# 5. Modal Analysis

This chapter describes a simple modal analysis capability. This capability will compute the lowest few vibration eigenmodes and values at the end of each model load step. This capability only works with solid uniform gradient hex elements and augmented Lagrange tied contact.

Warning

Modal analysis is still a capability in the early development stages. This capability is not recommended for general use, nor will any use of this capability currently be supported by Sierra/SM development.

    BEGIN LANCZOS EIGEN SOLVER
      MASS MATRIX = IDENTITY|LUMPED(LUMPED)
      NUMBER OF EIGENPAIRS = <integer>N
      DEBUG = OFF|ON(OFF)
    END

The command `NUMBER OF EIGENPAIRS` defines the number of eigenvalues and modes to compute. The lowest `N` modes will be computed. Significant expense is required to compute and store each mode, thus `N` should be kept relatively small (no more than 25 or so).

The `DEBUG` command turns on or off additional debugging outputs from the eigensolver.

The `MASS MATRIX` allows the user to selectively compute the eigenvalues of the tangent stiffness matrix when `IDENTITY` is set. Otherwise, the standard eigenvalue problem is computed with both the tangent stiffness matrix and the lumped mass matrix. A consistent mass matrix is not available at this time.