VARIABLEDT
From MohidWiki
User's manual
To activate a variable time step bounded between a minimum and maximum DT, in configuration file Model_x.dat
VARIABLEDT : 1 DT : 10. MAXDT : 100.
A log file DTLog.txt is stored in the results folder /res of the main domain if the option DT_LOG is set in the nomfich.dat file in /exe folder of the main domain.
DT_LOG : ..\res\DT_Log_2.log
An option can be checked in MOHID Studio or MOHID GUI that enables the logging of variable time steps.
An excerpt of a log file looks like this (Iterartion number, dt, i, j, k, model name, property name):
1, DT: 10.000, I: 177, J: 75, K: 43, Model - Property: Portugal - momentum
2, DT: 10.000, I: 37, J: 1, K: 43, Model - Property: Portugal - momentum
3, DT: 10.000, I: 177, J: 101, K: 43, Model - Property: Portugal - momentum
4, DT: 10.000, I: 62, J: 1, K: 43, Model - Property: Portugal - momentum
5, DT: 10.000, I: 156, J: 93, K: 25, Model - Property: Portugal - nitrate
6, DT: 10.000, I: 69, J: 1, K: 43, Model - Property: Portugal - momentum
7, DT: 10.000, I: 177, J: 84, K: 43, Model - Property: Portugal - momentum
8, DT: 10.000, I: 64, J: 1, K: 43, Model - Property: Portugal - momentum
9, DT: 12.000, I: 155, J: 106, K: 33, Model - Property: Portugal - nitrate
10, DT: 13.846, I: 65, J: 1, K: 49, Model - Property: Portugal - nitrate
11, DT: 16.364, I: 25, J: 4, K: 45, Model - Property: Portugal - nitrate
12, DT: 18.000, I: 32, J: 6, K: 43, Model - Property: Portugal - nitrate
13, DT: 20.000, I: 26, J: 6, K: 44, Model - Property: Portugal - nitrate
14, DT: 22.500, I: 27, J: 8, K: 44, Model - Property: Portugal - nitrate
15, DT: 25.714, I: 156, J: 119, K: 30, Model - Property: Portugal - nitrate
16, DT: 30.000, I: 32, J: 4, K: 42, Model - Property: Portugal - silicate acid
17, DT: 36.000, I: 30, J: 3, K: 42, Model - Property: Portugal - silicate acid
18, DT: 36.000, I: 27, J: 10, K: 44, Model - Property: Portugal - nitrate
19, DT: 36.000, I: 32, J: 6, K: 43, Model - Property: Portugal - silicate acid
20, DT: 36.000, I: 30, J: 5, K: 42, Model - Property: Portugal - silicate acid
Physical rationale
The main to compute an ideal time-step is to choose the largest time-step above which the numerical model iteration would overshoot. This is equivalent to make sure that no cell in any property can deplete more than its property mass in one time-step, given that it has a net flux with positive divergence. Mathematically speaking this would be equivalent to:
Failed to parse (unknown error): \Delta t = \min_{ \frac{p\;ijk} \left( m_{p\;ijk}}{\Phi_{p\;ijk}}\right) if Failed to parse (unknown error): \Phi_{p\;ijk} \gt 0 .
