# Reynolds Stresses in Fluent

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 June 5, 2010, 11:09 Reynolds Stresses in Fluent #1 New Member   Join Date: Jun 2010 Posts: 9 Rep Power: 15 Hello, I am working the Re=3900 2-D cylinder problem for a turbulence class in graduate school. We were to use the k-e and k-w turbulence models and plot uu Reynolds stress profiles to compare to Kravchenko's paper. How do I back out the Reynolds stress components I need from the turbulent quantities given by these two models? I imagine I just use the Bouss Approx, and write a UDF to plot them, but I've never used the UDF's. Can someone give me some idiot-proof step by step instructions on how to do this? Thanks!

 January 10, 2011, 14:31 #2 New Member   Smith, John Join Date: Nov 2010 Posts: 8 Rep Power: 14 i have the same question right now

 February 1, 2012, 09:50 Same doubt #3 New Member   Andre Novgorodcev Join Date: Jan 2012 Posts: 3 Rep Power: 13 I'm facing the same problem right now.

 February 1, 2012, 12:57 #4 New Member   Join Date: Mar 2011 Posts: 7 Rep Power: 14 You have to use Bous. Approx. Use the Define->Custom Field Functions to calculate delu=dx-velocity-dx + dy-velocity-dy + dz-velocity-dz then calculate strain rate tensor S_uu S_uu=dx-velocity-dx - 1 / 3 * dukxk Then use -u'u'=2*viscosity-turb*S_uu/density - 2/3*turb-kinetic-energy maphd likes this.

September 15, 2013, 07:29
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Quote:
 Originally Posted by dynamics You have to use Bous. Approx. Use the Define->Custom Field Functions to calculate delu=dx-velocity-dx + dy-velocity-dy + dz-velocity-dz then calculate strain rate tensor S_uu S_uu=dx-velocity-dx - 1 / 3 * dukxk Then use -u'u'=2*viscosity-turb*S_uu/density - 2/3*turb-kinetic-energy
What is dukxk ?

April 14, 2014, 16:26
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Quote:
 Originally Posted by dynamics You have to use Bous. Approx. Use the Define->Custom Field Functions to calculate delu=dx-velocity-dx + dy-velocity-dy + dz-velocity-dz then calculate strain rate tensor S_uu S_uu=dx-velocity-dx - 1 / 3 * dukxk Then use -u'u'=2*viscosity-turb*S_uu/density - 2/3*turb-kinetic-energy
Hi Dynamix,
I need to define Reynolds stresses in fluent with k-e model. I also could not understand the term dukxk and you define in the beginning something called delu but you do not use it anywhere. could you explain what is that one also?
Thank you very much, I really appreciate it.

 December 1, 2015, 10:16 Boussinesq aproximation solves the problem #7 New Member   Miloslav Dohnal Join Date: Jan 2014 Location: Brno, The Czech Republic Posts: 7 Rep Power: 11 Hello everyone Yesterday, I found this thread and spent almost whole day by searching for answer with some, let's say, proof or explanation. I found that the problem raises from the transport equation of k in two equation models; and it's one of the problems in turbulence modeling. In general, we have more unknowns to solve than available equations; and Boussinesq aproximation is used to enclose transport equation of k in one- or two- equation models. And now, back to problem: let's have the Boussinesq aproximation where is Kronecker delta and also have those two vectors: position vector and velocity vector if the flow is incompressible, then Now, it only depands what kind of Reynolds stress you want. For instance, if you want to know normal stress simply put and ; then, substitute first element in and to the Boussinesq aproximation and do the math. You will obtain: And if you want to know shear stress , just put and and you will get: Apply same principle for any other Reynolds stress you want to know and just simply put this to Fluent using Custom Field Functions Reference Wilcox, D. C., 2006, Turbulence Modeling for CFD, 3rd ed., Dcw Industries, Incorporated. masood135, Grigor Nikolov, srv537 and 4 others like this.

 Tags boussinesq, reynolds, stress