Inertial effects on the dynamics of rigid heavy fibers in isotropic turbulence

While fiber-turbulence interactions are common in industrial and environmental applications, little is known about inertial fiber dynamics in isotropic turbulence. Here, rotational and translational dynamics of rigid, heavy fibers in air isotropic turbulence were measured using two-orthogonal-view, holographic cinematography. Measurements were conducted in a turbulence chamber (Reλ = 115). Several batches of nylon fibers with different diameters and lengths were investigated, resulting in Stokes numbers ranging between 1.0 ≤ St ≤ 32.5, and fiber length to Kolmogorov length scale ratios ranging between 3.6 ≤L/ηk≤ 17.3. Ratios between fiber settling velocities in turbulence and quiescent conditions, V2/Vs, scaled with the ratio of the rms of air fluctuating velocities, u′, and Vs, similar as for spherical particles. Fiber inertia (as indicated by St) decreased the response of the fibers to the fluctuating air velocities, and ratios of the rms values of fluctuating fiber centroid velocities and air velocities dropped from 0.96 to ∼0.77 for the highest St and were well predicted by the model of Wang and Stock [J. Atmos. Sci. 50, 1897 (1993)JAHSAK0022-492810.1175/1520-0469(1993)050%3C1897:DOHPBT%3E2.0.CO;2]. Furthermore, with increasing St, probability density functions (PDFs) of fiber centroid velocities that were well described by a normal distribution narrowed in comparison to the those of the air velocity. PDFs of in-plane fiber rotation rates could not be described by a normal distribution. In the absence of significant length effects, fiber rotation rates were governed by St. Our results indicate that the fiber "tumbling" rate peaks at around St ≈ 4, most likely as a result of reduced fiber alignment with the vorticity vector compared to fibers having lower St. At the highest investigated St, decreasing "tumbling" rates are the result of increasingly limited response to the fluctuating flow field, well predicted by the slightly modified model proposed by Bounoua et al. [Phys. Rev. Lett. 121, 124502 (2018)PRLTAO0031-900710.1103/PhysRevLett.121.124502].