Session: 7.4.2 - Fluid-Solid Flows II

Paper Number: 156164

156164 - Slender Heavy Fibers in Turbulent Channel Flow 

Abstract: 

Turbulent suspensions of long slender fibres have wide-ranging applications in industry and nature, more evidently in the form of microplastics that plague our atmosphere and marine ecosystems of the modern era. We investigate the combined role of fluid inertial forces and torques, as well as fibre flexibility, on the orientational and deformation dynamics of flexible fibres suspended in wall turbulence. We do this by performing direct numerical simulations in the Euler-Lagrange framework of a turbulent channel flow laden with long flexible fibres. The suspended flexible fibres are constructed by linking sub-Kolmogorov rods using the rod-chain model. We incorporate the effects of fluid inertial torques on the orientational dynamics of the fibres by exploiting the model developed by Dabade et al., J Fluid Mech, vol. 778 (2015), pp. 133–188, in conjunction with Jeffery’s torque (Jefffery, Proc. R. Soc. Lond, 102.715 (1922), 161–179). The inclusion of fluid-inertial torques can alter the orientational dynamics of the heavy fibres as they drift away from Jeffery orbits and strengthen the influence of local stretching. What has never been explored is the effect that fluid inertial forces and torques, in conjunction with the role of varying fibre lengths and flexibility, have on the collective dynamics of the fibres suspended in wall turbulence. To these aims, we perform direct numerical simulations in the Euler-Lagrange framework of a turbulent channel flow with shear Reynolds numbers, Re_t = 300 and 600, laden with long flexible fibers. Through the simulations, we study how fiber inertia and fluid inertia modulate the dynamics of the suspended fibers possessing large density ratios - O(10²) − O(10³) - with respect to the suspending fluid. We show that the inclusion of fluid inertial torques in the equation of motion of the fibers can significantly alter their orientational dynamics provided that the fiber-to-fluid density ratio and the Stokes number of the fibers are sufficiently large, namely fall in the range of values that are typical of atmospheric microplastics. Otherwise, when the values fall in the range that is typical of marine microplastics (density ratios of order one and Stokes numbers O(10^-1) or less) no discernable difference can be seen with the inclusion of fluid inertia as the associated values of Rep are significantly smaller.

Presenting Author: Cristian Marchioli University of Udine

Presenting Author Biography: