Simulation of microparticle inhalation in rhesus monkey airways

The flow in anatomically accurate rhesus macaque airways is studied using large eddy simulation. The deposition of inhaled 1- to 10-$\mu \text{m}$-diameter particles in the flow is also presented. The geometries comprise the nasal cavity, maxillary sinus cavities, pharynx, larynx, and trachea. Simulations are carried out for constant inspiratory flow rates of 2, 4, and 6 L/min, corresponding to Reynolds numbers of 351, 702, and 1053 for a nominal tracheal diameter of 0.8 cm. Unsteady flow is seen at certain flow rates, with the onset of turbulence occurring at constrictions in either the pharynx or glottis. The computed particle deposition results are compared extensively with experimental measurements carried out in models of the same computed tomography-based geometries, showing good agreement in the majority of cases. We observe significant deposition in the nasal airways as well as in the pharyngeal and laryngeal jet regions which dominate deposition for most particle sizes and flow rates. Inclusion of the pharynx and larynx regions of the upper airway in simulations is critical for accurately capturing the unsteady flow entering the lower airways, as well as predicting the fraction of inhaled particles that is filtered by the upper airways. Variability in airflow features, total particle deposition, and deposition locations between three monkeys is presented. These findings will allow better characterization of exposure dosages of inhaled compounds in animal experiments.

Figure 1

Views of a 3D printed, CT-based geometry of Monkey 1 from the (a) front and (b) side and views of the numerical airway model from the (c) back, and (d) side, which includes a flow conditioning section.

Figure 2

Airway experiments were performed by placing the plastic airway replica into an aerosol chamber. Flow is driven through the airway and particle concentration at the airway outlet is recorded by an aerodynamic particle sizer.

Figure 3

Mesh refinement produces small $(<0.05)$ changes to particle deposition fraction in (a) Monkey 1 and (b) Monkey 2 when increasing mesh size from 4 to 13 million control volumes at all flow rates and particle sizes.

Figure 4

Comparison between right nasal airflow in (a, below panel label) Monkey 1, (b) Monkey 2, and (c) Monkey 3 at Re 702. The contour variable $w$ corresponds to the streamwise velocity normal to the slice while the superimposed vectors correspond to the in-plane velocity.

Figure 5

Comparison between airflow in the pharynx and glottis in (a) Monkey 1, (b) Monkey 2, and (c) Monkey 3 at Re 702. The contour variable $w$ corresponds to the mean streamwise velocity normal to the slice while the superimposed vectors correspond to the mean in-plane velocity.

Figure 6

Comparison between airflow in the trachea in (a) Monkey 1, (b) Monkey 2, and (c) Monkey 3 at Re 702. The contour variable $w$ corresponds to the mean streamwise velocity normal to the slice while the superimposed vectors correspond to the mean in-plane velocity.

Figure 7

(a) Magnitude of the mean flow velocity at nominal tracheal Re of 702 for (i) Monkey 1, (ii) Monkey 2, and (iii) Monkey 3. A high-velocity laryngeal jet forms in the narrow constrictions of the pharynx and glottis. (b) Magnitude of the root mean square of flow velocity reveals turbulence generation in the larynx of Monkey 1 at (i) Re 351, (ii) Re 702, and (iii) Re 1053.

Figure 8

Comparison between (a) LES and (b) DNS results in the larynx and trachea of Monkey 2 at Re 702. The contour variable $w$ corresponds to the mean streamwise velocity normal to the slice while the superimposed vectors correspond to the mean in-plane velocity. The parameter shown in the table is the dimensionless average root-mean-square of the velocity (${\overline{U}}_{\text{RMS}}/U_T$) for each slice made dimensionless with the nominal mean tracheal flow velocity ($U_T$) at Re 351, or 0.663 m/s. The percentage change between LES and DNS is shown as $\mathrm{\Delta }$. Slices are numbered from left to right.

Figure 9

Total deposition fraction predicted by simulations compared with experimental measurements in (a) Monkey 1, (b) Monkey 2, and (c) Monkey 3.

Figure 10

Nasal airway and trachea/larynx deposition fractions predicted by simulations in (a) Monkey 1, (b) Monkey 2, and (c) Monkey 3.

Figure 11

Local deposition of 6 and $10\mu \text{m}$ particles in the nasal passages at Re 702 show deposition hot spots in the nasal valve region, the top of the middle turbinate, and the nasopharynx.

Figure 12

Local deposition of 6 and $10\mu \text{m}$ particles in the larynx and trachea at Re 702. Differences in the pharyngeal/laryngeal jet features produce significantly different deposition patterns between monkeys.

Figure 13

Nasal airway deposition at all flow rates and monkeys when plotted by Stokes number. The characteristic length is defined as: $L_c=\sqrt[]{\frac{4A_{\text{cs}}}{\pi }}$ with $A_{\text{cs}}$ being the minimum cross-sectional area at the nasal valve.

Figure 14

Deposition fraction in the larynx and trachea at all flow rates and monkeys when plotted by Stokes number. The characteristic length is defined as: $L_c=\sqrt[]{\frac{4A_{\text{cs}}}{\pi }}$ with $A_{\text{cs}}$ being the minimum cross sectional area at the (a) pharynx and (b) glottis.