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# EOS9:FOLLOWING TWO STEPS THAT CONVERGED ON ITER = 1

Hello,

It looks like I am running into this exact issues described on page 10 of the Tough2 user manual (copied below). What suggestions do you have when this occurs?

Thanks

Zhouxiaoxiao

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From Tough2 manual

Another termination criterion is monitored when automatic time step control is in effect. In
this case execution will be stopped when convergence failure is encountered following two time
steps for which convergence on ITER = 1 was achieved. The rationale for this is as follows. As
conditions approach steady state, the rates of change in thermodynamic variables become small, and
time steps will grow to very large values when automatic time step control is in effect. The offdiagonal elements in the Jacobian matrix are proportional to time step size, and may become so
large that convergence of the linear equation system may no longer be achieved because of
numerical roundoff. Thus we may have a situation where for a somewhat reduced time step
convergence is achieved without requiring a linear equation solution (ITER = 1), while no
convergence at all can be obtained when a linear equation solution is required. The aforementioned
stopping criterion was designed to prevent such “no progress” calculations to go on and on for a
potentially large number of user-specified time steps.

From my output file

...ITERATING...  AT [1129,  9] --- DELTEX = 0.360000E+02   MAX. RES. = 0.101811E-05  AT ELEMENT A3455  EQUATION   1
+++++++++ REDUCE TIME STEP AT (1129,  9) ++++++++++++++++   NEW DELT = 0.900000E+01
A6U42(1129,  1) ST = 0.234675E+07 DT = 0.900000E+01 DX1= 0.000000E+00 DX2= 0.000000E+00 T =  15.000 P =        0. S = 0.384110E+00
A6U42(1130,  1) ST = 0.234677E+07 DT = 0.180000E+02 DX1= 0.000000E+00 DX2= 0.000000E+00 T =  15.000 P =        0. S = 0.384110E+00
...ITERATING...  AT [1131,  1] --- DELTEX = 0.360000E+02   MAX. RES. = 0.177317E-05  AT ELEMENT A6U42  EQUATION   1
...ITERATING...  AT [1131,  2] --- DELTEX = 0.360000E+02   MAX. RES. = 0.102159E-05  AT ELEMENT A3458  EQUATION   1
...ITERATING...  AT [1131,  3] --- DELTEX = 0.360000E+02   MAX. RES. = 0.101811E-05  AT ELEMENT A3455  EQUATION   1
...ITERATING...  AT [1131,  4] --- DELTEX = 0.360000E+02   MAX. RES. = 0.102005E-05  AT ELEMENT A3458  EQUATION   1
...ITERATING...  AT [1131,  5] --- DELTEX = 0.360000E+02   MAX. RES. = 0.101811E-05  AT ELEMENT A3455  EQUATION   1
...ITERATING...  AT [1131,  6] --- DELTEX = 0.360000E+02   MAX. RES. = 0.102005E-05  AT ELEMENT A3458  EQUATION   1
...ITERATING...  AT [1131,  7] --- DELTEX = 0.360000E+02   MAX. RES. = 0.101811E-05  AT ELEMENT A3455  EQUATION   1
...ITERATING...  AT [1131,  8] --- DELTEX = 0.360000E+02   MAX. RES. = 0.102005E-05  AT ELEMENT A3458  EQUATION   1
...ITERATING...  AT [1131,  9] --- DELTEX = 0.360000E+02   MAX. RES. = 0.101811E-05  AT ELEMENT A3455  EQUATION   1

+++++++++ CONVERGENCE FAILURE ON TIME STEP #1131 WITH DT = 0.360000E+02 SECONDS, FOLLOWING TWO STEPS THAT CONVERGED ON ITER = 1
STOP EXECUTION AFTER NEXT TIME STEP

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