[[Loss Coefficient]] $K_{SE} =\approx 2.61sin\theta\ \left(1-\frac{d^2}{D^2}\right) + f_{avg}\frac{L}{d_{avg}}$ for $\theta$ less than 22.5°. For more than that drop the 2.61 sine theta term and it goes to the equivalent of [[Sudden Expansion or Contraction#1. Sudden Expansion| sudden expansion]] loss 2-15° make the best diffusers. Above 15° there maybe flow separation