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

Superior CFD Program?

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

Like Tree3Likes
  • 3 Post By cfdnewbie

Reply
 
LinkBack Thread Tools Display Modes
Old   December 12, 2011, 21:20
Default Superior CFD Program?
  #1
New Member
 
Join Date: Sep 2010
Posts: 28
Rep Power: 0
zephyrus17 is on a distinguished road
Physics is physics, math is math, and these are always the same. Is there actually a difference in inherent 'accuracy' by using OpenFOAM compared to ANSYS Fluent, for example? Are they superior in different areas (turbine vs exterior flow)? Or are they just all the same but with different brands/setup methodology?
zephyrus17 is offline   Reply With Quote

Old   December 13, 2011, 18:53
Default
  #2
Senior Member
 
cfdnewbie
Join Date: Mar 2010
Posts: 551
Rep Power: 11
cfdnewbie is on a distinguished road
Interesting question, but I believe your argument in not fully valid. One could argue that physics is physics, math is math, so a Toyota Civic is the same as a Porsche 911.

In a way it is, they both do the same thing (take you from A to B), but in very different manners with different features, qualities and such.

Codes (commercial and research) across the board differ greatly in terms of two things:
quality and efficiency. I'm in no position to pass judgement on openfoam or fluent, but I'm very sure that they will differ significantly, like a Civic and a 911.

Most commercial codes are like plough horses: Sturdy, hard-working, a little bit chubby (i.e inefficient) and pretty good for a wide variety of tasks - but they do not excel in a certain area, and the result you get with a commercial code will - in general - be inferior to the one you get with a specialized code (in terms of both accuracy and efficiency).

Highly specialized codes (like research codes tend to be) are incredibly good at what they are designed to do, but they won't do much else - like a highly trained race horse.

What you are missing in your statement "math is math, physics is physics" is that CFD is casting math into applied maths, i.e. the numerics. Codes for the same physical problem (described by the same math) can differ so significantly in terms of numerics, that their results can be completely unrelated.

So please don't forget the sweat and brainpower of the numericist, who translates continunous maths into valid discrete numerics!
kwardle, doki and swetkyz like this.
cfdnewbie is offline   Reply With Quote

Old   December 13, 2011, 19:21
Default
  #3
New Member
 
Join Date: Sep 2010
Posts: 28
Rep Power: 0
zephyrus17 is on a distinguished road
That's very well pointed out. I was prompted to this issue when I was looking at the various commercial CFD packages like CFX, Fluent, OpenFOAM, etc and just wondered, when solving simple flow cases, whether any of them, having the exact same mesh/turbulence models/etc, would give the exact same results.

But you make a good point about the dedicate packages too.
zephyrus17 is offline   Reply With Quote

Old   December 14, 2011, 04:01
Default
  #4
Senior Member
 
cfdnewbie
Join Date: Mar 2010
Posts: 551
Rep Power: 11
cfdnewbie is on a distinguished road
I don't have much experience with commercial code (being a researcher), so I can only speak very generally. Even for simple test cases like flow over a flat plate (laminar even), I would expect to see different results, especially on the same grid. All the codes will use different discretization strategies (e.g. order of consistency), so the associated errors will be different as well.

You can argue that if you do a fully resolved simulation in each case with each code, you should get the same results - that's absolutely true, but all codes will need a different number of DOF to achieve this. So in the end you will be able to get the same results for simple cases, but the codes will differ in terms of efficiency and speed.

Keep also in mind that for the majority of flows, you will not be able to resolve it fully in a DNS sense, i.e. your approximation errors will play a role, and therefore the numerics will become "visible" in the solution.


So short answer: Ideally, fully resolved flow: Yes, no difference (if same eqn are solved in a consistent manner).
For all the rest (what we do most of the time): Differences due to different numerical approximations / errors.

hope this helps!
cheers!
cfdnewbie is offline   Reply With Quote

Old   December 18, 2011, 16:21
Default
  #5
otd
Member
 
private
Join Date: Mar 2009
Posts: 74
Rep Power: 8
otd is on a distinguished road
[QUOTE=cfdnewbie;335742]Interesting question, but I believe your argument in not fully valid. One could argue that physics is physics, math is math, so a Toyota Civic is the same as a Porsche 911.

Probably frivolous, but I own a Honda Civic, not a Toyota Corolla.
otd is offline   Reply With Quote

Old   December 21, 2011, 05:04
Default
  #6
Senior Member
 
cfdnewbie
Join Date: Mar 2010
Posts: 551
Rep Power: 11
cfdnewbie is on a distinguished road
[QUOTE=otd;336256]
Quote:
Originally Posted by cfdnewbie View Post
Interesting question, but I believe your argument in not fully valid. One could argue that physics is physics, math is math, so a Toyota Civic is the same as a Porsche 911.

Probably frivolous, but I own a Honda Civic, not a Toyota Corolla.

LOL, sorry.... only German cars here, never anything else
Owned a Chevy once, broke down all the time!
cfdnewbie is offline   Reply With Quote

Old   December 21, 2011, 10:08
Default
  #7
otd
Member
 
private
Join Date: Mar 2009
Posts: 74
Rep Power: 8
otd is on a distinguished road
[QUOTE=cfdnewbie;336584]
Quote:
Originally Posted by otd View Post


LOL, sorry.... only German cars here, never anything else
Owned a Chevy once, broke down all the time!
Sorry! I got a bit smart-a**y there. The honda corolla or the toyota civic are both good 'generic models' for everyman's car.

My '53 Chevy rusted out before it broke down - but I get your point.
otd is offline   Reply With Quote

Old   December 21, 2011, 17:27
Default
  #8
Senior Member
 
cfdnewbie
Join Date: Mar 2010
Posts: 551
Rep Power: 11
cfdnewbie is on a distinguished road
Hey otd,
no problem, you were well within your right to call me out on this
cfdnewbie is offline   Reply With Quote

Old   December 23, 2011, 07:04
Default
  #9
Member
 
Robert
Join Date: Feb 2010
Location: Germany
Posts: 31
Rep Power: 7
Robert@cfd is on a distinguished road
Very nice postings cfdnewbie.

Concerning this:
Quote:
Originally Posted by cfdnewbie View Post
So short answer: Ideally, fully resolved flow: Yes, no difference (if same eqn are solved in a consistent manner).
I thought even with "exact" resolving in time and space (DNS) and the same numerics (modelling, discretizations) results will be different if running on different machines (see butterfly effect). Even with double precision after long time simulations you will get different results.
Robert@cfd is offline   Reply With Quote

Old   December 24, 2011, 08:11
Default
  #10
Senior Member
 
cfdnewbie
Join Date: Mar 2010
Posts: 551
Rep Power: 11
cfdnewbie is on a distinguished road
Quote:
Originally Posted by Robert@cfd View Post
Very nice postings cfdnewbie.

Concerning this:

I thought even with "exact" resolving in time and space (DNS) and the same numerics (modelling, discretizations) results will be different if running on different machines (see butterfly effect). Even with double precision after long time simulations you will get different results.

Hallo Robert,
I have to admit this issue puzzles me as well, so all I can give you are some experiences / hunches on this:

1) The numerical dissipation of the scheme (unless one is using an non-dissipative one of course) will help you dissipate everything at the small scales, so especially round-off stuff that comes in at the smallest scale you can discretize

2) The nature of the flows I'm investigating (turbulence) also helps keeping round-off under control, since round-off error appear in a homogeneous isotropic way, as do the smallest scale turbulence fluctuations. In other words, the flip of a bit will look like dissipation at the smallest scale.

3) I've done a large number of DNS simulations, where the flow field at the smallest scale (influenced by round-off) is instantaneously different from a previous run, but the large scale statistics (mean values, skin friction, heat transfer) are the same. I don't want to go into to much detail, but the issue of "scale separation" helps us here in this case...

4) While I personally haven't seen a case where round-off indeed "butterflies" to the large scales, I think it might indeed be possible and can't be ruled out categorically.



Sorry for this rather loose collection of thoughts, I haven't been able to find a stringent way of looking at it for myself!

Very interesting topic, thanks a lot for your input,

Cheers and Merry Christmas to all!
cfdnewbie is offline   Reply With Quote

Old   December 29, 2011, 03:14
Post hi
  #11
kid
Senior Member
 
cfdkid
Join Date: Mar 2009
Posts: 133
Rep Power: 8
kid is on a distinguished road
hi guys,
Good generic talks here, but let us not forget OpenFOAM is free of cost and Fluent is commercial.

Regards,
kid
kid 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
Chemkin Forums? PSR.f program help? johnjohnmulley Main CFD Forum 2 August 5, 2013 07:53
Which opensource and free CAD program? Xwang Main CFD Forum 1 October 24, 2010 17:29
Update boundary conditions calculated by an external program CedricVH OpenFOAM 2 January 15, 2010 12:55
CAE program for heat conducting Rogerio Fernandes Brito FLUENT 0 February 3, 2008 12:22
*** CAE program for heat conducting *** Rogerio Fernandes Brito CFX 12 January 26, 2008 07:12


All times are GMT -4. The time now is 14:38.