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Hi,<br>
it seems that the gmsh readin in moab only supports linear elements,
am I right? I saw that in the newest version of moab, there is a
spectral Hex module, but I don't know if this is already supported
in Nek, and if moab is able to read / write spectral elements...?<br>
best<br>
Florian<br>
<br>
<br>
<br>
<div class="moz-cite-prefix">Am 28.04.2014 15:13, 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.7284.1398690804.3880.nek5000-users@lists.mcs.anl.gov"
type="cite">
<div dir="ltr">Dear all
<div>I have read on the list that you can read 3d gmsh meshes
into nek via moab. Gmsh can generate higher order elements.
Last time I checked, it was not possible to read 2d gmsh
grids, only 3d. This would be a great option if the 2d case
worked with gmsh.</div>
<div>Best</div>
<div>praveen</div>
</div>
<div class="gmail_extra"><br>
<br>
<div class="gmail_quote">On Mon, Apr 28, 2014 at 3:50 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 bgcolor="#FFFFFF" text="#000000"> 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>Am 28.04.2014 10:38, 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>
</div>
<div>
<div class="h5">
<blockquote 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
style="font-size:13px;line-height:19px;font-family:sans-serif">
<h3
style="border-bottom-width:initial;padding-top:0.5em;width:auto;background-color:initial;margin-bottom:0.3em;font-weight:bold;padding-bottom:0.17em;border-bottom-color:initial;border-bottom-style:none;margin-right:0px;overflow-x:hidden;overflow-y:hidden;font-size:17px;margin-left:0px;margin-top:0px;background-repeat:initial
initial;background-image:none">
<span>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">
<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">
<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"
target="_blank">nek5000-users@lists.mcs.anl.gov</a>
wrote:</div>
<br>
<blockquote type="cite">
<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>
<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 cols="72">--
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Dipl. Ing. Florian Hindenlang
Institut fuer Aerodynamik und Gasdynamik
Phone: 0049 (0)711-685 63413
office 1.14
Pfaffenwaldring 21
70569 Stuttgart
E-Mail: <a moz-do-not-send="true" href="mailto:hindenlang@iag.uni-stuttgart.de" target="_blank">hindenlang@iag.uni-stuttgart.de</a>
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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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