[Nek5000-users] prex mesh precision

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

Hi Paul,

Thanks for the explanation.  I have implemented something in "usrdat2"
that snaps the grid to the circle.

Cheers,
Frank


On Fri, 2010-07-09 at 20:01 -0500, nek5000-users at lists.mcs.anl.gov
wrote:
> Hi Frank,
> 
> The main objective of going high order is to minimize dispersion
> errors so that the error at the end of the simulation is at
> acceptable engineering accuracy.  Dispersion errors accumulate
> linearly with time and thus, for long time-integrations, one
> requires that the leading contribution to the (discretization)
> dispersion error be small.  Roughly speaking, for a final
> error of .01 in and a final integration time T=1000, one would
> want the error to be 1.e-5.   Such tight tolerances are fairly
> easy to realize with a high-order discretization.
> 
> That being said, there are times (e.g., stability computations)
> where you really want to control all the errors in order to 
> establish critical Re or Ra to several digits.   In such cases,
> I typically project the geometry from prex onto the true geometry.
> This is easy for spheres and cylinders and only modestly more
> difficult for other shapes.   I can help you with some code to
> do this if you like - it involves inserting lines of code in either
> usrdat() or usrdat2()  [ which in fact is why those stubs were
> originally created ].
> 
> Cheers,
> 
> Paul
> 
> 
> On Sat, 10 Jul 2010, nek5000-users at lists.mcs.anl.gov wrote:
> 
> > Hello all,
> 
> I was wondering what would be involved in getting "prex"/"prenek" to
> perform all calculations in double precision and to output a mesh in
> this precision.  In the cases we are looking at which involve
> understanding stability boundaries and growth rates, the high accuracy
> of NEK is the prime reason for using it.  There is some concern that an
> inaccurate representation of the boundary could potentially be a source
> of meaningful error, in particular in the course of grid refinement
> studies.  Putting my two cents in, it seems somewhat incongruous for the
> fundamental geometry definition for such a highly accurate method/code
> to be in single precision.
> 
> Cheers,
> Frank
> 
> 
> -- 
> Frank Herbert Muldoon, Ph.D. Mechanical Engineering
> Technische Universität Wien (Vienna Technical University)
> Inst. f. Strömungsmechanik und Wärmeübertragung (Institute of Fluid
> Mechanics and Heat Transfer)
> Resselgasse 3
> 1040 Wien
> Tel: +4315880132232
> Fax: +4315880132299 
> Cell:+436765203470
> fmuldoo (skype)
> http://tetra.fluid.tuwien.ac.at/fmuldoo/public_html/webpage/frank-muldoon.html
> 
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-- 
Frank Herbert Muldoon, Ph.D. Mechanical Engineering
Technische Universität Wien (Vienna Technical University)
Inst. f. Strömungsmechanik und Wärmeübertragung (Institute of Fluid
Mechanics and Heat Transfer)
Resselgasse 3
1040 Wien
Tel: +4315880132232
Fax: +4315880132299 
Cell:+436765203470
fmuldoo (skype)
http://tetra.fluid.tuwien.ac.at/fmuldoo/public_html/webpage/frank-muldoon.html