How to calculate the normal of a 3D surface mesh

General Discussion
1

Dear Freefem team, is there any way we can calculate the normal of the surface mesh? I also attached the files here. thank you so much.
9T_MOTOR_R.mesh (229.0 KB)

surfacemesh
surfacemesh1262×624 8.09 KB

surface_mesh.edp (216 Bytes)