CFD Online Logo CFD Online URL
www.cfd-online.com
[Sponsors]
Home > Forums > Software User Forums > ANSYS > CFX

How to get better convergence in transient simulation

Register Blogs Community New Posts Updated Threads Search

Like Tree2Likes

Reply
 
LinkBack Thread Tools Search this Thread Display Modes
Old   April 17, 2013, 01:44
Default How to get better convergence in transient simulation
  #1
Member
 
leo
Join Date: Jun 2012
Posts: 98
Rep Power: 14
sjtusyc is on a distinguished road
I'm doing a transient multi-phase simulation with time-dependent pressure inlet.
I found it was difficult to converge with my desired time step. If smaller time step was used ,it'll cost time. So what should i do to get better convergence with relatively bigger time step.
1.Will the initial value influence the transient convergence as i use the steady-state result as initial value? Should i start with a better converged steady result?

2. Should i start the simulation with smaller time step and then increase the time
step as my simulation is periodic ?


Thank you in advance.
aero_head likes this.
sjtusyc is offline   Reply With Quote

Old   April 17, 2013, 08:43
Default
  #2
Super Moderator
 
Glenn Horrocks
Join Date: Mar 2009
Location: Sydney, Australia
Posts: 17,854
Rep Power: 144
ghorrocks is just really niceghorrocks is just really niceghorrocks is just really niceghorrocks is just really nice
Yes, the initial conditions will affect convergence. The closer the initial condition is to the real condition, and the smoother it is the better the convergence.

And yes, starting with a small time step and increasing it as the simulation progresses is standard practise.
ghorrocks is offline   Reply With Quote

Old   April 17, 2013, 09:34
Default
  #3
Member
 
leo
Join Date: Jun 2012
Posts: 98
Rep Power: 14
sjtusyc is on a distinguished road
Thank you for your reply.
I have a few more questions.
1)I have done the mesh sensitivity analysis when i was doing steady state simulation. Should i do it again when i do the transient one as i use the steady result as the initial value for the transient simulation.
2)I use adaptive time step when i was doing the transient simulation. In the first several iterations it didn't converge to my criteria.Will this affect the overall accuracy? My simulation is periodic ,i just want to get data of one cycle.

Thank you.
sjtusyc is offline   Reply With Quote

Old   April 17, 2013, 19:45
Default
  #4
Super Moderator
 
Glenn Horrocks
Join Date: Mar 2009
Location: Sydney, Australia
Posts: 17,854
Rep Power: 144
ghorrocks is just really niceghorrocks is just really niceghorrocks is just really niceghorrocks is just really nice
It is up to your judgement as to whether the mesh sensitivity done on steady state is applicable to a transient run. It probably is, but you need to think through what has changed and if it makes a difference to the mesh. Of course, if you want to be safe you can repeat the mesh sensitivity on the transient simulation.

If all you are interested in is the repeating periodic pattern then it does not matter if a few initial time steps do not fully converge.
ghorrocks is offline   Reply With Quote

Old   April 17, 2013, 21:49
Default
  #5
Member
 
leo
Join Date: Jun 2012
Posts: 98
Rep Power: 14
sjtusyc is on a distinguished road
If all you are interested in is the repeating periodic pattern then it does not matter if a few initial time steps do not fully converge.[/QUOTE]

So i just select the later cycles that are fully converged?
sjtusyc is offline   Reply With Quote

Old   April 18, 2013, 07:14
Default
  #6
Super Moderator
 
Glenn Horrocks
Join Date: Mar 2009
Location: Sydney, Australia
Posts: 17,854
Rep Power: 144
ghorrocks is just really niceghorrocks is just really niceghorrocks is just really niceghorrocks is just really nice
Yes, that is correct.
ghorrocks is offline   Reply With Quote

Old   April 18, 2013, 07:45
Default
  #7
Member
 
leo
Join Date: Jun 2012
Posts: 98
Rep Power: 14
sjtusyc is on a distinguished road
Thank you so much.
sjtusyc is offline   Reply With Quote

Old   April 20, 2013, 04:26
Default
  #8
Member
 
leo
Join Date: Jun 2012
Posts: 98
Rep Power: 14
sjtusyc is on a distinguished road
Quote:
Originally Posted by ghorrocks View Post
Yes, that is correct.
Glenn,CFD is not easy, sometimes it exhausted me.
A few questions occur.
As i previous said, i wanna to select the latter cycles that are fully converged as my data. But i found as the frequency of the inlet pressure increase(i set the inet total pressure as "Press = 1.0[MPa]+0.3[MPa]*sin(2*pi *f[Hz]*t" ),the simulation will go wrong at the latter cycles. I monitored massflow of the atomizer as the massflow is the physical parameter i care about.

What causes to this problem? Will the timestep is the reason?
I first set the timestep as "timestep=(1/f/8)[s]" and use the adaptive timestep to find the suitable timestep.
As the f increase, few points are calculated in a cycle, as the timestep for the high frequency cycles is small.

Can i just select the cycles which seems reasonable as my data ?
sjtusyc is offline   Reply With Quote

Old   April 20, 2013, 04:39
Default
  #9
Member
 
leo
Join Date: Jun 2012
Posts: 98
Rep Power: 14
sjtusyc is on a distinguished road
10Hz.jpg

30Hz.jpg

100Hz.jpg

200Hz.jpg

500Hz.jpg
sjtusyc is offline   Reply With Quote

Old   April 20, 2013, 08:20
Default
  #10
Senior Member
 
OJ
Join Date: Apr 2012
Location: United Kindom
Posts: 473
Rep Power: 20
oj.bulmer will become famous soon enough
The periodic plots of higher frequencies are dodgy. Either the flow is chaotic, or the timescales haven't been adequately resolved.

Some clarifications: Are you doing separate solution for every frequency? If yes, do you start with 1/8f as the timestep, with f being frequency?

If both are yes, and if you are using adaptive timestep, then perhaps the smallest timestep value in adaptive settings need to be further reduced. How about keeping it 1e-8 s with largest timestep being say 1000 s? If smallest and largest timesteps are not smal/large enough, the adpative timestepping can not home in on adequate timestep that lies outside these values.

As long as you use adaptive timestepping, it doesn't matter what timestep you start with, as Solver quickly adjusts it to meet the given convergence criterai.

OJ
oj.bulmer is offline   Reply With Quote

Old   April 20, 2013, 08:26
Default
  #11
Super Moderator
 
Glenn Horrocks
Join Date: Mar 2009
Location: Sydney, Australia
Posts: 17,854
Rep Power: 144
ghorrocks is just really niceghorrocks is just really niceghorrocks is just really niceghorrocks is just really nice
Quote:
Can i just select the cycles which seems reasonable as my data ?
That is very unwise.

Your graphs seem to show that the flow is in fact not periodic for a few configurations modelled. This may well be real. There is a transition from periodic to non-periodic at some input value you have used.
ghorrocks is offline   Reply With Quote

Old   April 21, 2013, 04:19
Default
  #12
Member
 
leo
Join Date: Jun 2012
Posts: 98
Rep Power: 14
sjtusyc is on a distinguished road
Thanks i will try with a smaller starting timestep.
sjtusyc is offline   Reply With Quote

Old   April 21, 2013, 08:37
Default
  #13
Super Moderator
 
Glenn Horrocks
Join Date: Mar 2009
Location: Sydney, Australia
Posts: 17,854
Rep Power: 144
ghorrocks is just really niceghorrocks is just really niceghorrocks is just really niceghorrocks is just really nice
Rather than just arbitrarily reducing the tiem step, how about doing a time step sensitivity study so you work out what time step size you actually need. Or even better, use adaptive timestepping, homing in on 3-5 coeff loops per iteration and let the solver work it out for itself.
ghorrocks is offline   Reply With Quote

Old   April 22, 2013, 10:41
Default
  #14
Member
 
leo
Join Date: Jun 2012
Posts: 98
Rep Power: 14
sjtusyc is on a distinguished road
Thanks,this simulation seems tricky. I am doing the sensitive analysis, and confused by a few results.
I will try to sort it out and then ask you for help.
Thanks for help.
sjtusyc is offline   Reply With Quote

Old   April 27, 2013, 04:10
Default
  #15
Member
 
leo
Join Date: Jun 2012
Posts: 98
Rep Power: 14
sjtusyc is on a distinguished road
1)Hi,I come here to ask for your help again.
There some problems confuse me for some time, and i think i should explain it in detail.
1. Physical background
I am doing a simulation of a pressure-swirled atomizer where there is a air-cone in the outlet region.So it is a multi-phase problem.I want to study the dynamic response characteristic of the atomizer. I set the inlet total pressure as 1.0[Mpa]+0.3[Mpa]*sin*(2*pi*f*t),where f refer to the frequency. I care about the average mass flow rate of the atomizer in one cycle. How will it change with the pressure fluctuation frequency.
Homogeneous and free surface model was applied.
First i did the steay simulation, with the inlet total pressure as the 1.0Mpa.
vf.jpg
a slice of the water volume fraction

Last edited by sjtusyc; April 27, 2013 at 04:59.
sjtusyc is offline   Reply With Quote

Old   April 27, 2013, 04:23
Default
  #16
Member
 
leo
Join Date: Jun 2012
Posts: 98
Rep Power: 14
sjtusyc is on a distinguished road
2) When i was doing the steady state simulation, I did the sensitivity analysis of the mesh and convergence criteria and other things.
Based on the steady simulation, I start to do the transient simulation. I used the adaptive time step to let the solver to find appropriate time step.
But something weird happened.
1. surface tension
To my understanding, i think the surface tension in this problem is not important,
so i didn't take it in my consideration. I did confirmed it when i was doing steady state simulation. But has a big effect on the result in transient simulation.
Take f=10Hz as an example.
when the surface tension is considered, the transient mass flow rate is like QQ??20130427152223.png
sjtusyc is offline   Reply With Quote

Old   April 27, 2013, 04:27
Default
  #17
Member
 
leo
Join Date: Jun 2012
Posts: 98
Rep Power: 14
sjtusyc is on a distinguished road
3)As the picture tells, the averaged mass flow rate in the transient simulation is bigger than the steady simulation.
But when the surface tension is not considered it is not the case at all QQ??20130427152237.png
The averaged mass flow rate is almost equals the steady simulation.
sjtusyc is offline   Reply With Quote

Old   April 27, 2013, 04:41
Default
  #18
Member
 
leo
Join Date: Jun 2012
Posts: 98
Rep Power: 14
sjtusyc is on a distinguished road
4)double precision
And i found out the double precision have an big effect on the result too.
The previous data was got without double precision.
When i turned on the double precision and take the surface tension into consideration, the reulsts are:
QQ??20130427154027.png
And the averaged mass flow is again equals the steady state.
sjtusyc is offline   Reply With Quote

Old   April 27, 2013, 04:50
Default
  #19
Member
 
leo
Join Date: Jun 2012
Posts: 98
Rep Power: 14
sjtusyc is on a distinguished road
5)As the averaged mass flow rate is what i care about, these results really confused me.
Will the surface tension lead to error and double precision will alleviate it ?
And i check the pressure field.
1.When surface tension is considered with single precsion:
t=0s.png
t=0s
t=0.1.png
t=0.01s
t=0.2s.png
t=0.02s
t=0.5.png
t=0.05s
t=0.1.png
t=0.1s
sjtusyc is offline   Reply With Quote

Old   April 27, 2013, 04:56
Default
  #20
Member
 
leo
Join Date: Jun 2012
Posts: 98
Rep Power: 14
sjtusyc is on a distinguished road
6)
It seems that the pressure field is not good enough.
When double precision is turned on the free surface tension is considered too.
0s.png
t=0s
0.02s.png
t=0.02s
0.05s.png
t=0.05s

It seems that the pressure field is better.
sjtusyc is offline   Reply With Quote

Reply


Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Trackbacks are Off
Pingbacks are On
Refbacks are On


Similar Threads
Thread Thread Starter Forum Replies Last Post
the initial condition for transient simulation junc CFX 2 August 22, 2012 07:13
Boundary Conditions - Transient Simulation miki256 CFX 2 May 18, 2012 02:22
Transient Simulation: Initial Time blackbody CFX 0 April 18, 2010 09:19
how to identify transient simulation converged littlelz CFX 5 January 27, 2009 19:13
How to observe transient convergence? Angelo CFX 3 May 20, 2005 15:42


All times are GMT -4. The time now is 03:25.