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 mmahdinia July 2, 2010 02:17

Mean calculation in LES

Hi,

I am using LES to calculate a completely unsteady flow (not periodic or something predictable in mean). It is of need to calculate the turbulence quantities like kinetic energy and dissipation.

The problem is that I don't know how to calculate the means and fluctuations in the unsteady flow? I read somewhere about averaging on a time window about every point in time. Is this possible? If yes, how much should be the window size? I'd appreciate it, if you could mention any other method you might know.

Sincerely,
Maani

 rsin July 8, 2010 18:59

Is your turbulent kinetic energy has any compositions?

In the case of stirred reactors, turbulent kinetic energy has two compositions: 1) periodic component due to cyclic passage of the blade and 2) random component due to the irregularities and randomness of the real turbulence.

In that case, i can help? What are you working at? any theoretical equation which define your turbulence energy for LES.

 mmahdinia July 9, 2010 03:45

Thank you for the reply. I am simulating the lock-exchange density current flow. Here, a heavy lock fluid is released under an ambient light fluid. The dynamic Smagorinsky method is used for the simulation. Here is an animation:

http://mech.sharif.edu/~mahdinia/A1.htm

The simulation is completely unsteady (not even periodic or like that) and I need to calculate a mean and fluctuation component, to calculate the profiles of epsilon, k, production, ... in different sections. I have searched the web but found nothing about how to calculate the mean.

I'd really appreciate it if you could help me with this (it wouldn't be a problem if the physics of your simulations are different). I really am looking towards the basics.

Sincerely,
Mahdinia

 rsin July 11, 2010 23:00

By defination turbulent kinetic energy = 1/2 (u'u'+v'v'+w'w')

turbulent kinetic energy for LES = 1/2 * (statistical reynolds stresses uu + ...vv + ....ww)

or = 1/2 * (Velocity.Trnrms^2 -Velocity.Trnavg^2)

statistical reynolds stresses are automatically calculated for the LES/SAS-SST and DES models. While .Trnrms and .Trnavg files are transient result files that have to be turned on in the output section.
Try both methods, they are practically same 99.9 % same probably. BUt there are slight difference atleast in my case. Have a look
Cheers

 mmahdinia July 14, 2010 03:48

Transient Calculation

The problem is that I do not know how to calculate u',v',w'.

Yours sincerely,
Maani

 rsin July 15, 2010 03:11

Dear maani,

You don´t have to calculate the u´, v´, w´ etc.

u´u´ etc are known as the Reynolds stresses and are directly calculated by the LES model through Statistical Reynolds Stresses. So, you dont have to calculate them. Just look them up in the variable list and they will be there. and use the formula as given them. What i wrote before is pretty straight forward. There should not be any problem.

 cfd_noob November 3, 2010 18:18

Could you implement k in your solver?

 rsin November 4, 2010 03:28

Is your problem solved or still looking for solutions

Hi,
I suppose your problem of finding turbulent kinetic energy should have been solved by now cause what i wrote earlier was pretty straight forward. Earlier, i had no clue about how to find dissipation rate.
Now, i have learned something. It can help, let me know if you still need help.
Cheers

 cfd_noob November 4, 2010 14:16

rsin,

Nop, I haven`t been successful at implementing k into the LES solver. (icoFoam). Do you have any code example for k ? That would be awesome.

kind regards,
Antonio

 rsin November 4, 2010 19:25

No point in setting up k in the solver

In LES model, turbulence kinetic energy is obtained directly through the filtered veloicty. Similarly, epsilon is also obtained. Even in general sense if you look out, you will see that kinetic energy = 0.5 * mass * veloicty^2.
In case of LES, it is important that you set up velocity and all of its component, shear rate etc in transient file for both average values and rms values.

Look up the article, it explains well the conservation of Energy for LES
http://leskandepsilon.blogspot.com/2...y-for-les.html
If you still have problems, go and read the book Turbulent Flows by Stepehen B Pope. It explains well this concept. Also, in ANSYS, they explain how to obtain kinetic energy for LES.
Cheers

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