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

How to get better convergence in transient simulation

Register Blogs Members List Search Today's Posts Mark Forums Read

Like Tree1Likes

Reply
 
LinkBack Thread Tools Display Modes
Old   April 27, 2013, 04:11
Default
  #21
Member
 
leo
Join Date: Jun 2012
Posts: 98
Rep Power: 5
sjtusyc is on a distinguished road
6)
so in conclusion , i'm really confused.
Whether should the surface tension should be considered and the double precision turned on. Whether the averaged mass flow rate in the transient simulation is larger than that in steady simulation.
I'm really confused.

Thanks in advance.
sjtusyc is offline   Reply With Quote

Old   April 27, 2013, 07:03
Default
  #22
Super Moderator
 
Glenn Horrocks
Join Date: Mar 2009
Location: Sydney, Australia
Posts: 10,929
Rep Power: 85
ghorrocks has a spectacular aura aboutghorrocks has a spectacular aura aboutghorrocks has a spectacular aura about
Quote:
To my understanding, i think the surface tension in this problem is not important
You are modelling a spray atomiser are you not? Then isn't surface tension critical to this process? If you do not understand a flow then you have no hope of modelling it.

I suggest you do some background research on spray formation, droplet formation and Rayliegh stability of liquid columns (eg http://www.maths.bris.ac.uk/~majge/PHF00941.pdf)

The glitches you saw in single precision look like classic numerical round-off problems. The fix is to run double precision, as you have found.
ghorrocks is offline   Reply With Quote

Old   April 27, 2013, 07:07
Default
  #23
Member
 
leo
Join Date: Jun 2012
Posts: 98
Rep Power: 5
sjtusyc is on a distinguished road
I simulated the internal flow of the atomizer, not the droplet formation.
sjtusyc is offline   Reply With Quote

Old   April 27, 2013, 07:10
Default
  #24
Member
 
leo
Join Date: Jun 2012
Posts: 98
Rep Power: 5
sjtusyc is on a distinguished road
What the "glitches" do you mean?
This simulation really confuse me ,may i hope you read it a little more carefully if you have time although i have no right to ask you to do so.
Thanks in advance
sjtusyc is offline   Reply With Quote

Old   April 27, 2013, 07:13
Default
  #25
Member
 
leo
Join Date: Jun 2012
Posts: 98
Rep Power: 5
sjtusyc is on a distinguished road
Quote:
Originally Posted by sjtusyc View Post
I simulated the internal flow of the atomizer, not the droplet formation.
The air is sucked in the atomizer, so ,the internal flow of the pressure-swirled atomizer is multiphase flow and where the free surface is not important.
But i found later as i said previous the free surface tension had a big influence on the result, will it be the numerical reason.
sjtusyc is offline   Reply With Quote

Old   April 27, 2013, 07:16
Default
  #26
Super Moderator
 
Glenn Horrocks
Join Date: Mar 2009
Location: Sydney, Australia
Posts: 10,929
Rep Power: 85
ghorrocks has a spectacular aura aboutghorrocks has a spectacular aura aboutghorrocks has a spectacular aura about
The glitches are the weird spots on post #19.

It might help if you drew a picture of what you expect the flow to do. The images you have posted do not clearly show what is happening.
ghorrocks is offline   Reply With Quote

Old   April 27, 2013, 07:39
Default
  #27
Member
 
leo
Join Date: Jun 2012
Posts: 98
Rep Power: 5
sjtusyc is on a distinguished road
There is a interface between the air and water just like that in the open channel.

Yes, there are weird spots on post #19. But why that will not happen if the surface tension is not considered though just single precision is used.
I mentioned that in the previous posts.

100Hz_001.jpg
This is the computational field. Water flows in and cause low pressure near the atomizer exit and that lead to the air sucked in.
out.txt
sjtusyc is offline   Reply With Quote

Old   April 27, 2013, 07:47
Default
  #28
Member
 
leo
Join Date: Jun 2012
Posts: 98
Rep Power: 5
sjtusyc is on a distinguished road
Quote:
Originally Posted by sjtusyc View Post
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:
Attachment 21200
t=0s
Attachment 21201
t=0.01s
Attachment 21202
t=0.02s
Attachment 21203
t=0.05s
Attachment 21201
t=0.1s


Does my guess reasonable ?
sjtusyc is offline   Reply With Quote

Old   April 27, 2013, 07:56
Default
  #29
Super Moderator
 
Glenn Horrocks
Join Date: Mar 2009
Location: Sydney, Australia
Posts: 10,929
Rep Power: 85
ghorrocks has a spectacular aura aboutghorrocks has a spectacular aura aboutghorrocks has a spectacular aura about
I can make a key suggestions from your CCL: You are using incompressible air at room density, yet the inlet pressure is MPa. This does not sound very appropriate to me. Sounds like you need a compressible gas model to account for the large variation in density the air would undergo.

Surface tension is modelled as a momentum source on the element at the fluid surface. Small kinks in the surface then cause kinks in the surface tension and these can be spurious and lead to numerical problems. That is why surface tension models are more expensive to run.

So only use surface tension if you have to. And it is not clear to me at the moment what role surface tension plays in this device. Can you post an image of what the device looks like after it has been running for a while?
ghorrocks is offline   Reply With Quote

Old   April 27, 2013, 08:13
Default
  #30
Member
 
leo
Join Date: Jun 2012
Posts: 98
Rep Power: 5
sjtusyc is on a distinguished road
I don't know how to say many many many thanks in English,just thank you for your help.

It is the internal flow of the pressure-swirl atomizer.
This thesis may help you.
http://www.ilasseurope.org/ICLASS/il...papers/058.pdf

There is only water come in the atomizer through the inlet and air come in the atomizer through the outlet.

So the air is workind under the 1atm.
sjtusyc is offline   Reply With Quote

Old   April 28, 2013, 06:45
Default
  #31
Super Moderator
 
Glenn Horrocks
Join Date: Mar 2009
Location: Sydney, Australia
Posts: 10,929
Rep Power: 85
ghorrocks has a spectacular aura aboutghorrocks has a spectacular aura aboutghorrocks has a spectacular aura about
Glad to help.

OK, it you are confident the air does not change much from atmospheric pressure then incompressible air is good and will considerably simplify the simulation.

Sorry - another thing - are you using the reference pressure correctly? Your outlet should be 0 Pa with a reference pressure of 1 bar. It seems you might have an outlet pressure of 1bar and a reference pressure of 0. This will cause numerical round-off problems - like you have been seeing.
ghorrocks is offline   Reply With Quote

Old   April 28, 2013, 09:54
Default
  #32
Member
 
leo
Join Date: Jun 2012
Posts: 98
Rep Power: 5
sjtusyc is on a distinguished road
As i know, in this simulation,the pressure ranges from 1 bar to 1Mpa,so it will have little effect whether the reference pressure was set to 1bar or zero.

Is it right?
sjtusyc is offline   Reply With Quote

Old   April 28, 2013, 10:00
Default
  #33
Member
 
leo
Join Date: Jun 2012
Posts: 98
Rep Power: 5
sjtusyc is on a distinguished road
Glenn, did you see the pictures in post 16,17 and 18.
Why just a little change will lead to so much difference.
In post 16, the transient mass flow rate fluctuate upon 0.32(steady state mass flow rate), and its average mass flow is larger then the steady flow rate.And in post 17,18 transient mass flow rate fluctuate fluctuate from 0.24 to 0.38.
So its average mass flow equals the steady flow rate.
sjtusyc is offline   Reply With Quote

Old   April 28, 2013, 18:42
Default
  #34
Super Moderator
 
Glenn Horrocks
Join Date: Mar 2009
Location: Sydney, Australia
Posts: 10,929
Rep Power: 85
ghorrocks has a spectacular aura aboutghorrocks has a spectacular aura aboutghorrocks has a spectacular aura about
With such a large range in pressures in the water - but a small range of pressure in air - this is always going to be a tricky model for the numerics. So double precision is recommended. But if you use the reference pressure correctly then the small pressure range in air will be well resolved as it will be near zero, and yes, you have to compromise resolution on the water as it has such a large pressure range.

So yes, definitely use a reference pressure of 1 atm.

As for posts 16, 17, 18 - this just shows that single precision with the wrong reference pressure gives incorrect results. So you definitely need double precision and a correctly set reference pressure.
ghorrocks is offline   Reply With Quote

Old   April 28, 2013, 21:45
Default
  #35
Member
 
leo
Join Date: Jun 2012
Posts: 98
Rep Power: 5
sjtusyc is on a distinguished road
Thanks so much. Your help let me sort out the problem gradually.
I hadn't expected round-off errors will lead such a great problem.
1)I am little confused why the reference pressure helps to get better numerical result?
As i know in NS equations, there is only pressure difference term in momentum equation,so it seems that the pressure difference will not change whether the reference pressure set or not.

2)Judged from the post 17,if the surface tension is not considered, just single precision is enough. And from post 18, it may seem that if the surface tension is considered double precision should be used.
Is that a little weird?
The surface tension has a big influence on the result.
sjtusyc is offline   Reply With Quote

Old   April 28, 2013, 21:56
Default
  #36
Super Moderator
 
Glenn Horrocks
Join Date: Mar 2009
Location: Sydney, Australia
Posts: 10,929
Rep Power: 85
ghorrocks has a spectacular aura aboutghorrocks has a spectacular aura aboutghorrocks has a spectacular aura about
The reference pressure changes the magnitude of the pressure difference in the momentum equation. For instance, if the pressure range in the air is 1Pa, then with a reference pressure of 0 bar it is calculating the difference of 100001 - 100000 = 1Pa, but it took 6 digits of precision to do it. You only get 6 or 7 digits of precision in single precision so you have just about used up all your precision on this calculation. CFX does use some tricks to reduce this effect and it is actually a lot better than this, but the concept still holds.

The same situation with a 1 bar reference pressure set means that the pressure difference is 1-0=1Pa, but the difference is resolved with full accuracy of the precision of the numerics.

Your point 2 is not weird. This is exactly what you would expect. Surface tension tends to magnify small defects, so if the defect is numerical noise from round off errors it will get magnified. The defects in double precision are far smaller, so you get much better behaviour with surface tension.
sjtusyc likes this.
ghorrocks is offline   Reply With Quote

Old   April 28, 2013, 23:19
Default
  #37
Member
 
leo
Join Date: Jun 2012
Posts: 98
Rep Power: 5
sjtusyc is on a distinguished road
Glenn, thank you so much.
I figured out my problem under your great help.
I find CFD is not easy,but it is interesting!
It is a great joy to solve problem.

Wish you have a good holiday.
sjtusyc is offline   Reply With Quote

Reply

Thread Tools
Display Modes

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 On
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 06:13
Boundary Conditions - Transient Simulation miki256 CFX 2 May 18, 2012 01:22
Transient Simulation: Initial Time blackbody CFX 0 April 18, 2010 08:19
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 14:42


All times are GMT -4. The time now is 02:19.