<html><body><div style="font-family: georgia,serif; font-size: 12pt; color: #000099"><div>Hello,</div><div><br></div><div>Currently I am trying to solve Laplace equation in 3D with both some Neumann and Dirichlet boundary conditions</div><div>on different parts of the problem domain. This domain consists in an outer cube with a cubic hole at centre </div><div>(see attached file Laplace3D.geo).</div><div>The problem to be solved is the following :<br></div><div><div><br></div></div><div>- div(grad(u) = 0 in Omega </div><div><br></div><div>with both the following BC:<br></div><div>1) Dirichlet BC:<br></div><div> u = 0 on GammaOuter<br></div><div><br></div><div>2) Neumann BC:<br></div><div>\partial u / \partial normal = grad(u).normal = (gbx, gby, gbz).(normalX, normalY, normalZ) on GammaInner<br></div><div><br></div><div>where:<br></div><div>GammaOuter = Outer cube surface<br></div><div>GammaInner = Inner cube surface<br></div><div>Omega = volume limited by these two surfaces<br></div><div><br></div><div>I tried to modify Onelab examples for 2D Laplace equations as shown at http://onelab.info/wiki/Category:GetDP.<br></div><div>I have some known functions of space coordinates X[], Y[] and Z[] and of an order norder (=3 e.g.) for gbx[], gby[]</div><div>and gbz[].</div><div>As I do not know how to access the normal components (normalX, normalY and normalZ) within GetDP and as </div><div>my geometry is quite simple, I divided both GammerOuter and GammaInner into 6 individual Physical Surfaces: </div><div>top, bottom, left, right, front and back faces for each cube. There the normal vectors are known and I can therefore </div><div>write the Neumann boundary conditions on each face see the (Weak) Formulation space in the attached file </div><div>Laplace3D.pro. Note: how to proceed for more complicated surfaces is still a mystery to me but this is not my current</div><div>problem !<br></div><div><br></div><div>Gmsh (command: gmsh Laplace3D.geo -3) does its works and provides me with a 3D mesh of the domain of interest.</div><div>GetDP (command: getdp Laplace3D.pro -solve -pos) runs and outputs files, but solved potentials are null everywhere !<br></div><div><br></div><div>Does anyone have some clue on what I am doing wrong in this formulation ? I tried to solve the same problem<br></div><div>using FreeFem++ (see atached files Laplace-cube-cube.edp and MeshSurface.idp) and got results making some sense. </div><div>But I would prefer much more to solve it in Gmsh/GetDP as Gmsh is so powerful for postprocessing data.<br></div><div><br></div><div>Thanks a lot for any help,<br></div><div><br></div><div> Gilles<br></div><div><br><h3><span class="mw-headline" id="Computation_in_GetDP"> </span></h3></div><div><br></div></div></body></html>