Hi i am trying to visualize structure of CFD code. the point where i get stuck is 'how these commercial codes solve the matrix of discritised equations'. the only methods i know are GUASS SEIDEL and SOR etc... but i trying to understand what methods are being used by e.g CFX, FLUENT, PHOENICS etc thanks in advance

They use usually Multigrid. Fluent uses AMG (Algebraic Multigrid). Numeca uses Multigrid. Flow3D 8.0 uses SOR, but they improved their solver to GEMRES is version 9.0. But in anycase they use usually AMG for the new commercial solvers.

Thanks Nabi... what about CFX ?

Some codes also use conjugate gradient type of solvers.

But note that CG is not a general purpose solver! Those codes might use it for diffusion only cases.

not only diffusion BICG or CGSTAB can be implied for a non symetric and even non positive definition matrices but they both break down in some cases as it happened for me. and not to mention lots of codes use a ADI method specially for a 3d code. I prefer it while you can use it because of simplisity and efficiency.