Pressure excursions during oscillatory flow in a branching network of tubes

The pressure difference across individual branches of a four-generation network of branching tubes was measured with the objective of obtaining general laws to describe the pressure drop in the airways under conditions of oscillatory flow. Fourier decomposition showed that the pressure signals consisted of a dominant component at the excitation frequency ("fundamental") and a "first harmonic" of smaller magnitude. For values of the ratio Re/alpha less than 200, the fundamental mainly represented fluid acceleration, whereas the first harmonic reflected the effects both of viscous dissipation and the change in total cross-sectional area between parent and daughter generations. For values of Re/alpha greater than 200, the magnitude of the fundamental was considerably larger than that due to fluid acceleration alone, suggesting the possibility of onset of turbulence in the branching network. These pressure measurements were applied to a simple model of the dog lung to predict total airway resistance. The results are found to be in substantial agreement with physiological measurements.