Model mass imbalance in rotor dynamic analysis

1. If necessary, activate the dynamic subcase. In the Simulation Navigator, right-click the dynamic subcase node and choose Make Active.

2. Choose Home tab→Loads and Conditions group→Load Type list→Force .

3. In the Force dialog box, in the Model Objects group, next to Select Object, click Model Objects to Create Force On . In the graphics window, select the node along the axis of rotation of the rotor at the location of the mass imbalance.

4. In the Magnitude group, from the Force list, select Field.

5. In the Model Objects group, next to Specify Field, choose Table Constructor from the list.

6. In the Table Field dialog box, in the Domain group, from the Independent list, select Frequency.

7. In the Data Points group, enter the tabular data in the entry box.The tabular data you enter 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 specifying system-wide rotor dynamic solution options, see Define system-wide rotor dynamics solution options.

8. In the Table Field dialog box, click OK.

9. In the Force dialog box in the Direction group, next to Specify Vector, select XC-axis from the list.Because the z-axis of either the global or a local coordinate system is the axis of rotation for the rotor, direct the forces in the positive x- and positive y-directions.

10. Click Apply.

11. In the Model Objects group, next to Select Object, click Model Objects to Create Force On . In the graphics window, select the node you selected previously.

12. In the Magnitude group, from the Force list, select Field.

13. Next to Specify Field, choose Select Existing Field from List from the list.

14. In the Fields dialog box, select the table field you created previously.

15. Click OK.

16. In the Force dialog box, in the Direction group, next to Specify Vector, select YC-axis from the list.

17. Click OK.To make the two forces you just defined simulate a rotating force of constant magnitude, they need to be phased 90 degrees apart. To do this, you must edit the Simcenter Nastran input file. For more information, see Simulate a rotating force.

18. In the Simulation Navigator, right-click the dynamic subcase node and choose Edit.

19. In the Solution Step dialog box, in the Properties group, next to Forcing Frequencies, click Edit Forcing Frequencies .

20. In the Modeling Objects Manager dialog box, click Create.

21. In the Forcing Frequencies dialog box in the Properties group, next to Frequency List Form, choose FREQ1 from the list.

22. Enter values for First Frequency, Frequency Increment, and Number of Frequency Increments.These entries define the reference rotor speeds at which the frequency response is calculated.Note: The choice of whether to choose FREQ, FREQ1, FREQ2, FREQ3, FREQ4, or FREQ5 depends on the problem and personal preference.

23. Click OK.

24. In the Modeling Objects Manager dialog box, in the Selection group, select the modeling object you just created.

25. In the List group, click Add .

26. Click Close.

27. In the Solution Step dialog box, click OK.

Simulate a rotating force

1. In the Simulation Navigator, right-click the solution node and choose Solve.

2. In the Solve dialog box, next to Submit, choose Write, Edit & Solve Input File from the list.

3. Click OK.

4. In the text editor, identify the FORCE bulk entry that is directed in the y-direction.This FORCE bulk entry points to an RLOAD1 bulk entry. For this RLOAD1 bulk entry, enter either +90.0 or –90.0 in the fifth data field. The correct sign depends on whether you want to excite forward or backward whirl. To excite forward whirl, enter +90.0. To excite backward whirl, enter –90.0. An example is as follows:Before editing $234567812345678123456781234567812345678123456781234567812345678 RLOAD1         3       3                       2               0After editing (assume forward whirl) $234567812345678123456781234567812345678123456781234567812345678 RLOAD1         3       3            90.0       2               0

5. Choose File → Save.

6. Click Close to close the text editor and resume the Simcenter Nastran run.

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

Create a Campbell diagram

Learn more

Rotor dynamics

Rotor dynamics workflow

Supported solution types for rotor dynamic analysis

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Pre/Post video examples

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Model mass imbalance in rotor dynamic analysis, Simcenter 3D 2021.1 Series

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