Quad Mesh Fails during evaluation on coordinates

1

Hello everyone,

I’m trying to create a trial demo as it is showed in Cahn-Hilliard demo of FEniCS.

https://fenicsproject.org/docs/dolfin/2018.1.0/python/demos/cahn-hilliard/demo_cahn-hilliard.py.html

I’m trying quad elements in elasticity and when i try to compile u(coord_x,coord_y) in 2D it gives error as following:

*** DOLFIN encountered an error. If you are not able to resolve this issue
*** using the information listed below, you can ask for help at

*** fenics-support@googlegroups.com

*** Remember to include the error message listed below and, if possible,
*** include a minimal running example to reproduce the error.

*** -------------------------------------------------------------------------
*** Error: Unable to intersect cell and point.
*** Reason: Intersection is only implemented for simplex meshes.
*** Where: This error was encountered inside Cell.cpp.
*** Process: 0

*** DOLFIN version: 2018.1.0

but it works fine if I used triangular elements. Do you suggest any method to evaluate a results in terms of displacement (u) at a specific coordinates in quad mesh ?

Thanks.

2

Hi,
As the error says, this is not implemented for quadrilateral and hexahedral cells.
Most likely, this feature will be added to dolfinx later this year

3

@CMA
Quad collision has now been added to dolfinx making it possible to do evaluation for quadrilaterals.

4

Hi, dokken

I am using the C++ version of FEniCS. I noticed methods such as evaluate_basis are called when a Function object evaluate values. Are these methods neccessary when a function is evaluated? Are there some unimaginable errors if I bypass these methods and use coordinate_dofs and dofs to evaluate a function directly?
I do not know how to write evaluate_basis for hexahedron by myself but I want to evaluate a function on hexahedron mesh.

Thank you very much!

5

If you are only interested in the function values at physical coordinates where there are degrees of freedom, you can access the function vector directly.

u=Function(V)
arr= u.vector().get_local()

The order in the array follows
X= V.tabulate_dof_coordinates()
Evaluate basis is used because if you evaluate a function at a non-dof coordinate, a linear combination of dof values has to be made

6

Thank you very much.

It seems unavoidable. Hope evaluation for function in hexahedron mesh in the future.

7

After reading more source code, I tried to write a hexahedron evaluation function. My program dose not work inc the fashion of fenics and only implemented for box cell not all hexahedron cell. However, It is enough for me!