# Mean calculation in LES

 Register Blogs Members List Search Today's Posts Mark Forums Read

 July 2, 2010, 02:17 Mean calculation in LES #1 Member   Join Date: Mar 2009 Posts: 46 Rep Power: 8 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

 July 8, 2010, 18:59 Is your turbulent kinetic energy has any compositions? #2 New Member   Join Date: Sep 2009 Posts: 15 Rep Power: 7 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.

 July 9, 2010, 03:45 Unsteady LES #3 Member   Join Date: Mar 2009 Posts: 46 Rep Power: 8 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

 July 11, 2010, 23:00 #4 New Member   Join Date: Sep 2009 Posts: 15 Rep Power: 7 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

 July 14, 2010, 03:48 Transient Calculation #5 Member   Join Date: Mar 2009 Posts: 46 Rep Power: 8 The problem is that I do not know how to calculate u',v',w'. Yours sincerely, Maani

 July 15, 2010, 03:11 #6 New Member   Join Date: Sep 2009 Posts: 15 Rep Power: 7 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.

 November 3, 2010, 18:18 #7 New Member   Join Date: Jan 2010 Posts: 9 Rep Power: 7 Could you implement k in your solver?

 November 4, 2010, 03:28 Is your problem solved or still looking for solutions #8 New Member   Join Date: Sep 2009 Posts: 15 Rep Power: 7 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

 November 4, 2010, 14:16 #9 New Member   Join Date: Jan 2010 Posts: 9 Rep Power: 7 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

 November 4, 2010, 19:25 No point in setting up k in the solver #10 New Member   Join Date: Sep 2009 Posts: 15 Rep Power: 7 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

 Thread Tools Display Modes Linear Mode

 Posting Rules You may not post new threads You may not post replies You may not post attachments You may not edit your posts BB code is On Smilies are On [IMG] code is On HTML code is OffTrackbacks are On Pingbacks are On Refbacks are On Forum Rules

 Similar Threads Thread Thread Starter Forum Replies Last Post tumble FLUENT 2 June 17, 2010 07:16 laurencep OpenFOAM 2 April 22, 2010 04:53 vatant OpenFOAM Running, Solving & CFD 0 October 30, 2006 10:15 aiya FLUENT 0 June 27, 2006 12:30 AB CD-adapco 6 November 15, 2004 05:41

All times are GMT -4. The time now is 23:17.