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Hi Wei,<br>
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. <br>
<br>
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?<br>
<br>
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...<br>
<br>
regards<br>
<br>
Florian<br>
<br>
<br>
<br>
<div class="moz-cite-prefix">Am 28.04.2014 10:38, 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.7262.1398674298.3880.nek5000-users@lists.mcs.anl.gov"
type="cite">Hi Florian,
<div><br>
</div>
<div>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</div>
<div><span class="Apple-style-span" style="font-size: 13px;
line-height: 19px; font-family: sans-serif; ">
<h3 style="color: black; background-image: none;
background-attachment: initial; background-origin: initial;
background-clip: initial; background-color: initial;
font-weight: bold; margin-top: 0px; margin-right: 0px;
margin-bottom: 0.3em; margin-left: 0px; overflow-x: hidden;
overflow-y: hidden; padding-top: 0.5em; padding-bottom:
0.17em; border-bottom-width: initial; border-bottom-style:
none; border-bottom-color: initial; width: auto; font-size:
17px; background-position: initial initial;
background-repeat: initial initial; "><span
class="mw-headline" id="For_Multiple_Segmented_Geometries">For
Multiple Segmented Geometries</span></h3>
<ul style="line-height: 1.5em; list-style-type: square;
margin-top: 0.3em; margin-right: 0px; margin-bottom: 0px;
margin-left: 1.6em; padding-top: 0px; padding-right: 0px;
padding-bottom: 0px; padding-left: 0px; list-style-image:
url(data:image/png;base64,iVBORw0KGgoAAAANSUhEUgAAAAUAAAANCAMAAABW4lS6AAAAGXRFWHRTb2Z0d2FyZQBBZG9iZSBJbWFnZVJlYWR5ccllPAAAAAZQTFRFAFKM////QIUK9QAAAAJ0Uk5T/wDltzBKAAAAGklEQVR42mJgBAEGokgGBjBGBxBxsBqAAAMACHwALd5r8ygAAAAASUVORK5CYII=);
">
<li style="margin-bottom: 0.1em; ">This feature allows users
to enter a complex sequence of segments for each of the
x,y,z directions.</li>
<li style="margin-bottom: 0.1em; ">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.</li>
</ul>
<div style="margin-top: 0.4em; margin-right: 0px;
margin-bottom: 0.5em; margin-left: 0px; line-height: 1.5em;
"><b>Line 5:</b> The line following the string name, is the
number of segments, <i>nsegs</i>, in the x_direction</div>
<div style="margin-top: 0.4em; margin-right: 0px;
margin-bottom: 0.5em; margin-left: 0px; line-height: 1.5em;
"><b>Line 6:</b> The next line is the number of elements in
each segment, so there should be <i>nsegs</i> numbers.
(nelx_1,nelx_2...)</div>
<div style="margin-top: 0.4em; margin-right: 0px;
margin-bottom: 0.5em; margin-left: 0px; line-height: 1.5em;
"><b>Line 7:</b> The next line is the start(and end)
coordinates for each segment in this direction. There should
be <i>nsegs</i>+1. (x(0),x(1)...x(nsegs))</div>
<div style="margin-top: 0.4em; margin-right: 0px;
margin-bottom: 0.5em; margin-left: 0px; line-height: 1.5em;
"><b>Line 8:</b> The following line is the distribution of
each segment, uniform spacing corresponds to 1, otherwise a
geometric sequence is generated.</div>
<dl style="margin-top: 0.2em; margin-bottom: 0.5em; ">
<dd style="line-height: 1.5em; margin-left: 1.6em;
margin-bottom: 0.1em; margin-right: 0px; ">-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.</dd>
</dl>
<ul style="line-height: 1.5em; list-style-type: square;
margin-top: 0.3em; margin-right: 0px; margin-bottom: 0px;
margin-left: 1.6em; padding-top: 0px; padding-right: 0px;
padding-bottom: 0px; padding-left: 0px; list-style-image:
url(data:image/png;base64,iVBORw0KGgoAAAANSUhEUgAAAAUAAAANCAMAAABW4lS6AAAAGXRFWHRTb2Z0d2FyZQBBZG9iZSBJbWFnZVJlYWR5ccllPAAAAAZQTFRFAFKM////QIUK9QAAAAJ0Uk5T/wDltzBKAAAAGklEQVR42mJgBAEGokgGBjBGBxBxsBqAAAMACHwALd5r8ygAAAAASUVORK5CYII=);
">
<li style="margin-bottom: 0.1em; ">Repeat (Lines 5-8) for
dimensions 2 and, if applicable, 3.</li>
</ul>
</span>
<div><br>
</div>
<div>can anyone give us a sample?</div>
<div><br>
</div>
<div>regards</div>
<div><br>
</div>
<div>Wei</div>
<div>
<div>On Apr 25, 2014, at 10:17 PM, <a moz-do-not-send="true"
href="mailto:nek5000-users@lists.mcs.anl.gov">nek5000-users@lists.mcs.anl.gov</a>
wrote:</div>
<br class="Apple-interchange-newline">
<blockquote type="cite">
<meta http-equiv="content-type" content="text/html;
charset=ISO-8859-1">
<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">nek5000-users@lists.mcs.anl.gov</a>:<br>
<br>
</div>
<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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<pre class="moz-signature" cols="72">--
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Dipl. Ing. Florian Hindenlang
Institut fuer Aerodynamik und Gasdynamik
Phone: 0049 (0)711-685 63413
office 1.14
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