$$\zeta=\left(1 - \frac{A_1}{A_2}\right)^2 $$ $A_1$ and $A_2$ are inlet and outlet area of the element. The inlet area is calculated based on the profile of the element, where as the outlet area is calculated using the provided diamter.
Parameter | Symbol | Unit | Description |
diameter | \(D_2\) | m | (new) diameter after the extension |