[Nek5000-users] Project a solution onto a polynomial space

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

Hi Philipp,

I guess to answer your question: we are not sure if de-aliasing is needed, and my impression was that it is only needed for computation, since non-dealiased terms would contaminate the solution, but not so much for post-processing. But to be certain: we wanted to do both approaches and compare. It is very likely we will come to the same conclusions as you did long time ago.

I do not believe we were planning to increase the polynomial order at this point, but we might consider it if it is necessary for accurate higher-order statistics. 

Best regards,
Yulia


On Mar 22, 2017, at 2:10 PM, nek5000-users at lists.mcs.anl.gov wrote:

> Hi,
> just out of interest, are you sure that you need dealiasing for statistics? Some long time ago, I tried it as well, and compared it to non-dealiased statistics, and I could not really see a difference. I guess my argument at the time was that you are interested in the low-frequency content of the statistics anyway, so that the aliasing errors would not show up.
> 
> Are you then planning to increase the polynomial order such that you can completely eliminate the qudrature errors even for third order terms?
> 
> Best regards,
> Philipp
> 
> On 2017-03-22 18:21, nek5000-users at lists.mcs.anl.gov wrote:
>> Dear Paul,
>> 
>> 
>> Thank you for answering so quickly. Yes, the polynomial spaces I'm using
>> are the standard Nek5000 basis functions.
>> 
>> 
>> To understand the problem a little better:
>> 
>> I am conducting DNS of a shear flow and I need to compute higher-order
>> velocity moments. I've implemented some subroutines based on the "avg2"
>> and "avg3" subroutines from Nek, but for higher order moments, i.e.
>> <U^3>, <U^2 V>, etc. The problem is that I need to de-alias the
>> non-linear quantities, i.e. U^3, before doing the averaging. Since the
>> flow is planar, I also take the spanwise average using the "z_average"
>> subroutine.
>> 
>> 
>> I checked out the "convect.f" file to see how dealiasing is applied by
>> mapping onto the finer grid (lxd), but I'm not quite sure how to project
>> these results back onto the original grid.
>> 
>> 
>> Thank you,
>> 
>> 
>> Juan Diego
>> 
>> 
>> 
>> _______________________________________________
>> Nek5000-users mailing list
>> Nek5000-users at lists.mcs.anl.gov
>> https://lists.mcs.anl.gov/mailman/listinfo/nek5000-users
>> 
> _______________________________________________
> Nek5000-users mailing list
> Nek5000-users at lists.mcs.anl.gov
> https://lists.mcs.anl.gov/mailman/listinfo/nek5000-users