asymmetry dictates the different orientations for fibers. In particular, we observed that orientation of curved fibers in the near wall region, bears similarities with inertial disks preferential orientation. In this thesis, we also show that, non-axisymmetric fibers have higher probability to experience extreme motions in the near-wall region compared to the channel center. Statistics of both mean square spinning and tumbling rate of the fibers are reported in this study. We have been able to compare the tumbling rate of our long non-axisymmetric fibers with previous solutions for curved ellipsoids in simple shear flow. Finally, we report that, independent of fibers shape asymmetry rate and their length to flow Kolmogorov length scale ratio, fibers tend to spin more than tumble, ⟨ΩsΩs⟩/⟨ΩtΩt⟩ > 1 and magnitude of this ratio decreases by increasing the asymmetry in shape.