### Solving simple non-linear diffusion equation

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Hi everyone,

I am trying to solve a simple non-linear diffusion equation. I need to use solve(F==0, bc, V). But it returns the error:

Solving nonlinear variational problem.

Newton iteration 0: r (abs) = 7.263e-02 (tol = 1.000e-10) r (rel) = 1.000e+00 (tol = 1.000e-09)

Traceback (most recent call last):

File "guassian.py", line 44, in <module>

solve(F==0, u, bc)

File "/usr/lib/python2.7/dist-packages/dolfin/fem/solving.py", line 300, in solve

_solve_varproblem(*args, **kwargs)

File "/usr/lib/python2.7/dist-packages/dolfin/fem/solving.py", line 349, in _solve_varproblem

solver.solve()

RuntimeError:

*** -------------------------------------------------------------------------

*** 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 successfully call PETSc function 'MatSetValuesLocal'.

*** Reason: PETSc error code is: 63 (Argument out of range).

*** Where: This error was encountered inside /build/dolfin-yRhxwC/dolfin-2017.1.0/dolfin/la/PETScMatrix.cpp.

*** Process: 0

***

*** DOLFIN version: 2017.1.0

The same error appears when I try to execute the (very slight) modified version of the heat equation example found in the FEniCS tutorial (code attached)

from fenics import *

import time

T = 2.0 # final time

num_steps = 50 # number of time steps

dt = T / num_steps # time step size

# Create mesh and define function space

nx = ny = 30

mesh = RectangleMesh(Point(-2, -2), Point(2, 2), nx, ny)

V = FunctionSpace(mesh, 'P', 1)

# Define boundary condition

def boundary(x, on_boundary):

return on_boundary

bc = DirichletBC(V, Constant(0), boundary)

# Define initial value

u_0 = Expression('exp(-a*pow(x[0], 2) - a*pow(x[1], 2))',

degree=2, a=5)

u_n = interpolate(u_0, V)

# Define variational problem

u = Function(V)

v = TestFunction(V)

f = Constant(0)

F = u*v*dx + dt*dot(grad(u), grad(v))*dx - (u_n + dt*f)*v*dx

#a, L = lhs(F), rhs(F)

# Create VTK file for saving solution

vtkfile = File('heat_gaussian/solution.pvd')

# Time-stepping

u = Function(V)

t = 0

for n in range(num_steps):

# Update current time

t += dt

# Compute solution

solve(F==0, u, bc)

# Save to file and plot solution

vtkfile << (u, t)

plot(u)

# Update previous solution

u_n.assign(u)

# Hold plot

interactive()

Can anybody help me?

Zhen

I am trying to solve a simple non-linear diffusion equation. I need to use solve(F==0, bc, V). But it returns the error:

Solving nonlinear variational problem.

Newton iteration 0: r (abs) = 7.263e-02 (tol = 1.000e-10) r (rel) = 1.000e+00 (tol = 1.000e-09)

Traceback (most recent call last):

File "guassian.py", line 44, in <module>

solve(F==0, u, bc)

File "/usr/lib/python2.7/dist-packages/dolfin/fem/solving.py", line 300, in solve

_solve_varproblem(*args, **kwargs)

File "/usr/lib/python2.7/dist-packages/dolfin/fem/solving.py", line 349, in _solve_varproblem

solver.solve()

RuntimeError:

*** -------------------------------------------------------------------------

*** 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 successfully call PETSc function 'MatSetValuesLocal'.

*** Reason: PETSc error code is: 63 (Argument out of range).

*** Where: This error was encountered inside /build/dolfin-yRhxwC/dolfin-2017.1.0/dolfin/la/PETScMatrix.cpp.

*** Process: 0

***

*** DOLFIN version: 2017.1.0

The same error appears when I try to execute the (very slight) modified version of the heat equation example found in the FEniCS tutorial (code attached)

from fenics import *

import time

T = 2.0 # final time

num_steps = 50 # number of time steps

dt = T / num_steps # time step size

# Create mesh and define function space

nx = ny = 30

mesh = RectangleMesh(Point(-2, -2), Point(2, 2), nx, ny)

V = FunctionSpace(mesh, 'P', 1)

# Define boundary condition

def boundary(x, on_boundary):

return on_boundary

bc = DirichletBC(V, Constant(0), boundary)

# Define initial value

u_0 = Expression('exp(-a*pow(x[0], 2) - a*pow(x[1], 2))',

degree=2, a=5)

u_n = interpolate(u_0, V)

# Define variational problem

u = Function(V)

v = TestFunction(V)

f = Constant(0)

F = u*v*dx + dt*dot(grad(u), grad(v))*dx - (u_n + dt*f)*v*dx

#a, L = lhs(F), rhs(F)

# Create VTK file for saving solution

vtkfile = File('heat_gaussian/solution.pvd')

# Time-stepping

u = Function(V)

t = 0

for n in range(num_steps):

# Update current time

t += dt

# Compute solution

solve(F==0, u, bc)

# Save to file and plot solution

vtkfile << (u, t)

plot(u)

# Update previous solution

u_n.assign(u)

# Hold plot

interactive()

Can anybody help me?

Zhen

Community: FEniCS Project

### 1 Answer

4

Just remove the second definition of the function

`u`

in the section` # Time-stepping`

Thank you very much!!!

written
7 months ago by
zhen shao

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