Defining Unbalanced Mass in Rotordynamics — Lesson 4

How is unbalanced mass defined in Rotordynamics? 

The unbalanced mass is defined by a force, where the force is associated with the mass and eccentricity (distance from point to center or 'rotating radius' in Ansys Mechanical™ structural finite element analysis software terms). 

We do it this way because we use a stationary frame of reference, so the geometry must be axisymmetric since it is not actually 'rotating' in the simulation, but the effect of rotation is accounted for in the gyroscopic damping matrix. In a similar way, the unbalance force must be applied to the center, and based on the real and imaginary components, the effect of the unbalance (via forces) is applied to the system.