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Research Papers

Role of Alveolar Topology on Acinar Flows and Convective Mixing

[+] Author and Article Information
Philipp Hofemeier

Department of Biomedical Engineering,
Technion—Israel Institute of Technology,
Haifa 32000, Israel
e-mail: philipph@bm.technion.ac.il

Josué Sznitman

Department of Biomedical Engineering,
Technion—Israel Institute of Technology,
Haifa 32000, Israel
e-mail: sznitman@bm.technion.ac.il

Contributed by the Bioengineering Division of ASME for publication in the Journal of Biomechanical Engineering. Manuscript received July 7, 2013; final manuscript received March 20, 2014; accepted manuscript posted April 2, 2014; published online May 6, 2014. Assoc. Editor: Francis Loth.

J Biomech Eng 136(6), 061007 (May 06, 2014) (10 pages) Paper No: BIO-13-1302; doi: 10.1115/1.4027328 History: Received July 07, 2013; Revised March 20, 2014; Accepted April 02, 2014

Due to experimental challenges, computational simulations are often sought to quantify inhaled aerosol transport in the pulmonary acinus. Commonly, these are performed using generic alveolar topologies, including spheres, toroids, and polyhedra, to mimic the complex acinar morphology. Yet, local acinar flows and ensuing particle transport are anticipated to be influenced by the specific morphological structures. We have assessed a range of acinar models under self-similar breathing conditions with respect to alveolar flow patterns, convective flow mixing, and deposition of fine particles (1.3 μm diameter). By tracking passive tracers over cumulative breathing cycles, we find that irreversible flow mixing correlates with the location and strength of the recirculating vortex inside the cavity. Such effects are strongest in proximal acinar generations where the ratio of alveolar to ductal flow rates is low and interalveolar disparities are most apparent. Our results for multi-alveolated acinar ducts highlight that fine 1 μm inhaled particles subject to alveolar flows are sensitive to the alveolar topology, underlining interalveolar disparities in particle deposition patterns. Despite the simplicity of the acinar models investigated, our findings suggest that alveolar topologies influence more significantly local flow patterns and deposition sites of fine particles for upper generations emphasizing the importance of the selected acinar model. In distal acinar generations, however, the alveolar geometry primarily needs to mimic the space-filling alveolar arrangement dictated by lung morphology.

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References

Figures

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Fig. 1

Generic alveolar topology models. Top row: single alveolar models for local flow and convective mixing studies. Bottom row: corresponding multi-alveolar models for particle transport and deposition studies. ((a) and (e)) polyhedral-, ((b) and (f)) spherical-, ((c) and (g)) truncated spherical-, and ((d) and (h)) toroidal-alveolar model.

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Fig. 2

Relative streamlines along the 2D midplane of the polyhedral alveolus at peak inhalation (t = 0.25T): (a) proximal (QA/QD = 0.001), (b) medial (QA/QD = 0.01), and (c) distal acinar generation (QA/QD = 0.1)

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Fig. 3

1D velocity profiles presented relative to the breathing motion along the alveolar midplane at peak inhalation (t = 0.25T) for medial acinar generation (QA/QD = 0.01): (a) profiles of spanwise velocity (uy) and (b) streamwise velocity (ux)

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Fig. 4

Projection of passive tracer particle positions inside the polyhedral alveolus after t = 8T; (a) QA/QD = 0.001, (b) QA/QD = 0.01, and (c) QA/QD = 0.1. Particles are color-coded with respect to their final displacements (Δ/DD) at t = 8T, relative to their initial positions.

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Fig. 7

PDF for the relative particle displacements Δ/DD after t = 8T in the polyhedral model at QA/QD = 0.01. Inset: distribution for the upper quartile (i.e., 75% percentile).

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Fig. 5

Root-mean-square displacement ΔRMS of ensembles of passive tracer particles. (a) ΔRMS for different QA/QD ratios inside the polyhedral alveolus model. (b) ΔRMS for different geometric models at QA/QD = 0.01. Note that the small indents seen in the curves for ΔRMS both in (a) and (b) correspond to tracer particles exiting the computational domain or depositing at a wall within the end of the first inhalation phase.

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Fig. 6

RMS displacement ΔRMS of ensembles of passive tracer particles for proximal (QA/QD = 0.001), medial (QA/QD = 0.01), and distal acinar generation (QA/QD = 0.1); data are sampled at the end of each exhalation phases (i.e., t=1T,2T,...,8T)

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Fig. 9

Particle deposition in the polyhedral model at QA/QD = 0.01 for dp = 3 μm ((a) and (b)) and dp = 1 μm ((c) and (d)). ((a) and (c)) Deposition fraction over time and ((b) and (d)) breakdown of deposition patterns.

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Fig. 8

Particle deposition sites in the polyhedral model for t = 4T, QA/QD = 0.01 and dp = 1 μm: (a) z-view and (b) x-view. Color-coding: red = deposition in duct, blue = deposition in alveoli. Note that particles are not moved further upon deposition such that deposited particles can be located outside of the domain in the above rendering.

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Fig. 10

Root-mean-square displacement ΔRMS of massless tracer particle in a laminar oscillating pipe flow

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