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9 votes

Writing a single PDE from a gradient equation

Your equation is not well posed: For general functions $f_1,f_2$, there is no function $\Phi$ so that the equation can be satisfied. For example, if you had $f_1=f_2=x$, then you are looking for a $\...
Wolfgang Bangerth's user avatar
8 votes

Mesh ordering algorithms used by COMSOL Multiphysics

The idea of "ordering the nodes" in a finite element mesh to improve the computational time of the sparse solver originated in the large structural analysis FE codes of the 70's. Those codes typically ...
Bill Greene's user avatar
  • 6,119
6 votes

What is numerical damping in the context of time-dependent FEM solvers?

It is quite straightforward to demonstrate this for implicit Euler (aka backward Euler) on a scalar example. Consider the initial value problem $$\dot{y}(t) = i \alpha y(t), \ y(0) = 1$$ with solution ...
Daniel's user avatar
  • 1,273
5 votes
Accepted

Understanding Finite-Element Modal Analysis

The biggest problem in answering to this question is that it's not as straightforward as you might think to see the physics in the equations until you see them fully developed, and even then you're ...
CADJunkie's user avatar
  • 383
4 votes

Alternatives to Comsol Multiphysics

You might also want to have a look at: Elmer https://www.csc.fi/web/elmer Kratos http://www.cimne.com/kratos/ OpenFoam http://www.openfoam.com/ CaeLinux http://caelinux.com/CMS/
Stefan's user avatar
  • 141
4 votes

Simulation of a complicated projectile motion

You need to know which equations you need to solve inc initial conditions. IMO, a disadvantage of click&result software is that it's not transparent to what you are actually solving. Why don't ...
nluigi's user avatar
  • 277
3 votes
Accepted

A lot of identical staff in Comsol material database?

I don't see anything unusual here. You have a material property that is defined in terms of other material properties. In your particular case, the real and imaginary parts of the refractive index ...
Anton Menshov's user avatar
  • 8,672
3 votes

How does rigid body rotation affect resuts in a simulation?

You question is unclear. However, rotations in the context of linear mechanics are often confusing. Therefore a response is probably needed. 1) I will assume you have a deformable body and a rigid ...
Biswajit Banerjee's user avatar
2 votes

Understanding Finite-Element Modal Analysis

Finite Element Analysis is a mathematical tool very extended among engineers. However, after more than a year researching on the topic of computer simulation, where FEA plays such an important role, I ...
Rabindranath Andujar's user avatar
2 votes

Flux boundary condition in solute transport

The correct boundary condition is in fact $$ (D\nabla C - vC) \cdot n = g. $$ That is, it is not the vector-valued quantity in parentheses that you can describe, but only the normal component, ...
Wolfgang Bangerth's user avatar
2 votes
Accepted

Why FEM electric analysis gives only access to current density?

They are saying the same thing, at a fundamental level - it's just that the implementation is a bit different. The first equation you noted is valid throughout the system because it solves for the ...
cbcoutinho's user avatar
2 votes

Writing a single PDE from a gradient equation

For this problem to be well-posed the necessary and sufficient condition is $\partial_y f_1 = \partial_x f_2$. If that is satisfied then $\Phi$ is a potential field and it is given by $$ \Phi(\vec{r}) ...
Maxim Umansky's user avatar
2 votes

Does mass balance hold in convective diffusion

Mass is conserved always. That's the known fact. To show this holds true in convection-diffusion equation, I need to introduce material derivative to you. The material derivative of a scalar quantity ...
Mithridates the Great's user avatar
2 votes

What is the difference between non-linear elastic simulation and linear elastic simulation with plasticity?

I think the OP is asking why in comsol linear electricity can't be deleted. Linear elasticity is by default the material constitutive model. The user has to add material models, nonlinear, plastic etc ...
feynman's user avatar
  • 287
1 vote

What is the difference between non-linear elastic simulation and linear elastic simulation with plasticity?

TL;DR: as answered by @BillGreene as a comment a model with plasticity is nonlinear. Even when the stresses are not high enough the solution process is probably different from a linear one. Regarding ...
nicoguaro's user avatar
  • 8,525
1 vote

How to connect two cylinders to form a knee in Comsol Multiphysics?

As an answer, in case of simple cylinders, it is enough to place a sphere with the radius of the cylinder at intersection of the cylinder axis.
Bort's user avatar
  • 1,285
1 vote
Accepted

For a determined (known) Space charge density, what are the conditions to obtain the Electric potential/field distribution? (COMSOL, MATLAB)

I think you have misunderstanding about what the Gauss's law and subsequently Poisson equation means. You have these relations (Gauss's units): $$\nabla \cdot \mathbf{E} = 4 \pi \rho$$ $$\mathbf{E} =...
Mithridates the Great's user avatar
1 vote

How to set an initial guess for the iterative solver in Comsol?

I am not too familiar with the details of the iterative solve in Comsol; however, quick googling did reveal any easy way to set up a custom initial guess for an iterative linear solver. (I ready to be ...
Anton Menshov's user avatar
  • 8,672
1 vote

temperature profile on axial direction of a chamber

You can use an interpolation function (see interp3) to estimate the temperature at each position in a cross-section. Set the extrapolation value to NaN so that points outside your chamber do not ...
Daniel's user avatar
  • 11
1 vote
Accepted

Mesh ordering algorithms used by COMSOL Multiphysics

Based on the comment by @TylerOlsen, I've applied the boost implementation of the reverse Cuthill-McKee ordering on the same matrix from the original question and here is the result: The structure ...
Jakub Klinkovský's user avatar
1 vote

How does rigid body rotation affect resuts in a simulation?

Due to the Euler's rotation theorem a rigid body in 3D space has 3 rotational degrees of freedom, (plus 3 displacement degrees of freedom for a total of 6 degrees of freedom.). This typically means ...
Stefano M's user avatar
  • 3,829
1 vote

Understanding Finite-Element Modal Analysis

If you are familiar with the standard FEM analysis works, the idea of modal analysis is straightforward. In standard FEM analysis, you transfer the time-dependent elastic wave equation (ignoring ...
Christian Clason's user avatar
1 vote

COMSOL: get list of eigenfrequencies

Under results node, there are Export and Reports options. Right click on Export will give you some options for exporting your data.
Blue_Elephant's user avatar
1 vote

Different Meshes Different Maximum velocities

The finite element methods, i.e., what COMSOL uses internally, does not solve the problem exactly. It only (i) provides an approximation to the solution, and (ii) does so in a way for which we can ...
Wolfgang Bangerth's user avatar

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