KR126: What do the 'On' and 'Passive' options in the six DOF solver mean?
When the six DOF solver is activated in Dynamic Mesh -> Parameters, `On` and `Passive` check boxes appear under Six DOF Solver Options when specifying a rigid body motion in Dynamic Mesh -> Zones.
The user can turn off the six DOF solver if it is intended to specify the zone motion explicitly for any zone (e.g. using DEFINE_CG_MOTION or DEFINE_GRID_MOTION or transient profile) and turn on the six DOF solver if the rigid body motion is a resultant of fluid forces and moments.
By turning on the `Passive` check box for any rigid body zone the user instructs FLUENT not to include that particular zone in the computation for forces and moments.
For example, it is often necessary to maintain good mesh around the rigidly moving object to resolve the flow structure around it (this is especially important for six DOF solver where the drag forces are to be captured accurately).
In order to maintain a good mesh around the object it is customary to have a layer of good quality mesh cells around it (as shown by the quad cells in the figure attached) and have it move rigidly with the object.
Therefore, this layer should inherit the same rigid body motion as that of the object but should be excluded from the fluid force calculation (in other words, it should move passively with the object).
Thus, in the above example, six DOF solver is turned `On` on both the object wall and the moving layer of quad cells and additionally `Passive` is turned on for the moving layer of quad cells.