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

Estimating Turbulent Viscosity Ratio from velocity data

Register Blogs Community New Posts Updated Threads Search

Reply
 
LinkBack Thread Tools Search this Thread Display Modes
Old   September 25, 2017, 06:22
Smile Estimating Turbulent Viscosity Ratio from velocity data
  #1
New Member
 
Zachary MacChesney
Join Date: Jan 2017
Posts: 6
Rep Power: 9
macchesney is on a distinguished road
Hello,

I have a plot here test1.PNG that shows the modelled turbulent viscosity ratio in the IDDES formulation in a backwards facing step geometry. As you can see once the flow enters the separated region, the turbulent viscosity ceases to be modelled and its effects are instead captured by the turbulent fluctuations in the flow with LES. Here you can see the turbulent intensity profiles generated by calculating v' / vref. test2.PNG.

My question is: is it possible to estimate the turbulent viscosity (ie. the ratio of apparent viscous forces due to turbulence with molecular viscosity) given the turbulence intensity?

Thanks for you interest!
macchesney is offline   Reply With Quote

Old   September 25, 2017, 06:39
Default
  #2
Senior Member
 
Filippo Maria Denaro
Join Date: Jul 2010
Posts: 6,849
Rep Power: 73
FMDenaro has a spectacular aura aboutFMDenaro has a spectacular aura aboutFMDenaro has a spectacular aura about
Quote:
Originally Posted by macchesney View Post
Hello,

I have a plot here Attachment 58611 that shows the modelled turbulent viscosity ratio in the IDDES formulation in a backwards facing step geometry. As you can see once the flow enters the separated region, the turbulent viscosity ceases to be modelled and its effects are instead captured by the turbulent fluctuations in the flow with LES. Here you can see the turbulent intensity profiles generated by calculating v' / vref. Attachment 58612.

My question is: is it possible to estimate the turbulent viscosity (ie. the ratio of apparent viscous forces due to turbulence with molecular viscosity) given the turbulence intensity?

Thanks for you interest!

If you are using an LES with the (static or dynamic) Smagorinsky model, in your code is computed the eddy viscosity field. You just need to access the data. Turbulent intensity alone cannot provied a full field.
FMDenaro is offline   Reply With Quote

Old   September 25, 2017, 08:43
Default
  #3
New Member
 
Zachary MacChesney
Join Date: Jan 2017
Posts: 6
Rep Power: 9
macchesney is on a distinguished road
Hi there.. from my somewhat limited understanding, the idea behind the turbulent viscosity parameter is that turbulent fluids behave as if their viscosity has increased.. so in these models we substitute a region of turbulence with a fluid with an apparent viscosity. This is only done for the Smagorinsky RANS model however.. within DES we have RANS regions and LES regions.. therefore when I show the turbulent viscosity parameter in a plot, I can only see the turbulent viscosity parameter in the rans regions (inlet boundary layer). After the step LES takes over and the modelled turbulent viscosity disappears.. what I am actually interested in is what is the apparent viscosity of the entire turbulent region.. I am able to obtain the turbulence intensity from the LES region.. and was wondering if I can use this to estimate the apparent viscosity of the the turbulent region.. that ms for your reply.
macchesney is offline   Reply With Quote

Old   September 25, 2017, 08:48
Default
  #4
Senior Member
 
Filippo Maria Denaro
Join Date: Jul 2010
Posts: 6,849
Rep Power: 73
FMDenaro has a spectacular aura aboutFMDenaro has a spectacular aura aboutFMDenaro has a spectacular aura about
Quote:
Originally Posted by macchesney View Post
Hi there.. from my somewhat limited understanding, the idea behind the turbulent viscosity parameter is that turbulent fluids behave as if their viscosity has increased.. so in these models we substitute a region of turbulence with a fluid with an apparent viscosity. This is only done for the Smagorinsky RANS model however.. within DES we have RANS regions and LES regions.. therefore when I show the turbulent viscosity parameter in a plot, I can only see the turbulent viscosity parameter in the rans regions (inlet boundary layer). After the step LES takes over and the modelled turbulent viscosity disappears.. what I am actually interested in is what is the apparent viscosity of the entire turbulent region.. I am able to obtain the turbulence intensity from the LES region.. and was wondering if I can use this to estimate the apparent viscosity of the the turbulent region.. that ms for your reply.

No. In LES the SGS model can be based on the Bousinnesq as same as happens for the RANS formulation. If you have an LES with a Smagorinsky model you have a local eddy viscosity variable. What you have in the LES with the dynamic Smagorinsky model is that the eddy viscosity vanishes (correctly) in regions of laminar flow condition, for example close a wall for y+ =O(1).
FMDenaro is offline   Reply With Quote

Reply

Tags
turbulent intensity, turbulent viscosity


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
Problem with divergence TDK FLUENT 13 December 14, 2018 07:00
3D CFD simulation of propeller blade - Turbulent viscosity limited to viscosity ratio Quentin_C FLUENT 1 April 4, 2017 06:32
Turbulent viscosity Limited to viscosity ratio Adrian FLUENT 12 September 21, 2011 05:22
Turbulent viscosity limited to viscosity ratio of 1.0000000e+5 eespi002 FLUENT 3 June 30, 2009 14:24
turbulent viscosity limited to viscosity ratio mukkarum FLUENT 3 May 11, 2004 05:05


All times are GMT -4. The time now is 14:18.