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Hi Matt,<br>
I see that you found a similar approach by manually fixing the GLL
points. Do you have experience with the Moab interface in Nek? It
seems that the geometry of spectral elements is supported. Then
probably, the manual fix in nek could be replaced by the readin of
a curved mesh format via moab. What do you think?<br>
<br>
Florian<br>
<br>
<div class="moz-cite-prefix">Am 28.04.2014 12:16, schrieb
<a class="moz-txt-link-abbreviated" href="mailto:nek5000-users@lists.mcs.anl.gov">nek5000-users@lists.mcs.anl.gov</a>:<br>
</div>
<blockquote
cite="mid:mailman.7271.1398680182.3880.nek5000-users@lists.mcs.anl.gov"
type="cite">
<div dir="ltr">
<div>
<div>
<div>
<div>
<div>
<div>
<div>
<div>Hi Wei and Florian,<br>
<br>
</div>
In general you can first generate a 2D grid for
flow past a wing if it is a straight wing (of if
you represent the 3 dimensionality by a forcing in
the z direction to indicate a fixed sweep angle).
This is not optimal though for a larger problem
even if you have periodic boundary conditions
because your dz spacing will be fixed at a small
value by the BL region requirements. <br>
<br>
</div>
For DNS of the boundary layer region close to the
wing you generally want <br>
dx+~10<br>
</div>
dy+ ~ 0.5 next to the wall, less than dz+ away from
the wall<br>
</div>
dz+ ~ dx+/2<br>
<br>
</div>
<div>This gives a maximum aspect ratio of about 20 near
the wall (dx/dz). <br>
</div>
<div><br>
</div>
In the wake / separated flow region you generally want
max(dx,dy,dz)/eta < 4 where eta is the kolmogorov
lengthscale. <br>
<br>
The problem is that with a wake calculation such as an
airofil you generally also want the boundaries to be far
away. You can do this by growing dx and dy away from the
airfoil. However, you will eventually reach a point where
your aspect ratio is 10-1,000 far away from the wing as
well since dz is small. The problem with doing this is
that when you are far away from the wing then dz will
still be very small. With a structured mesh there appears
to be no way to fix this issue without a multiblock
method. For an unstructured mesh there is a possibility of
coarsening more in all 3 directions but this is quite
challenging to generate.<br>
<br>
Also, it is not as simple as just thinking about the
maximum value of (dx,dy,dz) divided by the minimum value
of (dx,dy,dz). The relative ratios of dx,dy,dz all matter
and with a wake you generally have regions where you have
all 6 cases of <br>
dx < dy < dz<br>
dx < dz < dy, <br>
dy < dx < dz<br>
dy < dz < dx<br>
dz < dx < dy<br>
dz < dy < dx<br>
<br>
</div>
<div>All these areas create problems for an iterative
solver. You can write a more robust / complex solver (such
as a semi-coarsening multigrid algorithm) to handle these
different aspect ratio regions but then much more work is
required per iteration. It is a trade-off between # of
iterations required and computing time per iteration.<br>
</div>
<div><br>
</div>
For generating the mesh itself.<br>
</div>
We have an inhouse generated method based on gridgen-c. <a
moz-do-not-send="true"
href="https://code.google.com/p/gridgen-c/">https://code.google.com/p/gridgen-c/</a>
. It is not ideal but it can be made to work. We also looked
into using cubit for unstructured meshes but we are still
testing that. With cubit you can either use moab to load in
the mesh to nek but then you have some limitations. For both
options you can write your own converter as well... we do this
for now. <br>
<br>
To curve the boundary layer elements we first generate the
element locations themselves and then the GLL points are
created on straight line segments. We go back in and manually
correct the location of the GLL points located closest to the
wall using a spline of the wing. Then we solve a laplace
equation to smooth out the GLL points in the rest of the
domain. <br>
<br>
Matt<br>
</div>
</div>
<div class="gmail_extra"><br>
<br>
<div class="gmail_quote">On Fri, Apr 25, 2014 at 10:17 PM, <span
dir="ltr"><<a moz-do-not-send="true"
href="mailto:nek5000-users@lists.mcs.anl.gov"
target="_blank">nek5000-users@lists.mcs.anl.gov</a>></span>
wrote:<br>
<blockquote class="gmail_quote" style="margin:0 0 0
.8ex;border-left:1px #ccc solid;padding-left:1ex">
<div dir="auto">
<div>Hi matt,</div>
<div>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. </div>
<div>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? </div>
<div>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?</div>
<div><br>
</div>
<div>Florian</div>
<div><br>
Am 25.04.2014 um 18:02 schrieb <a
moz-do-not-send="true"
href="mailto:nek5000-users@lists.mcs.anl.gov"
target="_blank">nek5000-users@lists.mcs.anl.gov</a>:<br>
<br>
</div>
<div>
<div class="h5">
<blockquote type="cite">
<div>
<div dir="ltr">·HI Matt,
<div><br>
</div>
<div>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!</div>
<div><br>
</div>
<div>Wei<br>
<div class="gmail_extra"><br>
<br>
<div class="gmail_quote">2014-04-25 17:00
GMT+02:00 <span dir="ltr"><<a
moz-do-not-send="true"
href="mailto:nek5000-users@lists.mcs.anl.gov"
target="_blank">nek5000-users@lists.mcs.anl.gov</a>></span>:<br>
<blockquote class="gmail_quote"
style="margin:0 0 0 .8ex;border-left:1px
#ccc solid;padding-left:1ex">
<div dir="ltr">
<div>
<div>
<div>
<div>Hello,<br>
<br>
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. <br>
<br>
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?<br>
<br>
</div>
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. <br>
</div>
1. limit the maximum aspect ratio
to a critical value<br>
</div>
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. <br>
<br>
</div>
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?<br>
<br>
Thanks,<br>
<br>
Matt<br>
</div>
<br>
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Institut fuer Aerodynamik und Gasdynamik
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