Questions tagged [finite-element]

A means of solving ordinary and partial differential equations. The domain of the problem is broken up into elements, and the solution in each element is expanded in a basis of functions. The Finite Element Method lends itself well to adaptive refinement, irregular geometry, and good error estimates.

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58 views

Solving Laplace equation with constraint on boundary

I have found the following PDE problem in a paper: Essentially, we have a rectangular domain where there is a unknown interface $z=\xi(x)$ (liquid-air interface) separating the domain into two medium ...
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1D Poisson equation and quadratic basis functions assembly

I'm solving the simple Poisson problem $$-u''(x)=1$$ in the interval $[0,1]$ with $u(0)=u(1)=0$. I discretised my domain as done here, i.e. with ...
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Discretization of a nonlinear boundary value problem

I am trying to use finite element method to discretize the following problem \begin{align} \min_{u \in H^1_0(\Omega)} \int \| \Delta u(x) - 0.5*[u(x) + \langle e, x \rangle + 1]^3 \|^2_2 \ d\Omega, \...
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82 views

Renumbering the nodes for quadratic basis functions for a 2D domain

I have a simple triangulation for a 2D domain, described by the connectivity matrix $T$ and by the point matrix $P$. For didactic purposes, I assembled the stiffness matrix for $-\Delta u = f$ by ...
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Differences in using Clausius-Duhem inequality vs Principle of Virtual Work/Power in derriving constitutive equations?

I am a novice getting my toes wet in continuum mechanics and nonlinear elasticity. I have seen papers that use both approaches in developing constitutive connections to compliment balance equations of ...
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Who uses finite elements with higher continuity?

Lagrange elements of any polynomial describe piecewise continuous functions. Typically, those functions are differentiable. Mixed finite element methods use vector fields of even less continuity, such ...
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52 views

Spline interpolation for vector-valued data in 3D space

I have output from a 3D linear elasticity finite element simulation which uses linear tetrahedral elements, such that the displacement is continuous over the nodes but the gradient is not ($C_0$ ...
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1answer
137 views

FEM solution for Poisson is not exact at nodes

Let's consider the usual Poisson problem for FEM $$-u''(x)=1 \quad x \in [0,1]$$ with homogeneous Dirichlet boundary conditions. The solution is $u(x)=-\frac{1}{2}x(x-1)$ I know that the FEM solution (...
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80 views

C.S.R method in finite element matrix assembly

I have solved the 2D Poisson equation using finite element method with simplex triangular element in MATLAB. First, I generated the triangular mesh using pdetool in ...
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2answers
117 views

diffusivity matrix assembly in nonlinear finite element analysis

I want to solve a diffusion analysis using finite elements. According to fick's law, governing equation is $$\frac{\partial h}{\partial t} = D \frac{\partial^{2} h}{\partial x^{2}}$$ . h is relative ...
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141 views

Poisson equation, stiffness matrix positive definiteness, Dirichlet boundary conditions

I have a question regarding the positive definiteness of the stiffness matrix. Specifically, I believe that it should be positive definite only when at least one Dirichlet point is given, so I would ...
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105 views

Finite element interpolation

I have a finite element solver that I implemented in MATLAB. I am calculating a specific potential function that is constant in each tetrahedral element. The question is, I want to be able to ...
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30 views

velocity in CFL condition

I am studying the evolution of the density and velocity field of a core in a molecular cloud in 1 D. I defined the radial grid (let us say x between 100 and 101) and the time grid. I am using the ...
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43 views

Fourier transform in finite element

I have a finite element solver where I am using tetrahedral elements. I am solving for electric potentials and then calculate the current densities in each element, which are constant in each element. ...
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Sparse linear solver in fortran working with REAL16

I need some (direct) sparse linear solver for fortran, which works with REAL16 data type. Any suggestions? Both Pardiso and MUMPS support only REAL8. (identical question: https://math.stackexchange....
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120 views

Finite elements with CFL condition - How to obtain correct order of convergence

I have discretized a PDE with continuous finite element method in spatial variable and with implicit Euler or Crank-Nicolson in temporal variable. In both cases, I have error estimates in $L_2$ norm ...
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141 views

L2 projection with bounds

In some problems we are currently working on, we are working with discontinuous functions that are defined on a finite element mesh and are established using Lagrangian particles. To obtain them on a ...
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79 views

relation between different tangent stiffness

I need to find a relation between the tangent stiffness $L_1$ of the first Piola-Kirchhoff stress tensor with the tangents stiffness $L_2$ of the second Piola-kirchoff stress tensor. They are defined ...
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141 views

How can I implement second order derivatives of shape functions of a 3D elements?

I am developing an Abaqus UEL with 3D 8 nodes brick elements and I need second order derivatives of the shape functions, I have already mapped the first order derivatives from the element coordinates ...
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Mesh partitioning with METIS

I am trying to use METIS-5.1.0 edition in order to partion a FE mesh. For demostration purposes I created 2x2 rectangle mesh and tried to partition it. However, I notice a weird behaviour in my code. ...
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Output solution vector in Petsc

I am using petsc to solve a linear elasticity problem discretized by finite elements.The initial mesh is read by a mesh file and the distribution in each processor is done using METIS.I am using only ...
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70 views

How to solve a Stiffness matrix?

I am quite new in this field. For my university I prepared a stiffness matrix to solve for a project group. This matrix consists of 450 equations with 450 unknows (it's a Matlab script) and I have the ...
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Fortran compiles for legacy Finite Element Fortran program (1980)

The version of Fortran used come from Montreal Ecole Polytechical in 1980. I need a compiler for Fortran for Windows 7 or Windows 8.
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Technique to find the CFL condition using the Galerkin method in space and finite-difference in time?

I am using the Galerkin method (Discontinuous to be precise) to discretize in space the scalar linear wave equation and the explicit second order centered finite difference scheme to discretize in ...
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Solving PDEs in parallel

I have read different approaches on how to solve pdes in parallel which are discretized using finite element method. For example: Non-overlapping domain decomposition approach as mentioned in https://...
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47 views

Finite element analysis software for acoustic and electrostatic

I need to do a simulation for my thesis project involving some piezoelectric nanoparticles in a fluid beamed with ultrasounds. I'm looking for a software for such simulation and for now it seems me ...
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182 views

How to compute gradient of each node in finite element method?

I have a problem with computing gradient of each node in finite element method. I can get the value of each node. But how can I get the gradient? I know $u = \sum u_i \phi_i$ where $\phi_i$ are the ...
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143 views

Finite Element Analysis based on contractions of a subset of edges in enterior of mesh

I am modelling a problem that is "driven", not by typical boundary conditions but, by contractions in its interior. In a finite element analysis, I can specify the new lengths (not ...
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1answer
80 views

Transition from 2D to 3D finite element code, what are the inevitable modifications to be implemented?

Imagine we have a simple 2D FEM solver (we are dealing with solid mechanics) and we would like to develop it to a 3D FEM solver (let's say for the same solid mechanics problem) in this case what are ...
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180 views

Problem with solving coupled ODE and DAE equations with mass matrix (Error using daeic12 (line 77) This DAE appears to be of index greater than 1)

I am trying to solve 6 ODE equations coupled with 1 DAE one. The ODE equations have been discritized in space domain and ode15s MATLAB solver is used to solve the equations in time domain. I have ...
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509 views

Gauss-Lobatto quadrature and nodal points for FEM

By using the Legendre-Gauss-Lobatto (LGL) quadrature formula (QF) and LGL nodal points one achives a diagonal mass-matrix for finite element problems. (More specifically, the spectral element method.) ...
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655 views

FEM Python book

Is there any book or site available with Finite element Method for partial differential equations with python code apart from Fenics?
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86 views

inclined/general Dirichlet boundary conditions

For simpilcity, consider a single quad linear elasticity finite element in 2D. The Dirichlet boundary conditions on node 1 and node 2 are easy to implement and can be handled in the standard way. ...
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Learning the art/science of structural idealization

I am a mechanical engineer working in the field of aerospace structures. During the course of my studies, I have studied a course on structural analysis in which I learned 3D Euler-Bernoulli beam ...
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99 views

Which 2D PDE with an exact solution can I use to test/verify my FEM-PDE code?

I have created a program to solve 2D, time-dependent PDEs with the finite element method and get reasonable looking results for the 2D acoustic wave equation. Now I would like to go further and solve ...
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How can I improve the accuracy of the calculation of the magnetic field in Gmsh/GetDP?

I need to calculate the magnetic field along a straight line in proximity of an array of 6 magnets. I used the tutorial files "magnets" included in Gmsh and I slightly modified the file in ...
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Mesh transition between beam and shell element types

I am using NASTRAN solver & FEMAP as preprocessor for reduce modelling of wing using 1D and 2D finite elements. Beside transition of 1D & 2D elements to 3D, I had not found any method/solution ...
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86 views

How to compute the $L^{2}$ error of the gradient in the Finite Element Method

Let $\Omega\subset \mathbb{R}^{2}$ and $\tau_{h} = \{\Omega_{k}\}_{k=1}^{N}$ be a triangulation of $\Omega$. The $L^2$ error for a FEM approximation $u_{h}$ is given by: $ || u-u_{h} ||_{L^2} = \sqrt{ ...
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trilinear hex elements

Do the faces of tri-linear hex elements have to be planar? Three nodes define a plane. If the fourth node does not lie on the plane, then the nodes are not planar and the face is not plane. In general,...
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Create a sparse matrix

I am writing a FE program which calculates the displacements under a uniform load. I want to store the stiffness matrix in sparse form(COO) without using an external library.Assume an upper-bound for ...
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Construction of Prolongation and Restriction Operator for Geometric Multigrid (2D-FEM): Resulting in a Decreasing Solution

Consider the following problem, $$ -\Delta u(x) = f(x), \qquad x \in \Omega \\ u(x) = 0,\qquad x \in \partial \Omega$$ with $\Omega = [0,1]\times [0,1]$ being the domain and $\partial \Omega$ being ...
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Abaqus approach to simulate many flexible elements

I would premise that i am not an expert on the domain (i am a programmer that usally work with DB and data, not structural problem), but i have just see the work of a friend of mine and i am curious ...
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59 views

Symmetry in P1 basis elements on a reference triangle in 2D-FEM

I am trying to understand the finite element method and want to apply it to a 2D equation with a triangular mesh. I have chosen the reference element to be the triangle with vertices $(0, 0), (0, 1)\...
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How can the choice of coarsening factor affect Multigrid's convergence?

The linear system $Ax=b$ is coming from the discretization of an elliptic PDE. Multigrid method is used in order to solve it. Suppose $c_0$ is the coarsening factor on level 0 and $c_m$ the coarsening ...
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151 views

how to Implement linear tetrahedral elements for finite element computations?

I am trying to implement 3D tetrahedral elements in my finite element code (which works fine for linear triangles and quadrangles in 2D). But my simulations are crashing with tetrahedral elements. My ...
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188 views

Heat equation in non-dimensional form behaving differently than in usual format

Starting from $$ c_p \frac{\partial u }{\partial t} = k \nabla^2 u $$ in a one dimensional domain [0,1] where $c_p$ and $k$ are modeling two different materials: $$ k = \begin{cases} 1 ~\text{if} ~x &...
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Parallel In Time with Multigrid

I am trying to solve the linear finite element equation $M\ddot{u}+Ku=F(t)$, where $M$ is the mass matrix ,$K$ the stiffness matrix and $F(t)$ the external load vector, parallel in time using XBraid ...
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Solution predictors for accelerating convergence in nonlinear FEM

I am looking for the details of commonly-used predictors for accelerating the convergence of iterations using Newton-Raphson scheme for nonlinear problems in FEM. I am looking specifically for static ...
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How to use FEniCS to calculate the electric field of an isolated charged sphere

Initially I thought that this is the kind of question which ought to have already been answered in the form of an example online, but so far I haven't found one. I will admit that I am very new to ...
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Numerical integration in time for finite elements

I am trying to solve $M\ddot{u}=-Ku+F_\text{ext}$ for a 2D linear elastic model with $M$ be the mass matrix,$K$ the stiffness matrix and $F_\text{ext}$ the external load vector coming from a uniformly ...

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