# How small is "small" for the under-relaxation factor?

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 March 5, 2011, 23:02 How small is "small" for the under-relaxation factor? #1 Member   Join Date: Mar 2011 Posts: 38 Rep Power: 8 I choose SIMPLEC as solver, and second-order upwind for all properties except STANDARD for pressure. I choose 1 for pressure under-relation factor (from what I found about SIMPLEC), and all other factors are in default, say 0.7 for momentum, except that I set 0.0001 for my contaminant species. In my problem, the experimental concentration data is on the order of 200mg/m3, and the species initial guess starts at 0, SI unit is used. My question is: can I set, say 10^(-10) for my contaminant species as the under-relaxation factor? Of'coz I will start with 1 in the beginning ,and monitor the continuity to make sure it goes down to a lower level and keeps steady there, then decrease the factor, and wait till it decreases to another lower level and doesn't change much there, the do it over and over... Same thing for the momentum under-relaxation factor. Is there any lower limit of this factor? Am I doing it right? I'm worried about this. Thanks!

 March 7, 2011, 01:45 #2 Senior Member   Join Date: Jun 2009 Posts: 100 Rep Power: 10 Hi, first of all, if you are using such low values for under-relaxation factor first of all the equation for which you are assignig that factor will stagnate. Secondly you cannot use the said strategy with the momentum equations because assuming you are using SIMPLEC they are coupled to your continuity equations and hence if you give a very low under-relaxation factor for your momentum equation, your continuity residuals are bound to stagnate. I think you can go as low as 10^-14, but I dont see the point of going so low since after you reduce under-relaxation factor to 10^-10 there would be virtually no change in the residuals of the equation to which you have assigned it, so why go lower? Also using 1 for pressure under-relaxation may make the solution unstable.

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