iteration in CFx

hfawad · September 7, 2009, 03:52

i am new user of CFX but i have used ANSYS a lot, there is a confusion between iteration of ansys and cfx. in ansys iteration determines how many attempts the solver should make to converge but in CFX one can also apply boundary condition like heat flux for a range of iterations which i cannot comprehend.
can someone explain

ghorrocks · September 7, 2009, 08:44

ANSYS and CFX are totally different in their numerical approach so the concept of an "iteration" in each is totally different. Yes, in CFX you can make boundary conditions a function of iteration number but the only time you would do this is to ramp an unstable boundary condition in to make the numerics easier - the fully converged solution should have all boundaries fully active.

In ANSYS structural a direct solver will not iterate at all, or an iterative solver will iterate in the linear solver to get a solution to the matrix. Structural non-linearities (eg contact) may introduce further layers of iterations on top of that.

In CFX you almost always have two levels of iterations. The lowest layer is the linear solver, which is solving the linear equations in matrix form. CFX uses a multigrid solver to do this. You do not see these iterations in the output file, just the final convergence obtained by default. The higher layer is the "iterations" you see which is where the non-linear nature of the Navier Stokes equations is captured. This is the iterations you see in the output file.

September 7, 2009, 03:52	iteration in CFx	#1
hfawad New Member Hasan Fawad Join Date: Sep 2009 Posts: 1 Rep Power: 0	i am new user of CFX but i have used ANSYS a lot, there is a confusion between iteration of ansys and cfx. in ansys iteration determines how many attempts the solver should make to converge but in CFX one can also apply boundary condition like heat flux for a range of iterations which i cannot comprehend. can someone explain

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September 7, 2009, 08:44		#2
ghorrocks Super Moderator Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 17,870 Rep Power: 144	ANSYS and CFX are totally different in their numerical approach so the concept of an "iteration" in each is totally different. Yes, in CFX you can make boundary conditions a function of iteration number but the only time you would do this is to ramp an unstable boundary condition in to make the numerics easier - the fully converged solution should have all boundaries fully active. In ANSYS structural a direct solver will not iterate at all, or an iterative solver will iterate in the linear solver to get a solution to the matrix. Structural non-linearities (eg contact) may introduce further layers of iterations on top of that. In CFX you almost always have two levels of iterations. The lowest layer is the linear solver, which is solving the linear equations in matrix form. CFX uses a multigrid solver to do this. You do not see these iterations in the output file, just the final convergence obtained by default. The higher layer is the "iterations" you see which is where the non-linear nature of the Navier Stokes equations is captured. This is the iterations you see in the output file.