|
[Sponsors] |
May 23, 2013, 16:45 |
epsilon and omega free stream BC's
|
#1 |
Senior Member
Ehsan
Join Date: Oct 2012
Location: Iran
Posts: 2,208
Rep Power: 27 |
in this page:
http://www.cfd-online.com/Wiki/Turbu...ary_conditions there are different formulas for epsilon and omega,I want to test with both kOmegaSST and k-epsilon-realizable(does anyone knows how to set it in OF?) how can find out which is appropriate for OF with your experience? for epsilon we have: or or and for omega we have: or or
__________________
Injustice Anywhere is a Threat for Justice Everywhere.Martin Luther King. To Be or Not To Be,Thats the Question! The Only Stupid Question Is the One that Goes Unasked. Last edited by immortality; May 25, 2013 at 16:01. |
|
May 25, 2013, 16:12 |
|
#2 |
Senior Member
Ehsan
Join Date: Oct 2012
Location: Iran
Posts: 2,208
Rep Power: 27 |
Hi again
I saw in page 25 of this thesis that he has used :(I managed this LaTeX formula at last!) the thesis: Simulation and validation of compressible flow in nozzle geometries and validation of OpenFOAM for this application is this work and change in formula by adding l is acceptable? http://s2.picofile.com/file/77781039...rbeit.pdf.html
__________________
Injustice Anywhere is a Threat for Justice Everywhere.Martin Luther King. To Be or Not To Be,Thats the Question! The Only Stupid Question Is the One that Goes Unasked. |
|
May 25, 2013, 17:17 |
|
#3 | |
Retired Super Moderator
Bruno Santos
Join Date: Mar 2009
Location: Lisbon, Portugal
Posts: 10,981
Blog Entries: 45
Rep Power: 128 |
Hi Ehsan,
Quick note - here's another link to said thesis: http://www.rw.ethz.ch/alumni/thesis/...sterarbeit.pdf From what I can figure out, the author of the thesis went along with the spirit of this whole turbulence stuff. If you look at the first sections of the wiki page you've mentioned earlier, you'll see in "Modified turbulent viscosity" that the "turbulent length scale" is used for the "modified turbulent viscosity". This is possibly what influenced the author's decision. As for the wiki, it does have some more helpful hints, namely the one about "Eddy viscosity ratio": http://www.cfd-online.com/Wiki/Eddy_viscosity_ratio In essence, if you are able to calculate the "turbulent length scale", it's easier to calculate through it, instead of through the "Eddy viscosity ratio". In case you haven't seen how to calculate the relevant parameters, here's a quote: Quote:
Best regards, Bruno
__________________
|
||
September 6, 2013, 09:49 |
|
#4 |
Senior Member
Eloïse
Join Date: Jul 2012
Location: Trondheim, Norway
Posts: 113
Rep Power: 14 |
Hi Ehsan,
I'm also interested in some free-stream turbulence problems. I run a simulation of an object in uniform flow and I would like to perform a sensitivity study on the inlet turbulence intensity level. I've tested several TI levels (3% to 7%), but in all cases the TI drops to about 1% a few cells after the inlet. So whatever TI level I put at the inlet, my object is always in a 1% TI level. Have you experienced something similar with your simulations? If not, what is the way to maintain TI through the domain? About my case: the velocity profile is steady and uniform, the mesh is uniform, I use k-omega SST turbulence model. Best regards, Eloïse |
|
September 7, 2013, 08:47 |
|
#5 |
Retired Super Moderator
Bruno Santos
Join Date: Mar 2009
Location: Lisbon, Portugal
Posts: 10,981
Blog Entries: 45
Rep Power: 128 |
Hi Eloïse,
A few questions regarding your case:
Bruno
__________________
|
|
September 7, 2013, 09:00 |
|
#6 | |
Senior Member
Ehsan
Join Date: Oct 2012
Location: Iran
Posts: 2,208
Rep Power: 27 |
Quote:
sorry for delay,how did you notice that TI in internal cells becomes low?
__________________
Injustice Anywhere is a Threat for Justice Everywhere.Martin Luther King. To Be or Not To Be,Thats the Question! The Only Stupid Question Is the One that Goes Unasked. |
||
September 9, 2013, 03:54 |
|
#7 | |
Senior Member
Eloïse
Join Date: Jul 2012
Location: Trondheim, Norway
Posts: 113
Rep Power: 14 |
Quote:
1. It is a 3D case 2. I ran the case in laminar and haven't had any specific problem. Do you think of something specific which should be checked in the laminar case? I'll have a look at the paper, thanks for the link. 3. It is a steady-state simulation 4. The TI decreases in the very first cells after the inlet. Around the object, the TI increases again. (see attached image, which is a cut of the domain) Ehsan: that's how I detected the issue. Regards, Eloïse |
||
September 9, 2013, 06:30 |
|
#8 |
Senior Member
Ehsan
Join Date: Oct 2012
Location: Iran
Posts: 2,208
Rep Power: 27 |
How did you define turbulence intensity field?is there any postProcessing utility for that?
__________________
Injustice Anywhere is a Threat for Justice Everywhere.Martin Luther King. To Be or Not To Be,Thats the Question! The Only Stupid Question Is the One that Goes Unasked. |
|
September 9, 2013, 07:53 |
|
#9 | |
Senior Member
Eloïse
Join Date: Jul 2012
Location: Trondheim, Norway
Posts: 113
Rep Power: 14 |
Quote:
Eloïse |
||
September 9, 2013, 08:33 |
|
#10 |
Senior Member
Ehsan
Join Date: Oct 2012
Location: Iran
Posts: 2,208
Rep Power: 27 |
how you have defined TI in inlet?
you should set it to a constant so that can define k in inlet not inverse.
__________________
Injustice Anywhere is a Threat for Justice Everywhere.Martin Luther King. To Be or Not To Be,Thats the Question! The Only Stupid Question Is the One that Goes Unasked. |
|
September 9, 2013, 09:47 |
|
#11 | |
Senior Member
Eloïse
Join Date: Jul 2012
Location: Trondheim, Norway
Posts: 113
Rep Power: 14 |
Quote:
Eloïse |
||
September 9, 2013, 12:28 |
|
#12 |
Senior Member
Ehsan
Join Date: Oct 2012
Location: Iran
Posts: 2,208
Rep Power: 27 |
is velocity constant in inlet?if it varies during run and you have set a fixed k and omega it might makes some error,I don't know how you set your BC's,could you attach them?
__________________
Injustice Anywhere is a Threat for Justice Everywhere.Martin Luther King. To Be or Not To Be,Thats the Question! The Only Stupid Question Is the One that Goes Unasked. |
|
September 9, 2013, 15:32 |
|
#13 |
Retired Super Moderator
Bruno Santos
Join Date: Mar 2009
Location: Lisbon, Portugal
Posts: 10,981
Blog Entries: 45
Rep Power: 128 |
Greetings to all!
@Eloïse: Well, the only way to maintain the turbulence intensity is if you introduce small obstacles, such as wires or something like that. Because there are a few details in play here and all of them are related to the solver doing what it was told to do. More specifically:
Which reminds me... have you ever seen those old fluid dynamics/mechanics videos showing how turbulence work and so on? I can't remember where I kept the link for them... Anyway, one of them shows the effect of a wire inducing turbulence. Best regards, Bruno
__________________
|
|
September 10, 2013, 04:38 |
|
#14 | |
Senior Member
Eloïse
Join Date: Jul 2012
Location: Trondheim, Norway
Posts: 113
Rep Power: 14 |
Quote:
@Bruno: Thanks for the advices on fvSchemes. I'll have a look at that too. I guessed that if there is nothing to create velocity gradients and maintain turbulence, then the turbulence would fade out. Or at least that's how I understood my problem. I asked about it here because I was wondering if others observed the same and if something could be done about it |
||
|
|
Similar Threads | ||||
Thread | Thread Starter | Forum | Replies | Last Post |
calculation of k, epsilon and omega | kroetenechse | OpenFOAM Pre-Processing | 31 | March 5, 2021 11:32 |
Bounding epsilon or bounding omega | Stylianos | OpenFOAM | 8 | February 23, 2018 14:41 |
Problem with K omega boundary conditions | malaboss | OpenFOAM Pre-Processing | 6 | April 20, 2015 00:25 |
problem from k epsilon to k omega | NJG | OpenFOAM Running, Solving & CFD | 0 | April 15, 2013 09:24 |
why omega instead of epsilon in turbulence modeling | pgoyal10 | Main CFD Forum | 1 | May 5, 2012 12:02 |