[Nek5000-users] 3-D Taylor-Green problem

Sun Dec 9 10:15:33 CST 2012

Hi Praveen

Filtering removes energy at the small scales (small length scales/high frequency components)
If you estimate the turbulent kinetic energy removed by the filtering and compare it to the resolved kinetic energy or the total (resolved+subgrid scale)
then your simulation :

(1) May be considered DNS  ; if the above ratio is small
(2) LES if it is relatively big

in (2) the hope is that subgrid scale motion does not affect the scales of interest in a given problem. 

Filtering stabilizes simulations since it provides a way for removing energy at the small unresolved scales. I ran turbulent wake simulations in the past
where filtering was absolutely necessary and the argument was that it did not affect the scales of interest.

So the bottomline is you need to at least be aware of what scales are impacted by filtering and if indeed you are interested in such scales. That is why
you see some comparisons in the paper for a given case with different values of the filtering parameter.  

so let me know  what happens when 32^3 is run with f=0 and also the comparison to Brachet et al. 

Ammar

On Dec 8, 2012, at 10:31 PM, nek5000-users at lists.mcs.anl.gov wrote:

> Hello Ammar
> 
> The filter refers to a stabilization technique and is not related to LES. It is explained here
> 
>  "Filter-Based Stabilization of Spectral Element Methods" 
> 
> I do not know the details of the spectral simulation. I took the data from the website I mentioned in my post and some explanation is given in the case description I linked to before.
> 
> I used 14^3 and 28^3 elements but within each element the velocity is a Q_9 function and pressure is Q_7 function.
> 
> Thanks for the reference. I have not seen it before and will check it out.
> 
> Thanks
> praveen
> 
> On Sun, Dec 9, 2012 at 2:42 AM, <nek5000-users at lists.mcs.anl.gov> wrote:
> Can you elaborate on the 512^3 case? it is impressive you are getting close with 32^3 and some filtering
> I was wondering :
> (1) what if you turn off filtering with 32^3  (i.e. set f=0), the question here if filtering is significantly contributing to the dissipation rate. if it does not make a big difference then I would say your LES with 32^3 and f=0.05 is pretty good. 
> (2) how does the time history in all cases compare to Brachet et al. (see my previous note) at the same Re. 
> 
> Ammar
> 
> On Dec 8, 2012, at 3:01 PM, nek5000-users at lists.mcs.anl.gov wrote:
> 
>> Your results make sense to me.  Note that you are not really "capturing" peak dissipation rate with filtering. 
>> you are only supplementing the resolved dissipation on a given grid by artificially damping energy.   that's why you get higher peak with f=0.05 and N=14^3 relative to  f=0  and N=14^3 
>> I wonder how much resolution N^3 you need (with f=0) to truly capture the peak dissipation rate?  
>> 
>> In order to better judge your simulations you should compare DNS (f=0) and LES f=nonzero for gradually increased resolution N^3 
>> 
>> if DNS is expensive , good news DNS is already been done , Check Brachet et al JFM article  Journal of Fluid Mechanics / Volume 130 / May 1983,	 pp 411-452
>> (
>> Small-scale structure of the Taylor–Green vortex
>> Marc E.  Brachet a1p1, Daniel I.  Meiron a1, Steven A.  Orszag a1, B. G.  Nickel a2
>> )
>> 
>> 
>>  and compare the time history of the dissipation rate
>> to what you get at the same Reynolds number. 
>> 
>> 
>> 
>> Ammar
>> 
>> 
>> 
>> 
>> 
>> 
>> On Dec 8, 2012, at 2:36 AM, nek5000-users at lists.mcs.anl.gov wrote:
>> 
>>> Hello
>>> I am solving the 3-D Taylor-Green problem as described here 
>>> 
>>> http://www.dlr.de/as/desktopdefault.aspx/tabid-8170/13999_read-35550/
>>> C3.5 Direct Numerical Simulation of the Taylor-Green Vortex at Re = 1600
>>> 
>>> using NEK.
>>> 
>>> I use
>>> 
>>> lx1 = 10
>>> lx2 = lx1-2
>>> ldx = 15
>>> 
>>> No of elements = 14^3 or 28^3
>>> 
>>> The evolution of dissipation rate with time is shown in attached pdf. 
>>> 
>>> f=0.0 indicates no filtering and f=0.05 means filter parameter P103.
>>> 
>>> Use of filter seems to capture peak dissipation rate better but still some difference is seen. The DG results with similar number of dofs in the problem description give good estimate of peak rate. So I expect NEK results to improve further and would like to ask for any advice. Should I try with larger amount of filtering ?
>>> 
>>> I have attached my SIZE and rea files.
>>> 
>>> Thanks
>>> praveen
>>> <box.rea><diss.pdf><SIZE>_______________________________________________
>>> Nek5000-users mailing list
>>> Nek5000-users at lists.mcs.anl.gov
>>> https://lists.mcs.anl.gov/mailman/listinfo/nek5000-users
>> 
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