# Motion: PASSIVE_RIGID_BODY

It is possible to define a rigid body motion in HyperWorks such that the body is freely interacting with the fluid (exchanging momentum and heat with the fluid).

In order to define such motion as generally as possible, a number of parameters have
to be set in `Imposed motions`.

PASSIVE_RIGID_BODY motion defines the standard 6-DOF motion and
constrains motion in a number of ways, and also includes linear and torsional spring
forces that act on the body. PASSIVE_RIGID_BODY offers parameters
that can be set for this type of motion. Some of the commands are simple and can be
considered self-explanatory, for example, `body_mass`,
`init_CoM`, `init_vel`, but others require
clarification.

Depending on the complexity of the case and the data you have available, you may need to specify 0, 1, or 2 additional coordinate systems.

The Cartesian coordinate system, which is commonly referred to as a global coordinate system or global reference frame, is the default coordinate system in which the code is operating and does not need a definition. It is assumed that it coincides with the inertial frame of reference.

`mom_principal_ax_x_i`"x y z"`mom_principal_ax_y_i`"x y z"`mom_principal_ax_z_i`"x y z"

Where, the “x y z” coordinates represent the unit vectors expressed in global coordinates. The suffix “i” at the end of the command stands for “inertial”, to indicate which coordinates to use when defining the unit vector components.

`prbcon_ax_x_i`"x y z"`prbcon_ax_y_i`"x y z"`prbcon_ax_z_i`"x y z"

The exact same principle can be applied to the constraint coordinate system if you, for example, want to constrain the motion of the body in the direction that is under 45 degrees with respect to the global x or y axes.

`prbcon_pt_i`command. Along with the new coordinate center you need to set the rotational constraints for the new “hinge point”. This is done using

`prbcon_ax_hinge_c`command, where it is a vector that says which rotational motions are locked (x, y or z axis in constraint coordinate system). By setting all three values to 0 you will essentially define a ball-joint. The simplified 2D setup of a hinge is shown in Figure 3.

Linear or torsional springs can also be defined. To illustrate the basic concept of
such a setup, refer to Figure 4. The example shows the setup of a body which is hanging on a spring. The initial
location of the body does not correspond to the equilibrium point of the spring,
that is to say that you have a pre-deformation of the spring
(`prbcon_linspr_p_c`) when starting the simulation. Along with
this you of need to set the stiffness coefficient of the spring
(`prbcon_angspr_k_c`). Between these two parameters you can
define a force that is initially acting on the spring. Same principles apply for
torsional springs, except that the deformation is expressed in [rad], and the
stiffness of the spring in [Nm/rad].

`prbcon_linlim_pls_candprbcon_linlim_mns_c`(or

`prbcon_anglim_pls_candprbcon_anglim_mns_c`for angular limits) to define positive and negative displacement, respectively. These commands are always defined with respect to the

`init_CoM`.

Rigid-rigid body or wall-rigid body interactions are not supported. It is possible to simulate more than one rigid body in the domain, but their interactions are not modeled.

Also, a rigid body cannot cross a PERIODIC boundary.

With passive rigid body motion it is also possible to prescribe constant force or
torque acting on a body by using `prbcon_cnstfrc_c` or
`prbcon_cnsttrq_c`.

Alternatively, you can prescribe either constant or varying linear or angular
velocities to each of the bodies by using `prbcon_linvel_*` and
`prbcon_angvel_*` sets of commands.