6 - Fluid dynamics in the respiratory tract

Abstract

This chapter discusses the fluid dynamics in the respiratory tract. The flow of a pure fluid can normally be considered incompressible if the Mach number is less than 0.3, and temperature differences in the fluid are small relative to a reference temperature. In the upper half or so of the conducting airways, convection dominates the fluid motion over most of the breath cycle. The principal terms in the Navier–Stokes equations are the pressure gradient term and the convective (nonlinear) term. The flow in the downstream bifurcation is more complex than what occurs in a single bifurcation. In particular, the vortices formed in the first bifurcation are convected asymmetrically into the second bifurcation and counter the formation of such vortices in one of the daughter branches, with the convected motion coming in from the upstream bifurcation actually dominating the secondary flow in the one daughter branch. In the daughter bifurcation, secondary flow development is much reduced due to the much lower velocity (and centrifugal force) compared to the parent bifurcation such that the secondary flow pattern that is carried into the daughter branch actually dominates the secondary flow there.