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Expose theta on SemiLagrangian DDt for BE/CN choice #187

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Merged
lmoresi merged 3 commits into from
May 14, 2026
Merged

Expose theta on SemiLagrangian DDt for BE/CN choice #187

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379cdde
Expose theta on SemiLagrangian DDt for BE/CN choice on AM flux
lmoresi May 14, 2026
290951b
Address Copilot review on PR #187: docstring + regression test
lmoresi May 14, 2026
4e9f8df
Merge development (post-#186 merge) — combine theta and monotone_mode…
lmoresi May 14, 2026
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17 changes: 15 additions & 2 deletions src/underworld3/systems/ddt.py
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Original file line number Diff line number Diff line change
Expand Up @@ -1172,6 +1172,7 @@ def __init__(
smoothing=0.0,
preserve_moments=False,
with_forcing_history: bool = False,
theta: float = 0.5,
):
super().__init__()

Expand All @@ -1185,6 +1186,18 @@ def __init__(
self.order = order
self.preserve_moments = preserve_moments
self.with_forcing_history = with_forcing_history
# Adams-Moulton θ for the implicit flux at order 1.
# The order-1 AM coefficients are ``[θ, 1-θ]``:
# θ=0.5 → Crank-Nicolson (A-stable, 2nd order accuracy on
# flux; NOT L-stable — stiff modes get amplification
# factor → -1, can ring on under-resolved sharp
# gradients in deformed cells)
# θ=1.0 → Backward Euler (L-stable, monotone for diffusion;
# 1st order accuracy on flux)
# Default 0.5 preserves the legacy SLCN behaviour.
# Settable after construction:
# ``adv_diff.DuDt.theta = 1.0``
self.theta = float(theta)

# Forcing-history storage. Allocated only if requested. Populated
# each step via update_forcing_history(forcing_fn) — used by ETD-2
Expand Down Expand Up @@ -1295,7 +1308,7 @@ def __init__(
self._exp_coeffs = _create_exp_coefficients(self.instance_number)
# Initialise to order-1 / viscous values
_update_bdf_values(self._bdf_coeffs, 1, None, [])
_update_am_values(self._am_coeffs, 1, 0.5)
_update_am_values(self._am_coeffs, 1, self.theta)
_update_exp_values(self._exp_coeffs, None, None)

# Working variable that has a potentially different discretisation from psi_star
Expand Down Expand Up @@ -1671,7 +1684,7 @@ def update_pre_solve(

# Update coefficient values for current effective_order and dt
_update_bdf_values(self._bdf_coeffs, self.effective_order, self._dt, self._dt_history)
_update_am_values(self._am_coeffs, self.effective_order, 0.5)
_update_am_values(self._am_coeffs, self.effective_order, self.theta)

## Progress from the oldest part of the history
# 1. Copy the stored values down the chain in preparation for the next timestep
Expand Down
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