Measurements of length effects on the dynamics of rigid fibers in a turbulent channel flow

We present experiments on rigid, nylon fiber translation, orientation, and rotation dynamics (one-way coupled) in a fully developed turbulent channel flow (Friction Reynolds number, Reτ=435). The experiments were performed using two-orthogonal view, digital inline Fraunhofer holographic cinematography that allowed us to track individual fibers and determine their three-dimensional (3D) position and orientation along tracks. The research focused on fiber length effects, and two fiber types having similar Stokes numbers but different mean lengths, L¯+ (=27.7 and 50.8), were investigated. Time-resolved data were acquired in the buffer layer, the log layer, and the wake region of the turbulent boundary layer. In the buffer layer irrespective of length, fibers moved faster than the fluid, presumably as a result of fiber accumulation in high-speed streaks. Beyond the buffer layer fibers lagged the fluid, more so for the longest fibers due to increased drag. Probability density functions of instantaneous components of fiber velocities showed that the longest fibers exhibit increased probabilities of "extreme"transverse and wall-normal velocities, mostly in the log layer and the wake region. This is attributed to their interaction with larger, more energetic turbulence structures. Significant fiber-wall interactions were absent, even for the longest fibers due to fiber preferential alignment with the streamwise direction resulting in limited fiber-wall interaction even when the ratio of fiber length to wall-normal distance is smaller than unity. Upon approaching the wall, fiber rotation rates strongly increased. In the wall-normal plane, in-plane fiber rotation rates as a function of wall-normal position were the same for both fiber types. However in wall-parallel planes, in-plane rotation rates of the shorter fibers were higher than those of the longer ones in the buffer layer and vice versa in the wake region. Measured mean-squared fiber tumbling rates strongly increased in the buffer layer for both fiber types, while they remained nearly constant in the log layer and the wake region. A clear length effect was apparent, and the longest fibers consistently tumbled at a higher rate than the shorter ones, surmised to be the result of their interaction with more energetic, larger turbulence structures.