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

[nek5000-users at lists.mcs.anl.gov]

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
>> 
>> _______________________________________________
>> Nek5000-users mailing list
>> 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
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://lists.mcs.anl.gov/pipermail/nek5000-users/attachments/20140425/49c4e0ed/attachment.html>