The solver prints the error message "GGI intersection error: The theta extent is too small", but none of the 3 suggested causes are applicable.

The reasons for this error message are typically:
1) surface with more than 360 degrees of revolution
2) surface intersects R=0
3) surface has faces normal AND parallel to the rotation axis
which are also printed in the error message. If none of these apply, then it might be the following reason.

This description is based on the assumption of an axial interface. The solver determines the pitch change based on the lowest radial line of the interface. The algorithm might fail when the radial thickness of the first element layer at this line is smaller than the radius differences along this line. The radius on the line might have small error due to mesh generation accuracy or geometry accuracy. For a very fine boundary layer resolution this error might be larger than the element height at this location.

Solution is:
1) correct solution: Check the geometry accuracy and the mesh generation accuracy and fix the mesh to obtain a constant radius at the low-radius edge of the interface.
2) workaround: Set the pitch angle on both sides of the interface explicitly in CFX-Pre. So in the Stage interface settings change the Pitch Change Option from "Automatic" to "Specified Pitch Angles".

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