PLCAMP

PLCAMP, Option, SLOPE, UNIT, FREQB, Cname, STABVAL, KeyAllFreq, KeyNegFreq
Plots Campbell diagram data for applications involving rotating structure dynamics.

POST1: Special Purpose

Option

Flag to activate or deactivate sorting of forward or backward whirl frequencies:

0 (OFF or NO)	—	No sorting.
1 (ON or YES)	—	Sort. This value is the default.

SLOPE

The slope of the line to be printed. This value must be positive.

SLOPE > 0

—

In the stationary reference frame (RefFrame = YES on the CORIOLIS command), the line represents the number of excitations per revolution of the rotor. For example, SLOPE = 1 represents one excitation per revolution, usually resulting from unbalance.

In the rotating reference frame (RefFrame = NO on the CORIOLIS command), the line represents the number of excitations per revolution of the rotor minus 1.

SLOPE = 0

—

The line represents the stability threshold for stability values or logarithmic decrements printout (STABVAL = 1, 2, or 3)

UNIT

Specifies the unit of measurement for rotational angular velocities:

RDS	—	Rotational angular velocities in radians per second (rad/s). This value is the default.
RPM	—	Rotational angular velocities in revolutions per minute (RPMs).

FREQB

The beginning, or lower end, of the frequency range of interest. The default is zero.

Cname

The rotating component name.

STABVAL

Flag to plot the stability values:

0 (OFF or NO)	—	Plot the frequencies (the imaginary parts of the eigenvalues in Hz). This value is the default.
1 (ON or YES)	—	Plot the stability values (the real parts of the eigenvalues in Hz).
2	—	Plot the inverse of the logarithmic decrements. A negative logarithmic decrement indicates stable motion.
3	—	Plot the logarithmic decrements. A positive logarithmic decrement indicates stable motion and is consistent with API (American Petroleum Institute) standards.

For more information about complex eigenmodes and corresponding logarithmic decrements, see Complex Eigensolutions in the Mechanical APDL Theory Reference.

KeyAllFreq

Key to specify if all frequencies above FREQB are plotted:

0 (OFF or NO)	—	A maximum of 10 frequencies are plotted. This value is the default.
1 (ON or YES)	—	All frequencies are plotted.

KeyNegFreq

Key to specify if the negative frequencies are plotted. It only applies to solutions obtained with the damped eigensolver (Method = DAMP on the MODOPT command):

0 (OFF or NO)	—	Only positive frequencies are plotted. This value is the default.
1 (ON or YES)	—	Negative and positive frequencies are plotted.

Notes

The following items are required when generating a Campbell diagram:

Activate the Coriolis effect (CORIOLIS command) in the solution phase (/SOLU).
Run a modal analysis using the QR damped (MODOPT,QRDAMP) or damped (MODOPT,DAMP) method. Complex eigenmodes are necessary (MODOPT,QRDAMP,,,,Cpxmod = ON), and you must specify the number of modes to expand (MXPAND).
Define two or more load step results with an ascending order of rotational velocity (OMEGA or CMOMEGA).

In some cases where modes are not in the same order from one load step to the other, sorting the frequencies (Option = 1) can help to obtain a correct plot. Sorting is based on the comparison between complex mode shapes calculated at two successive load steps.

At each load step, the application compares the mode shape to the loads at other load steps to determine whirl direction at the load step. If applicable, a label appears (in the plot legend) representing each whirl mode (BW for backward whirl and FW for forward whirl).

At each load step, the program checks for instability (based on the sign of the real part of the eigenvalue). The labels "stable" or "unstable" appear in the plot legend for each frequency curve.

The rotational velocities of a named component (Cname) are displayed on the X-axis.

For information on plotting a Campbell diagram for a prestressed structure, see Solving for a Subsequent Campbell Analysis of a Prestressed Structure Using the Linear Perturbation Procedure in the Rotordynamic Analysis Guide.

For a usage example of the PLCAMP command, see Campbell Diagram in the Rotordynamic Analysis Guide.

Damped modal cyclic symmetry (CYCLIC) analyses do not support the PLCAMP command.

Menu Paths

Main Menu>General Postproc>Rotor Dynamics>Plot Campbell