KNOWN ISSUE: Stream tubes/ribbons can appear twisted
In many cases, CFD-Post can produce stream-tube and stream-ribbon plots that show a flow rotation in the direction of the stream which is not present. The effect manifests itself as an abrupt necking of the tube or ribbon, even when there is irrotational flow.
There are several areas in the code where the local streamline orientation was being calculated incorrectly.
No workaround is currently available.
ANSYS CFX Release 12.1.