I am trying to simulate a flow of air at high pressures . I am using pressure based model my residuals dont converge upto the range of 10^-6 as mentioned in the fluent manual do i need to further dampen my resisuals . I am also getting reversed flow at my inlets and outlets
Convergence is something that all CFD Engineers talk about, but we must remember that the way we generally define convergence (by looking at Residual values) is only a small part of ensuring that we have a valid solution. For a Steady State simulation we need to ensure that the solution satisfies the following three conditions:
- Residual RMS Error values have reduced to an acceptable value (typically 10^-4 or 10^-5 or sometimes 10^-6)
- Monitor points for our values of interest have reached a steady solution
- The domain has imbalances of less than 1%.
(So it is not a golden thumb rule)
Secondly, getting flow reversal depends upon the type of your geometry. If it is nozzle or expanded pipe (as per your question), flow reversal takes place due to sudden expansion or constriction, which is similar to backward facing step problem. It is not bad to get flow reversal.
I agree about the control of the residuals of the equations (for steady state, the residuals are nothing but the time derivatives).
Just a note, if you want to have convergence at lower threshold values, don't forget to use double precision.
I agree with Mr. Musavir and Fillipo.
Additionaly, reason for better or worse convergence is always mesh and geometry. Try to refine mesh. Sometimes geometry need to be redefined.
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