# Mean velocity overprectied , LES Turbulent channel

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 August 16, 2008, 05:31 Mean velocity overprectied , LES Turbulent channel #1 pankaj saha Guest   Posts: n/a hi, i am working on LES chanel flow. i have set my first grid around y+=2 and took 5-6 points within y+=11. i am getting friction velocity right...but my mean velocity is overprectied by 30%. AS, my case , it should be 1.0 , but i am getting 1.36. So, every result is overeprected by some margin. Could, you face this type of situation. Please, tell what is the remedy?

 August 16, 2008, 06:16 Re: Mean velocity overprectied , LES Turbulent cha #2 Alexey Guest   Posts: n/a LES channel simulation is challenge. Result stongly depends on subgrid model. For example, classic Smagorinsky model is not good for this simulation. Usually dynamic model a la Germano is used. I use this more simple model E.Leveque, F.Toschi, L.Shao and J.-P. Bertoglio. Shear-improved Smagorinsky model for large-eddy simulation of wall-bounded turbulent flows, J. Fluid Mech, (2007) vol. 570, pp. 491-502. It gives good result.

 August 19, 2008, 03:06 Re: Mean velocity overprectied , LES Turbulent cha #3 Tom Guest   Posts: n/a Hi, Rule of thumb: first grid point at y+=1 and three grid points within y+=5. This is at least the requirement for DNS. What kind of subgrid model are using? Is the subgrid viscosity correctly zero at the wall?

 August 19, 2008, 18:31 Re: Mean velocity overprectied , LES Turbulent cha #4 pankaj saha Guest   Posts: n/a i am using dynamic model. and cs behaves correctly near the wall. only thing is that , my mean flow increases ... can u tell, what is the reason?

 August 22, 2008, 15:25 Re: Mean velocity overprectied , LES Turbulent cha #5 Alexey Guest   Posts: n/a Did you validate your code against some tests? For example, lid-driven cavity laminar problem?

 August 22, 2008, 18:08 Re: Mean velocity overprectied , LES Turbulent cha #6 agg Guest   Posts: n/a If you are overpredicting at the center, you must be underpredicting somewhere else, even if your wall profile is correct. I suspect it is your LES model. This can be tested easily. Just increase your grid resolution in the y-direction and see if it alleviates your problem. If it does, you should look at your LES model to see if it is ok.

 August 22, 2008, 19:11 Re: Mean velocity overprectied , LES Turbulent cha #7 pankaj saha Guest   Posts: n/a hi, I have validate my LES model, with square cylinder case and lid-driven case. As, agg, i will increase the resolution to check it. One more thing, i am using carseian rectangular grid and no tarnofrmation. so, i am using no-uniform grid in computational calculation. i am using hybdid scheme----3rd order upwind biased of kuwamura and 2nd order cd. with wt factor 0.15 and 0.85 respectively. is the problem is coming from scheme?? can anyone tell--what should the variation of instantaneous w velocity above 0.0. my case, i am getting (-0.1to 0.1) is it too large, because my time history shows, it very dense signal. so, i doubt, whethere, numerical oscullation comes into solution or not?

 August 22, 2008, 19:15 Re: Mean velocity overprectied , LES Turbulent cha #8 agg Guest   Posts: n/a Have your looked at your urms vrms and wrms plots? How do they compare with DNS results?

 August 23, 2008, 09:01 Re: Mean velocity overprectied , LES Turbulent cha #9 Tom Guest   Posts: n/a Hi, The combination of upwind (biased) schemes and LES hasn't been very successful I think. These schemes can introduce quite a lot of numerical viscosity. The combination of LES and CD appears to perform better.

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