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

[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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