Unable to define nonlinear problem

Errors
1

Hey there,

I just started with FENICS. I copied this program from a tutorial, but it doesn’t work for me:

PROGRAM

from fenics import *
from dolfin import *

dimensiones y numero de puntos en x y en y

L=25.
H=1.
Nx=250
Ny=10

mallado

mesh = RectangleMesh(Point(0., 0.), Point(L,H), Nx, Ny, “crossed”)

desplazamientos en 2d (viga plana) la derivada de los desplazamientos son las deformaciones (eps)

def eps(v):
return sym(grad(v))

parámetros ed la viga

E = Constant(1e5)
nu = Constant(0.3)
model = “plane_stress”

mu = E/2/(1+nu)
lmbda = Enu/(1+nu)/(1-2nu)
if model == “plane_stress”:
lmbda = 2mulmbda/(lmbda+2*mu)

def sigma(v):
return lmbdatr(eps(v))Identity(2) + 2.0mueps(v)

consideraremos polinomio de grado 2 para interpolar y una carga distribuida uniformemente de valores (0,-f)

carga = 1e-3
f = Constant((0, -carga))

cálculo de la solución

V = VectorFunctionSpace (mesh, ‘Lagrange’, degree=2)
du = TrialFunction(V)
u_ = TestFunction(V)
a = inner(sigma(du),eps(u_))*dx
l = inner(f,u_)*dx

#cálculo

def left(x, on_boundary):
return near(x[0], 0.)

bc = DirichletBC(V, Constant((0.,0.)), left)

u = Function(V, name=“Displacement”)
solve (a==1, u, bc)

plot(1e3*u, mode=“displacement”)

The error I get is

*** Error: Unable to define nonlinear variational problem F(u; v) = 0 for all v.
*** Reason: Expecting the residual F to be a linear form (not rank 2).
*** Where: This error was encountered inside NonlinearVariationalProblem.cpp.
*** Process: 0

I’ve read the problem is because I’m using a bad trial function, but how to fix it?

Thanks

2

as far as i can tell, this should be a==l, not a==1.

For any further questions please encapsulate your code with 3x`, such as

```python
solve(a==l, u, bc)
```
3

Sorry for not using 3x’, I didn’t know it before!

It seems like the problem is solved. Why can it be that I can’t plot the solution?

Object cannot be plotted directly, projecting to piecewise linear

Thanks again.

4

As the error message states, matplotlib only supports first order functions. To plot higher order functions, like P2, use XDMFFile.write_checkpoint and use paraview to open the resulting file