[BUG] -AttributeError: 'NaturalBC' object has no attribute 'fn_F' when using add_natural_bc

[Belay-129]

Hi uw3 support team,
I encountered an error when applying a natural boundary condition to the Poisson solver in Underworld3, which prevents the solve from running successfully.

poisson.add_dirichlet_bc([17.0], "Top")
q_bottom = 0.065 # W/m2
poisson.add_natural_bc([q_bottom], "Bottom")

poisson.solve()

When applying a natural boundary condition to a Poisson solver, the following error occurs:

AttributeError Traceback (most recent call last)
Cell In[18], line 1
----> 1 poisson.solve()

File src/underworld3/cython/petsc_generic_snes_solvers.pyx:872, in underworld3.cython.generic_solvers.SNES_Scalar.solve()

File src/underworld3/cython/petsc_generic_snes_solvers.pyx:194, in underworld3.cython.generic_solvers.SolverBaseClass._build()

File src/underworld3/cython/petsc_generic_snes_solvers.pyx:744, in underworld3.cython.generic_solvers.SNES_Scalar._setup_pointwise_functions()

AttributeError: 'NaturalBC' object has no attribute 'fn_F'

Thanks for your time and help

[gthyagi]

Hi @Belay-129,

Please attach the example script you are running so we can debug. It also looks like you’re using an older version of Underworld, please update to the latest release, as some of these issues are already fixed.

Cheers,
Thyagi

[Belay-129]

Hi Thyagi, Thank you for your response. Could you please confirm the current version of Underworld? I checked on GitHub and found the version listed as: __version__ = "0.99.0b" This matches the version I already have installed. Cheers, Belay

…

________________________________ From: Thyagarajulu Gollapalli ***@***.***> Sent: Tuesday, September 23, 2025 11:54 AM To: underworldcode/underworld3 ***@***.***> Cc: Belay Mino ***@***.***>; Mention ***@***.***> Subject: Re: [underworldcode/underworld3] [BUG] -AttributeError: 'NaturalBC' object has no attribute 'fn_F' when using add_natural_bc (Issue #25) [https://avatars.githubusercontent.com/u/44185316?s=20&v=4]gthyagi left a comment (underworldcode/underworld3#25)<#25 (comment)> Hi @Belay-129<https://github.com/Belay-129>, Please attach the example script you are running so we can debug. It also looks like you’re using an older version of Underworld, please update to the latest release, as some of these issues are already fixed. Cheers, Thyagi — Reply to this email directly, view it on GitHub<#25 (comment)>, or unsubscribe<https://github.com/notifications/unsubscribe-auth/BR5VEZYTWDBVDZVA7VR4PBT3UCR6XAVCNFSM6AAAAACHEFJ7TKVHI2DSMVQWIX3LMV43OSLTON2WKQ3PNVWWK3TUHMZTGMRSGA4TQMZRGI>. You are receiving this because you were mentioned.Message ID: ***@***.***>

[Belay-129]

'''
mesh = uw.meshing.UnstructuredSimplexBox(
minCoords=(423755, -10000),
maxCoords=(496255, 0),
cellSize=1000,
regular=False,
)
'''

'''
k_scalar = uw.discretisation.MeshVariable("k", mesh, 1)
'''

'''
T_soln = uw.discretisation.MeshVariable("T", mesh, 1)
'''

'''
Set up the Poisson system
poisson = uw.systems.Poisson(mesh, u_Field=T_soln)

poisson.constitutive_model = uw.constitutive_models.DiffusionModel

poisson.constitutive_model.Parameters.diffusivity = k_scalar.sym[0]

poisson.f = 0
'''

BCs

'''
poisson.add_dirichlet_bc([17.0], "Top")
q_bottom = 0.065 #W/m2
poisson.add_natural_bc([q_bottom], "Bottom")
'''

solve
poisson.solve()

[gthyagi]

Hi Thyagi,
Thank you for your response. Could you please confirm the current version of Underworld? I checked on GitHub and found the version listed as:
version = "0.99.0b"

This matches the version I already have installed.
Cheers,
Belay
…

The version number may look the same, but we’ve recently pushed many important changes.

[jcgraciosa]

Hello @gthyagi and @Belay-129, I found the cause of the issue and I'm investigating a fix.
I'll give you some updates soon.

Cheers,
JC

[Belay-129]

Thank you Juan for updates. Looking forward to hearing from you. Thanks

…

________________________________ From: jcgraciosa ***@***.***> Sent: Tuesday, September 23, 2025 2:12 PM To: underworldcode/underworld3 ***@***.***> Cc: Belay Mino ***@***.***>; Mention ***@***.***> Subject: Re: [underworldcode/underworld3] [BUG] -AttributeError: 'NaturalBC' object has no attribute 'fn_F' when using add_natural_bc (Issue #25) [https://avatars.githubusercontent.com/u/12045001?s=20&v=4]jcgraciosa left a comment (underworldcode/underworld3#25)<#25 (comment)> Hello @gthyagi<https://github.com/gthyagi> and @Belay-129<https://github.com/Belay-129>, I found the cause of the issue and I'm investigating a fix. I'll give you some updates soon. Cheers, JC — Reply to this email directly, view it on GitHub<#25 (comment)>, or unsubscribe<https://github.com/notifications/unsubscribe-auth/BR5VEZ5B3PTMN5G3HOA3BGL3UDCELAVCNFSM6AAAAACHEFJ7TKVHI2DSMVQWIX3LMV43OSLTON2WKQ3PNVWWK3TUHMZTGMRSGM2DQNBVGI>. You are receiving this because you were mentioned.Message ID: ***@***.***>