fix(constitutive_model): make flux_jacobian settable for custom Jacobian tangents#362
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Pull request overview
This PR makes Constitutive_Model.flux_jacobian writable so callers can provide a custom SymPy expression used specifically for Jacobian/tangent assembly (while leaving the residual flux unchanged), enabling more robust/faster Newton solves in cases where differentiating the exact flux is problematic.
Changes:
- Added a
_flux_jacobianbacking attribute initialized toNoneinConstitutive_Model.__init__. - Updated
flux_jacobiangetter to return the backing value instead of always returningNone. - Added a
flux_jacobiansetter to store a user-provided expression.
Comments suppressed due to low confidence (1)
src/underworld3/constitutive_models.py:619
- Typo in the _reset() docstring: "consitutive" should be "constitutive". Since this hunk is being touched, it’s a good opportunity to correct the spelling to avoid propagating it into user-facing docs/help.
"""Flags that the expressions in the consitutive tensor need to be refreshed and also that the
solver will need to rebuild the stiffness matrix and jacobians"""
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| @flux_jacobian.setter | ||
| def flux_jacobian(self, value): | ||
| """Set a custom Jacobian flux expression (or None to revert).""" | ||
| self._flux_jacobian = value |
…ian tangents The flux_jacobian property on Constitutive_Model was read-only and always returned None. This prevented models from providing a simplified flux expression for the Jacobian independently of the exact residual flux. Now it has a getter/setter backed by an instance attribute. Default remains None (identical behaviour) — setting it to a SymPy expression and enabling consistent_jacobian = True on the solver gives the solver a custom expression to differentiate for the Jacobian tangent while the residual keeps the exact flux. Use cases: - Viscoplastic models with sharp yield kinks: supply a smooth constitutive law for the tangent only (Newton avoids Min-composite derivative discontinuities). - Multicomponent diffusion with composition-dependent coefficients: supply a constant-coefficient flux so the Jacobian gets a clean SPD stiffness matrix (D_ii · ∇·∇) instead of the zero-G3 from UWexpression-wrapped coefficients, reducing Newton iterations from ~50 to ~3.
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Summary
Makes
Constitutive_Model.flux_jacobiansettable (was read-only, always returnedNone).Motivation
The
flux_jacobianproperty is consumed by_newton_fluxin the Cython solver base class (used by bothSNES_ScalarandSNES_Stokes_SaddlePt) whenconsistent_jacobian=True. Despite this, the base class had no way to store a custom expression — subclasses had to override the property, andGenericFluxModel(the most common flux model) couldn't provide one at all.Use cases
Multicomponent diffusion (composition-dependent
D_ijcoefficients): The exact flux wrapped in UWexpressions becomes opaque to SymPy diff, givingG3 = 0(no stiffness matrix). A constant-coefficientflux_jacobianrestores the SPD stiffness term, dropping Newton iterations from ~50 to ~3.Viscoplasticity with sharp yield kinks: Supply a smooth constitutive law for the Jacobian tangent while the residual keeps the exact sharp-Min yield surface, avoiding derivative discontinuities in Newton.
Change
self._flux_jacobian = Noneto__init__self._flux_jacobian(defaultNone— identical behaviour)Default remains
None. Zero impact on existing models.