Thanks. From your originial image, it appears that the cylinder has a top surface as well. Is this also 3 mm thick? Is the top surface connected to both of the inner and outer cylinder surfaces? There should be a continuous, undivided space inside the vessel, i.e. no internal walls between the surfaces.
I should also mention that for a hollow cylinder of 3 mm thickness, you will need to use quite a small mesh element size. Strictly, the element size should be no larger than the separation between the surfaces (3 mm). However, you can relax this requirement by turning on the Hydrodynamic Diffraction Analysis Settings option 'Ignore Modelling Rule Violations', which would allow you to increase the mesh element size a bit.
Mike
You are navigating away from the AIS Discovery experience