A Modeling Study of the Sensitivity of Natural
Frequency of Vibration to Geometric Variations
in a Turbine Blade
by
Daniel A. Snyder
A Project Submitted to the Graduate
Faculty of Rensselaer Polytechnic Institute
in Partial Fulfillment of the
Requirements for the degree of
MASTER OF ENGINEERING in MECHANICAL ENGINEERING
Approved:
_________________________________________
Ernesto Gutierrez-Miravete, Project Adviser
Rensselaer Polytechnic Institute
Hartford, CT
December 2010
(For Graduation August 2011)
© Copyright 2010
by
Daniel A. Snyder
All Rights Reserved
ii
Contents
Contents ............................................................................................................................ iii List of Tables ..................................................................................................................... v List of Figures................................................................................................................... vi Acknowledgement ........................................................................................................... vii Abstract........................................................................................................................... viii 1. Introduction and Background ...................................................................................... 1 1.1 High Cycle Fatigue ............................................................................................. 1 1.1.1 Zero-Mean Stresses................................................................................ 1 1.1.2 Non-zero Mean Stresses......................................................................... 2 1.1.3 Goodman Diagram ................................................................................. 2 1.2 Vibratory Response ............................................................................................ 3 1.2.1 Design Philosophy ................................................................................. 3 1.2.2 Frequency Prediction ............................................................................. 4 1.2.3 Campbell Diagram ................................................................................. 4 1.3 The Problem........................................................................................................ 6 1.3.1 Defining a Design Space........................................................................ 6 1.4 Previous Work on this Topic .............................................................................. 7 2. Theory and Methodology ............................................................................................ 9 2.1 Monte Carlo Simulation ..................................................................................... 9 2.1.1 Geometric Parameter Scheme ................................................................ 9 2.1.2 Generating Random Variable Combinations ....................................... 13 2.1.3 Explanation of Parameter Scheme ....................................................... 14 2.2 Modal Analysis................................................................................................. 18 2.2.1 Finite Element Implementation............................................................ 19 2.2.2 Execution of Computations.................................................................. 20 3. Results and Discussion .............................................................................................. 21 iii
3.1 Mode Shape Identification................................................................................ 21 3.1.1 Frequency Results ................................................................................ 21 3.2 General Trends in Response ............................................................................. 22 3.2.1 First Mode Shape ................................................................................. 22 3.2.2 Second Mode Shape............................................................................. 24 3.2.3 Third Mode Shape................................................................................ 25 3.2.4 Fourth Mode Shape .............................................................................. 26 3.2.5 Fifth Mode Shape................................................................................. 27 3.3 Regression Functions........................................................................................ 28 3.3.1 Linear Regression ................................................................................ 28 3.3.2 Pareto Analysis .................................................................................... 28 4. Conclusion ................................................................................................................. 39 4.1 Design Spaces and Parameter Schema ............................................................. 39 4.2 Future Work...................................................................................................... 39 5. References.................................................................................................................. 40 6. Appendices ................................................................................................................ 41 6.1 Appendix 1: Turbine Blade Parametric Model................................................. 41 iv
List of Tables
Table 1: Geometric Parameters and their Ranges............................................................ 14 Table 2: Statistical Data for Frequency Results .............................................................. 21 v
List of Figures
Figure 1: Goodman Diagram ............................................................................................. 2 Figure 2: Campbell Diagram ............................................................................................. 5 Figure 3: Airfoil Section Definition .................................................................................. 9 Figure 4: Airfoil Section Interpolating Splines ............................................................... 10 Figure 5: Parametric Model of Fictitious Turbine Blade................................................. 11 Figure 6: Interlocking Shrouds of Adjacent Blades ........................................................ 12 Figure 7: Airfoil Section Chord Illustration .................................................................... 15 Figure 8: Airfoil Parameters related to Stiffness ............................................................. 16 Figure 9: Mateface Gap ................................................................................................... 17 Figure 10: Mode Shape 1 at 268 Hz ................................................................................ 23 Figure 11: Mode Shape 2 at 827 Hz ................................................................................ 24 Figure 12: Mode Shape 3 at 1446 Hz .............................................................................. 25 Figure 13: Mode Shape 4 at 1705 Hz .............................................................................. 26 Figure 14: Mode Shape 5 at 4255 Hz .............................................................................. 27 Figure 15: Linear Response Coefficients for First Frequency......................................... 30 Figure 16: Linear Response Coefficients for Second Frequency .................................... 32 Figure 17: Linear Response Coefficients for Third Frequency ....................................... 34 Figure 18: Linear Response Coefficients for Fourth Frequency ..................................... 36 Figure 19: Linear Response Coefficients for Fifth Frequency ........................................ 38 Figure 20: Top View of Fictitious Parametric Turbine Blade Model ............................. 41 Figure 21: Generic Tip Shroud Design............................................................................ 41 Figure 22: View of Fictitious Turbine Blade Root Attachment Geometry ..................... 42 Figure 23: Close-up View of Turbine Airfoil Section Definition ................................... 42 Figure 24: Example of Random Turbine Blade – 1......................................................... 43 Figure 25: Example of Random Turbine Blade – 2......................................................... 44 Figure 26: Example of Random Turbine Blade – 3......................................................... 45 Figure 27: Example of Random Turbine Blade – 4......................................................... 46 Figure 28: Example of Random Turbine Blade – 5......................................................... 47 vi
Acknowledgement
Thanks to Jeff Beattie for help with ANSYS and for giving me the concept for this
project. Thanks to Grant Reinman for consultation on statistical methods such as partial
least squares regression and principal component analysis. Thanks to Pratt & Whitney
for providing me with an environment that fosters research and new ideas.
vii
Abstract
A combined Monte Carlo – Finite Element simulation was used to investigate the
effect of geometric variation in a turbine blade on its natural frequencies of vibration.
First, a parameter scheme is selected and implemented in a parametric 3D solid model.
The parameters chosen are those that affect section properties of the airfoil at three
different sections throughout the airfoil.
Other parameters control other geometric
features in the turbine blade. A total of 37 geometric parameters are independently
varied to test the response of the first 5 natural frequencies of the turbine blade model.
A random set of values for the parameters is generated using a Latin Hypercube function
implemented in Matlab.
The impact of each geometric parameter on each natural
frequency is quantified by calculating a linear regression.
The linear regression
function approximates each natural frequency as a linear function of each independent
variable (geometric parameter).
This result shows that the fundamental mode of
vibration is affected mainly by the moment of inertia of the blade about the fixity point.
All parameters that tend to increase the moment of inertia of the blade tend to decrease
the first natural frequency. The importance of this type of study is to explore and
understand the implications of varying parameters and combinations of parameters in the
design space. Further design studies like this could allow the design engineer to tune
certain frequencies out of undesirable ranges while leaving other modes frequencies
unchanged.
viii
1. Introduction and Background
This project deals with prediction of the effects of manufacturing/geometry
variation on turbine blade vibration. It is important to prevent resonant vibration of
turbine blades during operating conditions. Turbine blades experience unsteady forcing
functions at many different frequencies. If the blade experiences an excitation frequency
equal to its natural frequency, the blade will usually fail very quickly in a failure mode
known as high-cycle fatigue. This fatigue mode is characterized by relatively low
fluctuation in stress and very high frequency of fluctuation. This failure mode embrittles
the material and causes it to crack in regions of high steady stress. Characterization of
materials in this failure mode gives rise to the Goodman Diagram (see Figure 1).
1.1 High Cycle Fatigue
High cycle fatigue is the failure mode of a material experiencing fluctuating stress.
Compared to yielding or rupture failure in which stress is said to be steady or
unchanging, fluctuating stresses cause materials to fail by becoming brittle and cracking.
The characterization of materials in this failure mode involves testing materials subject
to alternating stresses of different means and different amplitudes. In an alternating
stress condition, the mean stress is defined as the average stress over a long period of
time. For stresses whose variation is sinusoidal over time, the mean stress is simply the
average of the maximum and minimum stress.
1.1.1 Zero-Mean Stresses
Testing materials with an alternating stress that has a zero mean allows the engineer
to measure how many cycles before the material fails. Typically, these data are plotted
on a chart with number of cycles on the horizontal axis and stress amplitude on the
vertical axis. The number of cycles is typically on a logarithmic scale for plotting
purposes since number of cycles to failure can become exponentially larger for low
stress levels. Some materials, such as steels, have a stress level below which fatigue
failure will never occur – this is called the endurance limit.
1
1.1.2 Non-zero Mean Stresses
In cases when the temporal average of an alternating stress is not zero, the fatigue
life is reduced compared to the same amplitude of alternating stress with a zero mean.
When data are collected from material tests in which the mean stress is not zero, the data
are plotted on a two dimensional plane with level curves of fatigue life. The plane of
interest shows the average, or steady stress on the horizontal axis and the alternating
stress amplitude on the vertical axis. There are several methods of data fitting a curve of
constant cyclic life. The most common is called the Goodman Line.
Figure 1: Goodman Diagram
1.1.3 Goodman Diagram
In the Goodman Diagram a line is plotted on the stress plane [4]. The line delimits
the state of stress below which the material will not fail (assuming a certain probabilistic
certainty). The Goodman line plotted on the x-y plane of mean stress and alternating
stress is described by the equation:
2
⎡ σ ⎤
σa = σe ⎢1 − m ⎥
⎣ σu ⎦
[1]
The subscript “a” denotes alternating. The subscript “e” denotes endurance or the
fatigue limit of a material for a given number of cycles. The subscript “m” denotes
€
mean stress or the constant component of the alternating stress signal. The subscript “u”
denotes ultimate tensile stress in uniaxial tension. The material in a vibrating object may
experience both a constant and alternating component of stress.
Therefore,
understanding material response to alternating stress with nonzero mean is required
when designing mechanical components that experience vibration. The next section
deals with vibratory response of turbine blades.
1.2 Vibratory Response
Turbine blades have curved surfaces used to redirect the flow of fluid in order to
extract work from it. Jet engines use turbine blades, positioned behind turbine vanes, to
extract work from the flow of very hot combustion products of fuel and air. The turbine
vanes are static and act as nozzles to direct the flow toward the blades at the proper
direction and flow rate. When the blades see the flow, its intensity fluctuates as is goes
between nozzles and wakes. In general, the frequency of excitation is related to the
rotational speed of the turbine. The excitation frequency is also related to multiplicative
factors related to number of disturbances in the airflow around the turbine. For example
such factors can be: number of upstream vanes (nozzles) adjacent to the blade row,
number of downstream vanes adjacent to the blade row, difference between number of
upstream and downstream vanes, number of fuel nozzles in the combustor, and many
other geometric features.
1.2.1 Design Philosophy
In the design of a turbine blade, the design engineer tries to minimize the number of
times when the blade will experience an excitation frequency equal to one of its natural
frequencies at a given running condition. One can never prevent all resonant excitations
but can try to place them at engine operating conditions that are not used for long
periods of time. The major operating conditions to at which the design engineer must
3
avoid resonance are idle, take-off, climb, and cruise. Those operating conditions may all
be associated with different engine operating speeds, each of which creates the potential
to excite a resonant mode of the turbine blades.
1.2.2 Frequency Prediction
The typical method of predicting natural frequency of a given blade design is to use
Finite Element Analysis. The natural frequencies and mode shapes (eigenvalues and
eigenvectors) can be numerically approximated and used in the design iterations to
predict and prevent resonant excitations. In this type of finite element analysis, the
equation to be solved is that of 3-dimensional vibration [6]. It is multidimensional
extension of the one dimensional vibration equation. The equation is of the form:
mx˙˙ + kx = 0
[2]
Where in the case of a single degree of freedom system, m is mass, k is stiffness,
and x is scalar displacement. In the case of multiple degrees of freedom, x is a vector of
€
displacements while k is a matrix of stiffness (between every mass). This topic is
continued in section 2.2, Modal Analysis.
1.2.3 Campbell Diagram
Using modal analyses at several operating conditions (with different temperatures
and rotation speeds) the engineer can produce a Campbell Diagram, Figure 2. The
Campbell diagram simply plots natural frequencies versus engine operating speed. The
horizontal axis shows the engine speed (in Rev/min). The vertical axis shows the modal
frequencies (in Hertz).
The Campbell Diagram shown uses fabricated data for
illustrative purposes. The horizontal, drooping lines are the natural frequencies of the
blade. The straight lines of various integer slopes are the excitation frequencies caused
at certain engine speeds. The slope of the line is called the Engine Order. This is an
integer factor equal to a rotational symmetry or repetition found somewhere in the path
of the turbine blades such as the number of nozzles before the blade or the number fuel
nozzles in the combustion chamber. Any repeated feature that the blade will see in its
travel around the engine represents a potential for resonant excitation. If an engine order
line crosses a natural frequency line at an operating speed it is said to be a “resonant
4
crossing.” This is a visualization tool that allows the engineer to easily see which modes
of vibration will be excited at which engine speeds. The following diagram shows an
example of a Campbell Diagram with 7 natural frequencies and 6 engine orders of
concern. A crossing between the 22E driver and 4th mode has been identified with a red
circle at the second engine operating condition.
Figure 2: Campbell Diagram
Again, the horizontal axis is the engine rotational speed (in RPM) and the vertical
axis is the vibration frequency in Hertz. The conversion from RPM to Hertz is 1/60
(revolutions per minute to cycles per second).
The natural frequency lines are not perfectly horizontal. Usually the lines are not
perfectly horizontal because natural frequency will vary depending on engine speed and
temperature. Although engine speed and temperature are not directly related, they are
often highly correlated. For the purposes of the Campbell Diagram, we assume a
constant known temperature for each operating condition. For turbine blades, the natural
frequency lines usually droop with higher engine speed because thermal softening
effects overtake stress-stiffening effects. For fan blades, the lines of natural frequency
5
typically increase slightly with respect to engine operating speeds because temperature
increase is small and stress stiffening effects overtake.
1.3 The Problem
The difficulty is that when a resonant crossing is predicted, it is up to the intuition of
the engineer to know what geometric properties of the blade to change in order to affect
the natural frequency desirably.
As an added complication, changing one natural
frequency desirably may adversely affect another natural frequency.
Without a
comprehensive analysis of the entire design space, one can never fully understand the
practical limitations of tuning turbine blade airfoils.
1.3.1 Defining a Design Space
The question posed in this study is: can one accurately and quantitatively
characterize the effect each geometric parameter, or combination thereof, has on natural
frequencies, or combinations thereof? In order to do this, one first needs to define the
design space. The design space is comprised of defined, controllable parameters that
affect the shape of the turbine blade. Some simple examples are: height, thickness,
aspect ratio, etc.
Every design feature in the turbine blade can have a numerical
parameter associated with it. In order to fully understand the design space, the engineer
must devise a way to test every region of the design space equally.
For a simple two-variable design space, assuming there are absolute maxima and
minima constraining each variable, the design space is rectangular and has four corners.
For higher dimensional design spaces, it is not immediately obvious how to explore the
boundaries and interior regions of the space.
6
1.4 Previous Work on this Topic
In a paper published by J. M. Brown and R. Grandhi, a similar study was performed
on fan blade airfoils (Brown & Grandhi, 2008). In this study, a population of fan blades
was measured using a coordinate measuring machine (CMM). The machine measures
the 3D Cartesian position of a point on the surface of the object given an approach
orientation. The machine can repeat this measurement for many different points around
the airfoil. The data collected was then made to have a zero-mean by subtracting the
mean value from each variable.
The interpretation of this zero-mean data is the
“deviation” from an average airfoil. Zero represents a point being equal to the average
position and positive or negative represents deviation from the average. The variations
to be measured were caused by random manufacturing variation. To study the effect of
this variation on the natural frequency of the airfoils, a large number of realistic sets of
deviation variables were to be generated.
Many of the deviation measurements of the airfoils would be highly covariant. This
is because the airfoil, while deviating from an average population, still remains smooth.
Points adjacent to one another on the airfoil surface had high covariance. The authors of
this paper projected the measured variable space of high covariance into an orthogonal
variable space by means of principal component analysis. This is a statistical technique
that determines orthogonal linear combinations of variables that most highly explain the
variance in the data not explained by precedent variables combinations. The technique
involves simply finding the eigenvectors of the covariance matrix of the dataset. The nth
eigenvector projects a data point in the old variable space into the nth new variable. A
matrix whose rows are the eigenvectors of the covariance matrix forms the
transformation matrix that transforms the old, highly covariant variable space into a new
set of independent (orthogonal) variables. In many cases, the majority of the variation in
the data is explained using a small number of orthogonal variables. The measured data
set may have thousands of dimensions but the majority of the variance can be explained
by a much smaller number of independent dimensions or variables. This is referred to as
“reduced order modeling.” Using this technique, Brown and Grandhi were able to
randomly create realistic combinations of variables that represented plausible airfoils. In
7
this case, plausible means that the deviations were random but the randomly generated
airfoils were still as smooth as the measured ones.
These randomly selected deviations representing realistic airfoils were then input
into a low fidelity finite element analysis to determine the perturbation of the natural
frequency of the airfoil. The result of the study was that the natural frequency of the
airfoils was significantly affected by manufacturing variation.
Brown and Grandhi’s paper illustrates that it is possible to characterize
manufacturing variation and to determine its effect on responses such as natural
frequency.
8
2. Theory and Methodology
2.1 Monte Carlo Simulation
In order to explore the design space affecting the modal response of a turbine blade,
Monte Carlo simulation will be used in this study. In this simulation, many geometric
parameters will be varied randomly to see their independent effect on the desired
response – frequency in this case. Required for this type of analysis is a 3D solid model
of a turbine blade using a certain parameter scheme. A scheme of parameters controlling
the shape of a turbine blade model is not unique. The size and shape of its features could
be defined in many different ways.
2.1.1 Geometric Parameter Scheme
For this analysis, the turbine blade will be constructed between two fixed points in
space representing the inner and outer flow path surfaces. An airfoil will be defined
between these two points using three cross-section curves as seen in Figure 3. There
will be a section at the inner radius, outer radius, and half way in between. Each airfoil
cross-section curve will be defined by its leading-edge and trailing-edge points. Other
parameters defining the airfoil will be its maximum thickness at the middle, section
curvature, leading edge diameter, trailing edge diameter, axial chord length, true chord
length, and several other parameters fully defining the airfoil section.
Figure 3: Airfoil Section Definition
9
Since there will be three airfoil sections, spline surfaces used to connect the sections
into a solid airfoil will be second-degree (quadratic) in the vertical direction as seen in
Figure 4. Using more sections could give extra flexibility to the airfoil but can also lead
to reversals in the airfoil shape. Using three sections allows for a maximum of one
reversal over the whole airfoil. A reversal is when one part of the airfoil reverses
direction on its way up the airfoil. A five-section airfoil could reverse direction four
times. Typically, the interpolation spline degree is limited to 3 making it a natural cubic
interpolating spline.
Figure 4: Airfoil Section Interpolating Splines
10
The following figure shows the final turbine blade.
Figure 5: Parametric Model of Fictitious Turbine Blade
11
2.1.1.1 Blade Root Geometry
The turbine blade will have a root at the bottom and a tip-shroud at the top. The
root will be defined by several parameters, not all of which will need to be varied in this
analysis. The main effect that the root will have on the mode frequencies will be due to
its mass. Its stiffness will not cause very much variation in the frequency. The airfoil
stiffness will be a significant driver of frequency variation.
2.1.1.2 Tip Shrouds
The tip of the airfoil will be attached to a tip-shroud (see Figure 21 and Figure 20).
This is a design feature typically used to reduce endwall losses in a turbine. Airfoils
without shrouds (unshrouded airfoils) exhibit differential motion between the outer gaspath surface and the airfoil tip. A shroud is like an outer gas-path that moves with the
airfoil because it is attached. There is no differential motion between the airfoil and the
endwall so the aerodynamic losses are reduced. The shroud can also be used as a
vibratory friction damper. Each shroud can be made to interlock with adjacent shrouds
and cause frictional damping (see Figure 6).
Figure 6: Interlocking Shrouds of Adjacent Blades
While I will include this design feature in the model, I will not be analyzing the
variation in damping effectiveness because it is beyond the scope of this analysis. In the
vibratory analysis, I will not model any displacement boundary condition at the shroud
12
interface. This vibratory analysis will be simulating the result of a single blade “ping”
test with the blade constrained in a root fixture.
2.1.2 Generating Random Variable Combinations
In order to generate random sets of parameters for the solid models, matlab will be
used to create a latin hypercube design space. Matlab implements this using the function
lhsdesign(N,P). N is the number of samples and P is the number of variables. The
output of the function is uniformly distributed from 0 to 1. Each variable is uncorrelated
to every other variable. While uniformly distributed data may not be as realistic as
normally distributed data, uniform sampling tests all regions of the design space equally.
Such is the goal of Latin Hypercube sampling.
I will define a maximum and minimum value for each parameter I want to vary.
Then I will transform the random numbers to be between the minimum and maximum
for each geometric variable that I have defined using a linear equation.
The
combinations of geometric parameters will be used to create unique 3-D models of
turbine blades representing that point in the design space. The parametric model must be
rigorously tested for robustness given highly variable input parameters. In the case that
a certain variable is causing a high instance of model failure, the parameter range is
reduced. The process of updating the parametric solid model can be done automatically
in Unigraphics NX6.
Many of the parameters in the model control airfoil section properties. Some also
control non-airfoil features such as tip shroud shape and size, root thickness, airfoil
length, and so on. The parameters used in the solid model are shown in Table 1 along
with their mean, minimum, and maximum values.
13
Mean
0
0
0
0
0
0
1.4
1.4
1.15
0.5
0.5
0.5
10
16
0.25
0.25
0.25
0.07
0.07
0.07
0.02
0.75
0.38
0.25
0.25
0.25
17
1.6
0.08
0.2
0.05
0.025
0.025
0.025
1.85
1.85
1.85
airfoil_x_position_mid
airfoil_x_position_root
airfoil_x_position_tip
airfoil_y_position_mid
airfoil_y_position_root
airfoil_y_position_tip
axial_chord_mid
axial_chord_root
axial_chord_tip
cmax_mid
cmax_root
cmax_tip
flowpath_id_rad
flowpath_od_rad
hmax_mid
hmax_root
hmax_tip
led_mid
led_root
led_tip
mateface_gap
neck_length
neck_width
peak_chord_percentage_mid
peak_chord_percentage_root
peak_chord_percentage_tip
platform_angle
rim_width
shroud_bump_height
shroud_bump_length
shroud_thickness
ted_mid
ted_root
ted_tip
true_chord_mid
true_chord_root
true_chord_tip
Min
-0.03
-0.03
-0.03
-0.03
-0.03
-0.03
1.3
1.3
1.1
0.4
0.4
0.4
9.5
15.5
0.2
0.2
0.2
0.04
0.04
0.04
0.01
0.65
0.36
0.2
0.2
0.2
14
1.5
0.05
0.15
0.04
0.015
0.015
0.015
1.8
1.8
1.8
Max
0.03
0.03
0.03
0.03
0.03
0.03
1.5
1.5
1.2
0.6
0.6
0.6
10.5
16.5
0.3
0.3
0.3
0.1
0.1
0.1
0.03
0.85
0.4
0.4
0.4
0.4
20
1.7
0.11
0.25
0.06
0.035
0.035
0.035
1.9
1.9
1.9
Table 1: Geometric Parameters and their Ranges
2.1.3 Explanation of Parameter Scheme
2.1.3.1 Airfoil Section Positioning Parameters
The first six are positioning parameters for the airfoil sections (see Figure 3). They
move the airfoil sections in the X and Y directions independently. There are six of them
because there are three sections in the airfoil.
14
2.1.3.2 Airfoil Section Axial Chord
The next parameters are airfoil axial chord (see Figure 7). This is the distance frontto-back of the airfoil section when viewed from the side. Axial refers to the direction of
the engine axis. There are three of these parameters; there is one for each airfoil section.
Figure 7: Airfoil Section Chord Illustration
2.1.3.3 Airfoil Camber Parameters
The next set of three parameters is called “cmax” here (see Figure 8). It stands for
“camber.” What is meant by camber is the curvature of the airfoil section. In the
defining section (See Figure 3), the parameter controls the distance from the center of
the airfoil max thickness circle to the line tangent to the leading edge and trailing edge
arcs. A larger camber distance causes the minimum moment of inertia to increase. The
maximum moment of inertia, which is perpendicular to the minimum, is not changed
very much.
2.1.3.4 Flowpath Radius Parameters
The next two parameters are flowpath inner radius and flowpath outer radius.
Airfoil length is the difference between these two parameters. Airfoil radial position is
related to the sum of these parameters. The frequencies should depend on airfoil length
15
but should be independent of airfoil radial position.
A more intelligent parameter
scheme would define airfoil length separately from airfoil radial position.
2.1.3.5 Airfoil Section Max Thickness Parameters
The next three parameters are “hmax.” This refers to airfoil max thickness for each
airfoil section. It is defined in the airfoil section as the diameter of the arc tangent to the
concave and convex side of the airfoil. Increasing this parameter stiffens the airfoil and
adds mass to the airfoil.
Figure 8: Airfoil Parameters related to Stiffness
2.1.3.6 Leading Edge Diamater
The next three parameters are “led”. This stands for Leading Edge Diameter. It is
the diameter of the leading edge arc for each section. It adds stiffness and mass to the
leading edge of the airfoil.
2.1.3.7 Mate-Face Gap
The next parameter is “mateface_gap” which refers to the clearance between the
platforms of two adjacent blades. Its range is set to be very small (between 0.010” and
0.030”). A view of two adjacent blades shows the mateface gap in Figure 9.
16
Figure 9: Mateface Gap
2.1.3.8 Blade Neck Parameters
The next two parameters are neck length and neck width. These control the length
and thickness of the neck of the blade. Neck length is the distance between the root and
the airfoil. Neck length is of practical concern because it is the isolation distance
between the hot flowpath and the cooler attachment region. The longer neck length is,
the cooler, and therefore stronger, the attachment will be.
The neck width is the
circumferential distance from one side of the neck to the other.
2.1.3.9 Airfoil Section Shaping Parameters
The next three parameters are “peak chord percentage”. This parameter moves the
max thickness arc forward and back in the section. When the parameter is low, the max
thickness in the airfoil section occurs nearer to the leading edge. When the parameter is
high, the arc is pushed back closer to the trailing edge of the airfoil section. This will
change the orientation of the principal axes of the section.
2.1.3.10 Platform Angle and Rim Width
The next two parameters are platform angle and rim width. Platform angle is the
angle of the sides of the platform relative to a plane passing through the engine axis and
17
the blade’s vertical axis. The effect on mass should be small and the expected impact on
frequency is small. The rim width is defined as the distance between the forward and aft
faces of the root attachment. A higher rim width will stiffen the root and should increase
the frequency of fundamental bending modes.
2.1.3.11 Shroud Stiffening Rail Parameters
The tip shroud seen in Figure 21 has a stiffening rail running circumferentially
across it. The parameters shroud bump height and shroud bump length control the
thickness and axial length of this rail. The thickness of the shroud is controlled by the
parameter shroud thickness. All of these parameters affect the mass and stiffness of the
tip shroud.
2.1.3.12 Trailing Edge Diameter
The next three parameters are “ted”. This stands for Trailing Edge Diameter. This
parameter controls the trailing edge thickness of the three airfoil sections. A thicker
trailing edge will stiffen the airfoil in bending.
2.1.3.13 Airfoil Section True Chord
The last three parameters are “true chord” (see Figure 7). This is the length of the
airfoil section measured on a line tangent to the leading edge and trailing edge arcs. The
ratio of axial chord to true chord is the sine of the stagger angle. In this parameter
scheme, the stagger angle is implicit. Another parameter scheme could define this angle
explicitly, but then one of the chord lengths would have to be implicit.
2.2 Modal Analysis
In this study, every blade will undergo modal analysis implemented using the Finite
Element approach. The solution will be calculated using ANSYS, a finite element
analysis software package widely used in the industry. Modal analysis in its most basic
sense solves the system of homogeneous differential equations:
[ M ]{ X˙˙} + [K ]{ X } = 0
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18
[3]
In this equation, X is a vector of the displacements of all the degrees of freedom of
the mass system. K is the stiffness matrix that relates the displacements to one another.
M is the mass matrix and it is a diagonal matrix that contains all the masses associated
with each displacement. In three dimensions, each mass will have three degrees of
freedom. Each entry in the displacement vector, X, is one of those displacements. The
solution to this system of differential equations can be found by assuming the solution is
harmonic and taking its derivative twice then inserting it into the original equation. The
result becomes the eigenvalue problem seen in Equation 4.
det [[K ] − ω 2 [ M ]] = 0
[4]
The eigenvalues, ω, are the characteristic frequencies of vibration.
The
displacement vectors, X, are the associated mode shapes.
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2.2.1 Finite Element Implementation
This equation can be solved for continuous materials using continuous displacement
gradients as solutions. Theoretically, there would be an infinite number of degrees of
freedom and an infinite number of eigenfrequencies. Analytic solutions only exist for
very simple geometric shapes.
For most real objects, mode shapes can only be
approximated by breaking up the volume of interest into finite volumes of simple
shapes. This is the approach taken in Finite Element Analysis.
The continuous material of the turbine blade will be discretized into a finite number
of elements that have mass and stiffness.
Inside each element, the continuous
displacement gradient is approximated as a simple, low order function continuous at the
element boundaries. Continuity in spatial derivatives of displacement is not necessarily
enforced at element boundaries.
The finite element software requires certain inputs from the engineer. The first is
the geometry of the object to be analyzed.
The engineer also inputs the material
properties, namely mass density, elastic modulus and Poisson ratio. For this analysis, all
the properties will be assumed as homogeneous and isotropic. Homogeneous means that
the properties are constant throughout the material. Isotropic means that the non-scalar
properties do not depend on orientation. Pertinent to this analysis, the elastic modulus
and Poisson ratio will not vary with respect to orientation. Some turbine blades are
19
made of anisotropic materials such as single crystal nickel superalloys. Changing from
isotropic to anisotropic material properties will significantly affect the vibratory
response of the turbine blade. In the scope of this analysis, we will assume the blades
are made of an “equiaxed,” isotropic material. This type of material has a random
crystal structure throughout. The anisotropy of any individual crystals is averaged out
over the entire sample of material.
2.2.2 Execution of Computations
The modal analysis was performed using Unigraphics NX6 Advanced Simulation
with ANSYS 12.0 as a solver. The model is prepared in NX Simulation. This includes
meshing the model into a finite element model, applying displacement fixity conditions
at the root, and defining material properties. The material used for this analysis was a
simple structural steel. The material does not actually matter since the desired outcome
is just the geometric sensitivity. Material variation affects all results uniformly and
therefore cancels out when comparing frequencies relative to one another.
The finite element modes were meshed using Unigraphics NX Simulation. The
meshes contained between 25,000 and 35,000 nodes. When the mesh was submitted to
ANSYS for solution, the solution took 30 to 60 seconds to solve the first 5 mode shapes.
The 50 parametric model updates and finite element solutions were performed manually
due to the quickness of the solution. A more investigative approach could explore more
combinations of parameters by using an automation macro.
The post processing was done within NX Simulation. The program is able to read
the ANSYS solution file and display mode shapes in various ways.
The solution
contains the mode shape solutions associated with each natural frequency.
contains derived results such as stress and strain.
20
It also
3. Results and Discussion
The parameter scheme chosen for the turbine blade proved to be robust. The
predetermined range of parameters produced stable, realistic turbine blade geometries.
Out of 50 parameter combinations of 37 parameters, 100% resulted in valid part
geometries.
This was due, in part, to the limited range of the parameters.
The
parametric blade model was tested extensively for robustness. Parameter ranges were
determined based on the tendency of the model to produce valid geometries. If a
parameter caused the model to fail, the parameter range was reduced. Examples of
failure would be: airfoil falling off platform; shroud falling off airfoil tip; airfoil to
platform blend failure.
3.1 Mode Shape Identification
Under special conditions of rotational symmetry, mode shape identification can be
difficult and natural frequencies can be very close to one another. In a disc shaped
object, there can be two mode shapes that are identical but offset by an integer fraction
of a rotation. This turbine blade was asymmetric enough that all of the mode shapes
were clearly differentiable. Also, the natural frequencies were spaced out enough that
there was no confusion about which mode was associated with which frequency.
For
the purposes of this analysis it is assumed that the mode shapes, when sorted by
frequency, are the same for every blade.
3.1.1 Frequency Results
The frequencies of each mode varied as expected. The ranges through which they
varied can be seen in the following table.
Mode Number
Average (Hz)
Standard Deviation (Hz)
Maximum (Hz)
Minimum (Hz)
1
302
35
405
224
2
863
78
1045
670
3
1391
133
1660
1047
Table 2: Statistical Data for Frequency Results
21
4
1970
217
2626
1545
5
3763
368
4614
2932
3.2 General Trends in Response
3.2.1 First Mode Shape
The first mode of vibration (see Figure 10) is a bending mode. The visualization is
showing the two opposite extremes of the deformed shape. The two deformed shapes
are separated in time by a phase shift of 180 degrees (half of a vibration cycle). This is
sometimes referred to as peak-to-peak displacement.
It is important to understand that this figure is plotting the relative magnitude of
displacements in the mode shape. Because mode shapes are eigenvectors, their absolute
magnitude is not important – only their relative magnitudes.
The maximum
displacement shown on the scale is completely irrelevant in a modal analysis.
Magnitude would only be meaningful in a harmonic analysis with a known forcing
function.
In this mode shape, the airfoil is tending to bend about the root section’s minimum
moment of inertia. The analysis shows that the parameters that affect root stiffness have
a large impact on the first mode frequency. The first mode frequency is also affected by
parameters that affect tip mass. Parameters that affect tip section stiffness have very
little impact on this mode frequency. The frequency associated with this mode shapes
ranges from
22
Figure 10: Mode Shape 1 at 268 Hz
23
3.2.2 Second Mode Shape
The second mode of vibration was also bending about the root (see Figure 11), but
in contrast to the first mode, the deflection was in the perpendicular direction. The blade
is tending to bend about the root section’s maximum moment of inertia. The frequency
is correspondingly higher due to the increased stiffness in that direction. The tip mass
affects this frequency similarly to the first mode frequency.
The chord length of the
airfoil root section also causes increased stiffness in this vibration mode thereby
increasing the frequency.
Figure 11: Mode Shape 2 at 827 Hz
24
3.2.3 Third Mode Shape
The third mode of vibration is a torsional mode (see Figure 12). This means that the
vibration is characterized by twisting of the airfoil tip. This plot is showing the blade
from the top down to emphasize the rotational motion of the tip. This frequency is
affected by the torsional stiffness of the root and midsection of the airfoil.
The
frequency is also affected by tip rotational moment of inertia. There is very little
displacement shown in the root area. This implies that variation in mass at the root will
not greatly affect the frequency of this mode of vibration.
Figure 12: Mode Shape 3 at 1446 Hz
25
3.2.4 Fourth Mode Shape
The fourth mode is a second-order bending mode (see Figure 13). It exhibits
motion in the tip shroud and the airfoil trailing edge. The two motions of the shroud and
the airfoil trailing edge are opposing one another as if to counterbalance each other’s
effect on the structure. The frequency is affected by many geometric factors. The linear
regression in the following section will reveal the trends more clearly than one can
discern from simply looking at the response matrix.
Figure 13: Mode Shape 4 at 1705 Hz
26
3.2.5 Fifth Mode Shape
See Figure 14 for a displacement plot of the fifth mode. The fifth mode shape
shows much of its deflection in the corner of one side of the tip shroud. That motion is
counterbalanced by motion in the trailing edge and leading edge of the airfoil. This
could be considered a third-order bending mode combined with some twisting of the tip
shroud.
Figure 14: Mode Shape 5 at 4255 Hz
27
3.3 Regression Functions
3.3.1 Linear Regression
The first step in interpreting the frequency results is to see which parameters most
highly affect the frequencies. One approach to this is to create a linear regression. The
geometric parameters will be the predictors while the calculated frequencies are the
responses. Matlab can generate coefficients for a linear regression for each response and
calculate the residual for each data point. The residual is the difference between the
actual response and the predicted response using the linear regression at an observation
(data point).
For N observations, there will be N residuals.
If the regression is
performed using the predictor variables as dimensional quantities, then the coefficients
will have units of response/predictor. In this case it would be Hertz/Inch. This means
that the response should change by a certain number of Hertz for every inch of variation
in the predictor variable. For some of the geometric parameters, one inch is a fairly
acceptable variation. For other parameters, one inch of variation is inconceivable and
would cause the model to fail.
To account for this, the predictors can be transformed to a new space depending on
their acceptable ranges of variation.
In this new space, zero would represent the
minimum value for the parameter and one would represent the maximum value for the
parameter. This is referred to as non-dimensionalizing. However, in this study the data
input to the regression will be dimensional. Therefore, the ranking of the influence
coefficients will not take into account usual and customary ranges for each parameter.
3.3.2 Pareto Analysis
The results of the linear regression are coefficients that represent idealized linear
response of the response with respect to the predictor variables. The significance of each
variable can be sorted by coefficient magnitude. When the variables are sorted in this
manner and the coefficient values are plotted on a bar chart, the result is often referred to
as a Pareto chart. The following section shows the Pareto charts of the regression
coefficients for each of the five responses.
28
3.3.2.1 First Mode Linear Response
The largest magnitude of influence is from the shroud thickness (See Figure 15). Its
coefficient value is about -700 Hz/inch. This says that a small variation in shroud
thickness has a large impact on the first natural frequency. Negative means that the
increase in the predictor decreases the response. A thicker shroud increases the shroud
mass and decreases the frequency.
The next three most influential parameters are
cmax_root, hmax_root, and neck_width. These parameters all increase the stiffness of
the root of the blade.
Unsurprisingly, they all have a large positively correlated
influence on the first mode of vibration. Positive means that increasing these parameters
increases frequency.
29
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85,=257/>/62?5@2:/453"
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Figure 15: Linear Response Coefficients for First Frequency
30
3.3.2.2 Second Mode Linear Response
The second mode response coefficients can be seen in Figure 16. The second mode
is a bending mode whose motion is primarily aligned with the stiff direction of the airfoil
and will therefore be called a stiff-wise bending mode. Shroud thickness has a very high
influence on this frequency. The second most influential factor was mateface gap. This
parameter controlled the clearance between the adjacent blades. It is not clear why this
parameter was a strong driver of this frequency. In practicality, the parameter will
always be set between .010” and .030”. Due to the small range of variation used in this
study, the confidence interval for this coefficient is very large; one should have low
confidence that mateface gap drives variation in frequency. Another driver of frequency
in this mode is neck width and airfoil maximum thickness at the root and middle
sections.
31
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Figure 16: Linear Response Coefficients for Second Frequency
32
%#!!"
3.3.2.3 Third Mode Linear Response
The third natural frequency responds to shroud thickness and to airfoil max
thickness at mid-span (hmax_mid). The first one, shroud thickness, has a large impact
on shroud mass. Shroud mass decreases the third mode’s frequency frequency. The
next parameter is airfoil max thickness at mid-span. This parameter increases the polar
moment of inertia of the middle airfoil section. A thicker airfoil will be stiffer and tend
to increase the frequency. The root section max thickness is less influential than that of
the mid section. The probable reason for that is the mid-span section experiences more
shear stress in this mode-shape than does the root section.
33
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Figure 17: Linear Response Coefficients for Third Frequency
34
3.3.2.4 Fourth Mode Response
According to this analysis, the fourth mode is most highly influenced by the shroud
thickness and the mate-face gap.
Again, mateface gap cannot be a big driver of
frequency because it does not change any geometry in the airfoil. The prediction of
influence is made with low confidence due to the limited range of variation in this
parameter. The next three parameters shown to have an effect on the fourth natural
frequency are ted_mid, cmax_mid, and led_mid. The parameters are all ones that tend to
increase the polar moment of inertia of the mid section of the airfoil. This implies that
much of the bending and twisting in this mode shape is occurring around the middle of
the airfoil.
35
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Figure 18: Linear Response Coefficients for Fourth Frequency
36
'!!!"
3.3.2.5 Fifth Mode Response
The fifth mode response is again showing high influence exerted by the shroud
thickness and the mateface gap. While the mateface gap influence prediction is not very
likely to be valid, the shroud thickness influence may be correct.
Much of the
displacement in this mode shape is associated with bending of the shroud. The thickness
of the shroud directly impacts the bending characteristics of it. Notice that for this
mode, the response depends positively on the shroud thickness. In all other modes, it has
depended negatively on shroud thickness. This is because the shroud stiffness is what
affects this mode and not the shroud’s mass. Some airfoil parameters that affect this
mode’s frequency are ted_tip, axial_chord_tip, and ted_mid.
37
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Figure 19: Linear Response Coefficients for Fifth Frequency
38
%#!!!"
4. Conclusion
4.1 Design Spaces and Parameter Schema
Parametric design studies can help in the mechanical design of turbine blades. The
results of this study show that with only 50 analyses realistic estimates of the influence
of geometric parameters can be found. It is also important to note that geometric
parameter schemes are not unique. There are many ways to fully define a shape and not
all of them will reveal useful information.
One must be sure to create the solid model with flexibility and independence in
mind. Flexibility means the model will give valid output for any input combination in
the defined parameter range. Independence means parameters should affect parts of the
geometry independently. This may help to prevent parameter interactions that will be
unseen in conventional linear regression.
One example of parameter interaction in this model is the way airfoil length is
defined.
The inner and outer flowpath radii are defined explicitly.
Many of the
fundamental frequencies depend on airfoil length. But none truly depend on the blade’s
position in space. A more thoughtful parameter scheme would have defined the airfoil
length and airfoil root radial position independently. It is likely that the regression
would show the frequencies having a strong correlation with airfoil length and a weak
correlation with airfoil radial position in space.
4.2 Future Work
When fitting a response surface to model data, a linear approach may not always be
appropriate. Interaction and quadratic terms can sometimes have a significant impact on
conclusions drawn from a regression analysis. Further analysis of this data could be
done to see if there is a better response surface model that describes this data. Some
examples of regression functions that could be used to fit this data are: products of two
or more variables, products of the same variables (quadratic terms), higher order
combinations of terms, or even non-linear functions of variables or combinations
thereof.
39
5. References
[1] Abdi, H. (2003). Partial Least Squares (PLS) Regression. In M. B. Lewis-Beck,
Encyclopedia of Social Sciences Research Methods. Thousand Oaks, CA: Sage
Publishing.
[2] Brown, J. M., & Grandhi, R. V. (2008). Reduced-Order Model Development for
Airfoil Forced Response. International Journal of Rotating Machinery , 2008, 112.
[3] Hastie, T., Tibshirani, R., & Friedman, J. (2008). The Elements of Statistical
Learning (2nd Edition ed.). Stanford, CA, USA: Springer.
[4] Norton, R. L. (2006). 6.11 - Designing for Fluctuating Uniaxial Stresses. In R. L.
Norton, Machine Design, an Integrated Approach 3rd Edition (pp. 356-360).
Upper Saddle River, NJ, USA: Prentice Hall.
[5] Petrov, E. P. (2008). A Sensitivity-Based Method for Direct Stochastic Analysis of
Nonlinear Forced Response for Bladed Disks with Friction Interfaces. Journal of
Engineering for Gas Turbines and Power , 130 / 022503-1.
[6] Rao, S. S. (2004). Mechanical Vibrations - 4th Edition. Upper Saddle River, NJ,
USA: Pearson - Prentice Hall.
[7] Richardson, M. H. (1997). Is It a Mode Shape or an Operating Deflection Shape?
Sound & Vibration Magazine (30th Anniversery).
40
6. Appendices
6.1 Appendix 1: Turbine Blade Parametric Model
Figure 20: Top View of Fictitious Parametric Turbine Blade Model
Figure 21: Generic Tip Shroud Design
41
Figure 22: View of Fictitious Turbine Blade Root Attachment Geometry
Figure 23: Close-up View of Turbine Airfoil Section Definition
42
Figure 24: Example of Random Turbine Blade – 1
43
Figure 25: Example of Random Turbine Blade – 2
44
Figure 26: Example of Random Turbine Blade – 3
45
Figure 27: Example of Random Turbine Blade – 4
46
Figure 28: Example of Random Turbine Blade – 5
47