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

Mon Apr 28 03:38:10 CDT 2014

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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