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Old   August 15, 2019, 18:26
Default Time units OpenFOAM
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Álvaro Pardo
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Hello everyone,

I have a very simple question to ask you all: the time units on OpenFOAM must be always in seconds? The OF User Guide states that the model can be used for any system of units, however I don't know if those units have to be always the base ones, as specified in the Guide for the IS and the USCS.

I would like to define the time control parameters in days or hours, so I need to know if it's indeed possible to do so rather than having to do it in seconds.

Thank you so much in advance!


Last edited by pardoa; August 16, 2019 at 11:20.
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Old   August 16, 2019, 01:24
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You can use whatever you want for the units. The input data just needs to be correct for the chosen set of units. Keep in mind that OpenFOAM uses some dimensioned physical constants in some cases.
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Old   August 16, 2019, 02:47
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Hi Álvaro:

Yes, OpenFOAM uses "dimensioned" scalars and vectors but these are restricted to basic SI units, i.e., kg, m, s, K, mol, A, and Cd (candela). I don't think you can change that.

I imagine you are interested in displaying these units in hours and days. The no-code solution is conversion, but if you want a more user friendly solution, I am afraid you will have to create your own solver for that.

Another issue concerns the postprocessed results. So far they will be sorted by the time elapsed in seconds. I don't know how much work it is to convert them to the time units you desire. Maybe (in Linux) a link (ln -s) from the original folders (in seconds) to new names (in hours / days), for which you can write a script to automate, would suffice?

Hope that helps, Gerry.
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Old   August 16, 2019, 12:55
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Thanks for the answers and help .

As far as I understand, the units that are used in the model are preset to the base/basic units of the International System (IS). Nevertheless, any other system of units should be object of application on OpenFOAM after modifying the chosen set of units and dimensioned physical constant values in the DimensionSets and DimensionedConstants subdictionaries of the main controlDict file of the OpenFOAM installation ($WM_PROJECT_DIR/etc/controlDict).

Therefore, since I aim to define all my time control parameters in days or hours, I should modify the scalar value of the basic unit "s" (seconds) to 3600 if my input data is in hours and to 86400 if it's in days. Likewise, all constants whose dimension set includes a temporal unit should be changed accordingly.

Does this approach sound reasonable for you? Thanks!

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