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 Tim April 25, 2002 09:15

Dynamic Smagorinsky model

I am having some problems with the implementation of the Germano Lilly dynamic smagorinsky model. I think I have the Lij tems done o.k. but am have a nightmare with the Mij terms. If anyone has experience of this model could they give me an outline of the procedure they used - do you know of any published data giving Lij , Mij & C values ???

Thanks

 Rajani Kumar Akula May 3, 2002 04:03

Re: Dynamic Smagorinsky model

Hi Tim,

there should not be any problem in finding the Mij terms. Mij is nothing but alphaij-(betaij )hat. tell me cleary in what way u r getting problem with Mij.

 Tim May 3, 2002 08:16

Re: Dynamic Smagorinsky model

Hi, thanks for replying. I have now got the Mij terms looking better - the problem I have is that I have no way of determining that I have the completely correct. i am trying to replicate the work done by Zhao & Voke applying the Dynamic model to channel flow. in the paper they have a table listing the value of C at a series of Yplus values - The values of C that I am getting are close(ish)but not the same. I am therefore trying to work out why they differ. How would you go about this ? After I get the channel case right I would like to apply the model to a backward facing step - the problem here is how to average in a statistically homogenous direction - do I use streamlines ? do you have any experience with this?

Thanks.

 Rajani Kumar Akula May 3, 2002 10:10

Re: Dynamic Smagorinsky model

Hi tim,

dynamic model act as a damping function. but it is better to use wall functions near the wall ,if your y+ is more(i mean first point placed little bit away from the wall). near the wall there may be a problem with comparision because of the wall effects. but away from the wall there should be no problem. time avarage is better choice.

 Li Yang May 4, 2002 09:12

Re: Dynamic Smagorinsky model

Hi Tim,

I am also interested in implementing dynamic model for LES. Which paper you reckon is the best for reference. You mentioned Zhao and Voke's paper. Do you think their paper is helpful. Could you please let me know which one you refer to.

Regards

Li

 Tim May 7, 2002 10:33

Re: Dynamic Smagorinsky model

Hi Li,

Obvious Reference for Germano Dynamic Smagorinksy Subgrid model is :

M.Germano, U.Piomelli, P.Moin & W.Cabot " A Dynamic subgrid scale eddy viscosity model" Phys Fluids A 3 (1991)

The original model was tweaked by Lilly:

D.Lilly " A proposed modification of the Germano subgrid scale closure method" phys Fluids A 4 (1992)

It is also worth checking out these papers

Meneveau & Lund " The dynamic Smagorinsky model and scale dependant coefficients in the viscous range of turbulence" Phys Fluids 9 (1997)

Zhao & Voke " a dynamic subgrid scale model for low Reynolds number channel flow" International Journal for numerical methods in fluids vol 23 (1996)

Good Luck

Tim.

 Li Yang May 8, 2002 09:47

Re: Dynamic Smagorinsky model

Tim,

Thank you very much indeed. Hope that your code will be soon working.

Regards

Li

 Ajay S. Parihar May 29, 2002 07:37

Re: Dynamic Smagorinsky model

Hi ALl.. I have worked on Dynamic subgrid stress model for the flow past square cylinder at high reynolds number. I also compared our results with experimental results of Lyn et al. and also with simulated result of SP Vanka. We are getting good agreement... if you need any help then plz let me know.

In case of dynamic model the value of C is very critical. So You can apply different averaging technique to get smooth value of C like 5 point aver or 9 point... near wall you have to apply less point averaging tech. Also check your grid spacing near the wall. It should be less that some critical value. Also you have to take care while applying test filtering. plane of test filter should be homogenous which is very rare in case of turbulent flow. Also the denominater part of formula which calculate C is very unstable. sometime it becomes 0/0 case. so avoid such situations.