Oscillating Flow of a Viscous Compressible Fluid Through a Rigid Tube: A Theoretical Model

The forced oscillation technique used to determine the impedance of the respiratory system has been modified by measuring, instead of pressure and airflow at the mouth of the subject, the transfer function over a rigid tube. An exact description of the aerodynamic characteristics of a cylindrical conduit is needed in this method. Therefore, a line model was developed, describing the tube as a pneumatic transmission line and taking into account the axial pressure losses, the gas compressibility, and the thermal conductivity of the tube wall. The quality of this model was verified by measuring velocity profiles at the end of an open tube at various frequencies. The agreement with the predictions of the model was satisfactory.