Number of iteration difference between PCG and GAMG

hr_nt · November 22, 2022, 05:52

I ran two interFOAM multiphase simulation in which one had the p_rgh solver set as PCG and one in which it was GAMG. The preconditioner for both was set to DIC. I observed that the number of iterations taken to converge to 1e-8 is higher for PCG (about 1000) and lower for GAMG (about 40). Why does this occur ?

dlahaye · November 22, 2022, 06:36

GAMG requires less iterations because it iterates on various levels of accuracy. See literature on Multigrid Methods, e.g., book of Moukalled-Darwish-Mangani or https://en.wikipedia.org/wiki/Multigrid_method.

I suggest using GAMG as preconditioner for CG to combine best of two worlds (zat least in case that number of processors is one or small).

November 22, 2022, 05:52	Number of iteration difference between PCG and GAMG	#1
hr_nt New Member HR Join Date: Nov 2022 Posts: 17 Rep Power: 3	I ran two interFOAM multiphase simulation in which one had the p_rgh solver set as PCG and one in which it was GAMG. The preconditioner for both was set to DIC. I observed that the number of iterations taken to converge to 1e-8 is higher for PCG (about 1000) and lower for GAMG (about 40). Why does this occur ?

November 22, 2022, 06:36		#2
dlahaye Senior Member Domenico Lahaye Join Date: Dec 2013 Posts: 736 Blog Entries: 1 Rep Power: 17	GAMG requires less iterations because it iterates on various levels of accuracy. See literature on Multigrid Methods, e.g., book of Moukalled-Darwish-Mangani or https://en.wikipedia.org/wiki/Multigrid_method. I suggest using GAMG as preconditioner for CG to combine best of two worlds (zat least in case that number of processors is one or small). joshwilliams and hr_nt like this.

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