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August 16, 2008, 05:31 
Mean velocity overprectied , LES Turbulent channel

#1 
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hi, i am working on LES chanel flow. i have set my first grid around y+=2 and took 56 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 
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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. Shearimproved Smagorinsky model for largeeddy simulation of wallbounded turbulent flows, J. Fluid Mech, (2007) vol. 570, pp. 491502. It gives good result. 

August 19, 2008, 03:06 
Re: Mean velocity overprectied , LES Turbulent cha

#3 
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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 
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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 
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Did you validate your code against some tests? For example, liddriven cavity laminar problem?


August 22, 2008, 18:08 
Re: Mean velocity overprectied , LES Turbulent cha

#6 
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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 ydirection 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 
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hi,
I have validate my LES model, with square cylinder case and liddriven 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 nouniform grid in computational calculation. i am using hybdid scheme3rd 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 tellwhat 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 
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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 
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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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