Rotordynamics Module

Analyse the Dynamics of Rotating Machinery with the Rotordynamics Module

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Predictive Rotordynamics Simulations

The study of rotordynamics is important in application areas that involve rotating machinery, such as the automotive and aerospace industries, power generation, and the design of electrical products and household appliances. The physical behaviour of rotating machines is greatly influenced by vibrations, which are exacerbated by the rotation and structure of the machines themselves. Perfectly symmetrical rotor assemblies exhibit different natural frequencies as a function of rotational speed, while imperfections and unbalances can excite these frequencies in intricate ways. When designing machinery with rotating parts, you need an efficient way to consider these behaviours and optimise operation and performance.

You can use the Rotordynamics Module, which is an expansion to the Structural Mechanics Module, to analyse the effects of lateral and torsional vibrations of rotating machinery in order to study rotor vibrations and contain their levels within acceptable design limits. Among the different design parameters you can evaluate with this module are critical speeds, whirl, natural frequencies, stability thresholds, and the stationary and transient responses of a rotor due to mass unbalances. You can also see how rotational behaviour may lead to stresses in the rotor itself, as well as additional loads on and vibration transmissions to other parts of the rotating machine's assembly.

With the Rotordynamics Module, you can take into account the effects of various stationary and moving rotor components, including disks, bearings, and foundations. You can also easily postprocess your results directly within the software environment, presenting them as Campbell diagrams, modal orbits, harmonic orbits, waterfall plots, and whirl plots.

Comprehensive Modelling Tools for Rotors and Hydrodynamic Bearings

With the COMSOL Multiphysics® simulation platform and its add-on modules, you have access to a set of predefined modelling tools called physics interfaces, which are tailored to specific areas of analyses. The Rotordynamics Module provides five dedicated physics interfaces that you can use to accurately model rotors and bearings:

  1. The Solid Rotor interface for modelling a rotor as a full 3D geometric model made with CAD software or using the built-in CAD capabilities of COMSOL Multiphysics®.
  2. The Beam Rotor interface for the approximate modelling of a rotor with 1D beams and accompanying components as points.
  3. The Hydrodynamic Bearing interface for detailed modelling of a bearing that includes the lubricant film within the bearing.
  4. The Solid Rotor with Hydrodynamic Bearing interface for the combined modelling of a 3D rotor and hydrodynamic bearings, as well as the interactions between them.
  5. The Beam Rotor with Hydrodynamic Bearing interface for the combined modelling of a rotor (defined as a beam) and hydrodynamic bearings, as well as the interactions between them.

By using the multiphysics couplings available in the Rotordynamics Module, you are able to capture the effect of oil whirl and whip through coupling the physics in the Hydrodynamic Bearing interface to those in the Beam Rotor and Solid Rotor interfaces.

Additionally, you can combine the Rotordynamics Module with other modules in the COMSOL product suite for coupled and multiphysics simulations. This means that you can examine how other types of physics affect your rotordynamics designs. For example, by combining the Rotordynamics Module with the Multibody Dynamics Module, you can perform a transient simulation to predict the vibrations in a geared rotor assembly for an externally applied torque.

The pressure distribution in the lubricant of the bearings (rainbow colour plot), von Mises stresses (blue colour plot), and displacement of the bearings (Orbit plot) resulting from a rotordynamics analysis.

Accurately Analyse Complete Rotating Machine Assemblies

To achieve the most accurate description of a rotating machinery assembly, you need to be able to consider all of its participating components. You can use the most general approach to modelling such assemblies, traditional finite element analysis, with the Solid Rotor interface in the Rotordynamics Module. With this approach, you represent the rotor with a 3D geometry and use solid elements to define the rotor assembly.

In the Solid Rotor interface, you can include full descriptions of geometrical asymmetries and unbalances in your analysis, as well as nonlinear geometric effects. You can also account for deformations in journals and mountings, consider gyroscopic effects, and capture the effects of spin softening and stress stiffening in your simulation. This interface is most useful if you need explicit simulation results for deformation and stresses in the rotor and its components.

Model Integral Bearings and Foundations in Your Rotor Assembly Design

Bearings and foundations are essential to a rotor assembly; they are the components that connect a rotor to its surrounding parts. Rotor system responses are very sensitive to the type of bearing or foundation, so you need to be able to accurately describe their behaviour. Dedicated interfaces and features within the Rotordynamics Module make these components easy to define.

Journal Bearings

Journal bearings restrict the translational motion of a journal in the lateral direction and its rotation about both lateral axes due to the finite length of the journal. There are two options for modelling journal bearings: either as a full hydrodynamic bearing, with a detailed description of the lubricant's pressure and flow, or by making approximations using lumped models.

 
Lumped Models

With lumped models, you can simulate the following varieties of journal bearings and their behaviour in the Rotordynamics Module:

  • No clearance bearings
    • These bearings are very stiff, so the motion of the journal in the bearing is very small and does not affect the overall response of the rotor.
  • Plain hydrodynamic bearings
    • Based on Ocvirk's theory, this bearing model works as a spring-damper system on the journal of the rotor. The dynamic stiffness and damping coefficients can be known or unknown. If unknown, you can evaluate them as a function of the journal motion in the bearing.
  • Bearing stiffness and damping coefficients
    • This model uses a spring-damper system with two translational stiffnesses and damping coefficients in the lateral directions of the rotor, and two rotational stiffnesses and corresponding damping coefficients about the lateral directions. These values are known either by performing experiments or through computer simulation, and can be entered as tabulated data as a function of journal motion.
  • Bearing forces and moments
    • Instead of simulating a bearing, you can also directly apply reaction forces and moments to the journal through experimental data or as functions of the journal's motion.
 
Hydrodynamic Bearings

You can model the behaviour of journal bearings in detail with the Hydrodynamic Bearings interface. The interface includes predefined physics that make it easy to model the oil between a journal and bushing that acts as a lubricant by solving the Reynolds equation.

You can use this interface to analyse a journal bearing and its characteristics in terms of stiffness and damping, or in a multiphysics coupling with the Solid Rotor or Beam Rotor interface to study the dynamics of the full rotor assembly. These interfaces provide built-in models for the following hydrodynamic bearing types:

  • Plain
  • Elliptic
  • Split-halves
  • Multilobe
  • Tilted-pad bearings
  • User defined

 

Thrust Bearings

To analyse thrust bearings, which restrict the axial motion of a rotor and the rotation about the lateral axes, you can use lumped parameters. You are able to model the following varieties of thrust bearings and their behaviour in the Rotordynamics Module:

  • No clearance bearings
    • You can use this model to completely constrain the axial motion of the rotor and rotation about two lateral axes. This is useful when the effect of the bearing is not significant on the dynamics of the rotor assembly.
  • Bearing stiffness and damping coefficients
    • This model uses a spring-damper system with one translation stiffness and damping coefficient along the axis of the rotor, and two rotational stiffnesses and corresponding damping coefficients about the lateral directions. These values are known either by performing experiments or through computer simulation, and can be entered as tabulated data as a function of collar motion.
  • Bearing forces and moments
    • Instead of simulating a bearing, you can also directly apply reaction forces and moments to the collar through experimental data or as functions of the collar's motion.
 
Foundations

Bearing foundations are the structural components on which bearings rest. You can model foundations in your rotor assembly design as:

  • Fixed foundations
    • Where the bearing's motion is rigid or does not affect the rotor response significantly.
  • Moving foundations
    • Where the foundation and bearing motion are subject to external vibrations. This can be applied as data, an equation, a function, or from results by solving these effects using other COMSOLMultiphysics® add-on modules.
  • Flexible foundations
    • A flexible foundation can alter the critical speed of the rotor and is captured by this model in cases where the equivalent stiffness of the foundation is known.

Different Analysis Techniques Made Possible by a Range of Study Types

With the range of study types included in the Rotordynamics Module, you can properly analyse the dynamics of a rotor assembly using different analysis techniques that are adapted to the characteristics of rotordynamics phenomena.

The Rotordynamics Module enables you to account for gyroscopic effects, including frame acceleration forces. In the module, the vibrational effects are modelled from the perspective of a corotating observer by using a coordinate system that is rotating in sync with the rotor. This simplifies your modelling process, as you do not need to know the actual physical rotation of the rotor in order to simulate the assembly.

From the perspective of a corotating frame, conventional intuition of what is a stationary versus dynamic force no longer applies. In a rotordynamics analysis, inertial effects may appear as stationary forces, whereas a gravity force, which is stationary in a traditional analysis, appears as a dynamic sinusoidally varying force seen from a corotating frame. In this way, a stationary study has a different interpretation in rotordynamics than in conventional analyses.

The vibrational modes of a rotor will travel in an orbit in either the direction of the rotor's rotation (forward whirl) or the opposite direction (backward whirl). This phenomenon can be analysed using various eigenfrequency and frequency domain studies, as well as by performing a fully transient analysis using a time domain study.

In the Rotordynamics Module, you can use the following study types for both static and dynamic analyses:

  • Stationary study
    • For cases where loads in the corotating frame do not change their magnitude and direction significantly or there is no time dependence in the material model of the rotor, such as viscoelasticity and creep. You can analyse parametric studies, such as a rotor's behaviour for different mass eccentricities, using a stationary study and stepping through the different parameters.
  • Eigenfrequency study
    • Solves for the natural frequencies and the corresponding mode shapes for undamped and damped systems, even in cases where the rotor is not fully constrained. You can use theEigenfrequency Study node to determine the stable operating ranges and critical speeds of the rotor through repeating an eigenfrequency analysis multiple times over a range of angular speeds of the rotor.
  • Frequency Domain study
    • Computes the response of the rotor if all loads are time harmonic when seen from the corotating frame.
  • Time Domain study
    • For cases where you cannot disregard the inertial effects from unbalances and their changes in time with respect to the corotating frame.
  • Transient with FFT study
    • Performs a parametric sweep over the rotor's angular speed, while also consisting of a time domain simulation followed by a fast Fourier transform (FFT). Since this study type is computationally expensive, you should mainly use it when the deformation of the rotor plays a significant role in determining the overall dynamics of the rotor assembly.

Product Features

  • Beam Rotor interface for the approximate modelling of a rotor as lines
  • Solid Rotor interface for modelling a rotor as a full 3D model
  • Hydrodynamic Bearing interface for detailed modelling of the lubricant film in a bearing
  • Beam Rotor with Hydrodynamic Bearing interface for the combined modelling of a rotor, described using beam elements, and hydrodynamic bearings, as well as the interactions between them
  • Solid Rotor with Hydrodynamic Bearing interface for the combined modelling of a rotor, described using solid elements, and hydrodynamic bearings, as well as the interactions between them
  • Lumped representation of bearings
  • Journal bearings
  • Thrust bearings
    • Hydrodynamic thrust bearings
  • Roller element bearings
    • Deep groove ball bearings
    • Angular contact ball bearings
    • Self-aligning ball bearings
    • Spherical roller bearings
    • Cylindrical roller bearings
    • Tapered roller bearings
  • Beam rotor mountings
    • Flywheels
    • Pulleys
    • Gears
    • Impellers
    • Rotor-blade assemblies
  • Fully asymmetric rotors based on 3D CAD models
  • Fixed, moving, and flexible foundations
  • Stationary study
  • Eigenfrequency study
  • Frequency Domain study
  • Time Domain study
  • Transient with FFT study
  • Campbell plots
  • Modal orbits
  • Harmonic orbits
  • Waterfall plots
  • Whirl plots

Application Areas

  • Powertrains
  • Drive trains
  • Jet engines
  • Steam turbines
  • Gas turbines
  • Turbochargers
  • Turbogenerators
  • Turbopumps
  • Internal combustion engines
  • Compressors
  • Propulsion systems
  • Electrical machinery
  • Household appliances
  • Disk drives

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