Simulations of the Motion of Arbitrarily Shaped Fibers in a Linear Shear Flow

ANDRIY ROSHCHENKO (1), Warren Finlay (1), Peter Minev (1)

(1) University of Alberta, Edmonton

     Abstract Number: 821
     Last modified: July 29, 2010

Abstract
Fibrous airborne particles when inhaled have long been known to have strong negative effects on the human respiratory system, leading to disease states such as lung cancer and asbestosis. In assessing risks of exposure to inhaled fibers, it is of prime interest to know where in lungs various types of fibers deposit.

Various factors influencing fiber deposition have been studied by numerous authors for a long time now. However, a complete mechanistic model of fiber dynamics in lungs which would account for all major factors affecting deposition including diffusion, gravitational sedimentation, inertial impaction and interception, as well as fiber shape, has not been developed yet.

One of the problems yet to be addressed is orientation of the real fibers with respect to the flow direction in peripheral airways. While analytical models for different orientations have been developed (Harris and Fraser, 1976; Asgharian and Yu, 1988; Balashazy et al., 1990), it is only for straight slender fibers that alignment with the flow can be predicted by Jeffery's theory (Jeffery, 1922). For particles of more general shape (such as chrysotile asbestos fibers) no attempts have been made in the literature to address dynamics in the lungs, whether analytically or numerically.

A two-grid fictitious domain method (Dechaume, et al., 2009) was used for direct simulations of high aspect ratio fibers in linear shear flow. Accounting for the complex spatial movement of fibers, we implemented an improved microscale grid resolution scheme. In our simulations the fiber is fixed with respect to the microgrid and an arbitrary Lagrangian-Eulerian approach (similarly to Duarte et al., 2004) used by transforming the Navier-Stokes equations from a laboratory coordinate system to one fixed with the microgrid.

Our simulations show the expected Jeffery orbits for straight, symmetric fibers. However, an interesting phenomena occurs when asymmetric fiber shapes are considered. In particular, in linear shear flow with fluid parameters reflecting conditions found in peripheral airways of human lungs, fibers with broken symmetry exhibit a surprising secondary rotation that is out of the shear plane.

In view of existing analytical deposition models for fibers, and particularly the different expressions for equivalent diameters applicable for different fiber orientations, our findings suggest that studies of deposition efficiencies of fibrous aerosols should account for possible increases in deposition due to asymmetrical aerosol particles or their aggregations.