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

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

Hi Yulia and Juan Diego,
thanks a lot for the answer. This is interesting, and let us know if it 
is really needed!
With "increase the polynomial order" I essentially meant the 
overintegration, sorry if I was unclear. I wanted to ask whether you 
plan to use 3 times lx1 for third-order terms.
Best,
Philipp

On 2017-03-22 22:50, nek5000-users at lists.mcs.anl.gov wrote:
> 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
>>>
>>>
>>>
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>>>
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