On intra- and intersubject variabilities of airflow in the human lungs

Abstract

The effects of intra- and intersubject variabilities in airway geometry on airflow in the human lungs are investigated by large eddy simulation. The airway models of two human subjects consisting of extra- and intrathoracic airways are reconstructed from CT images. For intrasubject study, airflows at two inspiratory flow rates are simulated on the airway geometries of the same subject with four different levels of truncation. These airway models are the original complete geometry and three geometries obtained by truncating the original one at the subglottis, the supraglottis, and the laryngopharynx, respectively. A comparison of the airflows in the complete geometry model shows that the characteristics of the turbulent laryngeal jet in the trachea are similar regardless of Reynolds number in terms of mean velocities, turbulence statistics, coherent structures, and pressure distribution. The truncated airway models, however, do not produce the similar flow structures observed in the complete geometry. An improved inlet boundary condition is then proposed for the airway model truncated at the laryngopharynx to improve the accuracy of solution. The new boundary condition significantly improves the mean flow. The spectral analysis shows that turbulent characteristics are captured downstream away from the glottis. For intersubject study, although the overall flow characteristics are similar, two morphological factors are found to significantly affect the flows between subjects. These are the constriction ratio of the glottis with respect to the trachea and the curvature and shape of the airways.

Figure 1

The respiratory tract for human subject 1. (a) Front view of the CT-based human airway model, (b) inside oblique view of the upper airways. LMB is the left main bronchus. RMB is the right main bronchus.

Figure 2

Cross-sectional views of the meshes at the glottis of (a) the original mesh and (b) the fine mesh, and at the trachea of (c) the original mesh and (d) the fine mesh.

Figure 3

(a) Normalized mean speed and (b) normalized rms velocity fluctuations along the jet centerline in the trachea. Lines: the original mesh. Symbols: the fine mesh.

Figure 4

Isosurface of normalized mean speed with ⟨u⟩∕U=1.65 for (a) case 1H and (b) case 1L.

Figure 5

Normalized mean speed ⟨u⟩∕U for (a) case 1H and (b) case 1L. Normalized TKE (u^rms∕U)² for (c) case 1H and (d) case 1L.

Figure 6

Normalized mean and rms fluctuations of the velocity along the jet centerline for (a) case 1H and (b) case 1L.

Figure 7

The first fluctuating eigenmode (POD-derived coherent vortical structure) identified by λ₂=−20. (a) Side and (b) front views of case 1H. (c) Side and (d) front views of case 1L.

Figure 8

Contours of (a) normalized mean speed ⟨u⟩∕U, (b) normalized TKE (u^rms∕U)² for case 0 (subject 2).

Figure 9

Contours of normalized mean speed ⟨u⟩∕U in a vertical plane. (a) Case 2H, (b) case 3H, and (c) case 4H.

Figure 10

Normalized mean speed along the jet centerline at the flow rate of (a) 15.2 and (b) 5.0 l∕min.

Figure 11

Normalized rms velocity fluctuations along the jet centerline at 15.2 l∕min. (a) Axial component and (b) nonaxial component.

Figure 12

Normalized rms velocity fluctuations along the jet centerline at 5.0 l∕min. (a) Axial component and (b) nonaxial component.

Figure 13

Flow statistics at the epiglottal level 4 marked in Fig. 1b. The cross-sectional views of: (a) mean speed normalized by the average velocity at the glottis ⟨u⟩∕U_g and (b) rms velocity fluctuations normalized by the average tracheal velocity u^rms∕U. The distributions of: (c) ⟨u⟩∕U_g and (d) u^rms∕U along A-A^′ and B-B^′.

Figure 14

Normalized mean speed ⟨u⟩∕U along the jet centerline in the trachea at the flow rate of: (a) 15.2 and (b) 5.0 l∕min.

Figure 15

Normalized rms velocity fluctuations along the jet centerline in the trachea at the flow rate of 15.2 l∕min. (a) Axial component and (b) nonaxial component.

Figure 16

Normalized rms velocity fluctuations along the jet centerline in the trachea at the flow rate of 5.0 l∕min. (a) Axial component and (b) nonaxial component.

Figure 17

Energy spectra of [(a), (c), and (e)] case 1H and [(b), (d), and (f)] case 4Hb at [(a) and (b)] the entrance of the glottis −0.5D, [(c) and (d)] 0.5D, and [(e) and (f)] 2.5D. S is the normalized frequency. Solid, dotted, and dash lines denote the slopes of −5∕3, −10∕3, and −7, respectively.

Figure 18

Distributions of pressure coefficient C_P from the midpharynx to the lower trachea. (a) Cases 1H, 2H, 3H, and 4H. (b) Cases 1L, 2L, 3L, and 4L.

Figure 19

Distributions of pressure coefficient C_P from the midpharynx to the lower trachea. (a) Cases 1H, 4H, 4Ha, and 4Hb. (b) Cases 1L, 4L, 4La, and 4Lb.

Figure 20

Contours of [(a)–(c)] normalized mean speed ⟨u⟩∕U and [(d)–(f)] normalized TKE (u^rms∕U)² in a vertical plane (front view) for cases: [(a) and (d)] 1H, [(b) and (e)] 2H, and [(c) and (f)] 4Hb.