 ## Structural Dynamics

#### How ANSYS translates the eigenmodes to displacement or stress values? When one change the unit system (say from m to mm) for the modal analysis, the displacement values change to a value that’s unrelated between the two unit-systems.

• FAQ
Participant

By default, the solver normalizes the eigenvectors using the mass matrix or: {phi}^T [M] {phi} = 1. Although, {phi} is reported in length units, its true units are 1/sqrt(mass). 1/sqrt(mass) is consistent with the units for {phi}^T [K] {phi} = omega^2: where omega is the circular natural frequency with the units of sqrt(k/m). If the units are changed in postprocessing, Mechanical just does a simple length unit conversion. If the units are changed before the solution, the eigenvalues must be ratioed by the square root of the masses to achieve consistent results. For example if the solution mass unit was kg and you want to obtain the equivalent result for a solution mass unit of slinch, the eigenvalue results must ratioed by the 1/sqrt(175.1 kg/slinch) or 13.23. 