<div>‐‐‐‐‐‐‐ Original Message ‐‐‐‐‐‐‐<br></div><div> Le 17 juin 2018 7:12 PM, Zuheyr Alsalihi <zuheyr@gmail.com> a écrit :<br></div><div> <br></div><blockquote class="protonmail_quote" type="cite"><div dir="ltr"><div>Is there a way to get the node ID's of  the elements that lie on a boundary surface in a 3d solid mesh where I mark the surface by creating a physical surface. <br></div></div></blockquote><div><br></div><div>I don't know how to do that directly in Gmsh, but for a (two-dimensional) example using Python to extract the nodes belonging to different Physical Lines, see <br></div><div><br></div><div>    <a href="https://github.com/kinnala/scikit-fem/blob/master/examples/ex14.py">https://github.com/kinnala/scikit-fem/blob/master/examples/ex14.py</a><br></div><div><br>The essential line is the dict-comprehension on line 52:<br><br>    boundaries <span class="pl-k">=</span> {bc:<br>                              np.unique(cells[<span class="pl-s"><span class="pl-pds">'</span>line<span class="pl-pds">'</span></span>][cell_data[<span class="pl-s"><span class="pl-pds">'</span>line<span class="pl-pds">'</span></span>][<span class="pl-s"><span class="pl-pds">'</span>gmsh:physical<span class="pl-pds">'</span></span>] <span class="pl-k">==</span><br>                                                                    field_data[bc][<span class="pl-c1">0</span>]])<br></div><div><br></div><table data-tab-size="8" class="highlight tab-size js-file-line-container"><tbody><tr></tr></tbody></table><div><span class="pl-k">                               for</span> bc <span class="pl-k">in</span> field_data <span class="pl-k">if</span> field_data[bc][<span class="pl-c1">1</span>] <span class="pl-k">==</span> <span class="pl-c1">1</span>}<br></div><table data-tab-size="8" class="highlight tab-size js-file-line-container"><tbody><tr><td class="blob-code blob-code-inner js-file-line" id="LC54"><br></td></tr></tbody></table><div>basically having loaded the mesh from Gmsh into Python with meshio, this filters the element-node connectivity array according to the Physical boundary entities, and then applies numpy.unique to each subset.<br></div><div><br></div>