60th Annual Meeting of the Divison of Fluid Dynamics
Volume 52, Number 12
Sunday–Tuesday, November 18–20, 2007;
Salt Lake City, Utah
Session FJ: Mini-Symposium II: Deformable Particle Suspensions and Solutions
8:00 AM–10:10 AM,
Monday, November 19, 2007
Salt Palace Convention Center
Room: 250 D
Chair: Dewei Qi, Western Michigan University
Abstract ID: BAPS.2007.DFD.FJ.2
Abstract: FJ.00002 : Unsteady Dynamics of free falling of multi flexible fibers in moderate Reynolds number flows
8:26 AM–8:52 AM
Preview Abstract
Abstract
Author:
Dewei Qi
(Western Michigan University)
The direct simulations of sedimentation of single and multi
flexible fibers are conducted in moderate Reynolds number
flows by using a newly developed method. In the method,
for fluid domain, the lattice Boltzmann equations are used to
solve the Navier Stokes equations. For solid domain, a fiber is
discretized as a chain of rigid segments. The segments are
connected through ball and socket joints and can be bent and
twisted. Constraint forces are introduced at each joint.
Translation and rotation matrix of fiber are linearized with
respect to the constraint forces up to a second order of
time step. Thus, motion of the fiber under the constraint and
hydrodynamic forces could be solved by using a modified
leap-frog algorithm. Effects of many body interaction on fiber
fluttering are studied. It is found that in the same conditions
initial fluttering may be damped by fluid viscosity for a
single flexible fiber while irregular and persistent
fluttering, rocking and oscillation may occur for a multi-
fiber system. It is evident that clusters, such as doublets and
triplets, are spontaneously formed and have a profound
impact on unsteady dynamics of fibers. Two mechanisms contribute
to an increase in unsteadiness. First the clusters have larger
local settling velocity than a single fiber. Second,
closely packed fibers become more ``fat'' or ``thick'' body and
have a lower effective aspect ratio. The flows behind the 'fat'
clusters tend to be more unsteady and induce vortex shedding
that causes fibers persistently fluttering, rocking or
oscillating. It is found that a fiber chain with a long vertical
dimension is not stable. They will break down and become more
flat structure. This property is directly related to that
the fiber is preferentially oriented in horizontal direction
due to inertia. In addition, the effects of flexibility on
unsteady dynamics of sedimentation of flexible fiber are studied
in a given range of Re. We find that when stiffness is very
large, the fiber behavior is similar to a stiff or rigid fiber.
It receives the largest drag force and results in the smallest
average terminal speed. As the fiber stiffness decreases and
becomes slightly flexible, a small shape flocculation without
reduction of effective fiber length may induce a drag reduction
and lead to a terminal speed increase. The mechanism behind the
drag reduction is that fiber flexibility changes the wake
structures and releases the tip vortexes to some extent.
As the stiffness continuously reduces, the fiber becomes more
flexible. Both the effective fiber length reduction and shape
flocculation contribute to drag reduction and result in the
largest terminal speed.
To cite this abstract, use the following reference: http://meetings.aps.org/link/BAPS.2007.DFD.FJ.2