Modelling airflow in the conducting airways of the human lungs is important to predict particle deposition of both contaminants and pharmaceuticals. In order to achieve good predictions of deposition the airflow has to be modelled correctly.
Because of the complexity of the bronchial tree structure, the using of the small airways and highly asymmetric of the structure, in vivo studies are difficult for obtaining the global airways effects and the detail of small airways downstream. While the experimental studies are crucial to obtain a somewhat satisfied detail by utilising physiologically scaled up model to study airflow in the lungs. However, those experimental studies are limited up to the 3rd generations of bifurcation and the smaller airways deep within the lungs are inaccessible to most experimental technique even in the scaled up model. CFD studies through the solutions of Navier-Stoke equations seem to be a better way to access the smaller airways and easy to show a large number of data in three dimensional physiologically realistic airway geometries.
In both experimental and CFD simulations the control and choice of boundary conditions are essential. A particular problem is the control of boundary conditions, since the complete lung models cannot be modelled and the experiment in vivo data are not available. The only data available is the flow rate at the mouth as a function of time. Because of the complexity of the airway geometries and the limitation of the present day computer power a truncated lung model has to be used to study in airflow dynamics in the lungs at the first stage. This results in a need to control the flow rates in each airway which causes a problem in the numerical boundary condition.
Thus in the current investigation aims to carry out the appropriate numerical boundary condition for controlling the flow rates in each airway. A two dimensional CFD model of an asymmetric single bifurcation has been used as a test case. This numerical airway model is an anatomic approximation based on Horsfield’s data [Horsfield et al., 1971]. The radii of curvature in the transition zone from the parent branch to daughter branches and a curved shape of carinal ridge have been taken into account on this model. Different flow boundary conditions have been used for respiratory flow. The most realistic result has been obtained by using numerical pistons attached into the end of most downstream airways. Each piston has been enlarged/contracted to generate the required oscillatory flow rate of air through the single bifurcation of the system. The two dimensional CFD model of the airways with numerical pistons along with finite-element mesh model has been shown in figure 1. These results are compared with the published CFD results using standard boundary conditions [Wilquem & Degrez, 1997].
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