Modal analysis

A modal analysis answers the question at which frequencies does this structure like to vibrate, and in what shape. Ashes computes the undamped natural frequencies and mode shapes of any model, and can repeat that computation over a range of rotor speeds to produce a Campbell diagram.

Both are reached from the Eigenmodes window, described in the Eigenmodes page of the User Manual. This section describes what they compute.

• Eigenvalue analysis covers the eigenvalue problem itself and the solver used for a structure at rest. This is what the Mode shapes tab shows and what is written to an eigenmodes file.
• Campbell diagram covers the sweep over rotor speed: centrifugal stiffening of the blades, and the conversion of the rotating blade modes into the ground-fixed frequencies that can be compared with the per-rev excitation lines.