How to separate a cell zone into two parts with an arbitrarily oriented plane

Sometimes it is necessary in Fluent to separate a cell zone into two cell zones
by splitting it with an arbitrarily oriented plane (i.e. a plane that is not
parallel or perpendicular to any coordinate direction).

This can be accomplished using separation by mark as follows.
First we will create the mark (i.e. adaption register) and then do the separation.

1. First determine the direction cosines of the vector perpendicular
to the cutting plane (i.e. the plane by which you want to split the cell zone).
If you know three points on the plane, then it is easy to determine the
direction cosines of the vector perpendicular to the plane. Let the points
be A B C. The vector cross product of vector AB and vector BC is a vector
perpendicular to the plane containing A B and C.
2. Under Define->CustomFieldFunctions, create a customfieldfunction:
custom_field_function_0 = ax+by+cz
Here a, b, c are the direction cosines of the perpendicular vector that
you have determined in step 1. x,y,z are Grid->X-coordinate, etc.
3. Using the coordinates of one of the points A B or C, calculate the
the value of ax+by+cz.
4. Make sure that the case is initialized. Solve->Initialize->Initialize
5. Go to Adapt->Iso-value. Select the custom field function that you have
created under "adapt by". Under "min" type a number close to negative infinity.
Under "max" type the value that you calculated in step 3.
5. Click on mark.
6. Go to Grid->Separate->Cells. Select the desired cell zone.
Select "separate by mark" Then select the adaption register created
and click apply. The cell zone will now be separated into the two
desired parts.

Additional notes:
a) You may plot contours of this custom field function on a surface bounding the
original cell zone before splitting. This will help you to make sure that
the contour lines are parallel to the desired cutting plane.
b) You can use Fluent to calculate the vector cross product required in step 1 above.
Go to Surface->Plane. Click on plane tool and bounded plane. Type coordinates
of the 3 points A B C in the fields for xyz coordinates of 3 points in the panel.
Then go to Define->BoundaryConditions. Open the set BC panel for for any
fluid zone. Click on "Porous zone" . Click on "Update from Plane Tool."
The Direction1 vector is same as the vector perpendicular to the cutting plane.
Normalize the coefficients of Direction1 to obtain the direction cosines.

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