Meysam Shahabi-Nejad you have important question. As far I can tell,

Fluent let do the heavy lifting and you have to do the calculations. check

https://www.afs.enea.it/project/neptunius/docs/fluent/html/udf/node86.htm

In fact, the program is lagging behind the theory and missing many elements that should be included. I would say that you better write your own code. The movement of center of rotation is your responsibility essentially. You should read my book stability of ship and other bodies to understand some the complexities involve in the problem. For example, the added moment of inertia was developed by me for change of rotation point is largely not public yet.

good luck.

The center of rotation can be changed by modifying the value of the "rotation_center" variable in the UDF. This variable is typically set to the initial position of the center of rotation and can be modified during the simulation by updating its value in the UDF.

1- Define the new center of rotation as a variable in the UDF:

#include "udf.h"

DEFINE_CG_MOTION(sdof_motion, dt, vel, omega, time, dtime)

{

/* Declare variables */

real x_new; /* New center of rotation */

/* ... */

2- Update the "rotation_center" variable with the new value during the simulation:

/* Update the center of rotation */

x_new = 0.5; /* Set the new center of rotation */

NV_S(rotation_center, =, x_new); /* Update the rotation_center variable */

/* ... */

3-Re-compile and re-load the UDF