[Nek5000-users] prex mesh precision
nek5000-users at lists.mcs.anl.gov
nek5000-users at lists.mcs.anl.gov
Tue Jul 13 05:39:55 CDT 2010
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
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