Hello Tim, Let your mesh object be called mesh, in this case FacetNormal(mesh) is the form expression that I think you need. Generally, one n = FacetNormal(mesh) and then just uses n in the form. Cheers, Miklos ________________________________ From: firedrake-bounces@imperial.ac.uk <firedrake-bounces@imperial.ac.uk> on behalf of Dodwell, Timothy <T.Dodwell@exeter.ac.uk> Sent: 25 August 2017 13:08:13 To: firedrake Subject: [firedrake] Normal to curved surface Hi all, New to firedrake, I am building a test problem for a new PhD which is poro-elastic consolidation of a sphere, I want to apply a pressure boundary (Neumann) condition to a curved surface. I.e \int (\sigma \dot n) \dot v\; ds Where n is the normal vector to the surface ds. Is there a function normal() which returns it for me. A look a documentation doesn’t look promising! Cheers Tim Dr Tim Dodwell Senior Lecturer in Engineering Mathematics Rm 221 - Harrison Building College of Engineering, Mathematics & Physical Sciences University of Exeter Exeter Devon EX4 4PY mail: t.dodwell(at)exeter.ac.uk<http://exeter.ac.uk/> tel: +44 (0)1392 725899 mob: +44 (0)7745 622870 web: http://emps.exeter.ac.uk/engineering/staff/td336 Papers and Pre-prints: @Research-Gate<https://www.researchgate.net/profile/Timothy_Dodwell> Citations: @Google-Scholar<https://scholar.google.co.uk/citations?user=lPpjRfUAAAAJ&hl=en>