
[Sponsors] 
April 2, 2013, 19:45 
Turbulent Prandtl number

#1 
New Member
Join Date: Jan 2013
Posts: 11
Rep Power: 6 
Hello,
I'm trying to run a given case and it is said that the turbulent Prandtl number is set to 0.9 (no more information). I've searched in Fluent and the definition of the turbulent Prandtl number is unclear to me, there are at least 5 different turbulent Prandtl numbers (with SST):  TKE inner  TKE outer  SDR inner  SDR outer  Energy  Wall I don't think the first four are related to the given turbulent Prandtl number in my paper. But the Energy and Wall turbulent Prandtl number have the same value, close to 0.9. Should I change both of them? Do you have any information about turbulent Prandtl number? I've looked at the definition of turbulent Prandtl number (wiki, cfdonline) and at the definition in Fluent Manual, but I can't really figure out which one is concerned here. The case is about hypersonic interactions and heat transfer. Thanks for your help, Ravenn 

April 3, 2013, 10:57 

#2 
Senior Member
OJ
Join Date: Apr 2012
Location: United Kindom
Posts: 475
Rep Power: 13 
Can you give more information about the paper you are referring to?
OJ 

April 3, 2013, 11:21 

#3 
Administrator
Peter Jones
Join Date: Jan 2009
Posts: 685
Rep Power: 10 
The turbulent Prandtl number is used to relate turbulent heat flux with turbulent momentum flux. The definition is given in for example this CFDWiki page:
http://www.cfdonline.com/Wiki/Favre...okes_equations By setting a constant turbulent Prandtl number you can compute the turbulent heat flux based on the turbulent eddyviscosity that a turbulence model predicts. Changing the turbulent Prandtl number is a way to tune heat transfer results. So always be skeptical about papers where it is not stated what turbulent Prandtl number has been used. 

April 3, 2013, 11:24 

#4 
New Member
Join Date: Jan 2013
Posts: 11
Rep Power: 6 
I only have hard copies of different papers:
 AIAA paper 930779: numerical simulation of crossing shock/turbulent boundary layer interaction at Mach 8.3 "The baldwinLomax and the Rodi kepsilon turbulence models are employed, the molecular viscosity was specified by Sutherland's law. The molecular Prandtl number is 0.73 (air) and the turbulent Prandtl number is 0.9."  AIAA Journal Vol 39 No 6 June 2001 p: 985995: Insights in Turbulence Modeling for CrossingShockWave/BoundaryLayer Interactions "The Sutherland's law is used to calculate the laminar viscosity,and a constant laminar Prandtl number of 0.72 is assumed. Central differencing is used to evaluate the viscous terms.The steady solution is obtained by applying a timemarching method based on the hybrid approximate actorization/relaxation algorithm. The first solutions presented here are computed with the k–x turbulence model by Wilcox and a constant turbulent Prandtl number of 0.9." Ravenn Edit: That's exactly the problem, I've seen in many papers "constant turbulent Prandtl number of 0.9" without more information, I suppose it is linked to the hypersonic characteristic of the flow, but it dos not seem like a "cheat" or something to obtain better results. Or someone proposed in the 50's to use this value for this case and ever since everybody uses it. Edit 2: In the cfdonline wiki page, it is said after equation 26 that: "Where Prt is a turbulent Prandtl number. Often a constant Prt = 0.9 is used." In that case, which one of the fluent turbulent Prandtl number is concerned? Is it possible that turbulent Prendtl numbers in fluent are not this turbulent Prandtl number Prt defined here, hence the Prt would be a results of the calculation and not a property defined before the calculation? Last edited by Ravenn; April 3, 2013 at 11:40. 

April 3, 2013, 11:50 

#5 
Administrator
Peter Jones
Join Date: Jan 2009
Posts: 685
Rep Power: 10 
In laminar flow the viscosity is used to compute the heatflux using Fourier's law and a laminar, well defined, Prandtl number as:
Most turbulence models just give a turbulent eddy viscosity . By setting a turbulent Prandtl number the turbulent heat flux can be estimated in the same way by just using the turbulent eddy viscosity that the turbulence mode predicts: Using a constant turbulent Prandtl number is a simplification and it is not fully correct. Experimentally a value of something close to 0.9 has been measured. You can find more information and further references here: http://en.wikipedia.org/wiki/Turbulent_Prandtl_number 

April 3, 2013, 17:18 

#6 
New Member
Join Date: Jan 2013
Posts: 11
Rep Power: 6 
So, I've postprocessed 2 different calculations, one with the default values for energy and wall Prandtl number, and the other with energy and wall Prandtl number set to 0.9.
I've read carefully fluent user manual and here is what I understood:    In Fluent, it is possible to obtain , and . With these values on a line normal to a surface, I've calculated and then . It appears that for the first calculation, outside the laminar part of boundary layer, which is the value of energy and wall Prandtl numbers. For the second calculation where I changed the value of energy and wall Prandtl numbers to 0.9, outside the laminar part of boundary layer. In the laminar part of boundary layer, , hence is undefined, or very high. I'm wondering now what is the value of in the laminar part of boundary layer for the papers where is said to be fixed to 0.9. In my opinion it depends on which parameter is used to calculate the other:  , fixed to 0.9, and are used to calculate (papers)  and are calculated by the turbulence model, and can be deduced with the same procedure I used. (Fluent) 

May 23, 2016, 13:06 

#7 
Senior Member

Dear Ravenn
I was facing the same issue. I used two turbulence models, kepsilon and komega SST. I was getting incorrect Nu from the komega. So I decreased very much the Prt and got my result close to experiment. But since I used very low value (0.35) I am still doubtful. Are you still working on that too? yuor post seems 3 years old 

May 24, 2016, 15:57 

#8 
New Member
Join Date: Jan 2013
Posts: 11
Rep Power: 6 
Hello Shamoon,
I'm sorry but it's been a while since I last worked on this subject (almost 3 years as you see) and I don't really remember everything, I've rediscovered the subject with your question. It appears I still receive the alerts for this post^^ Good luck for your research! 

July 18, 2016, 22:25 
how to change the Prt with ke

#9  
New Member
minqiangwu
Join Date: Jul 2016
Posts: 1
Rep Power: 0 
Quote:
would you mind telling me how to change the Prt with ke，I do really need it THX 

July 19, 2016, 11:36 

#10 
Senior Member

In the model> viscous select kepsilon then select realziable
On the right side there is a list of constants among which Energy and Wall Prandtal numbers are there 

July 19, 2016, 11:38 

#11  
Senior Member

Quote:
Dear Ravenn I was facing the same issue. I used two turbulence models, kepsilon and komega SST. I was getting incorrect Nu from the komega. So I decreased very much the Prt and got my result close to experiment. But since I used very low value (0.35) I am still doubtful. 

January 31, 2017, 05:00 

#12  
Member
Lorenzo Mazzei
Join Date: Dec 2010
Posts: 45
Rep Power: 8 
Be careful, you are just tuning your turbulent fluxes to match the experimental data. The constant Prt is usually a wrong assumption, as it varies within the domain.
Quote:


January 31, 2017, 06:55 

#13  
Senior Member

Quote:
So I rechecked my simulation and correct my tube length adn now its ok 

January 31, 2017, 10:11 

#14  
Member
Lorenzo Mazzei
Join Date: Dec 2010
Posts: 45
Rep Power: 8 
Quote:
That is however not representative of the reality. There are several publications in which turbulent fluxes are calculated with LES to produce the equivalent Prt/Sct distributions. In literature there are also several attempts to modify classical turbulence modeles to account for variable Prt/Sct number (see for example works by Goldberg et al. or Keistler). 

June 20, 2017, 11:13 

#15 
New Member
Vedamt Chittlangia
Join Date: Feb 2016
Posts: 4
Rep Power: 3 
Turbulent Prandtl number affects the heat transfer calculations as it is used to calculate turbulent heat flux.
A experiment based variable turbulent Pr number correlation can be used as given by Kays & Crawford. It can be added to Fluent using UDF: DEFINE_PRANDTL_T(Prt, c, t) { Prt = [some expression]; return Prt; } It can be added by compiling/interpreting this UDF and adding the new Prandtl number from Models>Viscous>ke (or any other) > drop down menu for Temperature Prandtl number 

Thread Tools  
Display Modes  


Similar Threads  
Thread  Thread Starter  Forum  Replies  Last Post 
compressible flow in turbocharger  riesotto  OpenFOAM  50  May 26, 2014 01:47 
Question regarding Reynolds Number and Turbulent Intensity  maxj  Main CFD Forum  4  March 5, 2013 07:16 
Prandtl Number  Thomas P. Abraham  Main CFD Forum  10  February 15, 2011 11:11 
BlockMeshmergePatchPairs  hjasak  OpenFOAM Native Meshers: blockMesh  11  August 15, 2008 07:36 