[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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> --
> -----------------------------------------------------------------------------
> 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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