Time step

[Siva]

Hi,

May I know why by default the time step is not allowed to exceed the initial step? 

And if we say it is to account for sudden changes, like high velocity flow etc. then it can also be sudden reduction of flow velocity. Consequently, the time step in the latter case can exceed the initial step.

Thanks in advance for the clarification.

Have a good day!

[Randy McDermott]

Just experience that it is safer.  If you don't want that constraint, simply set

RESTRICT_TIME_STEP=F

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[Siva]

Understood..thanks!

[Ellon Bernardes]

Hello!

Concerning the topic of the initial (and default upper limit) time step, I was wondering what is the rationale behind the formula FDS uses. I couldn't grasp it by myself and the manuals does not make it clear and I also saw that it was changed in 2020 ( https://github.com/firemodels/fds/commit/309333c8cfaf45f6ba4a0125323d53b38da66af2). I would appreciate to know the rationale of using it as initial step and upper limit.

Thank you in advance!

[fde]

@Ellon,

Does User guide  6.2.2 Special Topic: Controlling the Time Step  answer your question?

[Ellon Bernardes]

Actually, it does not. I am asking specifically about the initial time step estimation: (5(dx*dy*dz)^(1/3))/sqrt{gH}. I couldn't understand the physical explanation for it. As Randy explained, the upper limit can be turned off which is useful but, if possible, I would like to understand the physical rationale to get to the formula. That way I can assess better when to stick to it or even change my initial time increment.

[Randy McDermott]

The physical rationale is that it is the average velocity of a buoyant plume over the height H of the domain.  This is a reasonable first guess at setting a time step.

I changed the code to make the 10 more obviously the gravitational acceleration.

On Thu, Oct 20, 2022 at 10:47 AM 'Ellon Bernardes' via FDS and Smokeview Discussions <fds...@googlegroups.com> wrote:

Actually, it does not. I am asking specifically about the initial time step estimation: (5(dx*dy*dz)^(1/3))/sqrt{gH}. I couldn't understand the physical explanation for it. As Randy explained, the upper limit can be turned off which is useful but, if possible, I would like to understand the physical rationale to get to the formula. That way I can access better when to stick to it or even change my initial time increment.

Em quinta-feira, 20 de outubro de 2022 às 15:08:47 UTC+1, fde escreveu:

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[Ellon Bernardes]

Thank you for the feedback!