ON THE CONCEPT OF MODES

In order to better understand the concept of modes, the characteristics of modes are analyzed and compared with those of complex modes or nonlinear modes. The connotation of a mode includes the synchronicity of the modal vibrations, the invariance to the initial conditions, the orthogonality among modes, and the superposition of the modal vibrations into the response. A complex mode concerns with the synchronicity but not the invariance, while the orthogonality and the superposition hold only in the state space. A nonlinear mode concerns both with the synchronicity and the invariance, but not the orthogonality nor the superposition.