.. _conditions-centripetal:

*****************
Centripetal Force
*****************

.. code-block:: sierrainput

   BEGIN CENTRIPETAL FORCE
     NODE SET|NODESET = <string list>nodelist_names
     SURFACE|SIDESET|SIDE SET = <string list>surface_names
     BLOCK = <string list>block_names
     ASSEMBLY = <string list>assembly_names
     INCLUDE ALL BLOCKS
     REMOVE NODE SET = <string list>nodelist_names
     REMOVE SURFACE = <string list>surface_names
     REMOVE BLOCK = <string list>block_names
     #
     CYLINDRICAL AXIS = <string>cylindrical_axis
     ROTATIONAL VELOCITY FUNCTION = <string>rotational_vel_name
     ROTATIONAL VELOCITY SCALE FACTOR = <real>rvsf(1.0)
     FORCE SCALE FACTOR = <real>fsf(1.0)
     ACTIVE PERIODS = <string list>period_names
     INACTIVE PERIODS = <string list>period_names
   END [CENTRIPETAL FORCE]

The centripetal force boundary condition adds an external force to the body that is equivalent to the centripetal force on the body rotating at the defined rotational velocity. The centripetal force boundary condition can be used, for example, in a static analysis to calculate the deformation of a body spinning with constant rotational velocity around some axis.

The centripetal force boundary condition will be applied to all nodes selected within the command block. The equation for the centripetal force on a node is given by :eq:`eq:centripforce`, where :math:`F` is the magnitude of the centripetal force, :math:`m` is the mass of the node, :math:`R` is the current distance from the node to the rotational axis, and :math:`\omega` is the rotational velocity of the node about the rotational axis.

.. math::
   :label: eq:centripforce

   F = m R \omega^2

The ``CYLINDRICAL AXIS`` command defines the name of the axis of rotation. The name given is the name of a previously defined axis created with the ``DEFINE AXIS`` command. The rotational velocity as a function of time is defined by multiplying the function named by the ``ROTATIONAL VELOCITY FUNCTION`` by the ``ROTATIONAL VELOCITY SCALE FACTOR``. Additionally a scale factor on the final return force may be specified with he ``FORCE SCALE FACTOR`` command.

Assemblies may contain blocks, surfaces, nodesets, or assemblies of these.
