As I mentioned before, a contact card is required between the core SPH and the transition part. I used Tied-Node-to-Surface for both, the contact between the core SPH and the transition part and between the transition part and the outer finite elements. The Non-Reflecting-Boundary-Condition was also applied on the outer segments of the outer finite elements. it is working now somehow. I mean, with your suggested method, I'm able to somehow apply NRBC on the target now; however, I'm not sure if the Tied-Node-to Surface is an appropriate select for the contact between the mentioned parts? Please let me know what you think and find the new scenario below: