Flow partitioning in symmetric cascades of branches.

We have found markedly nonuniform partitioning of flow in idealized models of physiological cascades of branches. Liquids having viscosities of 0.8–7.0 cP were used to investigate this effect systematically in a regime characterizing a limited range of pulmonary flows, comprising inlet flow rates of 500–3,000 ml/min and branch diameters of 1.0 cm ID. Factors that affect the nonuniformity of inspiratory flow include inlet velocity profile and flow rate, cascade aspect (L/D) ratio, exit pressure distribution, and, to a lesser extent, kinematic viscosity (mu/rho). More qualitative observations using sinusoidally oscillating airflow revealed inspiratory and expiratory flow patterns to be quite dissimilar, emphasizing the inadequacy of a resistance model of flow partitioning based on Kirchhoff9s law. These results might suggest, in part, why regional ventilation in man is flow or frequency dependent and how bronchial smooth muscle could fine tune regional ventilation.