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 mp121209 March 12, 2013 14:17

Heat transfer on a cylinder

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Hello,

I study the problem flow around a cylinder with heat transfer. The cylinder surface is rough, its temperature is greater than the total temperature in the free stream at 10 K. The streamwise velocity is at about 10 m/s.
Comparing the results (CFX solver is used in this case, sst turbulence model, turbulent wall functions are used, too) with the experimental data and numerical data of some author gives good agree in the pressure coefficient and the skin friction profiles. About the Frossling number (Fr = Nu/sqrt(Re)) it's quite good, except the nose part. Near to the nose (about up to 7 degrees) there's a big, and on principle, difference between the result vs experimental data. According to the experimental data the Fr at nose is about equal to 1. In the numerical result, it's equal to about 0.5.

I can't figure out the problem. Will anybody help with this?

 ghorrocks March 12, 2013 18:41

Have you been through the basic accuracy checks? http://www.cfd-online.com/Wiki/Ansys..._inaccurate.3F

Your results look pretty good for a heat transfer model. It is challenging to get heat transfer much more accurate than you already have it. Many turbulent models have known issues with stagnation points and this may be the cause of this issue.

 mp121209 March 13, 2013 09:33

Can you direct me to some of refs on issues which turbulent models have with the area near to the stagnation point?
I'd used the sst model for my simulation. The result, I agree with you, is good. But I want to solve the issue near to the stagnation point.

 ghorrocks March 13, 2013 17:25

Any turbulence textbook should discuss this - my reference is Turbulence Modelling for CFD by Wilcox.

As far as I know (but I am not a turbulence modelling expert) all the common 2eqn models suffer from this. SST might be better than k-e here, but you had better check that. Not sure if RSM would help here, but you certainly will get convergence difficulties if you try - but worth a try never the less. So you may have to resort to LES/DES/SAS approaches.

 mp121209 March 13, 2013 19:48

Okay,
LES and DES in this case are meaningless. I've already tried SAS, but it gave me a strange result.
If anyone knows where can I find some works on the problem "Turbulent models and heat transfer near to the stagnation point", please direct me to.
Thank you so much!

 ghorrocks March 13, 2013 20:02

I have already directed you to a key reference. As I said this is a well known feature of k-e and other turbulence models so any turbulence modelling text will discuss it.

Can you explain why LES and DES are meaningless? LES especially should be perfect, but will require a major increase in simulation cost.

SAS is probably not ideally suited to this (it is more about vortices shed off the back of bluff bodies) so it does not do much for the stagnation point. So I am not surprised it did not help.

 aerothermal July 16, 2013 14:16

See the post, it may help you

Quote:
 Originally Posted by mp121209 (Post 413513) Hello, I study the problem flow around a cylinder with heat transfer. The cylinder surface is rough, its temperature is greater than the total temperature in the free stream at 10 K. The streamwise velocity is at about 10 m/s. Comparing the results (CFX solver is used in this case, sst turbulence model, turbulent wall functions are used, too) with the experimental data and numerical data of some author gives good agree in the pressure coefficient and the skin friction profiles. About the Frossling number (Fr = Nu/sqrt(Re)) it's quite good, except the nose part. Near to the nose (about up to 7 degrees) there's a big, and on principle, difference between the result vs experimental data. According to the experimental data the Fr at nose is about equal to 1. In the numerical result, it's equal to about 0.5. I can't figure out the problem. Will anybody help with this?
See the post, it may help you

http://www.cfd-online.com/Forums/ope...-m-0-07-a.html

and the paper

http://www.ats4i.com.br/en/publicati...11-38-0023.pdf

repository

http://code.google.com/p/ats4iopentools/

Besides other limintations common in CFD codes to model the similarity between momentum transfer and heat transfer in rough walls, your "nose" region issue may be related to transition. See that Pr_t analogy is not direct, you have a double layer effect, so an additional resistance must be taken into account in the form of a St_k. See Kays and Crawford book for the theory or the paper above.

For the transition, take a look at paper linked below:
http://arc.aiaa.org/doi/abs/10.2514/6.2010-7672

Regards,

Aerothermal

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