Study of Modal Behaviour of Viscoelastic Rotors Using Finite Element Method

Study of Modal Behaviour of Viscoelastic Rotors Using Finite Element Method
Study of Modal Behaviour of Viscoelastic Rotors Using
Finite Element Method
A THESIS SUBMITTED IN THE PARTIAL FULFILLMENT
OF THE REQUIREMENTS FOR THE DEGREE OF
Master of Technology
In
MECHANICAL ENGINEERING
[Specialization: Machine Design and Analysis]
By
Pavan R.Mutalikdesai
(211ME1345)
DEPARTMENT OF MECHANICAL ENGINEERING
NATIONAL INSTITUTE OF TECHNOLOGY ROURKELA
YEAR 2012-2013
Study of Modal Behaviour of Viscoelastic Rotors Using
Finite Element Method
A THESIS SUBMITTED IN THE PARTIAL FULFILLMENT
OF THE REQUIREMENTS FOR THE DEGREE OF
Master of Technology
In
MECHANICAL ENGINEERING
[Specialization: Machine Design and Analysis]
By
Pavan R.Mutalikdesai
(211ME1345)
Under the Supervision of
Prof. H. ROY
DEPARTMENT OF MECHANICAL ENGINEERING
NATIONAL INSTITUTE OF TECHNOLOGY ROURKELA
YEAR 2012-2013
National Institute Of Technology
Rourkela
CERTIFICATE
This is to certify that the thesis entitled, “Study of Modal Behaviour of Viscoelastic Rotors
Using Finite Element Method”, submitted by Mr. Pavan R.Mutalikdesai in partial
fulfillment of the requirements for the award of MASTER OF TECHNOLOGY Degree in
MECHANICAL ENGINEERING with specialization in MACHINE DESIGN AND
ANALYSIS at the National Institute of Technology, Rourkela (India) is an authentic Work
carried out by him under my supervision and guidance.
To the best of my knowledge, the matter embodied in the thesis has not been submitted to any
other University / Institute for the award of any Degree or Diploma.
Date:
Dr. HARAPRASAD ROY
Department of Mechanical Engineering
National Institute of Technology
Rourkela-769008
ACKNOWLEDGEMENT
First and foremost I offer my sincerest gratitude and respect to my supervisor and
guide Asst. Prof. (Dr.) HARAPRASAD ROY, Department of Mechanical Engineering, for
his invaluable guidance and suggestions to me during my study. I consider myself extremely
fortunate to have had the opportunity of associating myself with him for one year. This thesis
was made possible by his patience and persistence.
After the completion of this Thesis, I experience a feeling of achievement and
satisfaction. Looking into the past I realize how impossible it was for me to succeed on my
own. I wish to express my deep gratitude to all those who extended their helping hands
towards me in various ways during my tenure at NIT Rourkela. I greatly appreciate & convey
my heartfelt thanks to my colleagues, dear ones & all those who helped me in the completion
of this work. I am especially indebted to my parents for their love, sacrifice, and support.
They are my first teachers after I came to this world and have set great examples for me about
how to live, study and work.
I also express my sincere gratitude to Prof. (Dr.) K. P. Maity, Head of the
Department, Mechanical Engineering for valuable departmental facilities.
PAVAN R.MUTALIKDESAI
Roll No: - 211ME1345
INDEX
S. NO.
TOPICS
PAGE NO.
LIST OF FIGURES
I
LIST OF TABLES
I
ABSTRACT
II
1.
INTRODUCTION
1
1.1
BACKGROUND AND IMPORTANCE
2
1.2
VISCOELASTICITY
4
1.3
MODAL ANALYSIS
5
1.3.1.
VARIOUS METHODS OF MODAL ANALYSIS
6
1.3.2.
INTERNAL DAMPING
12
1.3.3.
RESONANT SPEED
12
1.3.4.
MODAL DAMPING FACTOR
13
1.3.5.
DIRECTIONAL FREQUENCY RESPONSE FUNCTIONS
13
(DFRF):
1.4
MOTIVATION OF THE WORK
15
1.5
LAYOUT OF THE PRESENT WORK
17
2.
VISCOELASTIC ROTOR AND ITS MODELING
19
2.1
EQUATION OF MOTION
20
2.2
FREE VIBRATIONAL ANALYSIS
24
2.3
DEVELOPMENT OF FREQUENCY RESPONSE FUNCTION
25
MATRIX
3.
RESULTS AND DISCUSSION
28
3.1
THE ROTOR-SHAFT SYSTEM
29
3.2
STABILITY LIMIT OF SPIN SPEED
30
3.2.1.
DECAY RATE PLOT
30
3.2.2.
MODAL DAMPING FACTOR
31
3.3
CAMPBELL DIAGRAM
32
3.4
THREE DIMENSIONAL MODE SHAPES OF SIMPLY
33
SUPPORTED ROTOR
3.5
DIRECTIONAL FREQUENCY RESPONSE FUNCTION
34
4.
CONCLUSIONS AND SCOPE FOR FUTURE WORK
36
4.1
CONCLUSIONS
37
4.2
SCOPE FOR FUTURE WORK
38
REFERENCES
39
LIST OF PUBLICATIONS
41
LIST OF FIGURES
S. No.
FIGURE NAME
Page No.
Figure 1.1.
Stress Strain Curve
5
Figure 2.1.
Displaced Position of the Shaft Cross Section
21
Figure 3.1.
Schematic Diagram of the Rotor
29
Figure 3.2.
Variation of Maximum real Part with the spin speed
31
Figure 3.3.
Variation of modal damping factor with the spin speed
32
Figure 3.4.
Campbell Diagram
33
Figure 3.5.
Mode shape of rotor for 1st Backward ( v =0)
34
Figure 3.6.
Mode shape of rotor for 1st Forward ( v =0)
34
Figure 3.7.
Mode shape of rotor for 1st Backward ( v  0 )
34
Figure 3.8.
Mode shape of rotor for 1st Backward (v  0 )
34
Figure 3.9.
dFRF ( H pg ) plot , at node 6
35
LIST OF TABLES
S. No.
TABLE NAME
Page No.
Table 1
Rotor Material and its Properties
29
Table 2
Disc parameters for 3 disc rotor
30
Study of modal behaviour of viscoelastic rotors using finite element method
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Study of modal behaviour of viscoelastic rotors using finite element method
Abstract:
The Property which combines elasticity and viscosity is called as viscoelasticity. Such
materials store energy as well as dissipate it to the thermal domain when subjected to
dynamic loading. The storage and loss of energy depends upon the frequency of excitation.
Modeling of elastic materials is easier, compared to the Modeling of viscoelastic materials.
This work attempts to study the influence of internal material damping on the modal
behaviour of a rotor shaft system. Rotary forces are generated due to the internal material
damping of rotor shaft system and are proportional to the spin speed and acts tangential to
the rotor orbit. These forces influence the dynamic behaviour of a rotor and tend to
destabilize the rotor shaft system as spin speed increases. Hence, the modal behaviour of rotor
shaft is studied by finite element method, to get better ideas about the dynamic behaviour of
the rotor shaft system.
The dynamic characteristics of rotor systems are closely related with the rotor spin
speed; hence the directivity of modes becomes very important in rotor dynamics. In the
approach used in this work, a natural mode of the rotor is represented as the sum of two sub
modes which are rotating to the forward and backward directions. This work explains the use
of directional information when the equation of motion of a rotor is formulated in complex
form. This methodology has an advantage of incorporating the directionality. Work
establishes the fact that directional frequency response functions (dFRF) relate the forward
and backward components of the complex displacement and excitation functions. The dFRFs
are obtained as the solution to the forced vibration solution to the equation of motion.
The finite element analysis is used for modeling the system. The rotor shaft system with
simply supported ends having three discs has been considered for the study. In this work the
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Study of modal behaviour of viscoelastic rotors using finite element method
effects of internal viscous damping have been incorporated into the finite element model. In
the analysis, the equations of motion are obtained to a good degree of accuracy by
discretizing the rotor-shaft continuum using 2-noded finite Rayleigh beam elements. These
equations are used for eigenanalysis. A finite element code is written in MATLAB to find out
the eigenvalues, eigenvectors and modal damping factor. Stability limit of spin speed and
effect of modal damping factor on the spin speed have been studied. This work establishes the
fact that directional frequency response functions (dFRF) relate the forward and backward
components of the complex displacement and excitation functions. The dFRFs are obtained as
the solution to the forced vibration solution to the equation of motion. The Campbell diagram
and the dFRF plots are obtained by using the Matlab.
Keywords: Internal Material Damping, Modal analysis, Modal Damping Factor, Stability
Limit of Spin Speed, directional Frequency Response Function (dFRF).
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Study of modal behaviour of viscoelastic rotors using finite element method
CHAPTER ONE
Introduction
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Study of modal behaviour of viscoelastic rotors using finite element method
1.1.
BACKGROUND AND IMPORTANCE:
Rotordynamics is a specialized branch of applied mechanics concerned with the
behavior and diagnosis of rotating structures. It deals with behavior of rotating machines ranging
from very large systems like power plant rotors, for example, a turbo generator, to very small
systems like a tiny dentist‟s drill, with a variety of rotors such as pumps, compressors, steam
turbines, motors, turbo pumps etc. as used for example in process industry. The principal
components of a rotor-dynamic system are the shaft or rotor with disk and the bearing. The shaft
or rotor is the rotating component of the system. Additionally, machines are operated at high
rotor speeds in order to maximize the power output. At its most basic level rotordynamics is
concerned with one or more mechanical structures (rotors) supported by bearings and influenced
by internal phenomena that rotate around a single axis. The supporting structure is called a stator.
Rotors are the main sources of vibration in most of the machines. At higher speeds of rotation,
the vibrations caused by the mass imbalance results in some serious problems (Rao [2]). So it
becomes necessary to limit the vibrations for operational safety and stability. It can be done by
proper assessment of dynamics of system. The dynamic behavior of a mechanical system must
be examined in its design phase so that you can determine whether it will present a satisfactory
performance or not in its condition of the planned operation. The natural frequencies, damping
factors and vibration modes of these systems can be determined analytically, numerically or
experimentally.
Unlike the viscoelastic structures (which do not spin) viscoelastic rotors are acted upon
by rotating damping force generated by the internal material damping, that tends to destabilize
the rotor shaft system by generating a tangential force proportional to the rotor spin speed. Thus
a reliable model is necessary to represent the constitutive relationship of a rotor-material by
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Study of modal behaviour of viscoelastic rotors using finite element method
taking into account the internal material damping for understanding the dynamic behaviour of a
viscoelastic rotor. Such a model is useful for getting an idea about safe speed ranges of rotation,
where the rotor is stable.In this work the effects of internal viscous damping have been
incorporated into the finite element model.
The study for rotary machines, however, requires a more careful and detailed analysis,
because the rotation movement of the rotor significantly influences the dynamic comportment of
the system, making the modal parameters dependent on the rotation of the machine (Lee [3]).
The gyroscopic effect couples the rotation movement and, as it‟s known, it is dependent on the
rotation speed of the rotor. Therefore, it is expected that the natural frequencies and vibration
modes of a rotating machine also depend on the system speed. Thus, unlike non rotating
structures, rotors have two different kinds of modes, known as forward and backward modes due
to the rotor spin.
In any individual Frequency Response Function (FRF) plot of a rotor the negative
frequency region of the FRF is merely a duplicate of the positive frequency region. Therefore, it
is only necessary to deal with one region of the FRF, conventionally the positive one (Mesquita
et al. [4]). By the use of traditional modal analysis, it becomes difficult to identify the directivity
of a mode, forward or backward. To overcome this problem, a new method called complex
modal analysis for the rotors has been introduced in this work. The equation of motion is
formulated by using complex variables representing displacement and excitation. The dFRFs
(Directional Frequency Response Function) are obtained as the solution to the forced vibration
solution to the equation of motion. The advantage of this methodology is that the directionality
can be incorporated, which was not possible by using the traditional modal analysis. The method
separates the backward and forward modes in the dFRF, so that effective modal parameter
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Study of modal behaviour of viscoelastic rotors using finite element method
identification is possible. The FRF obtained by the method is discussed with special attention to
its implied directional information. It is shown that the FRFs obtained as such relate forward and
backward responses to forward and backward excitations, and become the same functions that
were defined as the directional FRFs by (Lee [3]).
1.2.
VISCOELASTICITY:
Viscoelasticity is a property that combines elasticity and viscosity. These materials store
the energy as well as dissipate it under dynamic deformation. Thus, the stress in such materials is
not in phase with the strain. Due to these properties, it is extensively used in various engineering
applications for controlling the amplitude of resonant vibrations and modifying wave attenuation
and increasing structural life through reduction in structural fatigue (Dutt and Nakra [5]).
Williams [6] did the Structural Analysis of Viscoelastic Materials. The classical theory of
elasticity states that for sufficiently small strains, the stress in an elastic solid is proportional to
the instantaneous strain and is independent of the strain rate. In a viscous fluid, according to the
theory of hydrodynamics, the stress is proportional to the instantaneous strain rate and is
independent of the strain (Williams [6]). Viscoelastic materials exhibit both solid and fluid
behavior. Examples of Such materials are plastics, amorphous polymers, glasses, ceramics, and
biomaterials (muscle). Viscoelastic materials are characterized by constant-stress creep and
constant-strain relaxation. The Stress-Strain Curves for a purely elastic and a viscoelastic
material are shown in figure 1.1. There‟s a loss of energy during loading and unloading time.
Because of this reason, the stress strain curve for viscoelastic material is elliptic in nature. The
area enclosed by the ellipse is a hysteresis loop and shows the amount of energy lost (as heat) in
a loading and unloading cycle.
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Study of modal behaviour of viscoelastic rotors using finite element method
Figure 1.1 Stress Strain Curve
Some phenomena in viscoelastic materials are:
[1] If the stress is held constant, the strain increases with time (creep).
[2] If the strain is held constant, the stress decreases with time (relaxation).
[3] The effective stiffness depends on the rate of application of the load.
[4] If cyclic loading is applied, hysteresis (a phase lag) occurs, leading to dissipation of
mechanical energy.
[5] Acoustic waves experience attenuation.
[6] Rebound of an object following an impact is less than 100%.
[7] During rolling, frictional resistance occurs.
1.3
MODAL ANALYSIS:
Modal analysis is the process of determining the inherent dynamic characteristics of a
system in forms of natural frequencies, damping factors and mode shapes. It‟s used to formulate
a mathematical model for its dynamic behavior. The formulated mathematical model is known as
the modal model of the system and the information for the characteristics is known as its modal
data. Modal analysis has evolved into a standard tool for structural dynamics problem analysis
and design optimization (Chouksey et al. [11]]).
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Study of modal behaviour of viscoelastic rotors using finite element method
Modes are inherent properties of a structure. These are determined by the material
properties (mass, damping, and stiffness), and boundary conditions of the structure. Each mode
is defined by a natural (modal or resonant) frequency, modal damping, and a mode shape.
1.3.1.
VARIOUS METHODS OF MODAL ANALYSIS
Modal analysis is based upon the fact that the vibration response of a linear timeinvariant dynamic system can be expressed as the linear combination of a set of simple harmonic
motions called the natural modes of vibration (He and Fu [7]). The natural modes of vibration
are inherent to a dynamic system and are determined completely by its physical properties (mass,
stiffness, damping). Each mode is described in terms of its modal parameters: natural frequency,
the modal damping factor and mode shape. The mode shape may be real or complex. The degree
of contribution of each natural mode in the overall vibration is determined both by properties of
the excitation sourceand by the mode shapes of the system. Modal analysis involves both
theoretical and experimental techniques. (He and Fu [7]) gave details about the various methods
of modal analysis.
THEORETICAL MODAL ANALYSIS:
Physical model of a dynamic system comprising its mass, stiffness and damping
properties are used to study the dynamic behaviour of the system. Procedure involves finding of
mass, stiffness and damping properties of the system. Then the equation of motion of the system
is found out.
The solution of the equation of motion provides the natural frequencies and mode shapes
of the system considered and its forced vibration responses. More realistic physical model
consists of mass, stiffness and damping properties in terms of their mass, stiffness and damping
matrices. These matrices are incorporated into the equation of motion of the system. The
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Study of modal behaviour of viscoelastic rotors using finite element method
superposition principle of a linear dynamic system helps us to transform these equations into a
typical eigenvalue problem. The solution of the eigenvalue problem provides the modal data of
the system. Finite element analysis empowers the discretization of almost any linear dynamic
structure and hence has greatly enhanced the capacity and scope of theoretical modal analysis.
EXPERIMENTAL MODAL ANALYSIS:
Rotating machines appear in almost every aspect of our modern life for example cars,
aero-planes and steam-turbines.These all have many rotating structures whose dynamics need to
be modeled, analysed and improved. The stability and the response levels of these machines,
predicted by analytical models, must be validated experimentally. For this reason modal testing
needs to be performed. The practice of modal testing involves measuring the FRFs or impulse
responses of a structure. The FRF measurement can be done by using the response recorded by
the accelerometer and the data acquisition system. For small structures modal hammer can be
used for the excitation and for large structures exciter is used for applying the force.
Experimental modal analysis involves three steps: test preparation, frequency response
measurements and modal parameter identification.
1. Test preparation involves selection of a structure‟s support, type of excitation force(s),
location(s) of excitation, data acquisition system to measure force(s). Structure is divided
in to number of parts. Accelerometers are connected at the selected nodes.
2. Then the impact force is applied at some locations using the exciter and corresponding
response are noted using the data acquisition system. Using the responses and applied
force, the FRF matrix is obtained, which is then analysed to identify modal parameters of
the tested structure.
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Study of modal behaviour of viscoelastic rotors using finite element method
Modal analysis concepts have not been applied to rotor systems as extensively as they
have to other structural dynamic systems. Rotor systems force the reconsideration of some of the
basic assumptions applied in modal analysis of other structures. Jeffcott [1] provided a very basic
model of a rotor. Initial assumptions made by him are,: (i) No damping is associated with the
rotor, (ii) Axially Symmetric rotor, and (iii) The rotor carries a point mass. Later, the model was
expanded to take care of damping. Jeffcott rotor model is an oversimplification of real-world
rotors and retains some basic characteristics. It allows us to gain a qualitative insight into
important phenomena typical of rotor dynamics, while being much simpler than more realistic
models.
Between the work of Jeffcott [1] and the start of World War II there was much work in
the area of instabilities and modeling techniques culminating in the work of Prohl and Myklestad
which led to the Transfer Matrix Method (TMM) for analyzing rotors. Using this TMM technique
researchers found some difficulties by the peculiar approach developed for the study of the
attitude dynamics of spinning spacecraft. The wide diffusion of the finite element method (FEM)
deeply influenced also the field of rotor dynamics Strictly speaking; usual general purpose FEM
codes cannot be used for rotor dynamic analysis owing to the lack of consideration of gyroscopic
effects. It is true that a gyroscopic matrix can be forced in the conventional formulation and that
several manufacturers use commercial FEM codes to perform rotor dynamic analysis, but the
rotor dynamic field is one of these applications in which purposely written, specialized FEM
codes can give their best. Through FEM modeling, it is possible to study the dynamic behavior of
machines containing high-speed rotors in greater detail and consequently to obtain quantitative
predictions with an unprecedented degree of accuracy. Nelson and McVaugh [8] written
extensively history of rotor dynamics and most of the work is based on Finite Element Methods.
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Study of modal behaviour of viscoelastic rotors using finite element method
In particular, rotor systems do not in general obey Maxwell‟s reciprocity theorem; system
matrices are nonsymmetric. System matrices also depend upon rotor speed due to the presence of
gyroscopic effects which lead to skew-symmetry in the damping matrix; this is a linear function
of rotor speed. Also, support characteristics are nonsymmetric for commonlyused bearing types
and they vary widely with rotor speed. This work is aimed primarily to study the dynamic
behavior of fixed-base heavy rotors used in most of the power plant industries. Many Researchers
worked in the field of Rotordynamics. Some contributions are explained here.
Dutt and Nakra [5] carried out stability analysis of a rotor systems in which , rotor disc is
placed in the middle of a massless shaft, having linear elasticity and internal damping and
supported on Viscoelastic supports. They obtained results for cases with purely elastic or
viscously damped and flexible supports. They found that suitably chosen Viscoelastic supports
can increase the stability zones of the system considerably compare to the other type of the
supports. Nelson and McVaugh [8] presented a procedure for dynamic modeling of rotor bearing
systems which consisted of rigid disks, distributed parameter finite rotor elements, and discrete
bearings. They presented their formulation in both a fixed and rotating frame of reference. They
developed a finite element model including the effects of rotary inertia, gyroscopic moments,
and axial load. They utilized a coordinate reduction method to model elements with variable
cross-section properties. Their model included linear stiffness and viscous damping cases only.
The study included an overhung rotor with two sets of bearing parameters: (i) undamped
isotropic, (ii) undamped orthotropic. The comparison of results was made with an independent
lumped mass analysis. Zorzi and Nelson [9] provided a finite element model for a multi-disc
rotor bearing system. The model was based on Euler-Bernoulli beam theory. Their work
included viscous as well as hysteresis damping forms of material damping (internal damping) of
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Study of modal behaviour of viscoelastic rotors using finite element method
the system. They demonstrated that the material damping in the rotor shaft introduces rotary
dissipative forces which are tangential to the rotor orbit, well known to cause instability after
certain spin speed. Both forms of internal damping destabilize the rotor system and induce nonsynchronous forward precession. They also demonstrated the effects of anisotropic bearing
stiffness and external damping.
Ku [10] included the combined effects of transverse shear deformations and the internal
viscous and hysteretic damping in their analysis. Results of forward and backward whirl speeds
and damped stability are presented and compared with other previously published works. Better
convergence and high accuracy of the present finite element model are demonstrated with
numerical examples.
Chouksey et al. [11] studied the influences of internal rotor material damping and the
fluid film forces (generated as a result of hydrodynamic action in journal bearings) on the modal
behaviour of a flexible rotor-shaft system. Due to journal bearing and the internal material
damping introduce tangential forces increasing with the rotor spin speed and tend to destabilize
the system. Under this system of forces the modal behaviour of the rotor-shaft is studied to get
better ideas about the dynamic behaviour of the system. The stability limit speed is calculated
and the effect of modal damping factor on the spin speed is studied.The mode shapes of the rotor
are also found out.Laszlo [12] studied the stability analysis of self-excited vibrations of linear
symmetrical rotor bearing systems with internal damping using the finite element method. The
rotor system consists of uniform circular Rayleigh shafts with internal viscous damping,
symmetric rigid disks, and discrete isotropic damped bearings. The effect of rotary inertia and
gyroscopic moment are also included in the mathematical model. By doing analysis it‟s proved
that the whirling motion of the rotor system becomes unstable at all speeds beyond the threshold
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Study of modal behaviour of viscoelastic rotors using finite element method
speed of instability. It‟s found that the rotor stability is improved by increasing the damping
provided by the bearings, whereas increasing internal damping may reduce the stability
threshold.
Jei and Lee [13] studied the analysis of asymmetrical rotor-bearing systems. They
considered the effect of
rotary inertia, gyroscope, transverse shear deformation, internal
damping and gravity during finite element modeling. , the modal transform technique is applied
to reduce the size of the final resulting matrices. Finally the accuracy of the finite element model
and the modal transform technique is demonstrated. Agostini and Capello [14] worked on the
vibration analysis of vertical rotors by considering the gravitational and gyroscopic effects. Two
modes namely forward and backward are obtained separately by using complex modal analysis
in conjunction with the finite elements method in Matlab. Numerical simulations have been
satisfactory when compared with the existing literature.
Modal analysis is a useful tool to get an idea about the dynamic behavior of a system and
hence is considered to be important for dynamic design. Modal analysis of rotors differs from
modal analysis of structures due to additional forces induced by rotor spin like gyroscopic,
tangential and rotating damping forces. These forces change the nature of system matrices and
make them asymmetric and speed dependent. Therefore all modal characteristics of rotors are
closely related to rotor spin speed.
By doing the modal analysis we find out the resonant frequencies and the modal damping
factors. It‟s necessary to calculate these factors because of following reasons.
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Study of modal behaviour of viscoelastic rotors using finite element method
1.3.2.
INTERNAL DAMPING
All types of damping associated to the non-rotating parts of the structure have a stabilizing
effect on the system. On the other hand damping associated to rotating parts can result in
instability in supercritical ranges. Due to the Rotation of rotors rotary damping forces arise ,
which increase with the spin speed and acts tangential to the rotor orbit. These systems of forces
result in the instability of rotor-shaft systems. Thus a reliable model is necessary to represent the
rotor internal damping for correct prediction of stability limit of spin speed (SLS) of a rotor-shaft
system. Rotary machines, like motors, compressors and turbines are very common and widely
used. Now a days the designers of these machines have been required to meet very severe
specifications from the demands of high speed operating power or improvements in efficiency
and reliability for the design. In order to meet such requirements, we have to find some robust
and reliable mathematical models, along with special numerical solution procedures, which
enable designers to make an accurate assessment of the relevant parameters, the critical speeds
and the dynamic behavior of the system. Response of the system to an unbalance excitation is
considered to be of much importance in order to design for increased speeds of rotation, to
optimize weight and to improve reliability.
1.3.3. RESONANT SPEED
Many of the structures can be under resonance, i.e. to vibrate with excessive oscillatory
motion. Resonant vibrations arise due to the interaction between the inertial and elastic
properties of the materials within a structure. Resonance is often the cause for many of the
vibration and noise related problems that occur in structures and operating machinery. For the
better understanding of any structural vibration problem, its necessary to identify the resonant
frequencies of a structure. Today, modal analysis has become a widespread means of finding the
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Study of modal behaviour of viscoelastic rotors using finite element method
modes of vibration of a machine or structure . In the development of a new product, structural
dynamics testing is used to assess its real dynamic behavior.
1.3.4.
MODAL DAMPING FACTOR
In order to understand the importance of calculating the modal damping factor of a
system, consider an example of aeroplanes. Wings of airplanes can be subjected to similar flutter
phenomena during flight. Before the release of an airplane, flight flutter tests have to be
performed to detect possible flutters. The classical flight flutter testing approach is to expand the
flight envelope of an airplane by performing a vibration test at constant flight conditions. Curve
fitting method is used to estimate the resonance frequencies and damping ratios. Then these are
plotted against the flight speed. The damping values are then used to determine whether it is safe
to proceed to the next flight test point. When one of the damping values tends to become
negative, fluttering starts. Ground vibration tests as well as numerical simulations and wind
tunnel tests are done before starting the flight tests, to get some prior insight into the problem.
Hence by the modal analysis we find out the modal damping factors. These are plotted against
the spin speed. From this graph we can find out the stability of the system.
1.3.5. DIRECTIONAL FREQUENCY RESPONSE FUNCTIONS (DFRF):
The concepts of traditional modal analysis in stationary structures have been applied in
the analysis of rotating structures. However, the analysis for rotating structures requires a more
general theoretical development. Due to the rotation, gyroscopic effects appear resulting in nonsymmetric matrices in the equations of motion of the system and, as consequence the FRF
(Frequency Response Function) matrix does not obey the Maxwell‟s reciprocity theorem (Lee
[3]).
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Study of modal behaviour of viscoelastic rotors using finite element method
Rotors, unlike other stationary structures, exhibit a particular phenomenon in their modal
characteristics (Laszlo Forrai [12]). As rotor starts rotating, it gives raise to two different kinds of
modes known as the backward and forward modes. Although the presence of backward and
forward modes in rotor dynamics has been extensively investigated in the literature, the
directivity of the modes is often neglected in the usual formulations in the dynamic analysis of
rotors. If we neglect the important modal parameters such as the modal vectors and the adjoint
modal vectors, it becomes difficult to find out the mode shapes.
In any individual FRF plot of a rotor, the negative frequency region and positive
frequency region are merely duplicate of each other. Because of this reason it is only necessary
to deal with positive frequency region of FRF, because it yields some physical meaning. By
using the traditional modal analysis, the directivity of a mode whether forward or backward,
cannot be easily distinguishable in the frequency domain(Mesquita et al. [4]). The complex
modal analysis is nothing but, the application of classical modal analysis principles to rotating
systems, where complex variables are used to represent input and output of the system. This
methodology has the ability of incorporating directionality, which was not possible using the
traditional modal analysis(Kessler and Kim [15]).This method separates the backward and
forward modes in the dFRF (Directional Frequency Response Function), so that effective modal
parameter identification is possible. The frequency response function (FRF) obtained by this
method is discussed with special attention to its implied directional information. It is shown that
the FRFs obtained, relate forward and backward responses to forward and backward excitations,
and become the same functions that were defined as the directional FRFs (dFRFs) by Lee [3].
Complex modal testing theory developed in this project gives not only the directivity of the
backward and forward modes ,but also completely separates those modes in the frequency
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Study of modal behaviour of viscoelastic rotors using finite element method
domain so that effective modal parameter identification is possible. The concept of complex
modal analysis was first proposed by Lee[3].
Kessler and Kim [16] used a complex variable description of planar motion. It
incorporates directivity as inherent Information. They used the directional information explicitly
by formulating the equation of motion of a rotor in complex variables. Lee [3] proposed a new
modal testing method, termed complex modal testing, for the modal parameter identification of
rotating machinery and compared with the classical method. Complex modal testing allows clear
physical insight into the backward and forward modes. This also enables the separation of those
modes in the frequency domain so that effective modal parameter identification is possible.
Jei and Kim [17] proposed a new modal testing theory to separate the rotor vibration into
positive and negative frequency regions. The amplitude and directivity variations of frequency
response functions in positive and negative frequency regions are discussed using complex
modal displacement. A method to identify the directivity of modes such as forward and
backward is suggested using the frequency response function obtained by the proposed modal
testing theory.
Directional frequency response functions (dFRFs) provide modal directivity and
separation in the frequency domain. The work establishes the fact that dFRFs relate the forward
and backward components of the complex displacement and excitation functions.
1.4
MOTIVATION OF THE WORK:
Because of greater demands made on improving the performance of rotating machinery
the influence of rotordynamics on the day to day safe operation and long term health of the
equipment is becoming important. Increased power output through the use of higher-speed more
flexible, rotors has increased the need, at all stages of design. For the optimum and safe design of
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Study of modal behaviour of viscoelastic rotors using finite element method
these components, understanding of the interaction of the resulting static and dynamic forces
between the rotating element and stationary components becomes important.
The acceptable performance of a turbo machine depends on the adequate design and
operation of the bearing supporting a rotor. Turbo machines also include a number of other
mechanical elements which provide stiffness and damping characteristics and affect the
dynamics of the rotor-bearing system.
The adequate operation of a turbo machine is defined by its ability to tolerate normal (and
even abnormal) vibrations levels without affecting significantly its overall performance
(reliability and efficiency). The rotordynamics of turbo machinery encompasses the structural
analysis of rotors (shafts and disks) and design of bearings that determine the best dynamic
performance given the required operating conditions. The best performance is ensured when the
natural frequencies (and critical speeds) with amplitudes of synchronous dynamic response are
within required standards and the absence of sub synchronous vibration instabilities. A
rotordynamics analysis considers the interaction between the elastic and inertia properties of the
rotor and the mechanical impedances from the bearing supports.
The most commonly occurring problems in rotordynamics are Sub harmonic rotor instabilities.
Method to avoid Sub harmonic rotor instabilities may be avoided by:
1. Increasing the natural frequency of rotor system.
2. Eliminating the instability.
3. Introducing damping to increase the speed above the operating speed range.
Rotor dynamic instabilities have become more common as the speed and power of turbo
machinery increased. These instabilities can sometimes be erratic, resulting in increasing
vibration amplitudes for no apparent reason. So it becomes necessary to limit the vibrations for
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Study of modal behaviour of viscoelastic rotors using finite element method
operational safety and stability. It can be done by proper assessment of dynamics of system. The
purpose of modal analysis is to get an idea about the dynamic behaviour of the system. Using
this analysis we can find out the critical and stability limit speeds and from this we can minimize
the vibrations. Hence the modal analysis can be used as a tool to get an idea about the dynamic
behaviour of the system.
1.5
LAYOUT OF THE PRESENT WORK:
In this work, Finite element method is used for the analysis. Here the rotor–shaft system
is modeled by considering the Euler-Bernoulli beam theory and discretised by finite element
method to derive equations of motion (Rao [2], Zorzi and Nelson [9]). An example of three
discs rotor system is presented here. For the purpose of numerical analysis of the system, rotor
shaft system with the simply supported ends has been considered. Following Zorzi and Nelson
[9], the constitutive relationship is written, where Voigt model (2-element spring dashpot model)
is used to represent the rotor internal damping. A finite element code is written using MATLAB
to find out the eigenvalues and eigenvectors. Eigenvalues are calculated and the imaginary parts,
which show the natural frequencies of the system, are used to plot the Campbell diagram. Decay
rate is plotted by using the maximum value of the real part of all eigenvalues with the spin speed.
Eigen values are also used to study the effect of modal damping factor on the spin speed. From
these plots, the stability of the system can be studied and the stability limit of spin speed (SLS)
can be found out. Using the eigenvectors the mode shapes of the rotor are found out. Further this
code is extended to get the dFRF plot, from which the directivity of the modes can be found out.
Based on that review, the objective and scope proposed in this work are as follows.
1. Finding out the equations of motion and Development Finite Element formulation of the
viscoelastic rotor. Finite element method is used to discritize the rotor continuum.
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Study of modal behaviour of viscoelastic rotors using finite element method
2. Dynamic behaviour of a mild steel rotor is predicted through viscoelastic modeling of the
continuum to take into account the effect of internal material damping. Stability limit of
spin speed, effect of modal damping factor on the spin speed are studied. The mode
shapes are found out using the eigenvectors. Further dFRF plot is obtained by using the
eigenvectors in Mat Lab.
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Study of modal behaviour of viscoelastic rotors using finite element method
CHAPTER TWO
Viscoelastic Rotor and Its Modeling
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Study of modal behaviour of viscoelastic rotors using finite element method
This chapter forms the basis of the entire work as it presents the derivation of the
equations of motion of a viscoelastic rotor and Development Finite Element analysis procedure.
The equation of motion is further used to obtain the eigenvalue and eigenvector for various kind
of modal analysis.
Modal analysis is a useful tool to get an idea about the dynamic behaviour of a system
and hence is considered to be important for dynamic design. One of the characteristics of rotating
machinery is that they generally do not abide by the principle of reciprocity and are referred to as
„Non Self Adjoint Systems‟ (NSA). This property results in two sets of eigenvectors, referred to
as the „left hand‟ and „right hand‟ eigenvectors. The right hand eigenvectors are those generally
associated with the mode shapes, while the left hand eigenvectors are the set, which are obtained
by analysing the transformed equations and indicate the pattern of forces associated with a single
mode.
2.1
EQUATION OF MOTION:
In this section the mathematical modeling of viscoelastic rotor shaft is represented in time
domain. The finite element model of the viscoelastic rotor shaft system is based on the EulerBernoulli beam theory. The equation of motion is obtained from the constitutive relation where
the damped shaft element is assumed to behave as Voigt model i.e., combination of a spring and
dashpot in parallel.
Figure 2.1 shows the displaced position of the shaft cross section.  v, w Indicate the
displacement of the shaft Centre along Y and Z direction and an element of differential radial
thickness dr at a distance r (where r varies from 0 to ro ) subtending an angle d (t ) where  is
the spin speed in rad/sec and t varies from 0 to 2 at any instant of time ' t ' . Due to transverse
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vibration the shaft is under two types of rotation simultaneously, i.e., spin and whirl.
 is
the
whirl speed.
Figure 2.1 Displaced Position of the Shaft Cross Section
The dynamic longitudinal stress and strain induced in the infinitesimal area are  x and
 x respectively. The expression of  x and  x at an instant of time are given as Zorzi and Nelson
[9].


 x  E  v ;   r cos   t 
x
 2 R( x,t )
x 2
1
Where, E is the Young‟s modulus, v is viscous damping coefficient.
Following Zorzi andNelson [9] the bending moments at any instant of time about the y
and z-axes are expressed as:
2 ro
M     v  r cos  t   x rdrd (t )
zz
0 0
2 ro
M     w r sin   t   x rdrd (t )
yy
0 0
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 2
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Study of modal behaviour of viscoelastic rotors using finite element method
After utilizing equation (1) in equation (2) and following Zorzi and Nelson [9], the
governing differential equation for one shaft element is given as:

.. 
 M    M  q     K  G 
 T   R
v B
 



 . 
q    K B  v  KC 
 


 
q   B
 
 3
,  M  , G 88 ,  K B  and  KC  are the
In the preceding equation  MT 
88  R  88
88
88
translational mass matrix, rotary inertia matrix, gyroscopic matrix, bending stiffness matrix and
skew symmetric circulatory matrix, respectively. The expressions for those matrices are given
below.
l
 M     A  x   x T dx,
 T
0
l
 M     I  x   x T dx,
 R
0
l
0
0

G   2 I  x  1
 3a 
1
  x T dx,

0
l
T
 K    EI   x 
  x  dx,
 B
0
l
 0 1
 K    EI  ( x)
 ( x)T dx

 C
 1 0
0
Where, ρ is the mass density, ' I ' is the area moment of inertia,  I   y 2 dA  .

A

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


The Hermits shape function matrix,   x  , is given by   x   xy  x 

 

0


,
zx  x  

0


Where subscripts in the elements show the respective planes
The equation of motion for whole system is obtained by assembling the element matrix to
global matrix and it is rewritten as:
 M q  C q   K q  B
 4
Where  M  , C  and  K  are the global mass, damping and stiffness matrices, respectively and
is the external force applied. Their expressions are written as:
 M    MT  M R ;C   v  KB    G; K    K B   v  KC 
The disc mass is incorporated with the global mass matrix at appropriate node. The
global damping matrix contains the gyroscopic effects of shaft and disc, and effects of rotating
and non rotating damping.
Equation (4) once again is appended by an identity equation to constitute the states space
equation.
 A X    B X   P
 5
Where,
C
 A   M

M
,
0 
K
 B   0

0
0 
, X    ,

M 
q
 0 

P  


B






 6
Free vibration Equation of equation (5) is an eigenvalue problem and can be written by
assuming, u  et  y .
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  A X    B X   0
7
Where,  is system‟s complex eigenvalue, for which the stability can be predicted from the real
part and the imaginary part indicates the natural frequency.
2.2 FREE VIBRATIONAL ANALYSIS:
Free vibration analysis of the system of equations in (4), gives eigenvalues r , right
eigenvector ur and left eigenvector vr in real space and are connected by equations given in (8);
the subscript „r‟ denotes mode number.
(r 2M  r C  K )ur  0 and (r 2 M T  r CT  K T )vr  0
 8
Wherer=1to 4(N+1)
Where N is the number of elements
The rth eigenvalue problem associated with equation (5) may be written after following
Lee [3] as:
r A r  B r and r AT r  BTr
9
Where r=1 to 8(N+1), r is rth eigenvalue,  r and  r stand for the rth right hand and left hand
eigenvectors, respectively in state space and are given by –
 u 
 v 
 r   r r  , r   r r 
 ur 
 vr 
10 
 r And  r may be biorthonormalized so as to satisfy
iT A r  ir : iT B r   r ir
11
Wherei,r = 1 to 8(N+1), and  ir ,stands for the kronecker delta(i.e.= 1, when i = r and = 0, when
i  r)
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For r = 1 to 8(N+1),  r  r and r may be written in compactform as  , Φ and  respectively
and are given by
Ψ  ψ1 ψ2ψ8(N 1)  ,
 , ,   diag   

Φ   φ φ φ
8( N  1) 
8(N  1) 
 1 2
 1 2
12 
13
T A  I ; T B  
In equation (13) the symbol, „I‟ represents identity matrix of size 8(N+1) × 8(N+1).
2.3
DEVELOPMENT
OF
FREQUENCY
RESPONSE
FUNCTION
MATRIX:
Assuming a solution of X (t ) , the state vector in equation (5), as a linear combination of
the right hand eigenvectors ψ r multiplied by modal co-ordinates  r (t ) ,as given in equation (14):
8( N 1)
X (t )    rr (t )   (t )
r 1
14 
Substituting it into equation (5), pre multiplying throughout by T and incorporating the
bio-orthogonality conditions, the equation (15) is obtained.
.
 A  (t )  T B (t )  T F (t )
T
15
Using orthonormality relations of equation (13), the equation (15) may be transformed, as
shown below, into equation (16).
.
 (t )   (t )  n(t )
16 
Where, n(t)=T F(t) ,represents the modal excitation vector.
Equation (16) represents a set of independent modal equations, and may be written as in equation
(17):
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.
17 
 r (t )  rr (t )  nr (t )
r=1 to 8(N+1)
Where, nr (t )  r T F (t ) , is the generalized time varying forcing function exciting the r th mode.
Steady state response, of equation (17) under harmonic force F(t) of frequency „ω‟ is
given by:
r (t ) 
r T F (t )
j -r
Where
' j'
18
is the imaginary unit, i.e.
j
1 .
Substituting equation (18) into equation (14) gives
8( N 1) 
X (t )  
r 1
r r
T
j -r
19 
F (t )
Therefore, Frequency Response Function matrix (H) in state space may be written as:
H 
8( N 1)   T 4( N 1)   T 4( N 1)   T
r r 
r r  
r r


j -r
r 1
r 1 j -r
r 1 j -r
 20 
Where over bar   represents complex conjugate of a vector or scalar.
By using the relationship of  and  with
u
and
v
from equation (10), the Frequency
Response Function matrix  H  relating generalized displacement may be written as:
T
r ur r vr 
H 
8( N 1)  u  v 
  r  r 
j r
r 1
 r 2ur vr T r 2ur vr T

8( N 1)   2u v T
r 2ur vr T
   r r r
j -r
r 1




 21
Thus Frequency-displacement response functions (H), further referred to as Frequency
ResponseFunctions may be expressed as:
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H
 Hyy

 Hzy
Hyz 

Hzz 
 22
After following Lee [3], the directional Frequency Response Function (dFRF) and
reverse directional Frequency Response Function(r-FRF) can be written as:
2 H pg  H yy  H zz - i ( H yz - H zy );
2 H pgˆ  H yy  H zz  i ( H yz  H zy );
 23
2 H pg
ˆ  H yy - H zz - i ( H yz  H zy );
2 H pg
ˆ ˆ  H yy  H zz  i ( H yz - H zy );
Here
H pg
ˆ
H pg and H pg
ˆˆ
are directional Frequency Response Function(dFRF)matrix and
H pgˆ ,
are reverse directional Frequency Response Function(r-FRF)matrix (Lee[3]).Where p and g
represent the complex linear displacement and complex force at any particular node and are
given by equation:
p(t )  y(t )  jz (t ) and g (t )  f y (t )  jf z (t )
 24
p̂ and ĝ represent the complex conjugate of p and g respectively. Therefore H pg ( H pg
ˆ ˆ )is
defined as the frequency response of the rotor in the forward(backward)direction to rotating
excitation in the forward (backward)direction, and is termed as normal dFRF, whereas H pgˆ (
H p̂g )represents the frequency response of the rotor in the forward(backward)direction to
rotating excitation in the backward(forward)direction and is termed as the reverse dFRF(Kessler
and Kim[14].Following Lee we can write
H pg
ˆ ˆ (- j )
ˆ ( j )  H pgˆ (- j ) and H pg ( j )  H pg
 25
Using the Complex Response Functions the anisotropy/asymmetry and cracks in rotating
machinery can be detected.
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CHAPTER THREE
Results and Discussion
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This chapter includes the various numerical results based on the modal characteristic of a
simply supported viscoelastic rotor shaft system. The different modal study comprises the decay
rate plot, modal damping ratio, mode shape and DFRF. A finite element code is written in
MATLAB for numerical simulation of the theoretical work.
3.1
THE ROTOR-SHAFT SYSTEM
The schematic diagram for one such shaft system is shown in figure 3.1. A rotating shaft
system having three discs, with simply supported ends is considered. Table 1 shows the
dimension and material properties of the steel rotor. The dimensions of these three discs are
shown in Table 2.
Figure 3.1 Schematic Diagram of the Rotor
Material
Mild Steel
Density
Young’s
(kg/m3)
Modulus (GPa)
7800
200
Length(m)
Diameter(m)
Damping Coefficient
(N-s/m)
1.3
0.2
0.0002
Table 1 Rotor Material and its Properties
Disc
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Thickness(m)
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1
0.24
0.05
2
0.40
0.05
3
0.40
0.06
Table 2 Disc parameters for 3 disc rotor
3.2
STABILITY LIMIT OF SPIN SPEED:
3.2.1.
DECAY RATE PLOT
Figure 3.2shows the decay rate plot (i.e. the variation of maximum real part with the spin
speed) of two consecutive modes. After a certain speed the maximum real part line cuts the zero
line, the system becomes unstable and corresponding speed is called stability limit of spin speed
(SLS). From the graph SLS is 4353rpm. First mode cuts the zero line before the second mode,
this implies that the fist mode becomes unstable .If real part is negative, the amplitude decays in
time then the rotor has a stable behavior because the whirl motion tends to reduce its amplitude.
If real part is positive, the amplitude grows exponentially, the motion is unstable, as any small
perturbation can trigger this self-excited whirling.
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Figure 3.2 Variation of Maximum real Part with the spin speed
3.2.2.
MODAL DAMPING FACTOR
Figure 3.3 shows the variation of modal damping factor with the rotor spin speed. In case
of 1F and 2F modes, the modal Damping factor decreases as the spin speed increases. Whereas
the modal damping factor increases in case of 1B and 2B mode.Positive modal damping factor
indicates stability as vibratory energy is dissipated and negative modal damping signifies
instability as rotary energy supports rotor whirl by adding energy.This shows that forward mode
tend to destabilize due to internal material damping and backward mode does not have any effect
on instability.Like decay rate plot, the SLS can be obtained from this plot, till which none of the
modal damping factors is negative. From the graph, it can be seen that the 1F mode becomes
unstable at the 1st critical speed (Ωcr) of 4349 rpm.
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Figure 3.3 Variation of modal damping factor with the spin speed
3.3
CAMPBELL DIAGRAM
Figure 3.4 Shows the Campbell diagram of the rotor-shaft system, when the shaftsinternal
material damping is considered. The graph is plotted by using the whirl frequencies (obtained
from the imaginary part of the eigenvalues). There are 2 forward-whirling modes, marked
sequentially in the ascending order of frequency, by „FW(1 st mode) and FW(2nd mode) in which
the rotor whirls in the direction of the spin and there are two backward whirling mode „BW(1 st
mode)and BW(2nd mode), in which the rotor whirls opposite to the direction of spin. The
Synchronous Whirl Line is marked as „SWL‟. The speed corresponding to the first point of
intersection of „SWL‟ with the Campbell diagram is the critical speed shown as „Ωcr‟ in Figure
3.4. For this example Ωcr(1st FW)=4348 rpm and Ωcr(2nd FW)=15990 rpm .Hence the resonance
of the system can be found out using the Campbell diagram.
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Figure 3.4Campbell Diagram
3.4 THREE DIMENSIONAL MODE SHAPES OF SIMPLY SUPPORTED
ROTOR
The mode shapes of a rotating shaft indicate the locus of any point of the shaft during
whirling motion. First backward and forward Mode shapes of the simply supported rotor are
plotted using the eigenvectors. Figure 3.5 and 3.6 shows the first three dimensional mode shape
for undamped rotor v  0  and the figure 3.7 and 3.8 show the same plot for damped rotor
(considering internal damping i.e. ( v  0 ). „*‟ indicates the starting point of the whirl. In the
backward mode the whirl lines rotate clockwise direction whereas forward whirl lines rotate
counter clockwise direction. The mode shape is symmetric about y-axis and z-axis for undamped
rotor, but it is not maintained for damped rotor. It is due to the incorporation of skew symmetric
circulatory matrix in the system equation.
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Study of modal behaviour of viscoelastic rotors using finite element method
Figure 3.5 1st Backward ( v =0)
Figure 3.71st Backward ( v  0 )
Figure 3.61st Forward ( v =0)
Figure 3.8 1st Forward (v  0 )
3.5 DIRECTIONAL FREQUENCY RESPONSE FUNCTION
Figure 3.9 shows the plot of dFRF ( H pg ) at node 6 of the rotor Shaft system.In this plot
backward modes appear in the negative frequency region and forward modes appear in the
positive frequency region. Therefore we noted that the dFRF plot has the ability to separate the
forward and backward modes, while in FRF plot these modes are mixed, resulting in more
difficulty to the process of parameter estimation. The dFRF has the advantage of separating the
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Study of modal behaviour of viscoelastic rotors using finite element method
modes in the negative and positive excitation frequency region, which otherwise cannot be
identified through classical FRF due to its conjugate even property.
Figure 3.9dFRF ( H pg ) plot, at node 6,( K yy  7e7, K zz  5e7, c yy  0, czz  0,v  0 )
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Study of modal behaviour of viscoelastic rotors using finite element method
CHAPTER FOUR
Conclusionsand scope for future work
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Study of modal behaviour of viscoelastic rotors using finite element method
4.1
CONCLUSIONS
This work investigates the modal analysis of a rotor-shaft system with simply supported
ends, by considering the internal material damping of the Rotor. For this the finite element
method is used for modelling the rotor shaft. The following important conclusions are obtained
at the end.
1. During the forward whirl, damping decreases, as the spin speed increases and in
backward whirl damping increases, as the spin speed increases. Positive value of
damping factor indicates the stability of the system. Using this, the stability of the
system is found out.
2. The critical speed during the forward whirl and backward whirl are found out using a
Campbell diagram. The speed corresponding to the first point of intersection of „SWL‟
with the Campbell diagram is the critical speed. For the stability of the system, the
system must be operated at a speed less than the critical speed.
3. The mode shapes are plotted using the eigenvectors. The mode shapes are symmetric for
undamped rotor where as those are Unsymmetric for damped rotor.
4. The study also included the effect of shaft material damping on directional frequency
response characteristics. As rotor spin speed increases, the contribution of forward modes
in the directional frequency response increases and that of backward modes decreases.
This observation can also be used as an indicator for the presence of shaft material
damping.
Hence the Dynamic behaviour of the system is identified by performing themodal
analysis and which helps in the dynamic design of rotors. It can be concluded that the
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Study of modal behaviour of viscoelastic rotors using finite element method
complex modal analysis is a tool to get an idea about the dynamic behaviour of the
system.
4.2
SCOPE FOR FUTURE WORK:
This study has given birth to different other possibilities which may be taken up as future
research activities in this area.
1.
Sometimes the equations of motion of rotating shaft consist of higher order model. The
number of order depends on the nature of material. The study of complex modal
behaviour of a second order system is little straight forward. The present procedure can
easily be extended for obtaining the dynamic behaviour of viscoelastic rotor having a
higher order system.
2.
Obtaining the modes for the higher order system is cumbersome and complicated as
compare to general second order system. Because the eigen vector comprises the
displacement function as well as their higher order derivatives. Thus the present
complex modal analysis technique can be used to get the equivalent reduced form of
the higher order model.
3.
Using the Complex Frequency Response Functions the anisotropy, asymmetry and
cracks in rotating machinery can be detected. The complex modal analysis of complete
bladed disc will also help to detect the instant failure of any disc during operation.
4.
The modal analysis of non axy symmetric rotor is not available in the literature. To get
an insight idea of the dynamic characteristics of the non axy - symmetric rotor the
complex modal analysis is very important.
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REFERENCES:
1) Jeffcott H., 1919, “The Lateral Vibration of Loaded Shaft in Neighbourhood of a
Whirling Speed - The Effect of Want of balance”, Phil. Mag., vol. 37(6), pp. 304-314.
2) Rao J. S., 1996, ―Rotor Dynamics, New Age International Publishers.
3) Lee C.W., 1991, “Complex Modal Testing Theory for Rotating Machinery”, Mechanical
System and Signal Processing, vol. 5(2), pp.119-137.
4) Mesquita, M. D., Miranda U., 2002,”A Comparison between the traditional Frequency
Response Function(FRF) and the directional Frequency Response Function (dFRF) in
rotor dynamic analysis”, First South American congress on computational mechanics,
MECOM, Santa fe-parana, Argentina, pp. 2227–2246.
5) Dutt J.K. and Nakra B.C., “Stability of Rotor Systems with Viscoelastic Supports”,
Journal of Sound and Vibration”, vol. 153 (1), pp. 89-96.
6) Williams M.L., “Structural Analysis of Viscoelastic Materials”, AIAA journal, 1963.
7) He and Fu,”Modal Analysis”,A member of the Reed Elsevier group, 2001.
8) Nelson H.D. and McVaugh J.N.,1976, “ The Dynamics of Rotor-Bearing System using
Finite Elements”, Journal of Engineering for Industry, vol. 98, pp. 593-600.
9) Zorzi E.S. and Nelson H.D., 1977, “Finite Element Simulation of Rotor-Bearing Systems
with Internal Damping”, Journal of Engineering for Power, Transactions of the ASME,
vol. 99, pp. 71-76.
10) Ku D. M., 1998,”Finite element analysis of whirl speeds for rotor-bearing systems with
internal Damping”, Mechanical Systems and Signal Processing, Vol. 12, pp. 599–610.
11) Chouksey M., Dutt J. K., Modak S.V., 2012,” Modal analysis of rotor-shaft system under
the influence of rotor-shaft material damping and fluid film forces”, Mechanism and
Machine Theory,vol.48, pp. 81–93.
12) Laszlo Forrai, 2000, “A finite element model for stability analysis of symmetrical rotor
systems with internal damping”, Journal of computational and applied mechanics, vol.1,
pp. 37-47.
13) Jei Y.G. and Lee C.W., 1988, “Finite element model of asymmetrical rotor-bearing
systems”, KSME Journal. Vol. 2, No.2, pp. 116-124.
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Study of modal behaviour of viscoelastic rotors using finite element method
14) Agostini. C.E., Capello E.A, 2010,”Complex modal analysis of a vertical rotor by finite
elements method”, DINCON‟10,9th Brazilian Conference on Dynamics, Control and
their Applications.
15) Kessler C., Kim J., 2002,” Vibration analysis of rotors utilizing implicit directional
information of complex variable descriptions", J. Vibr. Acoustics. Trans. ASME, vol.124,
pp.340–349.
16) Kessler C. and Kim J., 1998, “Complex Modal Analysis and Interpretation for Rotating
Machinery”, Proceedings of the 16th Int. Modal Analysis Conf., pp.782-787.
17) Jei Y.G., Kim Y.J., 1993,”Modal Testing Theory of Rotor-Bearing Systems”, Journal of
Vibration and Acoustics. Trans. ASME, vol.115 pp. 165–176.
18) Genta,”Dyanmics of Rotating Systems”, Mechanical Engineering Series, Frederick F.
LingSeries Editor.
19) Kim D., Lee C. W., 1986,” Finite element analysis of rotor bearing systems using a
modal transformation matrix”,Journal of Sound and Vibration, pp.441-456.
20) Lee C.W., Kye-Si Kwon, Si-Kyoung Kim, 2001, “modal testing of rotors with rotating
asymmetry”, Proceedings of VETOMAC-I.
21) Lee C.W., kwon K.S. ,2001,”Identification of rotating asymmetry in rotating machines by
using reverse directional frequency response functions”, journal of Mechanical
Engineering Science, pp.215.
22) Dutt J.K., “Unbalance response and stability analysis of horizontal rotor systems mounted
on nonlinear rolling element bearings with viscoelastic supports”, Journal of vibration
and acoustics, vol.119 (4), pp.4-9.
23) Friswell M.I., Sawicki J.T.,Inman D.J., Lees A.W., “The Response of Rotating Machines
on Viscoelastic Supports.”, International Review of Mechanical Engineering, vol.1(1),
pp.32-40.
24) Crandall S.H., Brosens P. J.,1961,”On the Stability of Rotation of a Rotor with
Rotationally Unsymmetric Inertia and Stiffness Properties”, Journal of applied mechanics
,Vol. 28(4), pp. 567-571.
25) Lee C.W. AND Jei Y.G., 1992,”Modal Analysis of continuous Asymmetrical RotorBearing systems”, Journal of Sound and Vibration, vol.152 (2), pp. 245-262.
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Study of modal behaviour of viscoelastic rotors using finite element method
26) Suh J,H., Hong S.W. ,Lee C.W.,2005,” Modal analysis of asymmetric rotor system with
isotropic stator using modulated coordinates”, Journal of Sound and Vibration ,vol.284
,pp. 651–671.
27) Lee C.W., Han D.J., Suh J.H., 2007,” Modal analysis of periodically time-varying linear
rotor systems”, Journal of Sound and Vibration, vol.303,pp. 553–574.
28) Das A.S., Nikhil M.C., Dutt J.K., Irretier H., 2008,” Vibration control and stability
analysis of rotor-shaft system with electromagnetic exciters”, Mechanism and Machine
Theory, vol. 43, pp.1295–1316.
29) Khulief Y.A., Mohiuddin M.A., 1977,”On the dynamic analysis of rotors using modal
reduction”, Finite Elements in Analysis and Design, vol. 26, pp. 41-55.
30) Jei Y.G. and Lee C.W., 1988, “Finite element model of asymmetrical rotor-bearing
systems”, KSME Journal. Vol. (2), pp. 116-124.
31) Jei Y.G. and Lee C.W., 1990,”curve veering in the eigenvalue problem of rotor-bearing
systems”, KSME Journal, Vol. 4(2), pp.128-135.
32) Eman K. F., Kim K.J., 2001,” modal analysis of machine tool Structures based on
experimental data”.
LIST OF PUBLICATIONS:
 P. Mutalikdesai, S. Chandraker, H. Roy, 2013, “Modal Analysis of Damped Rotor using
Finite Element Method” Proceedings of (AMAAS), March 01-02, NIT Rourkela, India.
 S. Chandraker, P. Mutalikdesai, H. Roy, 2013, “Complex modal analysis of damped
rotor using finite element method” Proceedings of (ICAME), May 29-31, COEP Pune,
Maharashtra, India, Paper ID: S16/P4.
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