******************* This email originates from outside Imperial. Do not click on links and attachments unless you recognise the sender. If you trust the sender, add them to your safe senders list https://spam.ic.ac.uk/SpamConsole/Senders.aspx to disable email stamping for this address. ******************* Hello Kaloyan Kirilov, Thank you very much for providing those solutions. Unfortunately, I am still encountering the same error. I generated the mesh using gmsh, and for testing purposes, I simplified the mesh to the most basic case. Here is the content of the .geo file: SetFactory("OpenCASCADE"); Circle(1) = {0, 0, 0, 4, 0, Pi/6}; Circle(2) = {0, 0, 0, 14, 0, Pi/6}; Line(3) = {2, 4}; Line(4) = {1, 3}; Curve Loop(1) = {3, -2, -4, 1}; Plane Surface(1) = {1}; Transfinite Curve {3, 4} = 3 Using Progression 1.1; Transfinite Curve {1, 2} = 3 Using Progression 1; Transfinite Surface {1}; Recombine Surface {1};//+ Physical Curve("p1", 5) = {3}; Physical Curve("p2", 6) = {4}; Physical Curve("up", 7) = {1}; Physical Curve("up2", 8) = {2}; Physical Surface("plane", 9) = {1}; In this file, I want "p1" and "p2" to represent the periodic boundaries for rotation. Thank you once again for your guidance. Best regards, Yaguang.