Solid and Fluid motion with eigenfrequency study

Hello,

I made a very simple geometry in 2D, such as the cross-section of a microcapillary.

I want to find some eigenfrequency which can make a few modes on a microcapillary.

So, for the shell I used solid mechanics study and for the fluid in the shell, I chose the creeping flow.

To find eigenfrequency, I added pressure acoustics with frequency domain.

In study section, there was a problem because the creeping flow study couldn't be included in the eigenfrequency solver.

How can I use the creeping flow for a fluid which I want and can be used for a liquid in a microcapillary?

The most important thing is the eigenfrequency.