Impedance Relationship - Thermal Design

Obstructions in the airflow path cause static pressure within the enclosure. To achieve maximum airflow, obstructions should be minimized. However, obstructions in the form of baffles may be necessary to direct the airflow over the components that need cooling.

 
Impedance Relationship Curve for an Electronics Enclosure
Typical Relationship between Airflow and Static pressure for an Axial Cooling Fan

The left figure  shows the nonlinear relationship between airflow and static pressure for a typical fan.