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-   -   FloEFD solver, staedy-state & transient (https://www.cfd-online.com/Forums/floefd-floworks-flotherm/89446-floefd-solver-staedy-state-transient.html)

 Algoa June 13, 2011 07:15

FloEFD solver, staedy-state & transient

Hi there,
could anybody explain the difference between a steady state and a transient calculation. What criteria is the solver using to converge?

Regards
Algoa

 Kevin De Smet June 13, 2011 12:51

A steady state calculation takes initial conditions and iterates on a solution based on your boundary conditions until the values stop changing much (i.e. it has converged)

You can set the initial conditions close to what you expect the results to be, or run with the defaults. It uses internally defined parameters to converge on. However you can specify engineering goals, a goal might be a mass flow rate for a pressure boundary condition, as this is one of the unknowns it needs to solve.

Transient calculations require you give precise initial conditions, and it will run for a set time and the point here is not really to converge on "one solution" but rather to see the behavior over a certain amount of time of a particular system.

 Algoa June 15, 2011 03:30

let me ask my question with an example:
I run a steady-state calculation with mass flow rate as inlet boundary condition. As one goal pressure is set. The solver iterates until the criterion is reached/satisfied. I understand that. But does steady-state solving has any relation to time, because the inlet mass flow rate is a time depended variable?

 Kevin De Smet June 19, 2011 10:59

I think it does, it all depends on your initial conditions, when solving steady-state it will take practically whatever and just take longer or shorter until things balance out. Transient analyses require precise initial conditions so you get an accurate sense of how long something takes.

Basically, if you don't really care about the time, selecting steady-state makes your life easier. It's sort of a pseudo-time.

 Boris_M June 28, 2011 13:00

Hello to both of you,
last thing is not quite correct. If you define a mass flow rate in a steady-state it is not a time dependent variable. The unit of it might include the time such as kg/s but this only says that there is a constant flow of a certain amount.
A time dependent variable would be if the mass flow rate changes over the time for example if you start a pump the flow is zero at the beginning and rises over time to a "steady-state" to a point where it doesn't change anymore. Here are two examples: