[cfd-online]

I am trying to simulate the acoustics generated by an underwater vortex ring in a cylindrical tank. The vortex ring is formed by a water slug impinging from a 10 cm inlet. To model the acoustic behavior, we are using the Ffowcs Williams-Hawkings (FWH) model and have created a 15 cm cylinder named "fwh_surface" for this purpose.
The problems I am facing are:

1. The "fwh_surface" is categorized as an interior zone in the boundary conditions. Consequently, it does not appear in the source zone selection when activating the FWH acoustic model; only the outer walls are visible for selection.
2. When I change the "fwh_surface" from an interior zone to a wall, it becomes visible in the source zone. However, running the simulation in this configuration confines the vortex within the walls, preventing its expansion, as the "fwh_surface" acts like a wall. I attempted to address this by setting the wall boundary condition to free slip, which is defined as zero shear stress in all directions.
3. Additionally, I am unsure where to place the receivers for optimal results. Should they be placed inside or outside the "fwh_surface"?

I need guidance on three issues: first, how to define a volume region around the inlet that allows the FWH model to track the source movement. Second, how to ensure the "fwh_surface" does not act as a wall and interfere with the hydrodynamics of the vortex ring. Third, determining the best placement for the receivers to obtain accurate results.