Discussion Forum

Sometimes I need to perform frequency domain analysis of high-Q resonators. Those resonators are always operated at their resonance and have Q-factor of tens of thousands which is set as such it corresponds to measurement (isotropic or anisotropic material damping in the simulation) of different modes. The problem I have been always having is that at certain level of damping (Q-factor >1000's) the harmonic simulation stops converging (the relative error is larger than relative tolerance). I have tried various things including changing of direct and indirect solvers, different nonlinear methods with various damping of solvers, different scaling but I was not able to find a way how to make the solver converge. I was partially successful by ramping the load, however, the higher the Q-factor is, the larger the ramp must be and sometimes even very small boundary loads do not converge.

I know that when I use frequency-domain modal, there is no issue with converging. Essentially, it is possible to run simulations with Q-factor of millions. The interesting thing is that if I set initial conditions of the harmonic solver to the solution of frequency-domain modal I am able to increase the Q-factor for which the frequency domain simulations converge, however, when Q-factor reaches certain level (say Q-factor of 20 000), the simulations stop converging again. I also know that both simulations give very close results so what I don’t understand is when I give initial conditions to the solver which almost correspond to the solution, why the solver fails to converge. I have tried frequency-domain modal simulations with various number of eignemodes considered with very little effect on the convergence of subsequent frequency domain simulations. Due to the very low damping and simulating at exact resonance I suspect some numerical issues. However, why frequency domain does not work whereas frequency-domain does is something which I don’t understand.

I could always use frequency domain with low Q-factor or use frequency-domain modal with high Q-factor in case I don’t need any prestressing of the structure. Nonetheless, not being able to simulate what I want to has been bothering me for a while so I would be happy if anyone may have some advice or solution for me.

Mikulas