Self-excited, limit cycle, rotor lateral vibration caused by interaction between the fluid and the rotor. It occurs after a threshold of stability is exceeded, that is, when the direct and quadrature dynamic stiffness terms of the rotor/fluid system both equal zero. The fluid-induced vibrations are conventionally separated into two regimes called whirl and whip. Whirl has a frequency proportional to shaft rotative speed, with the coefficient of proportionality close to the fluid circumferential average velocity ratio in the bearing, seal or rotor periphery. Typically, whirl for bearings is less than one-half of shaft rotative speed, varying from 0.3X to 0.49X.
With an increase in rotative speed, whirl transforms into whip as the instability frequency asympatically approaches the high eccentricity natural frequency(ies) of the rotor system (at higher rotative speeds, fluid whirl and whip of the higher modes may also occur).
Fluid instability, characterized by a forward, almost circular, vibration orbit, is a property of the rotor/bearing/seal system. Thus, the mechanism is not limited to oil lubricated bearings. It can occur when any fluid (e.g., oil, steam, process gas, etc.) is enclosed within a small clearance area, between two body surfaces forming a cylinder within a cylinder, one of which is rotating and dragging the enclosed fluid into circumferential rotation.
