A Complex Orthogonal Decomposition for Wave Motion Analysis∗

A method is presented for decomposing wave motion into its principle components. The basic idea is a complex generalization of proper orthogonal decomposition. The method involves the representation of real oscillatory signals as complex analytic signals. The relationship between complex modes and wave motion is explored. From an ensemble of complex signals, a complex correlation matrix is formed, and its complex eigensolution is the basis of the decomposition (like a complex singular value decomposition). The complex eigenvectors contain standing and traveling characteristics. A traveling index is proposed to quantify the relative degree of traveling and standing in a waveform. A method of dissecting a wave mode into its traveling and standing parts is also proposed. From the complex modes and modal coordinates, frequencies, wavelengths, and characteristic wave speeds can be obtained. The method is applied to traveling and standing-wave examples.