Turbulent Prandtl number
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 
Can you give more information about the paper you are referring to?
OJ 
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. 
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? 
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 
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 
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! 
how to change the Prt with ke
Quote:
would you mind telling me how to change the Prt with ke，I do really need it THX 
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 
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. 
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:

Quote:
So I rechecked my simulation and correct my tube length adn now its ok 
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). 
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 
All times are GMT 4. The time now is 06:21. 