Numerical study of the impact of glottis properties on the airflow field in the human trachea using V-LES

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Highlights

  • Five glottis models with different shapes and cross area are considered.

  • The profile of the glottis has more impacts on the laryngeal shape.

  • The cross-sectional area has more impacts on velocity of the laryngeal jet and turbulent intensity.

  • The smaller and elliptic glottis tends to produce higher turbulent kinetic energy and higher turbulent intensity.

Abstract

The influences of the profiles and cross-sectional areas of glottal aperture on the upper respiratory airway are investigated using an idealized cast-based mouth-throat model and three dimensional computational fluid dynamics (CFD). The open source CFD code OpenFOAM is employed. The transient flows are modeled using the very-large eddy simulation with the Smagorinsky sub-grid scale (SGS) model. Five different shapes of glottis are considered, including circular glottis with 100 %, 75 % and 50 % cross-sectional area and elliptic glottis with 75 % and 50 % cross-sectional area. Both instantaneous and averaged flow fields are analyzed. It is found that the variations of glottis have great impacts on the properties of downstream flow fields such as the secondary flow, laryngeal jet, recirculation zone, turbulent kinetic energy, and vortex. Evident impacts are observed in the region within 6 tracheal diameters downstream of the glottis. The profile of the glottis has more impacts on the laryngeal shape, while the cross-sectional area has more impacts on velocity of the laryngeal jet and turbulent intensity. It is concluded that both the glottal areas and profiles are critical for an idealized geometrical mouth-throat model.

Introduction

Therapeutic aerosol drug delivered into human respiratory system via oral/nasal airway has become a popular way to treat different kinds of lung diseases such as asthma and chronic obstructive pulmonary diseases (COPD) (Kleinstreuer et al., 2008). The aerosol airway may not be able to reach the disease locations in the deep lung due to geometric complexity of the airway (Chow et al., 2007). Therefore, fully understanding the dynamic properties of airflow structure in the upper airway is crucial to increase the efficiency of aerosol drug delivery.

Computational Fluid Dynamics (CFD) method is an alternative tool to the in vivo imaging techniques (Conway, 2012; Inthavong, 2020) and in vitro experimental determinations (Corcoran and Chigier, 2000; Heenan et al., 2003) due to its efficiency in computational costs and time. It has been widely adopted in this research area. A suitable turbulent model is very important to capture the transitional airflow in the upper airway. Reynolds-Average Navier-Stokes (RANS) method has been widely used in this community (Shang and Inthavong, 2019; Yan et al., 2020). The low-Reynolds number (LRN) k-omega model was recognized as a suitable model to simulate the laminar-transitional-turbulent airflow by Zhang and Kleinstreuer (2012) based on their work to study the transitional flow in a constricted tube with four different RANS turbulence models. Cui and Gutheil (2011) demonstrated that the large eddy simulation (LES) method can better predict transitional flow structures in the same constricted tube than the RANS method. Comparing with the RANS and LES models, direct numerical simulation (DNS) resolves all time and length scales of turbulent eddies and then can better predict the turbulent flow structures correspondingly. However, few studies adopt DNS method due to its requirements of intensive computational resources and huge computational cost. LES has become more and more popular due to its moderate requirements for computational resources and its capability to capture more details about the turbulent properties.

There are two different strategies to construct a geometrical model of respiratory airways: (1) generating realistic geometrical models from medical images like computed tomography (CT) and magnetic resonance imaging (MRI) scans of specific patients (Brancatisano et al.,1983; Mylavarapu et al., 2013; Van der Velden et al., 2016; Tao et al., 2019; Farnoud et al., 2020a) and; (2) developing idealized configurations from different human populations that represent the 'average' geometric model of upper respiratory tract (Cui and Gutheil, 2011; Zhang and Kleinstreuer, 2012). Ahookhosh et al. (2021) developed an upper airway geometry model from computed tomography (CT) images of an adult healthy female. Although a realistic geometry from particular individual is very useful in clinical treatments and such geometries have been widely adopted in this community, a generalized model can be adopted when it is used to investigate the mechanism of the airflow dynamics. Several idealized geometrical models have been created in the past decades (Kleinstreuer et al., 2008; Ge et al., 2021). The circular mouth-throat model based on human cast is one of the most popular models, which has been widely validated and adopted in studying the airflow field in the upper airway.

In vivo studies have demonstrated that the glottis may have different shapes and sizes during a respiratory cycle. Brancatisano et al. (1983) measured 12 normal subjects, and reported the ratio of peak glottal area and minimum glottal area is 1.8. England et al. (1982) also reported this ratio as 1.16−1.54. To reveal the influence of varying glottis on the airflow field, experimental and numerical studies have been performed. Choi and Wroblewski (1998) compared triangular and circular glottal constriction in a straight pipe experiment, and proved the flow pattern is completely different. Brouns et al. (2007) studied the airflow field in an upper airway considering different glottal folder with different shapes and cross-sectional areas using RANS method. Their results show that the flow pattern is not obviously influenced by breathing capacity, but obviously influenced by the shape and cross-sectional area of glottal aperture. Xi et al. (2018) compared static and dynamic glottal apertures under either constant or sinusoidal breathing profiles using LES. Their results indicate that the deforming glottal aperture modifies the laryngeal jet instability and vortex generation by varying the main flow speed. Nevertheless, no research have been carried out to investigate the airflow pattern in a validated idealized upper airway model with different glottal shape and size using LES model.

In the present paper, the properties of the airflow field in the trachea of the cast-based mouth-throat models with different glottal shapes and areas are investigated numerically. The very-large eddy simulation (V-LES) with the Smagorinsky subgrid model in the platform of opensource CFD software OpenFOAM is employed to solve the laminar-transitional-turbulent flows. The objective of this study is to reveal the impacts of the glottal shapes and areas on airflow characteristics including the laryngeal jet, recirculation zone and secondary flow.

Section snippets

Geometry and mesh configuration

In the present study, a three-dimensional idealized mouth-throat model is adopted as a standard model seen in Fig. 1 and different kinds of glottal apertures are used. The standard mouth-throat model is built based on human-cast. Details about this model can be found in Zhang et al. (2002); Kleinstreuer et al. (2008), and Cui and Gutheil (2011). This model has been widely validated and applied in modeling the multi-phase flows in the respiratory airways. The glottal aperture is modified with

Result and discussion

Since modification of the glottis will mainly impact the airflow field in the trachea, we will focus on the properties of the airflow structures in the tracheal tube. To compare the properties of the airflow structures for the five cases, the non-dimensional velocities normalized by the velocity at the mouth inlet is adopted in the following sections for all cases.

Conclusion

In the present study, numerical studies are conducted to investigate the impacts of glottal aperture on the properties of the airflow fields using very-large eddy simulation method with the Smargorinsky sub-grid scale model. The opensource CFD code OpenFOAM is employed for this study. Five geometrical models, including standard, circular glottis with 75 % area of the standard case, elliptic glottis with 75 % area, circular glottis with 50 % and elliptic glottis with 50 % area, are considered.

Acknowledgements

The authors acknowledge the High-Performance Computing Center (HPCC) at Texas Tech University at Lubbock for providing HPC resources that have contributed to the research results reported within this paper.

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