[Nek5000-users] Rules of thumb for element aspect ratio limits

Mon Apr 28 08:13:02 CDT 2014

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> 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:
>
> 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 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:
>
>   ·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>:
>
>>   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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> Institut fuer Aerodynamik und Gasdynamik
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