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Rotordynamics with ANSYS
Mechanical Solutions
Pierre THIEFFRY
Product Manager
ANSYS, Inc.
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Agenda
General features
Generalized axisymmetric element
Rotordynamics with ANSYS Workbench
An ANSYS V12.0 example
Future plans
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General features
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Rotordynamics features
Pre-processing:
Appropriate element formulation for all geometries
Gyroscopic moments generated by rotating parts
Bearings
Rotor imbalance and other excitation forces (synchronous andasynchronous)
Rotational velocities
Structural damping
Solution: Complex eigensolver for modal analysis
Harmonic analysis
Transient analysis
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Rotordynamics features
Post-processing
Campbell diagrams
Orbit plots
Mode animation Transient plots and animations
Users guide
Advanced features:
Component Mode Synthesis for static parts
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Appropriate element formulation
The following elements are supported for rotordynamicsanalysis (stationary reference frame):
Mass MASS21
Beam BEAM4, PIPE16BEAM188, B
EAM189PIPE 288/289
Solid SOLID45, S
OLID95SOLID185, S
OLID186,
SOLID187Shell SHELL63SHELL181, SHE
LL281General
axisymmetric
elements
SOLID272, SOLID273
N e w i nA N S Y S12 .0
New inANSYS12.0
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Generalized axisymmetric element
The new 272/273 elements:
Are computationallyefficient whencompared to 3D solid
Support 3D non-axisymmetric loading
Allow a very fast setup ofaxisymmetric 3D parts:
Slice an axisymmetric 3DCAD geometry to get
planar model Mesh with 272/273
elements
No need to calculateequivalent beamsections
Can be combined with
full 3D models,including contact
2D axisymmetric mesh
3D representation
3D results (not necessarily axisymmetric)
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Bearings
2D spring/damper with cross-coupling terms: Real constants are stiffness and damping
coefficients and can vary with spinvelocity
Bearing element choice depends on:
Shape (1D, 2D, 3D)
Cross terms
Nonlinearities
Description Stiffness and Damping crossterms
Nonlinear stiffness anddamping characteristics
COMBIN14 Uniaxial spring/damper No NoCOMBI214 2-D spring/damper Unsymmetric Function of the rotationalvelocity
MATRIX27 General stiffness ordamping matrix
Unsymmetric No
MPC184 Multipoint constraintelement
Symmetric for linearcharacteristics - None fornonlinear characteristics
Function of the displacement
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Imbalance and other excitation
forces
Possible excitations caused byrotation velocity are:
Unbalance ( )
Coupling misalignment (2*)
Blade, vane, nozzle,diffusers (s* )
Aerodynamic excitations asin centrifugalcompressors (0.5* )
Input made as a force on themodel
yF
zF
2
0
2
b
FmrF ==z
y
m
tr
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Rotating damping
Considered if the rotatingstructure has:
structural damping (MP,DAMP orBETAD)
or a localized rotatingviscous damper(bearing)
The damping forces can induce
unstable vibrations.
The rotating damping effect isactivated along with theCoriolis effect (CORIOLIS
command).
Damper COMBI214
Beam BEAM4, PIPE16BEAM188, BEAM189
Solid SOLID45, SOLID95
SOLID185, SOLID186,
SOLID187General
axisymmetric
SOLID272, SOLID273
(new in V 12.0 )
Elements supporting rotating damping
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Campbell diagrams & whirl
Variation of the rotor naturalfrequencies with respectto rotor speed
In modal analysis performmultiple load steps atdifferent angular
velocities
As frequencies split withincreasing spin velocity,ANSYS identifies:
forward (FW) andbackward (BW)
whirl stable / unstable
operation
critical speeds
Also available for multispoolmodels
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Orbit plots
In a plane perpendicular to thespin axis, the orbit of a nodeis an ellipse
It is defined by threecharacteristics: semi axesA , B and phase in a localcoordinate system (x, y, z)where x is the rotation axis
Angle is the initial positionof the node with respect tothe major semi-axis A.
Orbit plots are available forbeam models
PRINT ORBITS
LOCAL y AX
0.0000
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Rotordynamics analysis guide
New at release12.0
Provides adetailed
description ofcapabilities
Providesguidelines forrotordynamics
model setup
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Sample models available
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Generalized axisymmetric element
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New Element Technology
General Axi-symmetric Element: 272/2733D elements generated based on 2D meshBoundary conditions applied in 3D spaceNonlinearities, Node to surface contact
BenefitsMultiple Axis can be defined in any directionTake advantage of axi-symmetry but deformationis general in 3D1 element in (hoop) direction
Struc
tura
lMech
an
ics
I
L
J
K
A
B
Y Z
X
3D view ofshaft
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Application to rotordynamics
The new 272/273 elements:
Are computationallyefficient whencompared to 3D solid
Support rotordynamicsanalysis
Support 3D non-axisymmetric loading
Allow a very fast setup ofaxisymmetric 3D parts:
Slice an axisymmetric 3DCAD geometry to getplanar model
Mesh with 272/273elements
No need to calculate
equivalent beamsections
2D axisymmetric mesh
3D representation
3D results (not necessarily axisymmetric)
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Rotordynamics with ANSYS
Workbench
An example
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Storyboard
The geometry is provided in form of aParasolid file
Part of the shaft must be reparametrized to
allow for diameter variations A disk must be added to the geometry
Simulation will be performed using thegeneralized axisymmetric elements, mixing
WB features and APDL scripting Design analysis will be made with variations
of bearings properties and geometry
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Project view
Upper part of the schematicsdefines the simulationprocess (geometry tomesh to simulation)
Lower part of the schematicscontains the designexploration tools
Parameters of the model aregathered in one location(geometry, bearing stiffness)
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Geometry setup
Geometry isimported inDesign Modeler
A part of the shaftis redesignedwith parametricdimensions
Model is sliced tobe used withaxisymmetricelements
Bearing locationsare defined
A disc is added tothe geometry
Initial 3D geometry
Final axisymmetric model
Bearings location
Additional disk
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Geometry details
Part of the original shaft isremoved and recreated with
parametric radius
3D Model sliced to createaxisymmetric model
Bearing locations and named selections are created (namedselections will be transferred as node components for the simulation)
Additional disk created withparameters (the outer diameter
will be used for design analysis)
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Mesh
The modelis meshedusing theWBmeshingtools
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Simulation
Simulation isperformed using anAPDL script thatdefines:
Element types
Bearings Boundary
conditions
Solutionssettings(Qrdamp
solver) Post-processing
(Campbellplots andextraction ofcritical
speeds)
Axisymmetric modelwith boundaryconditions
Expanded view
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APDL script
Spring1 componentcomes from namedselection
Mesh transferred asmesh200 elements,converted tosolid272
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Simulation results
The APDL scriptscan createplots andanimations
The results canalso beanalyzed withinthe MechanicalAPDL interface
Results areextracted using*get commandsand exposedas WBparameters(showing theperformance ofthe design)
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Mode animation (expanded view)
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Design exploration
The model has 2 geometryparameters (disc and shaftradius) as well as a stiffnessparameters (bearingsstiffness)
4 output parameters areinvestigated: first and secondcritical speeds at 2xRPM and4xRPM (obtained from
theCampbell diagrams and*get commands)
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Sample results
A response surface ofthe model is createdusing a Design ofExperiments
Curves, surfaces andsensitivity plots arecreated and thedesign can beinvestigated
Optimization tools arealso available
Sensitivity plots:the bearingstiffness has noinfluence on thefirst and secondcritical speeds, thedisc radius is thekey parameter
Evolution of criticalspeed with shaftand disc radius
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Optimization
A multi-objectiveoptimizationis describedand possiblecandidatesare found(usually, thereare multipleacceptableconfigurations)
Trade-off plotsgive anindicationabout theachievableperformance
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Future plans (V13 and beyond)
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Campbell diagrams
Multiple steps (modal)
Rotational velocityscoped onbodies( (multispoolanalysis) available inmodal analysis
OutputQuantities:frequencies or
stability values
X axis is rotationalvelocity
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Additional enhancements
Provide modal solver choice (QRDAMP, LANB)
The connection folder hosting bearings:
Location
Damping and stiffness (as functions ofw)
Coriolis option available from the Analysis settings(like the large deflection or inertia relief)
Orbit plots for beam models
Exposure of generalized axisymmetric elements
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Modal post-processing (already
available at V12)
For complex modes, tabular data display both
imaginary and real parts
Complex eigenshapes
Mode animation similar toANHARM
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Results parameterization
The user will probably want to be able toparameterize frequencies (real and/orimaginary part) but also the criticalfrequencies (from Campbell results)
Doing so, he will be able to perform DXanalyses :
to examine the variations of criticalfrequencies
To examine the evolution of the stability of amode wrt various parameters