Solution relaxation factors

SB_22 · November 26, 2020, 02:03

Hello,

I am simulating a steady state laminar flow inside a reactor operating at low Pressure and high temperature. If I use low value of momentum URF (0.2 in my case) my solution gets converged up to 10-14 and net fluxes are almost 0 and point monitors also gets steady. But if I increase the URF value from 0.2 to 0.3 and re run my solution gives me convergence only up to 10-1 and my fluxes are also not close to zero.
My question is can I trust the solutions obtained from low value of URFs?

Regards.

LoGaL · November 26, 2020, 05:39

If you decrease the relaxation factors of course the solution is going to change less iteration by iteration so the residuals are going to be lower. Unless you are dealing with very complicated flow (and yours is not), I would not change them, it will only make your simulation time longer

The point is that you shouldn’t use the residuals to judge convergence but rather set up some probes at 3-4 points in the domain and monitor quantities of interest (e.g velocity and temperature) there. If the values don’t change if you iterate more and the residuals are reasonably low (10-6) then you can say the solution is converged

November 26, 2020, 02:03	Solution relaxation factors	#1
SB_22 New Member Join Date: Nov 2020 Posts: 1 Rep Power: 0	Hello, I am simulating a steady state laminar flow inside a reactor operating at low Pressure and high temperature. If I use low value of momentum URF (0.2 in my case) my solution gets converged up to 10-14 and net fluxes are almost 0 and point monitors also gets steady. But if I increase the URF value from 0.2 to 0.3 and re run my solution gives me convergence only up to 10-1 and my fluxes are also not close to zero. My question is can I trust the solutions obtained from low value of URFs? Regards.

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November 26, 2020, 05:39		#2
LoGaL Senior Member Lorenzo Galieti Join Date: Mar 2018 Posts: 374 Rep Power: 12	If you decrease the relaxation factors of course the solution is going to change less iteration by iteration so the residuals are going to be lower. Unless you are dealing with very complicated flow (and yours is not), I would not change them, it will only make your simulation time longer The point is that you shouldn’t use the residuals to judge convergence but rather set up some probes at 3-4 points in the domain and monitor quantities of interest (e.g velocity and temperature) there. If the values don’t change if you iterate more and the residuals are reasonably low (10-6) then you can say the solution is converged