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= Why is my HPC model getting unstable right at the beginning? And why are my control numbers lower than they could be? =
If there are no wet cells at the beginning of the simulation, the adaptive timestep can get quite big. There might possibly be some NaNs in the model as well. Once the flow increases rapidly, twoinstabilities can develop, which leads to oscillations in variables that grow over time, eventually leading to NaNs in the solution. Two situations can occur:
* The solver can't reduce the big timestep to a sufficiently small timestep within the ten default trials and, the simulation gets unstable which stops the model.
* The solver is able to reduce the timestep to a sufficient timestep within the ten default trials and the simulation continues running. This however comes with a price of slower run time.:
** TheCurrently calculationif ofa adaptivecontrol timestepnumber isexceed quiteits difficulttarget by up to 20%, the step is still accepted but basicallythe next timestep is factored down by the same percentage. whenIf there are NaNs or therecontrol isnumber aexceeds bigits jumptarget inby anymore ofthan 20%, the controlstep is numbersrejected, the timestep getsis significantlycut to half, a repeat step is reducedperformed and the control number will be cut to 90% of the previous value. The control number is then allowed to increase only by 0.001% per timestep. It will take 10,000 timesteps to creep back up to the original value if there are no other reduced timesteps, keeping the model running slower than it could.
The below suggestions can be implemented to eliminate the instability and/or the decrease in control numbers:
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