Lagrangian Statistics of Long Flexible Fibers in Wall-Bounded Turbulence

We investigate the Lagrangian rotational and deformation dynamics of long flexible fibers in a turbulent channel flow at a shear Reynolds number equal to 300, using one-way coupled Euler-Lagrange direct numerical simulations. The fibers, modeled as chains of sub-Kolmogorov rod elements with prescribed bending stiffness, span more than one order of magnitude in length and are analyzed in three wall-normal slabs covering the near-wall, log-layer and bulk regions. We compute autocorrelations of the tumbling and spinning rates, power spectra of the fiber’s end-to-end vector, and waiting-time distributions of intense tumbling events. Spinning decorrelates on a fixed multiple of the local Kolmogorov time, independent of fiber length, whereas tumbling develops a memory that grows with fiber length by factors of two to five, reversing the ordering of the two timescales between the shortest and longest fibers. Flapping spectra separate into a high-frequency regime in which all fiber lengths carry the same energy in absolute terms and a low-frequency regime in which the energy increases with fiber length. Intense tumbling exhibits approximately exponential, weakly clustered waiting-time statistics whose rescaled form is universal across slabs, fiber lengths and bending rigidities, whereas the event rate depends on both fiber length and rigidity.