A triaxial accelerometer mounted on a test article registers the three orthogonal vibration signals when flow is initiated. The power spectra indicate a strong component in signals S0 and S2 (out of page) at 800 Hz.  The strength of the S1 signal is significantly less.

The phase spectra is used to deduce the relative motions. For frequencies below 1000 Hz the relative motion is “out of phase;” when S0 registers positive acceleration, S2 registers negative acceleration. For frequencies above 1000 Hz the signals are “in phase.”

Phase between SO & S2