[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 12:21:05 CDT 2014


Hi,
it seems that the gmsh readin in moab only supports linear elements, am 
I right?  I saw that in the newest version of moab, there is a spectral 
Hex module, but I don't know if this is already supported in Nek, and if 
moab is able to read / write spectral elements...?
best
Florian



Am 28.04.2014 15:13, schrieb nek5000-users at lists.mcs.anl.gov:
> Dear all
> I have read on the list that you can read 3d gmsh meshes into nek via 
> moab. Gmsh can generate higher order elements. Last time I checked, it 
> was not possible to read 2d gmsh grids, only 3d. This would be a great 
> option if the 2d case worked with gmsh.
> Best
> praveen
>
>
> On Mon, Apr 28, 2014 at 3:50 PM, <nek5000-users at lists.mcs.anl.gov 
> <mailto:nek5000-users at lists.mcs.anl.gov>> wrote:
>
>     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
>     <mailto: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
>>>>
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>>>>
>>>>
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>>
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>
>     -- 
>     -----------------------------------------------------------------------------
>     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  <mailto:hindenlang at iag.uni-stuttgart.de>
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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
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