Question on absorbing surfaces definition

[theinlinaung2010]

Hi there, I am learning how to use SPECFEM3D for local-scale (<100m) wave propagation problems.
I read through the user manual and could run the example simulations provided in the repo.
However, I'm still quite confused on how the (absorbing) boundary conditions are defined, especially on the definition of surface files.

The manual says we need to define an input file for each surface:

• free_or_absorbing_surface_file_zmax
• absorbing_surface_file_xmax
• absorbing_surface_file_xmin
• absorbing_surface_file_ymax
• absorbing_surface_file_ymin
• absorbing_surface_file_bottom

which corresponds to the block definitions in Cubit. My questions are:

1. Are those file names fixed in the code and must be present in every simulation? If a file is absent or its contents are empty, does it become free surface?
2. How to define absorbing/free surfaces if the model is not of rectangular topology? For example, coffee mug example does not have clear xmin, xmax surfaces, etc. Since SPECFEM3D code can be used for ultrasonic testing simulations, I think arbitary-shaped models should also be usable.

[homnath]

Hello: 1. Yes, they are fixed names. 2. For complex surfaces, we define surfaces as side sets in Cubit and use the exodus2specfem3d available in the MeshAssist package (https://github.com/homnath/MeshAssist) Best, Hom Nath

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________________________________ From: Thein Lin Aung ***@***.***> Sent: Monday, October 6, 2025 4:11 AM To: SPECFEM/specfem3d ***@***.***> Cc: Subscribed ***@***.***> Subject: [SPECFEM] Question on absorbing boundaries definition (Discussion #1838) Hi there, I am learning how to use SPECFEM3D for local-scale (<100m) wave propagation problems. I read through the user manual and could run the example simulations provided in the repo. However, I'm still quite confused on how the (absorbing) boundary conditions are defined, especially on the definition of surface files. The manual says we need to define an input file for each surface: * free_or_absorbing_surface_file_zmax * absorbing_surface_file_xmax * absorbing_surface_file_xmin * absorbing_surface_file_ymax * absorbing_surface_file_ymin * absorbing_surface_file_bottom which corresponds to the block definitions in Cubit. My questions are: 1. Are those file names fixed in the code and must be present in every simulation? If a file is absent or its contents are empty, does it become free surface? 2. How to define absorbing/free surfaces if the model is not of rectangular topology? For example, coffee mug example does not have clear xmin, xmax surfaces, etc. Since SPECFEM3D code can be used for ultrasonic testing simulations, arbitary-shaped models should also be usable. — Reply to this email directly, view it on GitHub<#1838>, or unsubscribe<https://github.com/notifications/unsubscribe-auth/ABMCQ4ULHGIM5TZSTYNPLLL3WIP4FAVCNFSM6AAAAACIMEQOJKVHI2DSMVQWIX3LMV43ERDJONRXK43TNFXW4OZYHE4TAOBQGA>. You are receiving this because you are subscribed to this thread.Message ID: ***@***.***>