Gravity Effects on Fiber Dynamics in Wall Turbulence

The present study investigated gravity effects on the dynamical behavior of inertial fibers suspended in a vertical channel flow. Direct numerical simulations were performed to obtain the turbulent flow field and the fibers were modelled as prolate spheroidal point particles. For each of the four fiber classes, three different gravity configurations were considered: upward flow with gravity opposing, downward flow with aiding gravity, and channel flow in absence of gravity. Results for the fiber distribution and the translational and rotational fiber motion were reported. In the near-wall region, the presence of gravity resulted in an increased fiber density in the downward flow but a nearly uniform distribution of fibers in upward flow. However, the preferential clustering of fibers in near-wall low-speed streaks was unaffected by gravity. The mean wall-normal or drift velocity of the fibers was higher in the downward flow and lower in the upward flow as compared to the case with no gravity. The suppressed drift velocity in the upward flow resulted in a more uniform fiber distribution throughout the channel in contrast to the near-wall accumulation of fibers in the two other cases. Overall gravity turned out to have negligible effects on some of the statistics of the least inertial fibers whereas the inclusion of gravity had a strong impact for heavier fibers.