Why we have Ueqn.relax() in pisoFoam

Houthuys · June 9, 2017, 04:19

Do any one know why we have Ueqn.relax() in pisoFoam which is a transient solver ?

Also when we should use equation relaxation (Ueqn.relax()) instead of explicit relaxation (U.relax) and vice versa?

Any contribution is highly appreciated

elones · June 9, 2017, 08:46

It may be needed in some cases to use under relaxation factor for momentum equations. Pressure is not relaxed and after at least two pressure corrections, you should obtain "not relaxed" solution.

Equation relaxation for momentum equations is used because we need to get diagonal dominant matrix. Similarly as in case of hyperbolic problems when with smaller time step, the stronger diagonal dominance is obtained. Aii = (something1 + someting2/timestep). In case of relaxation it is Aii = (something1 + something2/alpha_U).

June 9, 2017, 04:19	Why we have Ueqn.relax() in pisoFoam	#1
Houthuys New Member rik houthuys Join Date: Jan 2017 Posts: 27 Rep Power: 9	Do any one know why we have Ueqn.relax() in pisoFoam which is a transient solver ? Also when we should use equation relaxation (Ueqn.relax()) instead of explicit relaxation (U.relax) and vice versa? Any contribution is highly appreciated

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June 9, 2017, 08:46		#2
elones New Member Join Date: Mar 2014 Location: Czech Republic Posts: 29 Rep Power: 14	It may be needed in some cases to use under relaxation factor for momentum equations. Pressure is not relaxed and after at least two pressure corrections, you should obtain "not relaxed" solution. Equation relaxation for momentum equations is used because we need to get diagonal dominant matrix. Similarly as in case of hyperbolic problems when with smaller time step, the stronger diagonal dominance is obtained. Aii = (something1 + someting2/timestep). In case of relaxation it is Aii = (something1 + something2/alpha_U).