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-   -   Numerical dissipation/difusion in LES (http://www.cfd-online.com/Forums/fluent/29873-numerical-dissipation-difusion-les.html)

Ray June 7, 2002 06:47

Numerical dissipation/difusion in LES
 
Dear All,

Can any body talk about numerical dissipation or diffusion in CFD? what's the difference between these two? What's the numerical scheme used in FLUENT 5 for LES? is it highly dissipative?

And if finite-volume method and second-order temporal and upwind spatial discretization are employed, how to deteremine the numerical dissipation?

In large-eddy simulation, how to avoid the subgrid stress being overwhelmed by numerical dissipation/difusion?

Thanks in advance.

Ray

Steve June 9, 2002 18:31

Re: Numerical dissipation/difusion in LES
 
Hi Ray,

Dissipation and Diffusion are the same thing. Error that smears out sharp gradients. Dissipation is therefore associated with even order truncation terms usually i.e. a 2nd order derivative in a first order upwind scheme.

I don't know about FLUENT's implementation but LES should be used without any dissipative error - i.e. a central type scheme should be used. These schemes are dispersive i.e. error is spread in a wave like fashion which is evident as spurilous oscilations in the vicinity of steep gradients. These schems have a odd-order leading truncation term.

Error terms can be determined by writing the numerical scheme out as a taylors series expansion and looking at the largest term that has been truncated. You will have to do this both space and time.

Another option is to perform a grid and time step convergence study.

All the best, Steve

Ray June 10, 2002 05:15

Re: Numerical dissipation/difusion in LES
 
Many thanks! Steve.


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