# Numerical and experimental investigations of the flow in the human airways

Within the framework of the Protective Artificial Respiration (PAR) project, funded by the German Research Foundation (DFG), the unsteady flow in the human airways is investigated using numerical simulation methods as well as experimental measurement techniques. The objective is to improve the understanding of the complex gas transport mechanisms associated with a special artificial respiration technique applying a comparatively low tidal volume at rather high frequencies. This so-called high-frequency oscillatory ventilation (HFOV) may reduce ventilator associated lung injuries (VALI) from overdistension of lung tissue due to high tidal volumes (volutrauma), excessive alveolar pressure values (barotrauma) as well as cyclic alveolar re-collapse (atelektrauma) and resulting inflammatory response (biotrauma). Furthermore, HFOV-based ventilation strategies may improve outcomes from respiratory disease like acute lung injury (ALI) and acute respiratory distress syndrome (ARDS). However, optimising and advancing HFOV from being a rescue proceeding to an accepted alternative ventilation strategy certainly requires more detailed understanding of the governing gas transport mechanisms.

To perform reliable investigations of the air flow and the gas transport mechanisms in the upper (rhino-pharyngeal) and lower (especially tracheobronchial) airways, software tools are developed to reconstruct three-dimensional digital geometries of the human airways from medical image data based on computer tomography (CT) and magnetic resonance imaging (MRI).

This reconstruction tools can be used to generate realistic and patient specific volume meshes of the complex and irregular lung geometry, which are needed for numerical simulations of the air flow in the bronchial tree. We perform computational fluid dynamics (CFD) simulations of the oscillating flow in the human airways using finite volume (FV) methods to integrate the discretised Navier-Stokes equations on unstructured volume meshes. This also implies the development of one-dimensional simplified models of the unsteady air flow serving as boundary conditions to reproduce the physiological and pathological behaviour of the geometrically unresolved lower regions of the lungs which in turn highly affect the flow field in the resolved upper regions.

Figure 2: Colour encoded contour plot of the velocity magnitude of an oscillating flow in the upper region of the bronchial tree obtained by numerical simulation (left). Schematic of the one-dimensional model of the oscillating flow in the lower regions of the bronchial tree (right).

Figure 2: Colour encoded contour plot of the velocity magnitude of an oscillating flow in the upper region of the bronchial tree obtained by numerical simulation (left). Schematic of the one-dimensional model of the oscillating flow in the lower regions of the bronchial tree (right).

Furthermore, the reconstructed digital geometry is used to manufacture detailed cast models of the upper region of the bronchial tree made of polydimethylsiloxane (PDMS), which are needed for experimental investigations of the air flow. This includes flow field measurements in detailed lung models and generic bifurcations using conventional pressure and velocity sensors as well as non-invasive optical methods like laser-Doppler anemometry (LDA) and velocity encoded MRI measurements at the University Hospital in Mainz.

# References:

D. Feldmann & C. Wagner: Numerical Simulation of the Oscillatory Ventilation in Simplified Human Lung Models; Notes on Numerical Fluid Mechanics and Multidisciplinary Design, Vol. 121, p. 591-598, 2013

# Contact:

Prof. Dr. Claus Wagner

German Aerospace Center (DLR)

Institute of Aerodynamics and Flow Technology, Department Ground Vehicles

Göttingen

Phone: +49 551 709-2261