Response to the “Letter to the Editor”

Abstract

Please click here to find the Letter to the Editor to which this Response refers: http://dx.doi.org/10.1080/02786826.2012.684412

Copyright 2012 American Association for Aerosol Research

In our article Roshchenko et al. (2011) we study the influence of different aspects of particle geometries on particle rotational behavior in a linear shear flow, which locally approximates the flow deep in the lungs. Particularly, we were interested in certain geometries and mechanical properties that are representative of common fiber shapes found in industrial/indoor aerosols. Our goals, the scope of our research, and its outcomes were described in the introduction and discussion sections of our article.

The statements in the Letter to the Editor that question the relation and value of our results in the study of particle deposition in the lungs are not correct. In particular, before our publication appeared, there had not been any quantitative studies demonstrating the kind of results that we present, and we have been able to characterize certain behaviors of fibers that are not self-evident. First, we found that in a 2-D flow, real particles may gain significant inclination (we call it the out-of-plane angle) to the plane with unit normal parallel to the local vorticity in a relatively short time. 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.

Drs. Podgorski and Gradon also criticize our contribution for not giving enough attention to general aspects of fiber mechanics and they provide an overview of certain literature in the area, which in their opinion we should refer to. As we outlined in our paper, it was not our intent to study rotational behavior of elongated fibrous particles more generally outside of the context of rigid fiber deposition in the lungs. In their letter, they admit that the model of a rigid body is sufficient to study transport and deposition of common aerosols. Therefore, their overview of the methods for flexible or elastic particle is not relevant to our study. Their overview of methods for description of rigid particle motion is interesting from a historical, pedagogical point of view and we regret that we were not aware of some of the publications they list.

They also incorrectly conclude about our method: “The methodology, in essence, does not go further than the classical Jeffery's (1922) approach used for the straight still fiber.” In fact, our numerical method directly solves the full system of the Navier–Stokes equations coupled with the equations describing particle translation and rotation. In contrast to Jeffery's solution, our method can simulate any real particle geometry, it includes both fluid and particle inertia, and is not restricted by the assumption of vanishing Reynolds number. Obviously, this is the most realistic and general setting for studying such problems, and it can be resolved only through numerical solution as we did.