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peteraylett

HPC adaptive timestepping control numbers

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Hi folks,

For context, I've an HPC model that's not happy and is getting a lot of repeated timesteps before death, so I've set "Control Number Factor == 0.8" to make sure that everything is solidly staying below the theoretical limits even when not actually exceeding the repeat timestep criteria. All well and good.

The thing that's got me asking is that it looks as though timesteps are being repeated when the control number limit is exceeded by only 10%, not the documented 20%. Is this supposed to be the case? In particular, it's Nd that's controlling my timestep most of the time, and it was repeating timesteps at around and in excess of 0.331 (here's a full sample line: "Repeating step 3729 due to high control numbers (0.549011 0.856216 0.336456). Reducing target timestep to 1.0738"), or after I've applied the Factor of 0.8 it's repeating at just, for example, 0.264801. In which case I'd be safe with  "Control Number Factor == 0.9", unless it's still 20% on the Nu and Nc and only 10% on Nd?

Perhaps I've missed something in the manual/release notes, but I've had a good look! I'm currently running 2018-03-AB.

PHA.

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