Effect of Different Geometric Parameters on the Flow Characteristics in a Symmetric Bifurcation Model

Abstract

Abstract Steady divergent flow (inspiratory directed) is measured using Laser Doppler Velocimetry in large scale models carefully mimicing the morphometry of small human airways. We Evaluated the anatomical features which induced vorticity in the flow from a vorticity free entrance, under conditions of convective similitude (simultaneous Reynolds And Dean’s Numbers). Five symmetrical bifurcation models with different bifurcation angles and curvature ratios are tested. A flow separation (carina) in each model is shaped to anatomical measures and the conformation of the transition between parent and daughter branches representing the morphmetric mean shape defined from casts of human airways. The flow pattern in the daughter tubes is typical of laminar flow within the entrance (diameter/axial length < 5) to sharp bends (Dean number > 500), with rapid development of strong secondary flows (maximum secondary velocity is 40% of mean axial velocity) consisting of two main vortices, with two smaller and weaker secondary vortex activities towards the inner wall of curvature. These may be time dependent interrelation with these vortices causing warbling at specific flow conditions. The calculated vorticity transport along the flow axis showed interaction between the viscous forces at the new boundary layer development along the carinal wall and centrifugal forces of curvature, with a significant influence by the upstream flow prior to the entering the actual flow division. This interplay resulted in an overshoot of the calculated vorticity transport comparable to flow entering curved bends an a suppression for the tendency to separate at the inner wall of these tight bends. The maximum primary flow velocities are skewed towards the carinal side (outer wall of curvature) and development of a second peak occurred with convection of the higher velocity elements towards the inner wall of curvature by the strong secondary flow. The appearance of the second peak depends mainly on the curvature ratio of the daughter tubes. This study is the “baseline” observation for a airway structure to function analysis. The observed flow patterns have high influence on conductive mixing, particle deposition, volume flow distribution, gaseous scrubbing (absorption at the liquid wall surfaces), and conductive-diffusive axial dispersion mechanism; all critical features for respiration. Detailed velocity measurements are carried out in all models at different axial locations. The locations are chosen according to the expected rate of velocity alterations in the models. Figures 1 to 4 show some typical primary and secondary velocities in models 1 (1/7 curvature ration and 70° bifurcation angle) and 4 (1/14 curvature ratio and 70° bifurcation angle at Reynolds number of 1500 based on the parent tube diameter. As can be seen in Figures 1 and 2, the second peak is not observed in model 4 while it is clearly observed in model 1.