How to apply a joint and solve structures that are stuck together in IBFE method?

[陈奕纯]

I'm simulating a 3D fish swimming using IBFE method. I have 2 separate exodusII mesh which are the fish body part and the fish tail part. They are intertwined with a cylinder "joint" from the body part, as shown in the figure below (tail part is transparent). I use active PK1 strss function to rotate the tail part around the joint but the body part is always dragged to rotate together. 

1. I want the body part to stay still and only tail part to rotate around the cylinder "joint", is there any way to add a vertual joint instead of using this kind of cylinder "joint"? 

2. I tried removing the cylinder "joint" from body part and the corresponding hole from tail part. It turned out that when i apply active PK1 stress to rotate tail part, the body part is still dragged to rotate together, as if the two parts are stuck together and cannot have relative motion. How can i solve it?

3. I searched on the net and guess it may be due to the fact that the two structures are too close and the IB kernal requires some number of grid points between structures. For my case, the small gap between the two parts needs super high grid resolution to let the two parts to have relative rotation. Am I correct about it? Is there any better way instead of using super high grid resolution and super small DT?