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Save the assembly as a new part file. Use Combine → Subtract as shown in the first image below. If your object is hollow and closed, do not keep the volume associated with the inner enclosed region (see image 2 if applicable).
The cross-section should look have your object of interest subtracted from the prism in a similar manner to the image below:
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Save this part once again as a new .STEP file to be imported into ANSYS Fluent.
Setting up in ANSYS Fluent
Open the ANSYS License Management Center App on your computer (Allow Changes on pop-up) and start the license manager:
Once ANSYS is open, drag Fluent into the Project Schematic area.
