overflow problem

Marteusz · June 15, 2009, 16:16

Hello everyone,

At the beginning I want to mention that I have read all history about this error and the article at CFD-Wiki and all advices didn't solve that problem

Problem to solve:

http://student.agh.edu.pl/~marteusz/problemtosolve.png

simple pipe 6mm diameter, 1m long, inlet velocity 825m/s and static relative pressure 5bar

here you can see run definition settings:
http://student.agh.edu.pl/~marteusz/definition.txt

Domain: Ideal Gas
Heat Transfer: total energy
Turbulence model: K-epsilon

BC
Inlet - supersonic 825m/s and relative pressure 5bar
Outlet - supersonic

Initial values, 825m/s and temperature 20C

I had changed local timestep from 1E-3 to 1E7 to see if it help, but it doesn't.

Residuals target: 1E-05

Can anyone help me to get convergence, I will be very, very glad.

Problem can be cause because too small diameter and too big velocity.

Thanks,

Mateusz Kesek

ghorrocks · June 15, 2009, 21:23

Hi,

Try using Local Timescale Factor to get the thing started. Once it has converged for a bit using that for a while go back to a physical timescale.

Also consider using the high speed numerics option. It is an expert parameter which does a second continuity loop and that occasionally helps with high speed flows.

Glenn Horrocks

Timon · June 16, 2009, 04:22

Actually, the second continuity loop is activated by a separate expert parameter:

max continuity loops = 2

High speed numerics (found under compressibility control in the advanced solver control panel) does three other things. Copy-paste from the help:

"Firstly, it activates a special type of dissipation at shocks to avoid a transverse shock instability called the carbuncle effect (which may occur if the mesh is finer in the transverse direction than in the flow direction). Secondly, it activates the High Resolution Rhie Chow option to reduce pressure wiggles adjacent to shocks. Finally, for steady state flows, it modifies the default relaxation factors for the advection blend factor and gradients."

ghorrocks · June 16, 2009, 09:01

Thanks for the correction Timon, it's been a while since I used that option so I forgot the details!

Glenn Horrocks

Marteusz · June 16, 2009, 14:20

I still have this problem, I discovered that If I turn off the turbulance (laminar flow) or increase diameter of pipe the analysis gets convergence.

But I need to get convergence to that small tube

Timon · June 17, 2009, 04:21

Have you tried to initialize your solution with lower velocities, ie. gradually increasing your boundary conditions until you reach the desired values?

fab · June 17, 2009, 06:49

hi everybody,i am just a CFX-beginner,i have some questions about the Geometry,i have to design a Flowchanel!can some body help me please?
tanks lot

ckleanth · June 17, 2009, 09:14

Quote:

Originally Posted by fab

hi everybody,i am just a CFX-beginner,i have some questions about the Geometry,i have to design a Flowchanel!can some body help me please?
tanks lot

do the tutorials before asking any questions

LSC · June 19, 2009, 01:41

Hi

I do encountered convergence issue quite often (playing with the values for the past month). I realised that using Local timescale indeed helps a lot but for some of my simulations the residual graph is diving smoothly until in the 1e-4 to 1e-5 region it starts to oscillates. I tried to tune the solver fluid and mass relaxation in the expert parameters but it wont help much. Also noticed that physical time scale is much faster but oscillations are often encountered. Are there any best known methods to tackle this issue?

ghorrocks · June 19, 2009, 08:02

http://www.cfd-online.com/Wiki/Ansys...gence_criteria

LSC · June 19, 2009, 08:42

Hi Glenn,

many thanks for the link!

ghorrocks · June 20, 2009, 07:57

I wrote it too, years ago. Getting lots of questions on "my simulation is not accurate" lately, might write one about how to ensure your simulation is accurate someday soon.

LSC · June 20, 2009, 08:05

Hi Glenn,

my guess is correct! By the way, I have the beta features for V11 enabled. A lot of nice features. I have tried out the transition Roughness height and I am able to get nice convergence..Understand that the Roughness height for Wall B.C is based on Equivalent Sand Roughness Height and I am wondering what is the relationship for the transition roughness height found in the transition model (with SST) with the equivalent sand roughness height.

June 15, 2009, 16:16	overflow problem	#1
Marteusz New Member Join Date: May 2009 Posts: 10 Rep Power: 17	Hello everyone, At the beginning I want to mention that I have read all history about this error and the article at CFD-Wiki and all advices didn't solve that problem Problem to solve: http://student.agh.edu.pl/~marteusz/problemtosolve.png simple pipe 6mm diameter, 1m long, inlet velocity 825m/s and static relative pressure 5bar here you can see run definition settings: http://student.agh.edu.pl/~marteusz/definition.txt Domain: Ideal Gas Heat Transfer: total energy Turbulence model: K-epsilon BC Inlet - supersonic 825m/s and relative pressure 5bar Outlet - supersonic Initial values, 825m/s and temperature 20C I had changed local timestep from 1E-3 to 1E7 to see if it help, but it doesn't. Residuals target: 1E-05 Can anyone help me to get convergence, I will be very, very glad. Problem can be cause because too small diameter and too big velocity. Thanks, Mateusz Kesek Last edited by Marteusz; June 15, 2009 at 16:46.

June 17, 2009, 06:49	geometry	#7
fab New Member Join Date: Jun 2009 Posts: 13 Rep Power: 17	hi everybody,i am just a CFX-beginner,i have some questions about the Geometry,i have to design a Flowchanel!can some body help me please? tanks lot

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June 15, 2009, 21:23		#2
ghorrocks Super Moderator Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 17,872 Rep Power: 144	Hi, Try using Local Timescale Factor to get the thing started. Once it has converged for a bit using that for a while go back to a physical timescale. Also consider using the high speed numerics option. It is an expert parameter which does a second continuity loop and that occasionally helps with high speed flows. Glenn Horrocks

June 16, 2009, 04:22		#3
Timon Member Join Date: Mar 2009 Posts: 44 Rep Power: 17	Actually, the second continuity loop is activated by a separate expert parameter: max continuity loops = 2 High speed numerics (found under compressibility control in the advanced solver control panel) does three other things. Copy-paste from the help: "Firstly, it activates a special type of dissipation at shocks to avoid a transverse shock instability called the carbuncle effect (which may occur if the mesh is finer in the transverse direction than in the flow direction). Secondly, it activates the High Resolution Rhie Chow option to reduce pressure wiggles adjacent to shocks. Finally, for steady state flows, it modifies the default relaxation factors for the advection blend factor and gradients."

June 16, 2009, 09:01		#4
ghorrocks Super Moderator Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 17,872 Rep Power: 144	Thanks for the correction Timon, it's been a while since I used that option so I forgot the details! Glenn Horrocks

June 16, 2009, 14:20		#5
Marteusz New Member Join Date: May 2009 Posts: 10 Rep Power: 17	I still have this problem, I discovered that If I turn off the turbulance (laminar flow) or increase diameter of pipe the analysis gets convergence. But I need to get convergence to that small tube

June 17, 2009, 04:21		#6
Timon Member Join Date: Mar 2009 Posts: 44 Rep Power: 17	Have you tried to initialize your solution with lower velocities, ie. gradually increasing your boundary conditions until you reach the desired values?

June 19, 2009, 01:41		#9
LSC Member LSC Join Date: May 2009 Posts: 58 Rep Power: 17	Hi I do encountered convergence issue quite often (playing with the values for the past month). I realised that using Local timescale indeed helps a lot but for some of my simulations the residual graph is diving smoothly until in the 1e-4 to 1e-5 region it starts to oscillates. I tried to tune the solver fluid and mass relaxation in the expert parameters but it wont help much. Also noticed that physical time scale is much faster but oscillations are often encountered. Are there any best known methods to tackle this issue?

June 19, 2009, 08:02		#10
ghorrocks Super Moderator Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 17,872 Rep Power: 144	http://www.cfd-online.com/Wiki/Ansys...gence_criteria

June 19, 2009, 08:42		#11
LSC Member LSC Join Date: May 2009 Posts: 58 Rep Power: 17	Hi Glenn, many thanks for the link!

June 20, 2009, 07:57		#12
ghorrocks Super Moderator Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 17,872 Rep Power: 144	I wrote it too, years ago. Getting lots of questions on "my simulation is not accurate" lately, might write one about how to ensure your simulation is accurate someday soon.

June 20, 2009, 08:05		#13
LSC Member LSC Join Date: May 2009 Posts: 58 Rep Power: 17	Hi Glenn, my guess is correct! By the way, I have the beta features for V11 enabled. A lot of nice features. I have tried out the transition Roughness height and I am able to get nice convergence..Understand that the Roughness height for Wall B.C is based on Equivalent Sand Roughness Height and I am wondering what is the relationship for the transition roughness height found in the transition model (with SST) with the equivalent sand roughness height.