Crack detection in, and vibration characteristics of, a shaft with two open cracks rotating in a fluid medium are studied in this paper. The influence coefficient method is used to find the fundamental frequency of the cracked shaft. The frequency contours with respect to crack depths and locations are used to identify cracks. The effect of fluid on the cracked rotor is analyzed with the help of Navier-Stokes equations. The dynamic response of the rotating cracked shaft is compared with that of an uncracked one, and it is found that, when the cracked rotor rotates in a viscous fluid, there is change in the critical speeds and amplitudes of vibration.

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Vibration Analysis of a Cracked Rotor Surrounded by Viscous Liquid

Extraction of features of a specific signal in fluid–structure interaction is among the hottest problems in the field of mechanics. Yet, a comprehensive study of such problems remains a challenge d...

https://journals.sagepub.com/doi/pdf/10.1177/1687814018819341

Modelling and numerical simulation of vibrations induced by mixed faults of a rotor system immersed in an incompressible viscous fluid

Dynamic analysis of a combination of misaligned rotors, the disturbance of the Cardan joint and the rotor–stator rubbing within a restricted clearance space in a viscous fluid is complex and can result in persistent vibration anomalies that are often misunderstood. It becomes increasingly important to gain some insights into how the transmission of coupled motion responds dynamically under a variety of conditions. This paper introduces an efficient simulation of the misaligned multi-degree-of-freedom rotor’s model, which was developed to predict the transient dynamic behaviours of a driveshaft deflection. The model accounts for tight clearance as a function of contact deformation according to nonlinear Hertzian contact theory. The paper also examines recent research by considering the influence of parameters such as eccentric masses, applied torques and flexible coupling joint perturbation introduced in the proposed rotor system. The simulation results indicated that the viscous fluid surrounding the driveshaft had sufficient torsional flexibility to dampen the rubbing impact to the driven shaft displacement. In addition, the torsional fluctuations of the flexible coupling abruptly increased, and then significantly impacted the vibration of the submerged driveshaft. Parametric studies involving the interconnected rotor models indicated that the effects of fluid on a close-bounds contact area can create partial disturbance reduction. The high rubbing contact is shown to be lost through the Hooke’s joints during power transmission. The speed-frequency spectrum maps provide valuable information on all the modelled excitations over the frequency of the twice-running speed resonance in a viscous medium. Further, nonlinear characteristics are reconstructed through orbit shapes and can be adopted in the condition monitoring of rotors in engineering practice.

Vibrations of Misaligned Rotor System with Hysteretic Friction Arising from Driveshaft–Stator Contact under Dispersed Viscous Fluid Influences

Extraction of features of a specific signal in fluid-structure interaction is among the hottest problems in mechanic fields. Yet a comprehensive study of such problems remains a challenge due to their high non-linearity and multidisciplinary nature. Fluid-shaft interactions are strong nonlinear phenomena with wide applications in scientific and engineering disciplines. The vibration analysis of an oscillating vertical rotor carrying a breathing crack in an incompressible and inviscid fluid is studied in this paper. The governing equation of the coupled lateral and torsional vibrations of a cracked rotor system and the hydrodynamic forces are established based on Energy Principle. The unsteady vertical rotor-fluid motion is analyzed under various system operating conditions. It was found that the hydrodynamic forces are a source of attenuation of the severity of the faults. The fluid forces give more strongly softening signal spectrum when the rotating system is disturbed by the breathing of the crack. The obtained results are consistent with existing literature and could be adopted into rotor condition monitoring strategies.

Cracked Shaft Detection on Vertical Rotor System immersed in an Inviscid Fluid

Analytical results on the fundamental critical speed of a shaft in a viscous fluid are presented. When the fluid is set in motion by the shaft whirling, dynamic forces imposed by the fluid on the shaft are obtained by solving the two-dimensional Navier -Stokes equations under appropriate boundary conditions and on the assumption of small amplitude of the whirling motion. Added mass coefficients of the fluid and viscous damping coefficients are determined using these dynamic forces, and accordingly the equations of whirling motion of the shaft are derived. The critical speed of the shaft are found by solving these equations of whirl motion. In this analysis, configurations of the shaft are assumed to be a uniform elastic beam or a diskattached elastic beam, whose both ends are simply supported by bearings. For the latter case, an experimental study was made in order to confirm the analytical results. From this study, it was revealed that the critical speed decreases for large viscosity of the fluid, and when the fluid fills an annulus between the shaft and a cylindrical wall, the critical speed decreases for small annular gap width.

Yukihiko Kazao

Papers

Critical Speed of Rotor in a Liquid