Rotor Dynamics

The dynamic behavior of such structures is influenced by the type and angular velocity of rotating components and their locations within the model. Rotor dynamics is available in OptiStruct for modal frequency response, complex eigenvalue, static, linear direct transient and small displacement nonlinear direct transient analyses.

Motivation

When a component within the structure rotates, additional forces like the gyroscopic force and circular damping force act on it. It is important to determine the effects of rotating components on the system as a whole. The natural frequencies of a system usually change, if gyroscopic forces act on the model due to a rotating component. Circulating damping forces due to rotating components can lead to system instability. These forces are a function of the frequency of rotating component. In OptiStruct, they are included in the calculation of the response of the structure of interest when required in applicable subcases.

Figure 1. Application of Rotor Dynamics Analysis

In Figure 1, the rotating components of the structure are the shafts on which gears are mounted. The design of the rotors and their angular frequencies can affect the dynamic response of the structure. Any design will most likely lead to asymmetrical mass distribution about the rotor axes. This unbalanced mass, even if it is not significant, can result in deflection of the rotor depending on various factors. The magnitude of these deflections will be augmented when the rotating speed of the shafts equals the natural frequency of the structure (Resonance), and can lead to catastrophic failure of the system.

Implementation

The Rotor Dynamics functionality is activated in OptiStruct with the use of the RGYRO Subcase Information Entry (RGYRO=ID). This RGYRO entry references the identification number of a RGYRO Bulk Data Entry. Related Bulk Data Entries, RSPINR, UNBALNC, ROTORG and RSPEED are defined in the model for Rotor Dynamics. Parameters PARAM,GYROAVG, PARAM,WR3, and PARAM,WR4 are also used.

Whirl

A rotor is a structure that rotates about its own axis at a specific angular velocity. If a lateral force is applied to the rotor, it will deform in the lateral direction. This deformation is dependent on various factors, such as, magnitude of the applied force, rotor material properties, stator stiffness, and damping within the system. Due to rotor rotation, the deformed rotor will also whirl about an axis.

Synchronous and Asynchronous Analysis

The whirling speed can either be the same as rotor speed or it can be different from it. The type of analysis performed if the whirling speed and the rotor speed match is known as synchronous analysis. If the speeds do not match, then asynchronous analysis is used to determine the dynamic response of the model. In OptiStruct, the RGYRO Bulk Data Entry can be used to select synchronous/asynchronous analysis.

Figure 2. Types of Whirl and the Two Analysis Types that are Dependent on the Angular Frequency of a Rotor

Forward Whirl and Backward Whirl

The type of whirl depends on the spin direction of a rotor. If the rotor spin direction is the same as that of its whirl direction, then it is termed as forward whirl. If the rotor spin direction is opposite to the whirl direction, it is termed as backward whirl. In complex eigenvalue analysis, you can determine and differentiate between the modes of a structure undergoing backward whirl and forward whirl.