Numerical and Experimental Investigations to Verify the Existence of Backward Whirl Orbits in Rotating Dynamical Systems

  • Fatima Al Hammadi

    Student thesis: Master's Thesis

    Abstract

    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 of at least one of their critical speeds. Such systems exhibit recurrent startup and coast down operation in which the angular acceleration/deceleration is incorporated. During the passage through the critical speeds of such systems, an elevation in vibration whirl amplitudes is usually observed due to the effect of residual slight unbalance force in these real life application rotors. These critical resonance vibration whirl amplitudes theoretically appear as pairs of imaginary eigenvalues corresponding to the critical whirl rotational speeds. For each pair of these critical whirl speeds, forward whirl (FW) and backward whirl (BW) orbits are theoretically expected to appear. It is well-known in the literature and real life applications that the elevation of whirl vibration amplitudes usually occurs during the passage through the critical FW rotational speeds for both intact and cracked shafts. However, no robust experimental evidence was found to be available in the literature for capturing and localizing BW orbits either before or after the passage through the critical FW rotational speed during startup and coast-down operations. Some researchers believed that the appearance of BW orbits was not possible in intact rotor systems, while others believed that these BW whirl orbits, if exist, could be excited only before the passage through the critical FW speeds according to their theoretical and numerical studies. In this work, the zones of transition from FW orbit to BW orbits are investigated and demonstrated by robust numerical and experimental results for both symmetric and asymmetric rotor-disk-bearing systems (i.e., cracked rotor with an open crack model) under varying unbalance force angle and angular acceleration. The newly developed model that was used in this work incorporates the dynamic effect due to angular accelerations that occur during start-up and coast-down of rotordynamic systems. The dynamic behavior of the rotor system during the passage through the BW zone of rotational speeds was further studied via numerical and experimental investigations. The study was mainly focused on the startup operations of the rotor-bearing-disk systems where the effect of the angular acceleration is incorporated into the equations of motion. The existence of the zone/zones of BW rotational speeds of the shaft at which BW orbits are expected to appear has been investigated immediately before and after the passage through the critical FW speed. The effects of both the crack and the unbalance force vector direction on excitation of BW orbits and the location of the corresponding zone of shaft rotational speeds have been carefully studied during the project time. As reported in relevant works previously published in the literature, theoretical analysis and numerical simulation results, along with the related Campbell diagrams, suggest that the BW zone should precede the passage through the critical FW rotational speed/speeds of rotor systems. Contrary to that, it is found here that there exist zone/zones of the shaft rotational speeds at which BW orbits are experimentally captured where these new zones are localized immediately after the passage through the critical FW rotational speed during startup operations. The onset of the BW zone of whirl orbits is also found to be associated with a significant reduction in vibration whirl amplitudes rather than an elevation in these amplitudes. These BW zones are also found to be significantly affected by the unbalance force angle vector orientation, the shaft angular acceleration, and the crack appearance in the shaft. More importantly, the zones of BW orbits are not found to be preceding the fundamental critical FW zone for the considered intact and cracked shaft-disk configurations. These significant finding regarding to the BW zones of the frequencies are also verified here via the application of the full spectrum analysis method.
    Date of AwardDec 2018
    Original languageAmerican English

    Keywords

    • Dynamics systems
    • rotordynamics
    • crack detection
    • backward whirl
    • forward whirl
    • unbalance force angle

    Cite this

    '