HI Tangfei, In the attached test case which has all boundary conditions I have modified the PRIMCOEFF to a non-unit value and modified the exact value accordingly. It seems to run with a non-unit value. Cheers, Spencer. Spencer Sherwin FREng, FRAeS Head of Aerodynamics Section, Director of Research Computing Service, Professor of Computational Fluid Mechanics, Department of Aeronautics, s.sherwin@imperial.ac.uk<mailto:s.sherwin@imperial.ac.uk> South Kensington Campus, Phone: +44 (0)20 7594 5052 Imperial College London, Fax: +44 (0)20 7594 1974 London, SW7 2AZ, UK http://www.imperial.ac.uk/people/s.sherwin/ On 11 Jul 2020, at 13:16, zhangtengfei@sjtu.edu.cn<mailto:zhangtengfei@sjtu.edu.cn> wrote: This email from zhangtengfei@sjtu.edu.cn<mailto:zhangtengfei@sjtu.edu.cn> 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. Dear all, I am using the robin boundary condition in Nektar+ but have some problems understanding how it works. The b.c. I want to impose is D* gradient {u} + 0.5 * u = 0 in which D is the diffusion constant (same as the D value used in the adjacent element), u is the unknown field. To do this I normalize the b.c. into the form of gradient {u} + 0.5 / D * u = 0, and specify the robin boundary condition as <R VAR="u" PRIMCOEFF="0.5/D" /> This setting does not yield the correct answer, unless the special case that D = 1.0. Could anyone please tell me what is wrong here? Thanks, Tengfei Zhang </html_______________________________________________ Nektar-users mailing list Nektar-users@imperial.ac.uk<mailto:Nektar-users@imperial.ac.uk> https://mailman.ic.ac.uk/mailman/listinfo/nektar-users