sym网格问题

在CFD前处理时，总是提示下列信息：
ERROR #002100013 has occurred in subroutine Chk_Splane. |
| Message: |
| The symmetry boundary condition requires that the boundary patch |
| mesh faces form a plane or axis. However, face set 28 in the |
| symmetry boundary patch |
| |
| sym |
| |
| is not in a strict plane, which means that at least one of its |
| faces is not parallel to the others
请问有可能出现的原因以及该如何解决。面映射已经试过了。