Bulletin of the American Physical Society
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 |
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Chair: Dewei Qi, Western Michigan University Room: Salt Palace Convention Center 250 D |
Monday, November 19, 2007 8:00AM - 8:26AM |
FJ.00001: Single Molecule Studies of the Effect of Flow Type on the Dynamics of Polymer Solutions Invited Speaker: The dynamics of individual long chain polymers in solution depends critically on the local flow type (i.e. the relative balance between vorticity and rate of strain) even in the approximation that the flow is locally linear. Indeed, single molecule studies (including fluorescence microscopy of DNA as well as Brownian dynamics simulation) have elucidated the transition from stable extended conformation states for purely extensional flows to rotating ``vane-like'' motion in pure vortical flows. These studies have demonstrated most recently that \textbf{\textit{conformational fluctuations}} are critical in understanding the dynamics, whereas early theoretical literature focussed on only the mean polymer conformation. The effect of long range intra-chain interactions (hydrodynamic and excluded volume) on these conformational fluctuations also appears to be critical particularly at high molecular weights. In this talk, I will review progress in this area and then ultimately focus on specific studies of the effect of flow type on (a) conformation hysteresis in the coil-stretch transition, (b) tumbling motion in vortical flows and (c) interchain interactions in non-dilute solution. [Preview Abstract] |
Monday, November 19, 2007 8:26AM - 8:52AM |
FJ.00002: Unsteady Dynamics of free falling of multi flexible fibers in moderate Reynolds number flows Invited Speaker: 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. [Preview Abstract] |
Monday, November 19, 2007 8:52AM - 9:18AM |
FJ.00003: Fluid dynamics of confined polymer solutions:mechanisms and methods Invited Speaker: Micro- and nanofluidic implementations of DNA characterization methods provide a motivation to understand the dynamics of solutions of linear polymer molecules in flow fields at length scales where the polymer and process scales overlap. In this regime a number of effects come into play, particularly the steric interactions between polymer segments and microchannel walls and the segment-segment and segment-wall hydrodynamic interactions. Using theory and simulations, we describe and analyze the dynamics of DNA during flow in simple channels, where chain-wall hydrodynamic interactions lead to migration, as well as channels with a corrugated wall, where steric effects are important. (In both situations, phenomenological arguments based on the free energy of stretched vs. coiled chains would predict behavior opposite to what is actually observed.) Finally, we use simulation of chains near the dilute/semidilute crossover to illustrate how chain migration changes at finite concentration. These last computations are performed with a new O(N) particle-particle/particle-mesh method for calculation of hydrodynamic interactions between N point (or regularized point) particles in an arbitrary geometry. [Preview Abstract] |
Monday, November 19, 2007 9:18AM - 9:44AM |
FJ.00004: Lattice-Boltzmann simulations of flexible and semi-flexible polymers in solution Invited Speaker: I will describe ongoing research into the development of numerical methods to simulate the dynamics of polymer solutions. Our work is based on a fluctuating lattice Boltzmann model (FLBM), which can account for both the dissipative and correlated random forces between chain segments. I will outline a new approach to the FLBM, which emphasizes its statistical mechanical foundation. Polymer solutions can be most conveniently modeled as connected point particles, and I will present results for different methods to capture the polymer-fluid coupling. Finally, I will outline how the algorithm may be extended to semi-flexible polymers. [Preview Abstract] |
Monday, November 19, 2007 9:44AM - 10:10AM |
FJ.00005: LB-Based Simulation of Deformable Capsules, Particles and Fibers with Sharp Fluid-Solid Interface. Invited Speaker: The lattice-Boltzmann (LB) based DNS of rigid particles in fluid is now widely applied to suspension flows. With my students, J. Clausen, R. MacMeccan, and J. Wu, we have extended the original LB method for \textit{impermeable} rigid particles suspended in liquid (Aidun, et al., J. Stat. Phys. 1995, and JFM 1998) to include capsules with deformable membrane, deformable particles and fibers. For capsules with deformable membrane, the combination of the LB method for the fluid phase with finite element (FE) discretization of the solid membrane has shown to be most effective. I will also present a new solid-fluid coupling method for LB simulation of moving particles developed with my student, J. Wu, that is superior to the regular `bounce-back' method. The LB equation for the fluid is solved on a fixed lattice where the particles are fitted into a Lagrangian grid with sharp fluid-solid interface (IG) eliminating the inherent sudden movement/deformation of the interface from node to node through the lattice. For example, we apply this method to couple the LB with the lattice-Spring method. Compared to the regular ``bounce-back'', we will show that this method is more stable and smooth. I will present the effect of particle deformation on rheology of spheres, red blood cells, and fibers to demonstrate that these methods open the possibility for analysis of a class of deformable particle/fiber suspension rheology and microstructure. [Preview Abstract] |
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