How to write correct math expression for this body force in Fenics

Question

I am trying to imply the following body force on a cube. But the code gives me an error for what I have scripted. Is there something wrong with what I have written? Is there a more efficient/ correct way?

enter image description here

r1 = 5.0/6.0-DOLFIN_EPS;

if x[0] >= r1: 
 f= Expression(("0.0", "0.0", "-10*exp(2*x[2]+x[1])"))
else:
 f= Expression(("0.0", "0.0", "0.0"))

tianyikillua · Answer 1 · Sep 15, 2015

Best answer

Try this

f =  Expression(("0.0", "0.0", "x[0] >= 5.0/6.0 ? -10*exp(2*x[2]+x[1]) : 0.0"))

answered Sep 15, 2015 by tianyikillua FEniCS User (1,620 points)
selected Sep 16, 2015 by Chaitanya_Raj_Goyal

No, doesn't work. I have the entire code posted here: ( carvedcube.py)

https://github.com/ChaitanyaGoyal/Hollow_Cube_Problem

commented Sep 15, 2015 by Chaitanya_Raj_Goyal FEniCS User (4,150 points)

It is x[0] > = 5.0/6.0 and not x > 5.0/6.0...I corrected my answer 3 sec later...you were fast

commented Sep 15, 2015 by tianyikillua FEniCS User (1,620 points)

Yes, I tried the same thing at the posted link, didn't work :(

Debugging can be so irritating sometimes.

commented Sep 16, 2015 by Chaitanya_Raj_Goyal FEniCS User (4,150 points)

I ran your script. There was an error in your dimension definition. Replace w.cell().d by mesh.geometry().dim().

commented Sep 16, 2015 by tianyikillua FEniCS User (1,620 points)

Thanks tianyikilua, for taking the time to help me out.

Did the script run for you with that change?

I got this error:

Calling DOLFIN just-in-time (JIT) compiler, this may take some time.
In instant.recompile: The module did not compile with command 'make VERBOSE=1', see '/home/page/.instant/error/dolfin_compile_code_9a66ddcce42106682a98919ce5accc6b9fa05991/compile.log'
Traceback (most recent call last):
  File "carvedcube2.py", line 15, in <module>
    b = Expression(("x >= 0.8 ? -10*exp(2*x[2]+x[1]) : 0.0", "0.0", "0.0"))
  File "/usr/lib/python2.7/dist-packages/dolfin/functions/expression.py", line 602, in __new__
    mpi_comm=mpi_comm)
  File "/usr/lib/python2.7/dist-packages/dolfin/compilemodules/expressions.py", line 217, in compile_expressions
    mpi_comm=mpi_comm)
  File "/usr/lib/python2.7/dist-packages/dolfin/compilemodules/expressions.py", line 145, in compile_expression_code
    mpi_comm=mpi_comm)
  File "/usr/lib/python2.7/dist-packages/dolfin/compilemodules/jit.py", line 64, in mpi_jit
    return local_jit(*args, **kwargs)
  File "/usr/lib/python2.7/dist-packages/dolfin/compilemodules/compilemodule.py", line 458, in compile_extension_module
    **instant_kwargs)
  File "/usr/lib/python2.7/dist-packages/instant/build.py", line 563, in build_module
    recompile(modulename, module_path, new_compilation_checksum, build_system)
  File "/usr/lib/python2.7/dist-packages/instant/build.py", line 165, in recompile
    instant_error(msg % (cmd, compile_log_filename_dest))
  File "/usr/lib/python2.7/dist-packages/instant/output.py", line 85, in instant_error
    raise RuntimeError(text)
RuntimeError: In instant.recompile: The module did not compile with command 'make VERBOSE=1', see '/home/page/.instant/error/dolfin_compile_code_9a66ddcce42106682a98919ce5accc6b9fa05991/compile.log'

commented Sep 16, 2015 by Chaitanya_Raj_Goyal FEniCS User (4,150 points)

Try do a instant-clean in your terminal.

With f = Expression(("0.0", "0.0", "x[0] >= 5.0/6.0 ? -10*exp(2*x[2]+x[1]) : 0.0")) (note x[0]) and mesh.geometry().dim() I was able to run your script.

commented Sep 16, 2015 by tianyikillua FEniCS User (1,620 points)

Thanks a lot tianyikillua for your guidance!

commented Sep 16, 2015 by Chaitanya_Raj_Goyal FEniCS User (4,150 points)

hernan_mella · Answer 2 · Sep 16, 2015

Try something like this:

f = Expression(("0.0", "0.0", "-10*exp(2*x[2]+x[1])*(x[0]>=(5/6))"))

answered Sep 16, 2015 by hernan_mella FEniCS Expert (19,460 points)

How to write correct math expression for this body force in Fenics

2 Answers