Hello there! I have been studying a sample analysis which can be found in Abaqus verification manual-> element verification-> miscellaneous tests-> analysis of unbounded acoustic region. Specifically, I'm trying to understand the case duct_acin3d8.inp.  When I run the model it aborts due to uncomputed normals, and also gives a warning of element distortion. All these errors are caused by the infinite acoustic element. I have checked the normals through visualization module and it seems ok. How can I resolve this?  

in the first picture, I have removed the element below the acoustic infinite element just for clarity.