[Getfem-users] Assembly of 1D Poiseuille term

[Domenico Notaro]

Dear Users,

I am studying a coupled 3D-1D problem, for the sake of simplicity let's imagine a straight line immersed in a box.

I am wondering how to assemble the matrix corresponding to the weak formulation of the Poiseuille term in the 1D problem, -(p, du/ds), being p the pressure of the fluid inside the segment, u the velocity and s the arc length along the segment. I have to idea how to write the derivative inside the generic_assembly since the only way I know to compute derivate is "Gradient(#..)" that is a vector. 

Do I need to project the gradient along the direction of the segment? If this is the case, I don't know how to compute the tangent versor.

Thank you in advance.

Regards,

domenico_notaro