General Medical Information

Turbulent Flow

Thomas V. Mccaffrey

If the flow rate exceeds a critical value, the air molecules deviate from the streamline flow, and mixing occurs in the fluid. This is called turbulence. Turbulence occurs when the Reynolds number exceeds 2,000. This corresponds to a flow of approximately 250 to 500 cm3/s in the nose. Under conditions of turbulent flow, a simple relation between pressure and flow cannot easily be determined. Simplified theoretic considerations show that in turbulence, pressure is related to the square of flow. When turbulence occurs, resistance to airflow depends on factors other than the size of the conduit. During turbulence, factors such as wall roughness and flow separation can produce differences in the measured resistance of conduits of the same size.

In the airway, the relation between pressure and flow is more complex, and only an approximation of the ideal relation between pressure and flow occurs. The curve obtained is not a linear relation, as would occur if nasal airflow were laminar, but is a sigmoid curve. Under turbulent conditions, the flow curve departs from the laminar flow curve. This deviation increases with increasing flow rate. Conditions other than turbulence can account for the nonlinearity of the pressure-flow relation in the nasal airway. Regions of orifice flow or collapse of the airway can produce nonlinear pressure-flow relations.