# Discussion Forum

Discussion Closed This discussion was created more than 6 months ago and has been closed. To start a new discussion with a link back to this one, click here.

## Frequency Domain not Converging

Hello,

I'm trying to design a nonlinear MEMS energy harvester: * I am using a fine swept mesh. * Material: Silicone polycrystiline * physics: solid mechanics. * Boundary conditions: fixed at one end. * 1g gravity applied to the domain. * Study: eigenfreuquency and frequency domain.

I used the eigenfrequency study to find the first three modes, and it converged to the right soluction with no issues.

When I use the frequency domain study with conditions I stated above, I set the freuquency to the first mode freuncy that was found by the eigenfrequency study, and it coverges with no problems.

The problem When I specify a range of frequencies around the first mode/resonant frequency, the solver works for the frequencies up to the actual resonant frequency that is within the range specified, and it stops converging.

I am not sure to what I should attribute the problem to in my settings. I have tried running an Adaptive Mesh in the frequency domain study, a parameter sweep for the gravity to improve convergence with no luck.

I think what happens, is that the deflection is really high at the resonant frequency and that, somehow, messes the mesh up. as I have read something about how elements of the mesh can get inverted when the mesh deforms, which in my case, the mesh is moving and deforming for the boundary conditions I am using.

I have tried to solve the problem by adding damping to reduce deflection at the resonant frequency, that didn't work either.

I would really apreciate your input or pointer to my problem.

1 Reply Last Post Apr 30, 2019, 8:33 AM EDT

Posted: 1 year ago

Hi,

The most probable cause is that you have zero (or very low) damping. At resonance, this gives a singular matrix (analytically: infinte displacements).

Regards,
Henrik

-------------------
Henrik SÃ¶nnerlind
COMSOL

Note that while COMSOL employees may participate in the discussion forum, COMSOL® software users who are on-subscription should submit their questions via the Support Center for a more comprehensive response from the Technical Support team.