Lag label#228
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danshapero
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May 15, 2026
pbrubeck
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May 15, 2026
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I just noticed that #227 might potentially be useful for similar purposes.
I think it's fine to support both modes. |
pbrubeck
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May 17, 2026
| final_time = 10.0 | ||
| num_steps = int(final_time / float(dt)) | ||
| for step in range(num_steps): | ||
| stepper.advance() |
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Can we add an assertion here?
pbrubeck
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May 18, 2026
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Co-authored-by: Pablo Brubeck <brubeck@protonmail.com>
pbrubeck
reviewed
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Co-authored-by: Pablo Brubeck <brubeck@protonmail.com>
pbrubeck
reviewed
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Co-authored-by: Pablo Brubeck <brubeck@protonmail.com>
pbrubeck
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May 18, 2026
Comment on lines
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| # A :class:`ufl.Label` to mark nodes that are only evaluated at the start of | ||
| # the timestep. | ||
| lag_label = Label() | ||
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| def lag(expr): | ||
| """Mark a sub-expression to be evaluated only at the start of the | ||
| timestep during the implicit solve.""" | ||
| return Variable(expr, lag_label) | ||
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This file does not seem the right place to define this. I think we should be creating a new file.
pbrubeck
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May 18, 2026
pbrubeck
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May 18, 2026
pbrubeck
reviewed
May 18, 2026
Co-authored-by: Pablo Brubeck <brubeck@protonmail.com>
pbrubeck
reviewed
May 18, 2026
pbrubeck
reviewed
May 18, 2026
Co-authored-by: Pablo Brubeck <brubeck@protonmail.com>
pbrubeck
reviewed
May 18, 2026
pbrubeck
reviewed
May 18, 2026
pbrubeck
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May 18, 2026
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Resolves #221. This PR adds a function
lagwhich labels a single expression in a form for holding back to the start of a timestep. As @pbrubeck thought, implementing it amounted to modifyingirksome.tools.replace. I added a test based on lagging the conductivity in the Stefan problem. Without lagging, that fails because of the conditional.In addition to giving you an escape hatch, you can use lagging to define nonlinear preconditioners. I have a demo of this for the Allen-Cahn equation coded up just using petsc4py which I think is pretty compelling. @JHopeCollins and I are working on getting AuxiliaryOperatorSNES merged into Firedrake: firedrakeproject/firedrake#5119. Once that's merged I can write an Irksome demo showing that off, either in this PR or a separate one.