well, i tried to open only the cavity (without the tyre and the domain, because i have drawn the two meshes separately). When i apply the udf  and try to preview mesh motion, i got a black screen and the mesh disappears.

When the preview finished , i can re-see the cavity with the new length ( so i guess the udf is working properly but i dont know the error occuring when i have the whole system connected together and i don't know why the mesh disappers when previewing)!