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

[nek5000-users at lists.mcs.anl.gov]

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>_______________________________________________
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