<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 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 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 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 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 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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                            <br>
                          </blockquote>
                        </div>
                        <br>
                        <br>
                      </div>
                    </div>
                  </div>
                </div>
              </blockquote>
              <blockquote type="cite">
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    </blockquote>
    <br>
    </div></div><span class="HOEnZb"><font color="#888888"><pre cols="72">-- 
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Dipl. Ing. Florian Hindenlang
Institut fuer Aerodynamik und Gasdynamik 
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