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>Xcoordinate, 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>Isovalue. 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. 

