As show I have 2 body objects, and I wanna very fine mesh at the interface between the 2 bodies, and as we go into the bulk of each body the mesh gradually coarsened, so how to do that.
I can't tell from this image which condition you have.
Is this a small thin solid that is touching the flat face of a large solid?
Or is this a small surface body touching the flat face of a large solid?
Or is this a small thick solid embedded in a large solid?
In any case, you simply apply face sizing on the faces that touch. Set the Behavior to Hard to force the mesher to respect the element size. The mesh will grow from those faces out to the default mesh element size for the rest of the body.
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