smaller timestep leads not to converge

vovogoal · October 11, 2011, 00:47

Dear all,

I got a transient run with constant timestep 0.001s, smoothly done by CFX (ver 13.0).
Other I input BCs:
Laminar, isothermal.
inlet flowrate around 0.003 kg/s; outlet is about 10000Pa
For first two iteration , the courant number is about 200s, then reduced to the level of 50s.(I was noticed the CFX is fully implicit so greater courant number is not issue once converged. )
I monitor the outflow and inlet pressure, everything seems alright for this run.

Then I tried smaller timestep 0.0001s but the it couldn't get convergence for the first iteration.
The solver manager shows the linear solution failed in equations of U-Mom, V-Mom, and W-Mom; then fatal error, Floating point exception: Overflow.
I haven't checked the mesh yet.
I ticked the CFX-Pre->Solver control->Basic Setting->Timestep Initialisation-> Upper courant Number, and gave value to 10.
The first iteration just flow through and running and running.

I am wondering if this is right. Or there's another way around, I could use 0.0001s without divergence.

THANKS

mvoss · October 11, 2011, 07:53

hey,

really not sure about that but i think it could be a problem if the time step isn´t sufficient for letting the flow pass the very first volume with given velocity.

neewbie

ghorrocks · October 11, 2011, 18:38

You should set the outlet to 0Pa pressure and use the reference pressure to give the correct absolute pressure. Using a large outlet pressure causes large round-off errors which will get worse when you decrease the time step.

Also running double precision numerics might help.

vovogoal · October 12, 2011, 00:00

Quote:

Originally Posted by ghorrocks

You should set the outlet to 0Pa pressure and use the reference pressure to give the correct absolute pressure. Using a large outlet pressure causes large round-off errors which will get worse when you decrease the time step.

Also running double precision numerics might help.

I am doing FSI problem, so expect having the 'real' pressure.
I almost have the run finished I will check any diffrerence between two cases(0.001s and 0.0001s)

Thanks!

ghorrocks · October 12, 2011, 07:48

I am no expert on FSI but I am sure it is smart enough to be able to handle a reference pressure. This is really basic stuff. And very important for exactly the reason you have found.

juliom · October 17, 2011, 10:10

I I were you, I would use a steady state first.
I would define the reference pressure and I would defined the outlet pressure as a relative pressure, as a Glenn said.
Besides, I would start as a Steady state with and Auto time scale = 1 (conservative), and let the program to make some iteration, as a result you will get an "accurate" time step.
After you del with steady jump to transient....
Good luck!!

October 11, 2011, 00:47	smaller timestep leads not to converge	#1
vovogoal New Member Join Date: Mar 2009 Location: Dublin Posts: 11 Rep Power: 17	Dear all, I got a transient run with constant timestep 0.001s, smoothly done by CFX (ver 13.0). Other I input BCs: Laminar, isothermal. inlet flowrate around 0.003 kg/s; outlet is about 10000Pa For first two iteration , the courant number is about 200s, then reduced to the level of 50s.(I was noticed the CFX is fully implicit so greater courant number is not issue once converged. ) I monitor the outflow and inlet pressure, everything seems alright for this run. Then I tried smaller timestep 0.0001s but the it couldn't get convergence for the first iteration. The solver manager shows the linear solution failed in equations of U-Mom, V-Mom, and W-Mom; then fatal error, Floating point exception: Overflow. I haven't checked the mesh yet. I ticked the CFX-Pre->Solver control->Basic Setting->Timestep Initialisation-> Upper courant Number, and gave value to 10. The first iteration just flow through and running and running. I am wondering if this is right. Or there's another way around, I could use 0.0001s without divergence. THANKS

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October 11, 2011, 07:53		#2
mvoss Senior Member Matthias Voß Join Date: Mar 2009 Location: Berlin, Germany Posts: 449 Rep Power: 20	hey, really not sure about that but i think it could be a problem if the time step isn´t sufficient for letting the flow pass the very first volume with given velocity. neewbie

October 11, 2011, 18:38		#3
ghorrocks Super Moderator Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 17,870 Rep Power: 144	You should set the outlet to 0Pa pressure and use the reference pressure to give the correct absolute pressure. Using a large outlet pressure causes large round-off errors which will get worse when you decrease the time step. Also running double precision numerics might help.

October 12, 2011, 07:48		#5
ghorrocks Super Moderator Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 17,870 Rep Power: 144	I am no expert on FSI but I am sure it is smart enough to be able to handle a reference pressure. This is really basic stuff. And very important for exactly the reason you have found.

October 17, 2011, 10:10		#6
juliom Senior Member Julio Mendez Join Date: Apr 2009 Location: Fairburn, GA. USA Posts: 290 Rep Power: 18	I I were you, I would use a steady state first. I would define the reference pressure and I would defined the outlet pressure as a relative pressure, as a Glenn said. Besides, I would start as a Steady state with and Auto time scale = 1 (conservative), and let the program to make some iteration, as a result you will get an "accurate" time step. After you del with steady jump to transient.... Good luck!!