Very stiff problem solver about mpet HOT 7 OPEN

Does initialization succeed for a smaller applied current? How much overpotential is required?

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The initialization seems working for small currents (0.1C) but the first time step is not computed by the solver ("Miscellanous exception in function: dae::solver::daeIDASolver::Solve, source file: .\ida_solver.cpp, line: 1159
Sundials IDAS solver cowardly failed to solve the system at TIME = 0.00121158; time horizon [2.30984]; IDA_LSETUP_FAIL
IDAS solver error in module 'IDAS' in function 'IDASolve': At t = 0.00121158, the linear solver setup failed unrecoverably. [IDA_LSETUP_FAIL]")
To be more clear about the code here is an example:
alpha = 0.5
sqrt_eq_K = 1e-5
kinterlayer = 0.0000001
gamma_ts = 1./((1 - c1)**2 * (1 - c2)**2)
interLayerRxn = (kinterlayer/gamma_ts)(sqrt_eq_Kact1R-act2R/sqrt_eq_K)
# interLayerRxn = 0
RxnTerm1 = -interLayerRxn
RxnTerm2 = interLayerRxn
RHS1 += stech_1RxnTerm1
RHS2 += stech_2RxnTerm2

It's a simple modification of the reaction term of the graphite, but in this case the mu_0 of the two phases is different so we have this sqr(K) that is quite high in one direction. This leads to a very high reaction rate which is not well computed if k_0 is more than 1e-7, when physically it should be at least 1e-1 or more. I can try to rethink about the physics of the system, but in general this seems a good occasion to understand the limits of the solver and possible solutions.
From what I know a backward solver should be stable also for very stiff problems, but in this case it seems differently.

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Hi @Ombrini, in your example how are the variables sqrt_eq_Kact1R, stech_1RxnTerm1, and stech_2RxnTerm2 defined? Sorry for the delay, finally getting caught up after vacation.

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Hi @d-cogswell, Copying and pasting the "" was cancelled.
The equations are:
interLayerRxn = (kinterlayer/gamma_ts)(sqrt_eq_Kact1R-act2R/sqrt_eq_K)
RHS1 = stech_1*RxnTerm1 (where stech_1 = 0.5).

It seems to depend on the characteristics time of the system. In particular the ratio between the t0 given by the cathode length and 1/k0, so the reaction time. If this ratio is too big the program seems not able to reduce the stepsize efficiently when the reaction is starting. I guess it's mostly a matter of steptime tentative.
Maybe there are some daetools setting that can help for this kind of problems.
Thank you.

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So is this the model you are trying to solve below? Is k0 defined by the variable sqrt_eq_K?

            sqrt_eq_K = 1e-5
            kinterlayer = 0.0000001
            gamma_ts = 1./((1 - c1)**2 * (1 - c2)**2)
            interLayerRxn = (kinterlayer/gamma_ts)*(sqrt_eq_K*act1R-act2R/sqrt_eq_K)
            stech_1 = 0.5
            stech_2 = 0.5
            RxnTerm1 = -interLayerRxn
            RxnTerm2 = interLayerRxn
            RHS1 += stech_1*RxnTerm1
            RHS2 += stech_2*RxnTerm2

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Yes it is the model,
The k0 is kinterlayer and the sort_eq_K is a constant which multiplies the activity 1 act1R and divide the activity 2 act2R.
Those lines would be posed in the line 261 of mod_electrodes of the current main.
It's basically solving the CHR2 model with a strong interlayer diffusivity (due to the fact that in this case it's an olivine structure).
To be more clear I can rewrite the line as:

interLayerRxn = ((1 - c1)**2 * (1 - c2)**2)kinterlayer(1e-5act1R-1e5act2R)

When act1R will grow a lot, and c1 will be close to 1 the interLayerRxn becomes positive, otherwise it's negative.
This basically state equilibrium between the act1 and act2.

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Using smaller error tolerances such as relTol = 1e-8 and absTol=1e-8 helps a bit. My simulation almost finishes but encounters a liner solver error.

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