# General Discussion **URL:** https://community.freefem.org/c/general-discussion/7.md?page=49 [Latest](https://community.freefem.org/latest.md) · [Categories](https://community.freefem.org/categories.md) **Page:** 50 --- ## [The stiffness matrix](https://community.freefem.org/t/the-stiffness-matrix/1591)
**Author:** [@Qianrui](https://community.freefem.org/u/Qianrui)\ **Replies:** 1\ **Last updated:** [March 9, 2022, 8:58pm UTC](https://community.freefem.org/t/the-stiffness-matrix/1591 "2022-03-09T20:58:45Z")
When calculating the spatial error and convergence order, the determinant of the stiffness matrix is non-zero, but the following error and convergence order are obtained L2error nan H1error nan L2error nan H1error nan … --- ## [Mesh dissapears](https://community.freefem.org/t/mesh-dissapears/1592)
**Author:** [@SamuelJosephs](https://community.freefem.org/u/SamuelJosephs)\ **Replies:** 0\ **Last updated:** [March 9, 2022, 8:56pm UTC](https://community.freefem.org/t/mesh-dissapears/1592 "2022-03-09T20:56:10Z")
So for some reason my mesh vanishes after the first iteration. Any ideas why? For some context I am trying to solve Maxwells equations using the Yee scheme for time discretisation: My aim is to get a wave simulation, … --- ## [Mixed dimensional finite element](https://community.freefem.org/t/mixed-dimensional-finite-element/1588)
**Author:** [@sebaubal](https://community.freefem.org/u/sebaubal)\ **Replies:** 2\ **Last updated:** [March 9, 2022, 3:41pm UTC](https://community.freefem.org/t/mixed-dimensional-finite-element/1588 "2022-03-09T15:41:30Z")
Hello. I would like to formulate a problem that couples physics in a 2D domain and a different physics in part of the boundary of the 2d domain (thus, a 1d domain). Is there a mixed dimensional finite element type alread… --- ## [What causes a length interpole warning?](https://community.freefem.org/t/what-causes-a-length-interpole-warning/1589)
**Author:** [@SamuelJosephs](https://community.freefem.org/u/SamuelJosephs)\ **Replies:** 3\ **Last updated:** [March 8, 2022, 5:27pm UTC](https://community.freefem.org/t/what-causes-a-length-interpole-warning/1589 "2022-03-08T17:27:52Z")
I have the following weak form for a vector wave equation -\\int\_{\\Omega} \\nabla \\cdot \\vec{v} \\nabla \\cdot \\vec{E}^m d\\Omega + \\int\_{\\Omega} (\\nabla \\times \\vec{v})\\cdot (\\nabla \\times \\vec{E}^m) - \\frac{1}{dt^2} \\int… --- ## [Solution not updating in loop](https://community.freefem.org/t/solution-not-updating-in-loop/1587)
**Author:** [@SamuelJosephs](https://community.freefem.org/u/SamuelJosephs)\ **Replies:** 1\ **Last updated:** [March 8, 2022, 5:27pm UTC](https://community.freefem.org/t/solution-not-updating-in-loop/1587 "2022-03-08T17:27:06Z")
So I want to have a simulation where I have a small circular source where the vector field oscillates up and down to see how the rest of the field responds over time. There is a second order time derivative, I am using … --- ## [Magnetostatic Simulation of Dipole](https://community.freefem.org/t/magnetostatic-simulation-of-dipole/903)
**Author:** [@bazzil](https://community.freefem.org/u/bazzil)\ **Replies:** 1\ **Last updated:** [March 7, 2022, 7:02pm UTC](https://community.freefem.org/t/magnetostatic-simulation-of-dipole/903 "2022-03-07T19:02:43Z")
Hello Everyone. I have been learning how to use FreeFEM++ for the last few weeks to solve a research problem but first I am attempting to validate a model to gain confidence in my implementation. I have done lots of read… --- ## [Error in 1D integration](https://community.freefem.org/t/error-in-1d-integration/1554)
**Author:** [@Loic](https://community.freefem.org/u/Loic)\ **Replies:** 2\ **Last updated:** [March 7, 2022, 3:22pm UTC](https://community.freefem.org/t/error-in-1d-integration/1554 "2022-03-07T15:22:38Z")
coarse\_grid.mesh (266 Bytes) Plane\_1.mesh (8.7 KB) Hello all, I am running this code: verbosity=0; load "Element\_PkEdge"; load "msh3"; meshS TH=readmeshS("coarse\_grid.mesh"); fespace Tri(TH,P0); // P0 on coarse mesh… --- ## [How to deal with second order time derivatives?](https://community.freefem.org/t/how-to-deal-with-second-order-time-derivatives/1580)
**Author:** [@SamuelJosephs](https://community.freefem.org/u/SamuelJosephs)\ **Replies:** 1\ **Last updated:** [March 7, 2022, 3:02pm UTC](https://community.freefem.org/t/how-to-deal-with-second-order-time-derivatives/1580 "2022-03-07T15:02:10Z")
Looking through the forum (Approximation of the second order derivative) it seems as though P1 is inappropriate for second order and higher time derivatives. So how would I go about dealing with these? The examples only … --- ## [When can you multiply two fespaces?](https://community.freefem.org/t/when-can-you-multiply-two-fespaces/1579)
**Author:** [@SamuelJosephs](https://community.freefem.org/u/SamuelJosephs)\ **Replies:** 2\ **Last updated:** [March 7, 2022, 10:32am UTC](https://community.freefem.org/t/when-can-you-multiply-two-fespaces/1579 "2022-03-07T10:32:18Z")
I’m new to FreeFEM and have the following error for the following code snippet: fespace Uh(Th,P1); Uh Ex, Ext; Uh Exold; Uh Extold; Uh Ey,Eytold, Eyt; Uh fx, fy; problem Maxwell(\[Ex,Ey\],\[fx,fy\]) = int2d(Th)( (dx(Ex)+d… --- ## [Read a simple 2D mesh](https://community.freefem.org/t/read-a-simple-2d-mesh/1569)
**Author:** [@Loic](https://community.freefem.org/u/Loic)\ **Replies:** 2\ **Last updated:** [March 7, 2022, 6:36am UTC](https://community.freefem.org/t/read-a-simple-2d-mesh/1569 "2022-03-07T06:36:59Z")
grid2D.mesh (1.4 KB) Hello all, I am trying to read a simple 2D mesh done using SALOME, classically: mesh TH=readmesh("grid2D.mesh"); However, I get an error. The log file is: Dimension Vertices Nb of Vertice… --- ## [How to save solution in a loop?](https://community.freefem.org/t/how-to-save-solution-in-a-loop/1576)
**Author:** [@lietvo](https://community.freefem.org/u/lietvo)\ **Replies:** 2\ **Last updated:** [March 5, 2022, 7:21pm UTC](https://community.freefem.org/t/how-to-save-solution-in-a-loop/1576 "2022-03-05T19:21:18Z")
Hello everyone, I want to save solutions at each time iteration of heat/Stokes equation. I know how to save solution for the Poisson equation with function ‘ofstream’ but I don’t know how to storage the solutions of the… --- ## [Definition of the subdomain error](https://community.freefem.org/t/definition-of-the-subdomain-error/1572)
**Author:** [@rayhanai](https://community.freefem.org/u/rayhanai)\ **Replies:** 1\ **Last updated:** [March 4, 2022, 8:24pm UTC](https://community.freefem.org/t/definition-of-the-subdomain-error/1572 "2022-03-04T20:24:10Z")
Hello fellow users, I’m looking to create a mesh using a boundary from the alphabet R. I have successfully created the border geometry using respective parametric equations as shown in the picture and plot it. My probl… --- ## [Self teaching : book on variational formulation for FEM](https://community.freefem.org/t/self-teaching-book-on-variational-formulation-for-fem/1568)
**Author:** [@mbletry](https://community.freefem.org/u/mbletry)\ **Replies:** 2\ **Last updated:** [March 4, 2022, 5:03pm UTC](https://community.freefem.org/t/self-teaching-book-on-variational-formulation-for-fem/1568 "2022-03-04T17:03:52Z")
Hello, I hope this si the right forum to ask my question ! I am looking for recommendations of books for self study of FEM, including an initiation to variational formulation… I more or less have to teach it to students… --- ## [Computing numerical solution with parallel version](https://community.freefem.org/t/computing-numerical-solution-with-parallel-version/1567)
**Author:** [@Qianrui](https://community.freefem.org/u/Qianrui)\ **Replies:** 3\ **Last updated:** [March 4, 2022, 6:09am UTC](https://community.freefem.org/t/computing-numerical-solution-with-parallel-version/1567 "2022-03-04T06:09:42Z")
Trying to write a parallel version, in fact, is it wrong? load “PETSc-complex” // PETSc plugin macro dimension()2// EOM // 2D or 3D include “macro\_ddm.idp” real omega=1,lambda=0.5;// real … --- ## [Changing PETSc communicator](https://community.freefem.org/t/changing-petsc-communicator/1570)
**Author:** [@Markov](https://community.freefem.org/u/Markov)\ **Replies:** 1\ **Last updated:** [March 4, 2022, 6:08am UTC](https://community.freefem.org/t/changing-petsc-communicator/1570 "2022-03-04T06:08:10Z")
Hello everyone, I would like to run PETSc on a subset of MPI\_COMM\_WORLD, hence I would like to create a smaller communicator and I was wondering where to add it in the source files (since It can not be modified in a scr… --- ## [How to change graphics](https://community.freefem.org/t/how-to-change-graphics/1564)
**Author:** [@Qianrui](https://community.freefem.org/u/Qianrui)\ **Replies:** 2\ **Last updated:** [March 3, 2022, 2:47am UTC](https://community.freefem.org/t/how-to-change-graphics/1564 "2022-03-03T02:47:24Z")
According to the explanation on YouTube, using the same command,the teacher’s graph can be changed,while mine can only be this graph. --- ## [Atom compiles .edp file](https://community.freefem.org/t/atom-compiles-edp-file/1566)
**Author:** [@Qianrui](https://community.freefem.org/u/Qianrui)\ **Replies:** 0\ **Last updated:** [March 2, 2022, 2:11am UTC](https://community.freefem.org/t/atom-compiles-edp-file/1566 "2022-03-02T02:11:41Z")
When atom compiles .edp file, which cannot generate TXT . for example, … ofstream stiffnessmatrix(“matrix,P2,03.02,”+omega+","+lambda+","+Tf+","+R+","+dt+".txt"); … stiffnessmatrix\<\<“matric”\<\ **Author:** [@larismartins](https://community.freefem.org/u/larismartins)\ **Replies:** 6\ **Last updated:** [March 1, 2022, 8:46pm UTC](https://community.freefem.org/t/lagrange-multipliers/1541 "2022-03-01T20:46:29Z") Is there an example to solve the Laplace equantion with Neumann boundary conditions using lagrange multipliers but in a scenario that it’s not 1D? I did the follwing but I’m getting errors. fespace VhK(Th\[i\], Pk); fes… --- ## [Assertion fail (iseuil) when trunc](https://community.freefem.org/t/assertion-fail-iseuil-when-trunc/1565)
**Author:** [@ffeppon](https://community.freefem.org/u/ffeppon)\ **Replies:** 0\ **Last updated:** [March 1, 2022, 11:57am UTC](https://community.freefem.org/t/assertion-fail-iseuil-when-trunc/1565 "2022-03-01T11:57:35Z")
Dear developper, I encounter an assertion fails with some 2D meshes that I didn’t have before. Again these problems with the trunc function (which should be combinatorial, not ?)! Th.o.mesh (197.0 KB) The following co… --- ## [How to switch the graph generated by plotdmesh](https://community.freefem.org/t/how-to-switch-the-graph-generated-by-plotdmesh/1563)
**Author:** [@Qianrui](https://community.freefem.org/u/Qianrui)\ **Replies:** 0\ **Last updated:** [March 1, 2022, 5:58am UTC](https://community.freefem.org/t/how-to-switch-the-graph-generated-by-plotdmesh/1563 "2022-03-01T05:58:35Z")
I used the following program to get only one picture. plotDmesh(Th,cmm=“decomposition”) In fact, there is a divided grid. I can’t switch with “P” and “Enter”. --- ## [The time convergence order of parallel version FreeFem++-mpi is different from that of freefem + +](https://community.freefem.org/t/the-time-convergence-order-of-parallel-version-freefem-mpi-is-different-from-that-of-freefem/1561)
**Author:** [@Qianrui](https://community.freefem.org/u/Qianrui)\ **Replies:** 0\ **Last updated:** [February 28, 2022, 1:42pm UTC](https://community.freefem.org/t/the-time-convergence-order-of-parallel-version-freefem-mpi-is-different-from-that-of-freefem/1561 "2022-02-28T13:42:11Z")
The following is a parallel version of the program I wrote about the order of time convergence, but the output results are inconsistent with those of the program I first wrote with freefem + +. example.edp (2.0 KB) --- ## [The error and convergence order with PETSc](https://community.freefem.org/t/the-error-and-convergence-order-with-petsc/1560)
**Author:** [@Qianrui](https://community.freefem.org/u/Qianrui)\ **Replies:** 0\ **Last updated:** [February 28, 2022, 6:35am UTC](https://community.freefem.org/t/the-error-and-convergence-order-with-petsc/1560 "2022-02-28T06:35:46Z")
Is there an example of calculating the error and convergence order with PETSc？I tried to change an example of a wave equation, but the convergence order result and error result are different from the results of the progr… --- ## [Dominant Eigen Value Computation](https://community.freefem.org/t/dominant-eigen-value-computation/1493)
**Author:** [@D\_N\_Sarkar](https://community.freefem.org/u/D_N_Sarkar)\ **Replies:** 13\ **Last updated:** [February 25, 2022, 9:12am UTC](https://community.freefem.org/t/dominant-eigen-value-computation/1493 "2022-02-25T09:12:24Z")
For eigen value computation in FreeFem++, some guess shift eigen value needs to be specified. If I am working with a system, where there is no idea about dominant eigen value, what shift eigen value should be used for c… --- ## [The picture is white and has nothing](https://community.freefem.org/t/the-picture-is-white-and-has-nothing/1557)
**Author:** [@Qianrui](https://community.freefem.org/u/Qianrui)\ **Replies:** 0\ **Last updated:** [February 25, 2022, 3:17am UTC](https://community.freefem.org/t/the-picture-is-white-and-has-nothing/1557 "2022-02-25T03:17:02Z")
Run the following program with the command（C:\\Users\\weiqi\>FreeFem++ square.edp or C:\\Users\\weiqi\>FreeFem+±mpi square.edp -wg), but the picture is white and has nothing. mesh Th=square(20,20); plot(Th); load “metis” /… --- ## [Changing label of the mesh](https://community.freefem.org/t/changing-label-of-the-mesh/1555)
**Author:** [@zese](https://community.freefem.org/u/zese)\ **Replies:** 4\ **Last updated:** [February 24, 2022, 11:29am UTC](https://community.freefem.org/t/changing-label-of-the-mesh/1555 "2022-02-24T11:29:39Z")
Dear all I have a meh with defined lables. now I want to change part of the boundary’s labels. for example I have inlet as 1 and wall as 2, outside as 3. now I want to change the label of some part of wall and keep the … --- ## [Issue to define FE space of MeshS type](https://community.freefem.org/t/issue-to-define-fe-space-of-meshs-type/1504)
**Author:** [@Loic](https://community.freefem.org/u/Loic)\ **Replies:** 10\ **Last updated:** [February 23, 2022, 4:07pm UTC](https://community.freefem.org/t/issue-to-define-fe-space-of-meshs-type/1504 "2022-02-23T16:07:51Z")
Hello All, I build a 2D mesh on the SALOME Plateform (a grid in the plane OXY). Then, I import this mesh in FreeFEM like this meshS TH=readmeshS("coarse\_grid.mesh"); However, it seems that I cannot do fespace P0P0(TH… --- ## [Cut a 2D domain or a 2D mesh with a curve](https://community.freefem.org/t/cut-a-2d-domain-or-a-2d-mesh-with-a-curve/1351)
**Author:** [@Loic](https://community.freefem.org/u/Loic)\ **Replies:** 13\ **Last updated:** [February 23, 2022, 11:13am UTC](https://community.freefem.org/t/cut-a-2d-domain-or-a-2d-mesh-with-a-curve/1351 "2022-02-23T11:13:26Z")
Hello all, I am trying to make some perforations (circular) on an existing 2D meshes that I have previously built. For information, I can consider this mesh or only the triangular domain (without the elements inside)… --- ## [FreeFem++-mpi.exe ended prematurely and may have crashed. exit code 3](https://community.freefem.org/t/freefem-mpi-exe-ended-prematurely-and-may-have-crashed-exit-code-3/1549)
**Author:** [@Qianrui](https://community.freefem.org/u/Qianrui)\ **Replies:** 0\ **Last updated:** [February 23, 2022, 5:48am UTC](https://community.freefem.org/t/freefem-mpi-exe-ended-prematurely-and-may-have-crashed-exit-code-3/1549 "2022-02-23T05:48:04Z")
Hi, why? C:\>“Program Files\\Microsoft MPI\\Bin\\mpiexec.exe” -n 1 FreeFem+±mpi.exe -jc DDM-Schwarz-Lame-3d.edp – FreeFem++ v4.1 (Sat, Nov 13, 2021 7:40:01 PM - git v4.10) file : DDM-Schwarz-Lame-3d.edp Load: lg\_fem lg\_… --- ## [Non linear time dependent problem - Natural Convection](https://community.freefem.org/t/non-linear-time-dependent-problem-natural-convection/1546)
**Author:** [@D\_N\_Sarkar](https://community.freefem.org/u/D_N_Sarkar)\ **Replies:** 0\ **Last updated:** [February 22, 2022, 11:14am UTC](https://community.freefem.org/t/non-linear-time-dependent-problem-natural-convection/1546 "2022-02-22T11:14:43Z")
This is with respect to the code: Non linear time dependent problem - #2 by frederichecht In this code, natural convection within sqaure cavity (bottom wall hot, top wall cold) is being simulated. For such flows, convec… --- ## [MUMPS or UMFPACK64](https://community.freefem.org/t/mumps-or-umfpack64/1437)
**Author:** [@Qianrui](https://community.freefem.org/u/Qianrui)\ **Replies:** 16\ **Last updated:** [February 21, 2022, 1:53pm UTC](https://community.freefem.org/t/mumps-or-umfpack64/1437 "2022-02-21T13:53:39Z")
Hi,Are there any appropriate solutions to the following problems? why? load “MUMPS\_seq” … problem kg2(psai2,u,solver=sparsesolver,master=-1)=int2d(Th1)(psai2\*u/(dt^2))- … 0 : Allocation error for id%IS( 101830817 … [Previous page](https://community.freefem.org/c/general-discussion/7.md?page=48) [Next page](https://community.freefem.org/c/general-discussion/7.md?page=50)