[Nek5000-users] element order range

Mon Sep 20 11:07:31 CDT 2010

Let's discuss this off-list.

-Stefan

----- Original Message -----
From: nek5000-users at lists.mcs.anl.gov
To: nek5000-users at lists.mcs.anl.gov
Sent: Monday, September 20, 2010 9:53:43 AM GMT -06:00 US/Canada Central
Subject: Re: [Nek5000-users] element order range

Stefan,

I'm about to start some comparison simulations on this problem.  Are you still interested in looking over the input files for potential improvements?  

Thanks.
--Mike

On Sep 14, 2010, at 12:43 PM, <nek5000-users at lists.mcs.anl.gov> <nek5000-users at lists.mcs.anl.gov> wrote:

> Stefan,
> 
> Thanks for the offer.  If you'd like to look, I put the relevant files in a .tar.gz file at
> 
> http://michaelasprague.com/couette_scale.tar.gz
> 
> It's fairly simple.  The dimensions are (Lx,Ly,Lz) = (6,5,1) with periodic BCs in x and y.  At z = 0, (ux,uy,uz) = (0,0,0) and at z = 1, (ux,uy,uz)=(1,0,0).  
> 
> The domain is discretized as (Nx,Ny,Nz)=(24,16,16), and N = 13.
> 
> Please let me know if you have any suggestions for parameter improvements and/or need more information.
> 
> --Mike
> 
> On Sep 14, 2010, at 11:37 AM, <nek5000-users at lists.mcs.anl.gov> wrote:
> 
>> Mike:
>> 
>> If you're ready with the NEK case we're more than happy to have a look at it.
>> Sometimes it can get a little bit tricky to find the right solver parameters to get the most out of it.
>> 
>> We're very interested into comparisons like this!
>> 
>> -Stefan
>> 
>> 
>> ----- Original Message -----
>> From: nek5000-users at lists.mcs.anl.gov
>> To: nek5000-users at lists.mcs.anl.gov
>> Sent: Tuesday, September 14, 2010 12:31:40 PM GMT -06:00 US/Canada Central
>> Subject: Re: [Nek5000-users] element order range
>> 
>> Paul,
>> 
>> Thanks for that info and for the document.  Both are very helpful.
>> 
>> There's interest here in using OpenFOAM (second-order finite-volume), and I'm interested to see how Nek does in a head to head comparison.  We're using turbulent planar Couette flow as our comparison test case with Re = 8000 (See, e.g. Sullivan et al. (JFM 404, 2000).  We're looking at accuracy for a given number of grid points and scalability over 16-1024 cores.
>> 
>> I'll let you know how things turn out with the comparison.
>> 
>> --Mike
>> 
>> 
>> On Sep 14, 2010, at 10:33 AM, <nek5000-users at lists.mcs.anl.gov> wrote:
>> 
>>> 
>>> Hi Mike,
>>> 
>>> I've run with N=120 in the past and just this past
>>> week was running w/ N=64.  (I had to bump up a couple
>>> of parameters past the 40 or 50 mark to do so.)
>>> 
>>> Stefan has run the Pn-Pn variant with N=1 (lx1=2).
>>> 
>>> lx1=6-12 is the general sweet spot for the code because
>>> of the way the operators are evaluated...
>>> 
>>> When comparing to a low-order code it's generally best
>>> to compare for an equal number of gridpoints - i.e.,
>>> roughly the same resolution.  The main reason for this is that
>>> higher order does not buy you increased resolution. (The
>>> Nyquist sampling theorem dictates at least 2*k points are
>>> required to resolve a signal of wavenumber k.)   What the
>>> high-order method gains is improved accuracy for the modes
>>> that _are_ resolved.   My goal is to try to ensure that
>>> the performance is comparable to a low-order code - i.e.,
>>> that costs are not inordinate w/ N.
>>> 
>>> What type of comparison did you have in mind ?  You might
>>> be interested in:  www.mcs.anl.gov/~fischer/users.pdf
>>> which is a work-in-progress.
>>> 
>>> Paul
>>> 
>>> 
>>> 
>>> On Tue, 14 Sep 2010, nek5000-users at lists.mcs.anl.gov wrote:
>>> 
>>>> Hello All.
>>>> 
>>>> I am doing some tests of Nek5000 against a low-order code.  Under "Performance tips", it is recommended to "design your resolution (mesh) for N=7 or N=9"
>>>> 
>>>> Are there any limitations on the range of values for N?  For example, what is the smallest N and largest N that the code can/has been run with?    What are the limitations with running at, say, N = 16?
>>>> 
>>>> Thanks.
>>>> --Mike
>>>> 
>>>> 
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