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Under "Tree Outline", expand "3 Parts, 3 Bodies" and select "Surface Body". Then set the type "Fluid/Solid" to Fluid.
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Close Design Modeler and Save Project
To save the project, first close the design modeler and at the workbench window, go to files -> archive. If you choose "save", it will save it as two items: a file and a folder whereas the archive option saves it as one integrated item.