Orientation, distribution, and deposition of elongated, inertial fibers in turbulent channel flow

Abstract

In this paper, the dispersion of rigid, highly elongated fibers in a turbulent channel flow is investigated. Fibers are treated as prolate ellipsoidal particles which move according to their inertia and to hydrodynamic drag and rotate according to hydrodynamic torques. The orientational behavior of fibers is examined together with their preferential distribution, near-wall accumulation, and wall deposition: all these phenomena are interpreted in connection with turbulence dynamics near the wall. In this work a wide range of fiber classes, characterized by different elongation (quantified by the fiber aspect ratio, λ) and different inertia (quantified by a suitably defined fiber response time, τp) is considered. A parametric study in the (λ,τp)-space confirms that, in the vicinity of the wall, fibers tend to align with the mean streamwise flow direction. However, this aligned configuration is unstable, particularly for higher inertia of the fiber, and can be maintained for rather short times before fibers are set into rotation in the vertical plane. A more complex situation is observed in the spanwise and wall-normal flow directions, where fiber inertia and elongation destabilize near-wall alignment in a nontrivial fashion. Fiber orientational behavior and fiber translational behavior are observed to influence the process of fiber accumulation at the wall. Comparing the behavior of fibers with that of spherical particles, it is observed that the aspect ratio has little or no effect on clustering, preferential distribution, and segregation; yet it does affect the wallward drift velocity of the fibers in such a way that longer fibers tend to deposit at higher rates. No preferential orientation and no significant segregation is observed in the channel centerline, confirming that the role of inertia and, in particular, of elongation becomes less important in and beyond the logarithmic layer.