Stability Diagram
In addition to the Campbell diagram, a stability diagram, also known as a stability map, can be a useful tool for addressing the stability of a rotor-bearing system. In particular, this applies to rotors supported by hydrodynamic bearings, which characteristics are well-known for being highly dependent on the operational conditions.
The stability can be addressed based on an eigenfrequency analysis. If the damping ratio associated with one of the eigenfrequencies is negative, which corresponds to an eigenfrequency with negative imaginary part, the static equilibrium will be unstable. The speed at which either of the modes becomes unstable is called the stability threshold, also known as the stability limit, or the onset of instability.
The stability diagram shows the damping ratio as function of the rotational speed. The first intersection between either of the damping ratios and the horizontal (rotational speed) axis constitutes the stability threshold.
A Stability Diagram is available in Result Templates for each of the rotor interfaces. The result template contains a Color Expression node for indicating the stability of a specific mode. A green or red color indicates whether the corresponding mode is stable or unstable, respectively.
The Stability Diagram defines an intermediate interval in which the eigenmodes are interpreted as undamped. The default intermediate interval is defined as damping ratios in the range ζ ∈[-10-5, 10-5].