Guilin Sun
Ansys Employee

When BFAST is used, it automatically override the original "priodic boudary conditions" with its own special boundaries to keep the incident wavefront to be planar. From your result it is not oviously wrong. But make sure you chose the correct order. Please check if there is any higher-order diffraction. If large diffraction angles are there, the PML performance may be degraded. If so, you may need to use "Steep Angle PML", and use more number of PML layers if mesh size is small. you can set roughly the PML half wavelength thick.

In addition, please check the resonance. If there is strong resonance, the simulation my need longer simulaiton time and smaller autoshutoff min.