If you are solving a moving frame problem (as opposed to a transient moving mesh problem, where the mesh actually rotates in time), the gravity vector must be counter-rotated (to keep it pointing in in a constant direction). This step is necessary to consider the proper rotational effect of gravity acceleration. Example: a box which is rotating (about z axis) with gravity pointing in the y-direction. Gravity is not aligned with rotational axis, so it must be counter-rotated at the rotation rate of the box. As of Fluent V 18.0, there is a TUI option to do this automatically: /define/operating-conditions> gravity-mrf-rotation gravity-mrf-rotation: Enable/disable rotation of gravity vector in moving reference frame simulations. This option is also documented in the TUI Manual : “gravity-mrf-rotation” Enable/disable rotation of gravity vector in moving reference frame simulations. If enabled, the gravity vector will rotate with respect to the moving reference frame such that the direction of gravity in global coordinates remains fixed.”