Capturing BW zone in an intact rotor system

Aircraft engines, aerospace rotating equipment, gas turbines, compressors, and rotors in several industrial and aerospace applications approach their nominal operational speeds after the passage through at least one of their critical rotational speeds. During the passage through the critical speeds, elevation in vibration amplitudes is usually observed due to the effect of residual unbalance in these real-life applications rotors. In all of the reported literature, the theoretical and numerical simulation results and the related Campbell diagrams suggest that the backward whirl (BW) zone should precede the passage through the critical forward whirl (FW) speed/speeds of such systems. Here, the existence of zones of rotational speeds at which BW orbits are expected to appear will be investigated immediately before and after the passage through the critical FW speed. Accordingly, startup operations of two different configurations of crack-free rotor-disk systems are considered in this numerical and experimental study. It is found out that there exist zone/zones of the shaft rotational speeds at which BW orbits are experimentally captured where these zones are localized immediately after the passage through the critical FW rotational speed during the startup operations. These BW zones are strongly affected by the acceleration of the shaft during the transient startup operations. These findings suggests that the BW should not necessarily precede the critical FW speed as suggested by the related Campbell diagrams.