# Transient Convergence

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 September 23, 2019, 14:40 Transient Convergence #1 Member   Soumitra Vadnerkar Join Date: Aug 2018 Posts: 66 Rep Power: 3 If after sometime, every subsequent timestep is converging in mere 2-3 iterations (i.e. very quickly), can we say the solution not variables are not drastically changing, meaning the solution is nearly converged already?

September 23, 2019, 14:42
#2
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Filippo Maria Denaro
Join Date: Jul 2010
Posts: 5,007
Rep Power: 53
Quote:
 Originally Posted by soumitra2102 If after sometime, every subsequent timestep is converging in mere 2-3 iterations (i.e. very quickly), can we say the solution not variables are not drastically changing, meaning the solution is nearly converged already?

No, the convergence to a steady state solution must be confirmed by the residuals on the equations, that is the time derivatives magnitude.

 September 23, 2019, 15:13 #3 Member   Soumitra Vadnerkar Join Date: Aug 2018 Posts: 66 Rep Power: 3 I have kept the residual criteria for all equations--continuity, velocity, energy, etc. in my transient simulation as same as they are for a typical steady-state simulation. And with these set targets, each of the residual criteria is satisfying for every time-step within no more than 2-3 iteration calculation per time-step; and this is happening for almost all subsequent time-steps.

September 23, 2019, 15:24
#4
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Filippo Maria Denaro
Join Date: Jul 2010
Posts: 5,007
Rep Power: 53
Quote:
 Originally Posted by soumitra2102 I have kept the residual criteria for all equations--continuity, velocity, energy, etc. in my transient simulation as same as they are for a typical steady-state simulation. And with these set targets, each of the residual criteria is satisfying for every time-step within no more than 2-3 iteration calculation per time-step; and this is happening for almost all subsequent time-steps.

If the residuals correspond to the time derivatives and the thresholds for the convergence criteria is adequate, you can assume that the computed solution is steady.

 September 24, 2019, 00:04 Residual plot for a transient simulation #5 Member   Soumitra Vadnerkar Join Date: Aug 2018 Posts: 66 Rep Power: 3 Sorry to ask again, but, just to be clear, Can we say about the attached image of residual plots of a transient simulation indicate that the solution is nothing but a converged steady-state solution?tube_fluid-1-01001.png In other words, is it so that, maybe if a converged steady-state solution (with same residual target criteria) was available would seem no different than this transient simulation ?

 Tags convergence, timestep, tranient