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

RANS model

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

Reply
 
LinkBack Thread Tools Search this Thread Display Modes
Old   September 4, 2015, 09:45
Default RANS model
  #1
New Member
 
Join Date: Sep 2015
Posts: 5
Rep Power: 10
Cris2015 is on a distinguished road
Hi everyoone,

I'm using a RANS model, with k-epsilon turbulence model (IH2VOF).

One of the options in the model setup is to 'activate turbulence' and then specify 3 parameters (turbulence seed parameter, eddy viscosity and a parameter for boundary layer turbulence).

My question is if I choose not to include turbulence what is happening to the equations? they became NS?

Many thanks
Cris2015 is offline   Reply With Quote

Old   September 11, 2015, 00:40
Default
  #2
Member
 
Jingchang.Shi
Join Date: Aug 2012
Location: Hang Zhou, China
Posts: 78
Rep Power: 13
aerosjc is on a distinguished road
  • NS can describe turbulence, so if turbulence not included, the equations are still themselves.
  • If turbulence not included, you assume flow is always laminar whenever flow is turbulence.
  • You are using Reynolds Averaged Model and you inlcude no turbulence, I guess the model does no function.
  • Corrent me if I’m wrong.
aerosjc is offline   Reply With Quote

Old   September 11, 2015, 04:40
Default
  #3
Senior Member
 
Filippo Maria Denaro
Join Date: Jul 2010
Posts: 6,756
Rep Power: 71
FMDenaro has a spectacular aura aboutFMDenaro has a spectacular aura aboutFMDenaro has a spectacular aura about
Quote:
Originally Posted by aerosjc View Post
  • NS can describe turbulence, so if turbulence not included, the equations are still themselves.
  • If turbulence not included, you assume flow is always laminar whenever flow is turbulence.
  • You are using Reynolds Averaged Model and you inlcude no turbulence, I guess the model does no function.
  • Corrent me if I’m wrong.

that's not always true... without any turbulence model you have the original NS equations and turbulent flows obey to such equations...and DNS allows us to simulate turbulence from NS without any "artificial" inclusion of turbulence in the equations
FMDenaro is online now   Reply With Quote

Old   September 11, 2015, 06:39
Default
  #4
New Member
 
Join Date: Sep 2015
Posts: 5
Rep Power: 10
Cris2015 is on a distinguished road
Hi,

Thanks for the reply.

The model is RANS but I'm not activating the turbulence option.

The model runs well and of course no turbulence output files are created.
However, when I plot the velocity files (sqrt(U^2+V^2)), figure in attach, vortices clear appear near the structure that I'm modeling.

So my question is, since I didn´t activate the turbulence, these vortices are only due to molecular diffusion?
I'm supposing that the equations remain RANS but the velocity components will not include the fluctuations (Reynolds tensor will be zero)?
Another doubt:the molecular diffusion is not part of the fluctuations (I´m not sure on this)?

Finally, even if vortices appear in the figure probably I'll have to conclude that these velocities are overestimated since dissipation due to turbulence was not included in the simulation?

Thanks in advance for your help!
Attached Images
File Type: jpg velocity.jpg (28.4 KB, 11 views)
Cris2015 is offline   Reply With Quote

Old   September 11, 2015, 06:44
Default
  #5
New Member
 
Join Date: Sep 2015
Posts: 5
Rep Power: 10
Cris2015 is on a distinguished road
Quote:
Originally Posted by FMDenaro View Post
that's not always true... without any turbulence model you have the original NS equations and turbulent flows obey to such equations...and DNS allows us to simulate turbulence from NS without any "artificial" inclusion of turbulence in the equations

Please check my reply above/below
Cris2015 is offline   Reply With Quote

Old   September 11, 2015, 06:45
Default
  #6
Senior Member
 
Filippo Maria Denaro
Join Date: Jul 2010
Posts: 6,756
Rep Power: 71
FMDenaro has a spectacular aura aboutFMDenaro has a spectacular aura aboutFMDenaro has a spectacular aura about
Quote:
Originally Posted by Cris2015 View Post
Hi,

Thanks for the reply.

The model is RANS but I'm not activating the turbulence option.

The model runs well and of course no turbulence output files are created.
However, when I plot the velocity files (sqrt(U^2+V^2)), figure in attach, vortices clear appear near the structure that I'm modeling.

So my question is, since I didn´t activate the turbulence, these vortices are only due to molecular diffusion?
I'm supposing that the equations remain RANS but the velocity components will not include the fluctuations (Reynolds tensor will be zero)?
Another doubt:the molecular diffusion is not part of the fluctuations (I´m not sure on this)?

Finally, even if vortices appear in the figure probably I'll have to conclude that these velocities are overestimated since dissipation due to turbulence was not included in the simulation?

Thanks in advance for your help!

if you are using the steady 2D RANS formulation you have necessarily activated a turbulence model.... otherwise your solution has no physical meaning
FMDenaro is online now   Reply With Quote

Old   September 11, 2015, 06:49
Default
  #7
New Member
 
Join Date: Sep 2015
Posts: 5
Rep Power: 10
Cris2015 is on a distinguished road
Quote:
Originally Posted by FMDenaro View Post
that's not always true... without any turbulence model you have the original NS equations and turbulent flows obey to such equations...and DNS allows us to simulate turbulence from NS without any "artificial" inclusion of turbulence in the equations
Quote:
Originally Posted by aerosjc View Post
  • NS can describe turbulence, so if turbulence not included, the equations are still themselves.
  • If turbulence not included, you assume flow is always laminar whenever flow is turbulence.
  • You are using Reynolds Averaged Model and you inlcude no turbulence, I guess the model does no function.
  • Corrent me if I’m wrong.
Please check my reply above/below
Cris2015 is offline   Reply With Quote

Old   September 11, 2015, 06:50
Default
  #8
Member
 
Jingchang.Shi
Join Date: Aug 2012
Location: Hang Zhou, China
Posts: 78
Rep Power: 13
aerosjc is on a distinguished road
According to definition, vortices are generated due to velocity gradients, instead of diffusion.
aerosjc is offline   Reply With Quote

Old   September 11, 2015, 07:41
Default
  #9
Senior Member
 
Filippo Maria Denaro
Join Date: Jul 2010
Posts: 6,756
Rep Power: 71
FMDenaro has a spectacular aura aboutFMDenaro has a spectacular aura aboutFMDenaro has a spectacular aura about
Quote:
Originally Posted by Cris2015 View Post
Please check my reply above/below

what should I check? Again, a 2D steady RANS solves for a statistically steady state and that requires a supplied turbulence modelling (no matter of turbulent intensity prescribed at inflow or similar).
FMDenaro is online now   Reply With Quote

Old   September 11, 2015, 07:50
Default
  #10
Member
 
Jingchang.Shi
Join Date: Aug 2012
Location: Hang Zhou, China
Posts: 78
Rep Power: 13
aerosjc is on a distinguished road
Quote:
Originally Posted by FMDenaro View Post
what should I check? Again, a 2D steady RANS solves for a statistically steady state and that requires a supplied turbulence modelling (no matter of turbulent intensity prescribed at inflow or similar).
I guess FMDenaro means that you need to supply some terms like mixing length scale to close the equations since RANS introduces new terms.
aerosjc is offline   Reply With Quote

Old   September 11, 2015, 07:50
Default
  #11
New Member
 
Join Date: Sep 2015
Posts: 5
Rep Power: 10
Cris2015 is on a distinguished road
Dear Filippo, I have posted my second reply (with a figure) but I guess you have already seen it.
If turbulence has to be supplied I still don't understand why in the model interface is available the option 'activate turbulence'
Cris2015 is offline   Reply With Quote

Old   September 11, 2015, 07:58
Default
  #12
Senior Member
 
Filippo Maria Denaro
Join Date: Jul 2010
Posts: 6,756
Rep Power: 71
FMDenaro has a spectacular aura aboutFMDenaro has a spectacular aura aboutFMDenaro has a spectacular aura about
Quote:
Originally Posted by Cris2015 View Post
Dear Filippo, I have posted my second reply (with a figure) but I guess you have already seen it.
If turbulence has to be supplied I still don't understand why in the model interface is available the option 'activate turbulence'

I don't know the code you are using, it depends on what "activate turbulence" means in the user guide ... that's for sure that RANS without any turbulence modelling has no physical meaning
FMDenaro is online now   Reply With Quote

Reply

Tags
rans modelling, turbulence model

Thread Tools Search this Thread
Search this Thread:

Advanced Search
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 Off
Pingbacks are On
Refbacks are On


Similar Threads
Thread Thread Starter Forum Replies Last Post
Superlinear speedup in OpenFOAM 13 msrinath80 OpenFOAM Running, Solving & CFD 18 March 3, 2015 06:36
coalChemistryFoam: "Foam::error::printStack(Foam::Ostream&) at ??:?" musabai OpenFOAM Running, Solving & CFD 2 February 20, 2015 15:07
manualInjection model in sprayFoam Mentalo OpenFOAM Running, Solving & CFD 1 April 2, 2014 10:29
RANS vs.LES turbulence model usker Siemens 1 April 3, 2008 19:38
Advanced Turbulence Modeling in Fluent, Realizable k-epsilon Model Jonas Larsson FLUENT 5 March 13, 2000 04:27


All times are GMT -4. The time now is 04:26.