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is it possible to predict how long it takes to reach steady state solution in unstead

is it possible to predict how long it takes to reach steady state solution in unsteady approach based on the residence time? (my favorite monitors shows constant or periodic shape)

For Example if residence time of a problem is 7*10^-3 [s], and it will be solved unsteady, after how long it will become steady if it has steady state solution.

I know it depends on the initial condition, but I want an estimation for it.
for example if its initial condition is its steady state or if we start from the first point?

I have a problem that is in laminar regime and I think its impossible to solve it in steady state mode, and papers also have unsteady approach, to have convergence. I choose dt=2.5*10^-7 [s] and it becomes converged in each time step (10^-5 for continuity and 10^-7 for velocity), it is 3D and I have started from steady state solution as initial condition. I want to know after how many time steps it becomes steady. I know it has steady state solution.

it is kind of instability flow.

Thank you very much

 ghorrocks November 11, 2013 05:18

No, you cannot predict in general how long steady state will take at all, let alone based on residence time. Different classes of flows require different times so obviously different times are required.

You might be able to estimate how long it will take for your class of flow, but it will only work for your class of flow. It will not be general.

Note that if you are marching a transient simulation out to steady state then you do not care too much about the time history of the flow. So use first order time differencing and do not converge the timesteps too tightly. Loose convergence is OK, and then go tighter as you approach your final solution. You just want to quickly and approximately march it out in time and only worry about accuracy at the end.

Thank you very much for your consideration.