3 Defining NX Nastran Input for Rotor Dynamics
3.4 Bulk Data Section
For a rotor dynamics analysis, you define all relevant rotor dynamics data on the ROTORD bulk data entry. With the ROTORD entry, the main card and the first continuation card contain information that’s common to all the rotors present in the system. To specify rotor-specific information, you can use one or more continuation cards. The field assignments are as follows:
1 2 3 4 5 6 7 8 9 10
ROTORD SID RSTART RSTEP NUMSTEP REFSYS CMOUT RUNIT FUNIT +
+ ZSTEIN ORBEPS ROTPRT SYNC ETYPE EORDER
+ RID1 RSET1 RSPEED1 RCORD1 W3_1 W4_1 RFORCE1
+ RID2 RSET2 RSPEED2 RCORD2 W3_2 W4_2 RFORCE2
…
+ RID10 RSET10 RSPEED10 RCORD10 W3_10 W4_10 RFORCE10
For example:
ROTORD 998 0.0 5000.0 58 fix -1.0 cps +r0
+r0 No +r1
+r1 1 11 1 0.0 0.0 1 +r2
+r2 2 12 1 0.0 0.0 +r3
+r3 3 13 1 0.0 0.0 +r4
+r4 4 14 1 0.0 0.0 +r5
+r5 5 15 1 0.0 0.0 +r6
+r6 6 16 1 0.0 0.0 +r7
+r7 7 17 1 0.0 0.0 +r8
Defining NX Nastran Input for Rotor Dynamics Chapter 3
NX Nastran Rotor Dynamics User’s Guide 45
+r8 8 18 1 0.0 0.0 +r9
+r9 9 19 1 0.0 0.0 +r10
+r10 10 20 1 0.0 0.0 10
Field Contents SID Set identifier for all rotors. Must be selected in the case control deck
by RMETHOD=SID.
RSTART Starting value of rotor speed. (Real ≥ 0.0) RSTEP Step-size of rotor speed. (Real > 0.0)
NUMSTEP Number of steps for rotor speed including RSTART. (Integer > 0) REFSYS Reference system. (Default = ‘ROT’)
= ‘FIX’ analysis is performed in the fixed reference system.
= ‘ROT’ analysis is performed in the rotating reference system.
CMOUT Rotor speed for complex mode output request. (Real ≥ 0.0)
= -1.0 output complex modes and whirl direction for all RPM.
= 0.0 no complex modes and no whirl direction are output. (Default)
> 0.0 rotor speed value for which complex modes will be calculated and written to F06 or OP2.
RUNIT Units used for rotor speed input (RSTART and RSTEP) and output (units for output list and Campbell’s diagram output).
= ‘RPM’ revolutions per minute. (Default)
= ‘CPS’ cycles per second.
= ‘HZ’ cycles per second.
= ‘RAD’ radians per second.
FUNIT Units used for frequency output.
= ‘RPM’ revolutions per minute. (Default)
= ‘CPS’ cycles per second.
= ‘HZ’ cycles per second.
= ‘RAD’ radians per second.
ZSTEIN Option to incorporate Steiner’s inertia terms. (Default = NO)
= 'YES' Steiner’s inertia terms are included.
= 'NO' Steiner’s inertia terms are not included.
Chapter 3 Defining NX Nastran Input for Rotor Dynamics
46 NX Nastran Rotor Dynamics User’s Guide
ORBEPS Threshold value for detection of whirl direction. (Real > 0.0; Default
= 1E-6)
ROTPRT Controls .f06 output options. (Integer)
= 0 no print. (Default)
= 1 print generalized matrices.
= 2 print eigenvalue summary and eigenvectors at each RPM.
= 3 combination of 1 and 2.
SYNC Option to select Synchronous or Asynchronous analysis for frequency response. (Integer)
= 1 Synchronous (Default)
= 0 Asynchronous ETYPE Excitation Type (Integer)
= 1 Mass Unbalanced (Default). Specify mass unbalance = m r on DLOAD card and the program will multiply by Ω2.
= 0 Force Excitation. Specify force = m r Ω2 on DLOAD Card EORDER Excitation Order (Real)
1.0 (Default) - Modes crossing with 1P line in the fixed system 0.0 Forward Whirl (Modes crossing with 0P line, rotating system) 2.0 Backward Whirl (Modes crossing with 2P line, rotating system)
RIDi Rotor ID (Integer; Default = i); must be unique with respect to all other RIDi values.
RSETi Refers to a RSETID value on a ROTORG bulk entry. Each
ROTORG bulk entry specifies the grids for a single rotor. (Defaults to all grid points if only one rotor; no Default for multiple rotors.) (Integer)
RSPEEDi Relative rotor speed for rotor i. (Real; Default = 1.0).
RCORDi Points to CORD** entry specifying rotation axis of rotor i as z.
(Integer; Default = 0 for global z axis)
W3_i Reference frequency for structural damping defined by PARAM G for rotor i. (Real; Default = 0.0).
W4_i Reference frequency for structural damping defined on material card for rotor i. (Real; Default = 0.0).
Defining NX Nastran Input for Rotor Dynamics Chapter 3
NX Nastran Rotor Dynamics User’s Guide 47
RFORCEi Points to RFORCE bulk data entry for rotor i. (Integer; Default = 0 for no rotational force applied; a rotational force is required for differential stiffness to be calculated.)
Remarks:
1. Any entries where defaults exist are optional. Thus, if the defaults are acceptable for a particular model, then a continuation card would not be needed.
2. There is a maximum limit of 10 rotors (i.e., 11 continuation cards).
3. The Steiner’s term option (ZSTEIN) should only be used for analyzing the “stiff’
part of the rotor. In the rotating system analysis, there is a stiffening effect of the centrifugal forces. The gyroscopic matrix is calculated in the fixed system.
4. The W3 parameter defines the reference frequency for the structural damping defined by PARAM G.
5. The W4 parameter defines the reference frequency for the structural damping defined on the material cards.
6. The W3 and W4 parameters are required for all direct solutions. In the modal solutions, the eigenvalues are used. If the W3 and W4 parameters are defined for modal analysis they will be used. These parameters are not recommended for modal solutions
7. The static centrifugal force is calculated for unit speed measured in rad/sec. On the RFORCE card, the unit of Hz is used, thus the conversion 1/(2π) = 0.159155 must be used.
8. For calculating the modal frequency response using synchronous analysis, the rotation speeds are defined by RSTART, RSTEP and NUMSTEP entries of ROTORD card. The frequencies corresponding to these rotation speeds are computed and the dynamic loads are calculated accordingly.
9. For calculating the frequency or transient response using asynchronous analysis, the unique rotation speed is defined by RSTART entry of ROTORD card. The RSTEP and NUMSTEP entries in this case will be ignored. The frequency and dynamic load definitions are taken from the standard FREQ, DLOAD, RLOAD etc cards in the frequency response case and by TSTEP, DLOAD, TLOAD etc. cards in the transient response case.
10. The rotor speed defined by RSTART, RSTEP and NUMSTEP are called the nominal rotor speed. Rotors with relative speed defined by RSPEED will spin at the defined factor multiplied by the nominal rotor speed.
Select the rotating part of the model through ROTORG bulk data entry. The field assignments for this entry are as follows:
1 2 3 4 5 6 7 8 9 10
ROTORG RSETID GRID1 GRID2 … … GRIDN
Chapter 3 Defining NX Nastran Input for Rotor Dynamics
48 NX Nastran Rotor Dynamics User’s Guide
OR
1 2 3 4 5 6 7 8 9 10
ROTORG RSETID GRID1 THRU GRIDN BY M
By default, all grid points are rotating. The RSETID entry is referred to by RSETi entry or entries of the ROTORD bulk data entry. If model contains multiple rotors, use different ROTORG cards (one ROTORG card for each rotor).
Select the grid points of the bearings in case of multiple rotors. This is important for the calculation of the damping matrices in order to distribute the right bearing damping to the right rotor.
1 2 3 4 5 6 7 8 9 10
ROTORB RSETID GRID1 GRID2 … … GRIDN
OR
1 2 3 4 5 6 7 8 9 10
ROTORB RSETID GRID1 THRU GRIDN BY M
The RSETID must be equal to that of the ROTORG card. The grid points represent the non-rotating bearing points.
Between the grid point of the rotor and the grid point of the bearing there must be a RBE2 element or an MPC connection in order to get the correct partitions of the rotating and the non-rotating part of the model.
3.5 Parameters