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## Re: [Getfem-users] Assembly of 1D Poiseuille term

 From: Yves Renard Subject: Re: [Getfem-users] Assembly of 1D Poiseuille term Date: Mon, 27 Jul 2015 16:23:28 +0200 (CEST)

```Dear Domenico,

In the two assembly languages, the gradient has indeed the dimension of the
mesh, i.e. a 3D one in this case. It is along the tangent direction of the line
in this case.
One way to obtain the tangent vector is to use "element_K" from the high-level
generic assembly (gradient of the geometric transformation).
The assembly string
should work. May be "element_K" have to be transposed.

Additionally, if you need to interpolate some field from the 3D mesh to the 1D
mesh and perform some matrix assembly, the use of the high-level generic
assembly is far more simple (see

Yves.

----- Original Message -----
Sent: Sunday, July 26, 2015 7:30:42 PM
Subject: [Getfem-users] Assembly of 1D Poiseuille term

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

Regards,

domenico_notaro

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