Gu Xuan

Thank you for your reply.

>May I know the fluent version you are using.

Yes! I used ver. 2021 R2.

I studied these parts before. My case may be different.

In my model, the mediums a and b are the same.  As described, it seems that the refractive index   and   were important to change.

And if the wall is 0 thickness, the refractive index is not important.

If the wall has thickness, the absorption coefficient can be set to simulate raditation transmission and absorption.

But my case, the wall is 0 thickness, and medium a , b is the same. So how to do it?