Rotor dynamics

You can use Pre/Post to create rotor dynamic models for Simcenter Nastran and to post-process rotor dynamic analysis results. With Pre/Post you can create rotor dynamic models for complex eigenvalue rotor dynamic analysis, frequency response rotor dynamic analysis, and transient response rotor dynamic analysis in Simcenter Nastran.

When you solve the model, Pre/Post writes the following rotor dynamic-related instructions to a Simcenter Nastran input file as needed:

RMETHOD case control command

Use the RMETHOD case control command to select the ROTORD bulk entry to use in the rotor dynamic analysis and optionally specify complex modal reduction when SOL 107 is selected as the solution sequence. The presence of the RMETHOD case control command in the Simcenter Nastran input file activates the rotor dynamics capability.

ROTORD bulk entry

Use the ROTORD bulk entry to specify the system-wide and rotor-specific rotor dynamic analysis solution options.

ROTORG bulk entry

Use a ROTORG bulk entry to define a rotor. If the model contains multiple rotors, you need one ROTORG bulk entry for each rotor.

ROTORB bulk entry

Use ROTORB bulk entries to define the grids that represent the stationary portion of the bearing supports for each rotor. You need one ROTORB bulk entry for each rotor in the model.

ROTPARM bulk entry

Use ROTPARM bulk entries to define solution control parameters for complex eigenvalue rotor dynamic analysis like the parameters that define the strain and kinetic energy-based mode filtering criteria.

ROTSE bulk entry

Use ROTSE bulk entries to define the superelement reduction of a rotor.

CBEAR bulk entry

Use CBEAR bulk entries to define bearing connections. You need one CBEAR bulk entry for each bearing connection.

PBEAR bulk entry

Use a PBEAR bulk entry to define the stiffness and viscous damping for a bearing connection that is defined by a CBEAR entry. The stiffness and viscous damping can optionally be speed-dependent.

RESVEC parameter

Use the RESVEC parameter to turn off residual vectors.

MODTRK parameter

Use the MODTRK parameter to specify the mode tracking method.

ROTCSV parameter

Use the ROTCSV parameter to specify that the results of a complex eigenvalue rotor dynamics analysis are written to a .csv file.

ROTGPF parameter

Use the ROTGPF parameter to specify that the results of a complex eigenvalue rotor dynamics analysis are written to a .gpf file.

ROTSYNC parameter

Use the ROTSYNC parameter to skip synchronous analysis in a complex eigenvalue rotor dynamics analysis.

The workflow to perform a frequency response analysis, transient response analysis, or complex eigenvalue analysis for a system containing rotating components is only slightly different from the workflow to perform a standard frequency response analysis, transient response analysis, or complex eigenvalue analysis, respectively. The differences in the workflow allow you to:

• Create a rotor dynamic solution.

• Request complex eigenvalue rotor dynamic analysis results to be written to .csv and .gpf files.

• Define bearing supports with frequency-dependent and unsymmetric stiffness and viscous damping.

• Define rotors.

• Define the superelement reduction of a rotor.

• Specify system-wide rotor dynamic solution options.

• Specify rotor-specific rotor dynamic solution options.

• Filter unimportant modes from being mode tracked.

• Account for centrifugal stiffening of the rotors.

• Use the shaking force that results from mass imbalance of a rotor as the excitation in a rotor dynamic frequency response analysis.

• Create a Campbell diagram from complex eigenvalue analysis results.

Creating a rotor dynamic solution

When you create the FEM and Simulation files, select one of the following Simcenter Nastran solution sequences:

• For a frequency response rotor dynamic analysis, select either SOL 108 Direct Frequency Response or SOL 111 Modal Frequency Response.

• For a transient response rotor dynamic analysis, select either SOL 109 Direct Transient Response or SOL 112 Modal Transient Response.

• For a complex eigenvalue rotor dynamic analysis, select either SOL 107 Direct Complex Eigenvalues or SOL 110 Modal Complex Eigenvalues.

If you select SOL 107, you can optionally specify complex modal reduction. Complex modal reduction increases the efficiency of a SOL 107 solve. For more information on complex modal reduction, see Reduction to the Analysis Set for the Direct Methods in the Simcenter Nastran Rotor Dynamics User’s Guide.

A number of parameters are typically relevant in a rotor dynamic analysis.

• For any rotor dynamics analysis, you must turn off residual vectors. To do so, set the RESVEC parameter to NO.

• For any rotor dynamics analysis, if your model has any combination of unsymmetric stiffness, unsymmetric viscous damping, and structural damping, or if the default method fails to adequately track the modes, switch to the mode tracking method by setting the MODTRK parameter to 3.

• For either SOL 107 Direct Complex Eigenvalues or SOL 110 Modal Complex Eigenvalues, you can optionally do the following:Skip synchronous analysis. To do so, set the ROTSYNC parameter to NO.Request Simcenter Nastran write a .csv file. The .csv file contains the complex eigenvalue rotor dynamic analysis results needed to create a Campbell diagram. To request a .csv file, enter the FORTRAN unit number for the ROTCSV parameter. For your convenience, this option can also be selected from a check box.Request Simcenter Nastran write a .gpf file. The .gpf file contains the complex eigenvalue rotor dynamic analysis results needed to create a Campbell diagram. To request a .gpf file, enter the FORTRAN unit number for the ROTGPF parameter.

You can specify these parameters when you create the FEM and Simulation files.

Requesting CSV and GPF files

If you select either direct or modal complex eigenvalues as the solution sequence, you can optionally request that Simcenter Nastran write a .csv file and a .gpf file. The .csv and .gpf files contain the complex eigenvalue rotor dynamic analysis results needed to create a Campbell diagram. You can use the .csv file to create a Campbell diagram in this software. You can use the .gpf file to create a Campbell diagram in other post-processors. For more information, see The CSV File for Creating Campbell Diagrams and The GPF File for Additional Post-Processing in the Simcenter Nastran Rotor Dynamics User’s Guide.

Defining bearing supports

You define bearing supports in the FEM. Depending on how you model the rotors and whether or not you model the stationary portion of the structure, you may need to create nodes prior to defining the bearing supports. For more information on preparing your model for bearing support definition, see Modeling Bearings in the Simcenter Nastran Rotor Dynamics User’s Guide.

As a best practice, use CBEAR elements to model bearing supports. By using CBEAR elements, you can define the stiffness and viscous damping of CBEAR elements as speed-dependent, unsymmetric, and so on. For more information, see the CBEAR and PBEAR bulk entry listings in the Simcenter Nastran Quick Reference Guide.

Defining rotors

Each distinct rotating portion of the structure constitutes a rotor. You can define up to ten rotors in a rotor dynamics model. You define rotors in the Simulation file using the Rotor Dynamics Definition command by selecting the nodes that define the FE mesh of each rotor. As a best practice, when meshing the model, create a distinct mesh for each rotor. Doing so simplifies selection of the nodes that constitute each rotor. For more information, see the ROTORG bulk entry listing in the Simcenter Nastran Quick Reference Guide.

Superelement reduction of rotors

When you define the superelement reduction for a rotor, you specify whether the modal reduction is real or complex, and the corresponding eigenvalue extraction parameters. You must also select the nodes in the rotor mesh to retain after the modal reduction. Typically, the retained nodes are the nodes that connect the rotor to the supporting structure, the nodes where loads are applied to the rotor, and nodes that provide a better representation of mass distribution for the rotor.

You can define the superelement reduction for a rotor in any type of rotor dynamic analysis. When you do so, the software writes a ROTSE bulk entry in the Simcenter Nastran input file for each rotor for which you define the superelement reduction. However, the superelement reduction of rotors is valid for SOL 107–109 solution types only. For SOL 101 or 110–112 solution types, Simcenter Nastran ignores the ROTSE bulk entries and does not reduce rotors to superelements.

For more information, see the ROTSE bulk entry listing in the Simcenter Nastran Quick Reference Guide.

Associate CBEAR elements to rotors

You must associate CBEAR elements to rotors in the Simulation file. Each CBEAR element you define should be associated with a rotor. You can assign the bearing supports to rotors when you define the rotors.

Defining stationary nodes for CBEAR elements

If your model contains CBEAR elements, you must identify the nodes that represent the stationary portion of the CBEAR elements. The only exception is if your model contains only one rotor. Then any portion of the model that is not defined as the rotor is considered stationary by default and this step can be skipped. For more information, see the ROTORB bulk entry listing in the Simcenter Nastran Quick Reference Guide.

Specifying system-wide rotor dynamic solution options

System-wide rotor dynamic solution options apply to all rotors in the system. System-wide rotor dynamic solution options include the following:

• The starting reference rotor speed, speed step, number of speed steps, and reference rotor speed units.

• Whether to perform the analysis in the fixed or rotating reference system.

• Whether to perform a synchronous or asynchronous frequency response analysis.

• Whether to include Steiner’s inertia terms.

• The excitation type and excitation order for frequency response analysis or transient response analysis.

• The results to output.

• The threshold value for the detection of whirl.

For more information on system-wide rotor dynamic solution options, see the Simcenter Nastran Rotor Dynamics User’s Guide and the ROTORD bulk entry listing in the Simcenter Nastran Quick Reference Guide.

Specifying rotor-specific rotor dynamics solution options

Rotor-specific rotor dynamic solution options are defined for each rotor in the system. Rotor-specific rotor dynamic solution options include the following:

• The speed multiplier that relates the rotor speed to the reference rotor speed.

• The reference frequencies for structural damping conversion.

• The centrifugal force to assign to the rotor.

For more information on rotor-specific rotor dynamic solution options, see the Simcenter Nastran Rotor Dynamics User’s Guide and the ROTORD bulk entry listing in the Simcenter Nastran Quick Reference Guide.

Mode filtering

When you perform a SOL 107 Direct Complex Eigenvalues or SOL 110 Modal Complex Eigenvalues rotor dynamic analysis, you can use strain energy-based and kinetic energy-based criteria to exclude modes that contribute little to the dynamic response of the rotor from being tracked throughout the analysis. Because these modes are not tracked, the analysis results produce a less cluttered and potentially more meaningful Campbell diagram.

The strain energy-based and kinetic energy-based criteria you can use to filter modes are as follows:

• The ratio of the strain energy of a reference rotor to the total strain energy of the system for a given mode.The software does not track the mode if this ratio falls below the threshold value.

• The ratio of the kinetic energy of a reference rotor to the total kinetic energy of the system for a given mode.The software does not track the mode if this ratio falls below the threshold value.

• Both the ratio of the strain energy of a reference rotor to the total strain energy of the system and the ratio of kinetic energy of a reference rotor to the total strain energy of the system for a given mode.The software does not track the mode if both ratios fall below the threshold values.

Caution:

For meaningful results, the speed multiplier for the reference rotor must be 1.0.

Caution:

The reference rotor for mode filtering is not necessarily the reference rotor specified with the system-wide rotor dynamic solution options.

Modeling centrifugal stiffening and softening

If you use shell elements or solid elements to model a rotor, and perform the rotor dynamic analysis in the rotating reference frame, you can account for centrifugal stiffening and softening of the rotor. To do so, in your solution, create a static subcase that precedes the dynamic subcase. In the static subcase, include a centrifugal force for each rotor that you want to account for centrifugal stiffening and softening. When you create the centrifugal forces, you must enter 1 radian/sec as the angular velocity in order for Simcenter Nastran to use the actual rotor speed during each solve. The centrifugal forces are assigned to the rotors when you define the rotor-specific solution options.

Modeling mass imbalance in rotor dynamic analysis

You can use the shaking force that results from mass imbalance of a rotor as the excitation for a direct or modal frequency response analysis. To model the shaking force, define two orthogonal forces that have the same magnitude and frequency. Orient the forces in the radial plane of the rotor and define a phase angle of 90 degrees between them. Include the forces in the dynamic subcase of the solution.

Because the Z-axis of either the global or a local coordinate system is the axis of rotation for the rotor, as a best practice, direct the forces in the positive x- and positive y-directions.

Define the magnitude of the forces on a table field with frequency as the independent domain, where frequency is the frequency of rotation of the rotor. The tabular data you enter to define the magnitude depends on the setting for ETYPE on the ROTORD bulk entry.

• If ETYPE = 1, the tabular data you enter is the mass imbalance as a function of frequency, where frequency is the frequency of rotation for the rotor. Mass imbalance is the product of eccentric mass and the radial eccentricity. Because the mass imbalance is independent of angular velocity, the tabular data you enter must be independent of frequency. During the solve, Simcenter Nastran will look up the value for the mass imbalance and multiply it by the square of the appropriate angular velocity to create the shaking force.

• If ETYPE = 2, the tabular data you enter is the shaking force as a function of frequency, where frequency is the frequency of rotation for the rotor. Shaking force is the product of eccentric mass, radial eccentricity, and the square of angular velocity. During the solve, Simcenter Nastran will look up the value for the shaking force at the appropriate frequency and use it directly.

ETYPE is set when you specify the system-wide rotor dynamic solution options. For information on system-wide rotor dynamic solution options, see the Simcenter Nastran Rotor Dynamics User’s Guide and the ROTORD bulk entry listing in the Simcenter Nastran Quick Reference Guide.

To define the reference rotor speeds at which the frequency response is calculated, edit the dynamic subcase and create a modeling object that contains the frequencies.

When this software writes the Simcenter Nastran input file, an RLOAD1 bulk entry is written for each FORCE bulk entry. To phase the two forces, edit the Simcenter Nastran input file. In the text editor, add the 90 degree phase angle to the RLOAD1 bulk entry that corresponds to the FORCE bulk entry that is directed in the y-direction. The sign of the 90 degree phase angle depends on whether you want to excite forward or backward whirl. If the forces are directed in the positive x- and positive y-directions, enter +90.0 to excite forward whirl, and enter –90.0 to excite backward whirl.

Note:

If the forces are not directed in the positive x- and positive y-directions, this rule may not apply.

Post-processing rotor dynamic results

After Simcenter Nastran has completed the solve, you can use standard post-processing capabilities to examine results from frequency response and transient response rotor dynamic analyses.

You can use a Campbell diagram to examine results from a complex eigenvalue rotor dynamic analysis. To assist you in creating a Campbell diagram from Simcenter Nastran results, you can use the NX Open application. The NX Open application converts a .csv file to an .afu file. You can plot the Campbell diagram directly from the AFU file.

Note:

You can find the NX Open application at %UGII_BASE_DIR%\UGOPEN\SampleNXOpenApplications.NET\CAE\CampbellCsvToAfu.

How do I

Create a rotor dynamic solution

Define bearing supports between coincident nodes with CBEAR elements

Assign physical properties to CBEAR elements

Define rotors

Define the superelement reduction for a rotor

Map CBEAR elements to rotors

Define stationary nodes for CBEAR elements

Define system-wide rotor dynamics solution options

Define rotor-specific rotor dynamics solution options

Specify mode filtering

Model centrifugal stiffening and softening in rotor dynamic analysis

Model mass imbalance in rotor dynamic analysis

Create a Campbell diagram

Learn more

Rotor dynamics workflow

Supported solution types for rotor dynamic analysis

Quick links

Command reference

Pre/Post video examples

Bulk Entry Descriptions

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Rotor dynamics, Simcenter 3D 2021.1 Series

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