The Aerodynamic Behavior of Fibers in a Linear Shear Flow

Abstract

Lung deposition behavior for straight versus curved aerosol fibers is known to be different based on existing experimental data. However, our understanding of actual fiber dynamics in the respiratory system remains far from being complete. In particular, it is not clear how fiber shape influences particle motion in the lungs. This article presents the results of direct numerical simulations in a linear shear flow of the rotational dynamics of high aspect ratio fibers with complex shapes such as elliptic rods, torus segments, and helices. Our findings show that as expected the rotational behavior of complex fibers is different from that observed for straight symmetrical particles. In particular, we observe secondary rotation for our particles, which is perpendicular to the shear plane (a plane with the unit normal parallel to the local vorticity) as well as more frequent flipping (for helical particles) than observed for straight fibers with the same lengths and diameters. In view of our results, it can be suggested that respiratory tract deposition for particles with complex shapes will exhibit enhanced interception compared with the deposition of particles with simpler shapes.

Response to the “Letter to the Editor”

“…In the context of lung deposition, this finding means that a particle, while traveling through airways, has enough time to develop (or to increase) the out-of-plane angle even without secondary flow expected on top of the linear translation in axial direction. Second, our study is the first to report that helical particles exhibit more frequent flipping in ordered periodic rotation (angle φ Figure 8, Table 2 and Discussion in Roshchenko et al [2011]), which may lead to increased interception for particles close to the walls or alter their sedimentation velocity closer to the duct axis.…”

Simulation of Enhanced Deposition Due to Magnetic Field Alignment of Ellipsoidal Particles in a Lung Bifurcation

“…However, the present particles are known to remain aligned with the flow streamlines for periods much greater than the transit time through the airway. (29) For the parameters used in this study, the calculated average period of rotation (9,34) is 7 and 17 times greater than the average particle residence time for an aspect ratio of 6 and 20, respectively. The rotational Peclet number, which is the ratio of the hydrodynamic rotation and the Brownian rotation, is much greater than 1 for an aspect ratio of 6 and 20.…”

“…The algorithm has been described in full detail by others, so only a brief summary will be given here. (26)(27)(28)(29) As the scale of the particle is much smaller than the scale of the airways, a two-grid discretization is used to resolve properly both scales. A largescale (macro) grid discretizes the bifurcating pipe of the airways, and a micro grid fit to the particle discretizes the particle and its vicinity.…”

“…(27) Note that as the micro grid moves together with the particle, we need to combine the fictitious domain formulation with the Arbitrary LagrangianEulerian formulation. (28,29) The key idea of the FDM is that the fluid equations are extended over the whole domain, including the particle domain, (25) and the rigid motion of the particle is imposed as a side constraint using a noniterative method. (26) The incompressible Navier-Stokes equations are approximated by a second-order pressure-correction algorithm, (28) combined with the method of characteristics for the advection operator.…”

“…Therefore, the value of 3 is chosen for the factor of safety F s to take into account that the refinement factor r is always smaller than 2. (29) The e rms value is calculated from the velocity relative error between 1,000 points inside the bifurcation. The refinement factor is the ratio between the number of nodes in the refined grid and coarse grid.…”