https://www.math.nyu.edu/faculty/shelley/papers/TS2004.pdf

Simulating the dynamics and interactions of flexible fibers in Stokes flows

Non-Brownian fibers commonly flocculate in flowing suspensions at relatively low concentrations (<1% by weight) We have developed a particle-level simulation technique modeling fibers as chains of rods connected by hinges to probe fiber flocculation The model incorporates fiber flexibility, irregular fiber equilibrium shapes, and frictional fiber interactions Model fibers reproduce known orbits of isolated rigid and flexible fibers in shear flow Simulation predictions of first normal stress differences in homogeneously dispersed, dilute flexible fiber suspensions agree with experimental data Fiber features such as flexibility and irregular equilibrium shapes strongly impact single fiber and suspension behavior Fibers aggregate in simulations with interfiber friction, in the absence of attractive forces between fibers Strong flocculation is observed in suspensions of stiff fibers with irregular equilibrium shapes Flocs contain many fibers with three or more contact points, and derive cohesiveness f

/pdf/simulations-of-fiber-flocculation-effects-of-fiber-29wn5qoqnv.pdf

Simulations of fiber flocculation: Effects of fiber properties and interfiber friction

We employ a particle-level simulation technique to investigate the rheology of non-Brownian, flexible fiber suspensions in simple shear flow. The model incorporates a variety of realistic features including fiber flexibility, fiber deformation, and frictional contacts. The viscosity of fiber suspensions is strongly influenced by the fiber equilibrium shape, interfiber friction, and fiber stiffness. The viscosity of the suspension increases as the fiber curvature, the coefficient of friction, or the fiber stiffness is increased. The yield stress of fiber suspensions scales with the volume fraction in a manner similar to that observed experimentally. Fiber suspensions that flocculate exhibit a shear thinning regime that extends to shear rates lower than those observed for homogeneous suspensions.

/pdf/rheology-of-sheared-flexible-fiber-suspensions-via-fiber-1mnasvvhu2.pdf

Rheology of sheared flexible fiber suspensions via fiber-level simulations

Rheology of corn stover slurries at high solids concentrations--effects of saccharification and particle size.

The relationship between the stress and the fiber orientation distribution in semi-dilute (nL3⪢1,nL2d 1) fiber suspensions was investigated. Here, n is the fiber number density, L the length, and d is the diameter. A highly viscous, index-matched suspension was developed to permit measurements of both the microstructure and rheology using the same suspension. By removing the ambiguity of comparing data taken using different suspending fluids and fibers, a more accurate evaluation of available stress–structure models was made possible. The measured period of rotation and the distribution among Jeffery orbits were compared to the results of a theory for hydrodynamic fiber interactions and a simulation incorporating mechanical contacts. At low concentrations, the period increased above the dilute, Jeffery value. As the fiber loading was increased, the period peaked and decreased to approach the dilute result. The distribution of orbits shifted slightly towards the vorticity axis with increasing concentration. The inclusion of a nematic potential in the hydrodynamic theory provided a possible explanation for the decrease in the period of rotation. Measurements of the viscosity and first normal stress differences of the same suspensions were compared to theoretical predictions based on the orientation results. The measured viscosity was in good agreement with the mechanical contact simulation results but was much larger than predicted by hydrodynamic theories. The high viscosity and the measurement of significant first normal stress differences are suggestive of an enhanced stress resulting from the presence of fiber–fiber contacts.

An experimental determination of the stress–microstructure relationship in semi-concentrated fiber suspensions

The main purpose of this study was to examine the viscous and elastic properties and capillary flow of fiber suspensions in Newtonian fluids as well as in polymer solutions. The fillers used were glass, carbon, nylon and vinylon fibers. Glycerin was used as a Newtonian suspending medium and HEC and Separan solutions as viscoelastic suspending media. The viscosity and the first normal-stress difference were measured using a coaxial cylindrical rotating viscometer and a parallel-plate rheogoniometer respectively. The influence of the concentration, aspect ratio, diameter and flexibility of the fibers on the viscous and elastic properties of the fiber suspensions was investigated. Empirical equations were obtained for the relative viscosity and first normal-stress difference for the fiber suspensions in glycerin. The capillary flow of these suspensions is discussed in part II.

The flow behavior of fiber suspensions in Newtonian fluids and polymer solutions.

This is the second part of a study examining the mechanical properties and capillary flow of fiber suspensions in Newtonian fluids and in polymer solutions. In part I results for the viscous and elastic properties of the fiber suspensions were presented and it was shown that the fiber suspensions exhibited normal stresses in Newtonian as well as in viscoelastic suspending media. It was thus expected that circulating secondary flows would occur near the entrance to a capillary. Four types of fillers (glass, carbon, nylon and vinylon fibers) suspended in glycerin, HEC solutions and Separan solutions were investigated. The entrance flow patterns were visualized and the pressure fluctuations measured. The visualization enabled the eddies occurring in the fiber suspensions in Newtonian fluids to be analysed and classified into two tpyes. The results from the flow visualization were correlated with the pressure fluctuations. Empirical equations for the tube length correction factor due to the elasticity were obtained.

H. Nagazono

Papers

The flow behavior of fiber suspensions in Newtonian fluids and polymer solutions.

The flow behavior of fiber suspensions in Newtonian fluids and polymer solutions. II: Capillary flow