A rigid body is a solid object that moves through a fluid without itself deforming. Its motion is dictated by the fluid forces and torques acting upon it, plus any external forces (such as gravity) and external torques.
Within Ansys CFX, a rigid body can be modeled in two ways:
A rigid body can be defined by a collection of 2D regions that form its faces. When a rigid body is modeled in this way, the rigid body itself does not need to be meshed. Mesh motion is used to move the mesh on the rigid body faces in accordance with the solution of the rigid body equations of motion.
Alternatively, an immersed solid can be defined to be a rigid body. In this case the motion of the immersed solid is dictated by the solution of the rigid body equations of motion.