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If you're forcing liquid into the domain the gas has to go somewhere. Either it compresses or there has to be an outlet: to a point the solver error (poor convergence) will lose some mass, but that may then lead to the divergence you're seeing. nRe the mesh, have a look at dynamic adaption and VOF predefined criteria in the 2020R2 or 2021R1 versions. That may allow a coarser mesh overall but the solver can then refine the interface. n