Medical Engineering & Physics 31 (2009) 1148–1153 Contents lists available at ScienceDirect Medical Engineering & Physics journal homepage: www.elsevier.com/locate/medengphy Large eddy simulation of high frequency oscillating flow in an asymmetric branching airway model Martin A. Nagels, John E. Cater ∗ Department of Engineering Science, The University of Auckland, Private Bag 92019, Auckland 1142, New Zealand article info Article history: Received 5 March 2009 Accepted 11 July 2009 Keywords: Bifurcating flow Human lung Three-dimensional modelling HFOV Artificial ventilation Large eddy simulation abstract The implementation of artificial ventilation schemes is necessary when respiration fails. One approach involves the application of high frequency oscillatory ventilation (HFOV) to the respiratory system. Oscil- latory airflow in the upper bronchial tree can be characterized by Reynolds numbers as high as 10 4 , hence, the flow presents turbulent features. In this study, transitional and turbulent flow within an asymmetric bifurcating model of the upper airway during HFOV are studied using large eddy simulation (LES) meth- ods. The flow, characterized by a peak Reynolds number of 8132, is analysed using a validated LES model of a three-dimensional branching geometry. The pressures, velocities, and vorticity within the flow are presented and compared with prior models for branching flow systems. The results demonstrate how pendelluft occurs at asymmetric branches within the respiratory system. These results may be useful in optimising treatments using HFOV methods. © 2009 IPEM. Published by Elsevier Ltd. All rights reserved. 1. Introduction Artificial respiration techniques have been clinically applied to patients suffering from respiratory disorders since the 1950s. One effective method involves applying forced high frequency oscilla- tory ventilation (HFOV) artificially to the lung. Studies have been made into the dynamics of fluid flow within lung-like geometries at rapid ventilation frequencies, in order to better comprehend and analyse the properties of HFOV. The results of such studies are paramount to the success and efficiency of future treatments in this field. During the ventilation process, air enters the trachea and trav- els through the bronchial tree before exhalation reverses the bulk flow via the same geometry. The upper bronchial tree comprises the trachea, where the main air intake enters the lung, and two branches—the left main bronchus and the right main bronchus. The left main bronchus is the longer, more slender, and less ver- tical of the pair. These bronchi in turn branch out into two and three daughter bronchi respectively, which branch out sequentially ad dimunitum. The generation of the bronchial tree has been stud- ied, and its dimensions have been found to be extremely variable in humans, with the total airway length being the most irregular parameter [1]. At high ventilation frequencies (5–25 Hz), forced artificial res- piration is termed HFOV. The ventilation process is typically ∗ Corresponding author. Tel.: +64 9 373 7599. E-mail address: j.cater@auckland.ac.nz (J.E. Cater). approximated by applying sinusoidal velocity functions at the trachea entrance to represent inhalation and exhalation [2–4]. Analysing the system in this way demonstrates the phenomenon of pendelluft (transient movement of fluid at the end of an inhalation or exhalation cycle), which is thought to occur when the bronchial passages fill at differing rates. Studies of branching geometries sub- ject to HFOV have shown that pendelluft is a universally important mechanism under these conditions, and must be considered in models of the respiration process [3–6]. Previous studies have simplified the geometry of the bronchial tree for analysis. It has been represented by a single bifurcation, both symmetric [2–4,7] and asymmetric [8], while sequentially bifurcating geometries have also been analysed [9,10]. The usage of such geometries has been justified and validated in each case; how- ever, sequentially bifurcating, asymmetric geometries are a more accurate representation of the physical bronchial tree. A variety of methods have been utilised to model flow behaviour within an airway. Initially, the system was interpreted as a number of parallel pathways, each with a designated compliance, resis- tance and inertance. Flow could then be analysed by using the equations governing analogous inductor–capacitor–resistor (LCR) circuits [2,3]. However, these analyses did not consider the non- linear resistances and variable pressures that occur during the ventilation process. It is more common recently to utilise finite discretisation methods coupled with commercial CFD software [7,9,10]. Comparisons with experimental data have also verified the correctness of finite volume-based models [11]. One division of the methods used in CFD is the field of large eddy simulation (LES). LES is a family of methods used to solve the 1350-4533/$ – see front matter © 2009 IPEM. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.medengphy.2009.07.013