Figure 2: Calculating the fraction of the area of the rotor circle (red) contained between two pressure levels (green). In this example, the hub of the rotor lies between the pressure levels. The area above the hub is sum of two triangles and two sectors. The area of the triangles sum to . The area of the two sectors sum to In this example, the area below the hub makes a similar positive contribution. If the hub is not between the layers, the total area is given by the subtraction of two areas calculated from the hub. Likewise, if , then is replaced by in the calculation.