[Nek5000-users] Rules of thumb for element aspect ratio limits
nek5000-users at lists.mcs.anl.gov
nek5000-users at lists.mcs.anl.gov
Mon Apr 28 05:20:12 CDT 2014
Hi Wei,
I don't think genbox is the right tool to mesh airfoils, just because
you need the curved elements for high order computations. We are using
high order meshes for discontinuous Galerkin schemes. One technique is
to construct them from agglomeration of fine structured grids (ICEM,
Poitwise, Gridgen...), leading to coarse grids with the intermediate
points used for the curved element mapping.
But then I really don't know how this fits into the nek mesh format,
since I think there is no possibility to define intermediate
interpolation points for the elements. The only thing I saw is to
prescribe a curvature of an edge, but I think this is restricted to 2D
meshes and circular geometries, no?
Our workaround for an airfoil computation was more or less a hack
directly in Nek, where we read Gauss-Lobatto high order points of the
elements from file (we produced ourselves, together with the Nek format
with linear edges) and replaced (overwrote) the linear elements with the
curved ones during grid setup. Maybe somebody knows a better way to do
this...
regards
Florian
Am 28.04.2014 10:38, schrieb nek5000-users at lists.mcs.anl.gov:
> Hi Florian,
>
> For the 2D airfoil profile, I guess, the profile in nek5000 is
> connected by small straight lines not the curved lines. But even this,
> I have no idea to make a mesh for 2d airfoil flow. in the Genbox
> introduction there is something words on segments mesh without a sample
>
>
> For Multiple Segmented Geometries
>
> * This feature allows users to enter a complex sequence of segments
> for each of the x,y,z directions.
> * Each segment set is defined in x,y,z sections. So, Lines 6-8 would
> all pertain to x-dimension, 9-12 to y-dimension, ect.
>
> *Line 5:* The line following the string name, is the number of
> segments, /nsegs/, in the x_direction
> *Line 6:* The next line is the number of elements in each segment, so
> there should be /nsegs/ numbers. (nelx_1,nelx_2...)
> *Line 7:* The next line is the start(and end) coordinates for each
> segment in this direction. There should be /nsegs/+1.
> (x(0),x(1)...x(nsegs))
> *Line 8:* The following line is the distribution of each segment,
> uniform spacing corresponds to 1, otherwise a geometric sequence is
> generated.
>
> -In conclusion, a segment between x(e-1) and x(e) is filled with
> nelx_e elements determined by the geometric ratio given for that
> segment.
>
> * Repeat (Lines 5-8) for dimensions 2 and, if applicable, 3.
>
>
> can anyone give us a sample?
>
> regards
>
> Wei
> On Apr 25, 2014, at 10:17 PM, nek5000-users at lists.mcs.anl.gov
> <mailto:nek5000-users at lists.mcs.anl.gov> wrote:
>
>> Hi matt,
>> Sorry that I cannot fully answer your question, I know that at least
>> the smallest edge length in the mesh is a measure for the stiffness
>> of the full problem, so maybe you should avoid too small element
>> heights in the boundary layer.
>> However, I also would like to know how you are generating the
>> airfoil mesh, since the mesh has to be coarser than a standard meshes
>> and the boundary layer elements need to have curved boundaries, no?
>> Which mesh generator you use and how do you convert the mesh to Nek
>> format?
>> The 3d problem should boil down to a 2d problem, since I assume that
>> you want simulate a small part of the wing with periodic boundary
>> conditions in spanwise direction... But wei, for the 2d mesh, did you
>> resolve the issue to curve the boundary layer elements?
>>
>> Florian
>>
>> Am 25.04.2014 um 18:02 schrieb nek5000-users at lists.mcs.anl.gov
>> <mailto:nek5000-users at lists.mcs.anl.gov>:
>>
>>> ·HI Matt,
>>>
>>> Till now I have no experiments on 3D problem, what I am interested
>>> in is how you generate the 3D or 2D airfoil mesh for nek5000? I
>>> spend 2 weeks in generated a 2d airfoil flow mesh without any good
>>> results. would you like tell me some informations? thank you a lot!
>>>
>>> Wei
>>>
>>>
>>> 2014-04-25 17:00 GMT+02:00 <nek5000-users at lists.mcs.anl.gov
>>> <mailto:nek5000-users at lists.mcs.anl.gov>>:
>>>
>>> Hello,
>>>
>>> I am looking to do simulations of flow past a wing in 3D using
>>> nek5000 and I have been thinking more about potential issues
>>> with high aspect ratio elements. In general we have very fine
>>> resolution near the wing and then as we get further away the
>>> wall normal and wall parallel spacing increases. As a first try
>>> we will extend the domain in the cross stream direction which
>>> will result in small dz values. I know that in general the best
>>> performance is obtained with elements having dx=dy=dz and that
>>> as the aspect ratio increases the performance will degrade.
>>>
>>> I'm wondering if there are general rules of thumb for the
>>> performance degradation with increased aspect ratio. For
>>> example, is an aspect ratio of 10 ok but an aspect ratio of 100
>>> unacceptable? Is this even something we can estimate in general
>>> or does it vary so much problem to problem that no general
>>> estimate is possible?
>>>
>>> I saw an earlier post that referred to the paper "An Overlapping
>>> Schwarz Method for Spectral Element Solution of the
>>> Incompressible Navier-Stokes Equations", P. Fischer JCP 1997.
>>> From the paper I see two general strategies.
>>> 1. limit the maximum aspect ratio to a critical value
>>> 2. design a grid for our case, run it for a short time and then
>>> iteratively add more grid points to decrease the aspect ratio
>>> until optimal performance is achieved.
>>>
>>> Does anyone have a general or specific suggestion regarding how
>>> we should handle the grid generation in terms of selecting the
>>> largest aspect ratio possible with low computational cost?
>>>
>>> Thanks,
>>>
>>> Matt
>>>
>>> _______________________________________________
>>> Nek5000-users mailing list
>>> Nek5000-users at lists.mcs.anl.gov
>>> <mailto: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 <mailto: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 <mailto: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
--
-----------------------------------------------------------------------------
Dipl. Ing. Florian Hindenlang
Institut fuer Aerodynamik und Gasdynamik
Phone: 0049 (0)711-685 63413
office 1.14
Pfaffenwaldring 21
70569 Stuttgart
E-Mail: hindenlang at iag.uni-stuttgart.de
---------------------------------------------------------------------------
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://lists.mcs.anl.gov/pipermail/nek5000-users/attachments/20140428/bff2eb5d/attachment.html>
More information about the Nek5000-users
mailing list