Turn-over times of microplastic fibers in wall-bounded turbulence*

The spinning and tumbling turn-over times of microplastic fibers have been measured in wall-bounded turbulence. 

The experiments were performed in the TU Wien Turbulent Water Channel at a Shear Reynolds number of 720. The used Polyamide fibers are 1.2 mm long and 0.01 mm in diameter (aspect ratio 120). 

Their tip-to-tip length ranges between 3 and 10 times the Kolmogorov length scale and they are inertial-less, neutrally buoyant, and rigid. 

Their motion in the near-wall region and channel center is observed with six high-speed cameras. 

An established methodology, which involves the time-resolved tomographic reconstruction of each fibre and their subsequent tracking, is employed and improved upon.

Their curved shape is leveraged to uniquely identify the temporal evolution of their orientation, enabling measurements of spinning and tumbling turn-over times.

The analysed converged statistics reveal that on average turn-over times close to the wall are shorter than those at the channel center, which can be explained by a decreasing turbulent kinetic energy dissipation rate with increasing wall-normal position.

These results are original, as previous studies are restricted to measuring the effect of fiber length on turn-over times in homogeneous isotropic turbulence only.