Coupling of anisotropic fiber tracking with instantaneous volumetric flow field in turbulent channel flows.

Time-resolved and instantaneous volumetric measurements are coupled with Lagragian tracking of anisotropic curved fibers in the TU Wien Turbulent Water Channel. The experiments are conducted at friction Reynolds number ranging from 180 to 720 in the viscous wall region and in the channel center. The length of the fiber (1.2mm) and the spatial resolution of the velocity vector field (0.5mm) are comparable to the Kolmogorov length scales (0.3-0.8 mm). Full rotation rate tensor of small slender fibers is simultaneously captured with instantaneous velocity gradient tensor of the flow, which allows the determination of statistical and instantaneous velocity and rotational slip between the fiber and the flow structures (laden).

The measurement technique adopted in this work represents an improvement in respect to previous experimental investigations (Alipour et al., 2021 and 2022). As results, the different behaviors of the fibers in terms of transnational and rotational motion in different regions of the channel are correlated to the homogeneous turbulent structures in the center of the channel and to the coherent structures within the viscous wall region, i.e., hairpin packets and transverse and longitudinal vortices.