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Fix for NS solver #83

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Fix for NS solver #83

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0fc0132
Fix for NS solver
bknight1 Mar 18, 2026
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bknight1 Mar 18, 2026
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54 changes: 35 additions & 19 deletions src/underworld3/systems/solvers.py
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Original file line number Diff line number Diff line change
Expand Up @@ -2737,20 +2737,22 @@ def __init__(
### sets up DuDt and DFDt
## ._setup_history_terms()

# If DuDt is not provided, then we can build a SLCN version
# If DuDt is not provided, build it here. We prefer the Eulerian_DDt
# formulation over the SemiLagrangian alternative to keep the solver
# initialisation simpler and easier to maintain.
if self.Unknowns.DuDt is None:
self.Unknowns.DuDt = uw.systems.ddt.SemiLagrangian(
self.Unknowns.DuDt = uw.systems.Eulerian_DDt(
self.mesh,
self.u.sym, # Symbolic expression - SemiLagrangian evaluates this at each update
self.u.sym,
vtype=uw.VarType.VECTOR,
self.u,
vtype=self.u.vtype,
degree=self.u.degree,
continuous=self.u.continuous,
V_fn=self.u.sym,
varsymbol=self.u.symbol,
verbose=self.verbose,
bcs=self.essential_bcs,
order=self._order,
smoothing=0.0001,
smoothing=0.0,
verbose=self.verbose,
)

# F (at least for N-S) is a nodal point variable so there is no benefit
Expand All @@ -2760,19 +2762,33 @@ def __init__(
# Maybe u.degree-1. The scalar equivalent seems to show
# little benefit from specific choices here other than
# discontinuous flux variables amplifying instabilities.
if self.Unknowns.DFDt is None:
# self.Unknowns.DFDt = uw.systems.ddt.SemiLagrangian(
# self.mesh,
# sympy.Matrix.zeros(self.mesh.dim, self.mesh.dim),
# self.u.sym,
# vtype=uw.VarType.SYM_TENSOR,
# degree=self.u.degree,
# continuous=self.u.continuous,
# varsymbol=rf"{{ F[ {self.u.symbol} ] }}",
# verbose=self.verbose,
# bcs=None,
# order=self._order,
# )
self.Unknowns.DFDt = uw.systems.ddt.Eulerian(
self.mesh,
sympy.Matrix.zeros(self.mesh.dim, self.mesh.dim),
vtype=uw.VarType.SYM_TENSOR,
degree=self.u.degree,
continuous=self.u.continuous,
varsymbol=rf"{{ F[ {self.u.symbol} ] }}",
bcs=None,
order=self._order,
smoothing=0.0,
verbose=self.verbose,
)


self.Unknowns.DFDt = uw.systems.ddt.SemiLagrangian(
self.mesh,
sympy.Matrix.zeros(self.mesh.dim, self.mesh.dim),
self.u.sym,
vtype=uw.VarType.SYM_TENSOR,
degree=self.u.degree,
continuous=self.u.continuous,
varsymbol=rf"{{ F[ {self.u.symbol} ] }}",
verbose=self.verbose,
bcs=None,
order=self._order,
)

## Add in the history terms provided ...

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