Unstedy behavior
 

I try to solve an unsteady state of a problem having a initial steady state. I have determined the residuals at 1e-4. When start unsteady analysis the value of residuals produce and after reduce until reach the value of residual 1e-4. Then, time is changed. This behavior occur for 20-30 time steps but after stopped and by each time step the solution converged immediately and it is not appeared change in flow field.... Why occur that?? Is the problem by nature steady or occur something else?

Please, print here what appear in your Fluent console so we can better understand what happen.

Have a good day.

Micaël

So... I load the picture of residuals. As it is appeared, initialy there is a state in which each of time step the residual is increased and after reduce. However, after 49000 iterations the line of residual is changed and all of time steps converged directly in 1 iteration. With other words, like as dont occur change of flow field by run of time. Is it realy steady state the physical problem or reach in steady problem as a result of diffusion of solute?

That behavior might be normal. If you disturb the system right at the beginning of the transient simulation, then it is normal that first time steps take many iterations. Then, after a while, the system might tend to a steady state and hence requires less iterations.

In other words:
If you start your transient simulation in non-equilibrium state and that physically your system should tend to return at equilibrium, then it is possible that your case is normal.

Micaël

Therefore, the physical problem is steady due to this result of residual. If was unsteady will be followed similar bahavor by start behavor.
But as a steady problem why dont appear the final solution by steady state analysis and it need to run with unsteady state analysis??


Help for this below:
Consequently, if we want to take a right solution by a physical steady problem, we must try a steady state analysis for initial contition and after an unsteady analysis to take the right solution. If we stay at steady state the solution isnt right. Right?