[juies]

Define_cg_motion specifies the velocity of dynamic zone,but it is actually the location of the mesh that has changed,and I want to know the relationship between the displacement and the specified velocity.
Thanks

[AlexanderZ]

you may return velocity in [m/sec] though define_cg_motion macro
however, from manual:

Depending on the mesh smoothing models used and the dynamic mesh setup, ANSYS Fluent
can call mesh motion UDFs multiple times during a single time step. Therefore, UDFs must
be written so that the specified motion is an absolute function of time or so that the motion
is not incremented incorrectly if the UDF is called multiple times.

for more information and example read ansys fluent customization manual