This type of FSI component can be used to model a rigid reed valve or other torsionally activated moving body. The pressure load is integrated over the selected boundary to provide a torque value about the specified axis with which an angular version of Hooke's Law (T = k𝛉) will be used to compute an angular applied torque. A resistive torque proportional to the angular velocity of the body is also applied, with a user-supplied proportionality constant.

Which gives the body's angular acceleration:

The body's new angular position is then marched in time explicitly.

A displacement threshold may be specified to control when the FSI part would start to move. The FSI part would not move geometrically until the tracked displacement would exceed the displacement threshold. A maximum displacement may also be specified, and an option may be set to limit the spring motion to travel in only the positive direction with respect to the specified axis of rotation.

From the physical properties of the spring mechanism controlling the valve, the spring constant, mass moment of inertia of the body about the rotating axis, damping coefficient, and rotation axis must be provided.