Check these:
(1) Is the calculation implicit or explicit ?
(2) Did you change operating density from default ?
(3) Are you considering surface tension ?
(4) Is the gravity direction set properly ?
(5) Is the volume fraction at the boundaries (also backflow VF at outlets) set properly ?
(6) Which p-v scheme you are using ?
(7) What time step size have you set ?
(8) Have you checked the patched domain before calculation ?
(9) Did you change the advanced stability controls (velocity limiting etc) ?
(10) And finally, are you trying to simulate something unphysical ?
Also these might help: User's guide ; Theory guide
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