Hi Soha, Sorry for a slow response. I believe Robin (cc’ed) has been doing something similar recently so may be able to help with this question. Cheers, Spencer. On 8 May 2019, at 17:12, Soha Nirumandi <soha_nirumandi@yahoo.com<mailto:soha_nirumandi@yahoo.com>> wrote: Dear all, I would like to implement a kind of boundary condition on wall. I need to calculate the velocity gradients over wall elements. The same as the high order pressure boundary condition, I want to calculate the velocity gradient on each wall elements with the following command: ( elmt>PhysDeriv(MultiRegions::DirCartesianMap[j],velocity,grad)) then I extract the gradients value of the boundary with the following command: ( elmt->GetEdgePhysVals(boundary,bc,grad,grad boundary ), consider that I just brought the main sentence and these variables should be define. Due to the coordinates, I know that I could reach the wall nodes with these commands, The only problem is with the value of velocity gradients. Regarding the 3d mesh elements, tetrahedron or hexahedron, the values can not get their correct value. For example in a hexahedron mesh I get the wrong values of gradients on the boundaries of the wall elements. Besides, in the tetrahedron mesh, in some elements the gradients can be calculated correctly and in some can not and the boundary problem still exists. Is there any problem with the above commands? Is there any special method or library available for correcting the wall elements boundary values? Can the problem stem from the mesh type? Regards, Soha _______________________________________________ Nektar-users mailing list Nektar-users@imperial.ac.uk<mailto:Nektar-users@imperial.ac.uk> https://mailman.ic.ac.uk/mailman/listinfo/nektar-users Spencer Sherwin FREng, FRAeS Head, Aerodynamics, 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/