Prediction of Axial Compressor Blade Vibration

by Modelling Fluid-Structure Interaction

Faculty of Engineering at Stellenbosch University

Department of Mechanical and

Mechatronic Engineering

by J. D Brandsen

Supervisors:

Dr S. J. van der Spuy

Prof G. Venter

1/19

Overview

• Background

• Goal of Project

• FSI Modelling Approaches

• Experimental Work

• Numerical Modelling

• Results

• Conclusions

2/19

Background

Flutter in Turbomachinery

Background

Goal of Project

FSI Modelling

Approaches

Experimental

Work

Numerical

Modelling

Results

Conclusions

• Flutter is the vibration of a mechanical system:

At or near natural frequencies of system.

Caused by instability. Does not require disturbance.

Aerodynamic forces feed energy into system. Amplitude

increases with time.

• Cause of high cycle fatigue failure in turbomachinery.

• Project FUTURE initiated to improve methods used to model

and design for flutter.

• Project FUTURE is coordinated by Kungliga Teknista

Högskolan in Sweden.

• Also has 25 other partners, including Stellenbosch University

and the Council for Scientific and Industrial Research (CSIR).

3/19

Background (continued)

Vibration Excitation System

Background

Goal of Project

FSI Modelling

Approaches

Experimental

Work

Numerical

Modelling

Results

Conclusions

• As part of Project FUTURE, the CSIR have developed a

vibration excitation system:

Designed to excite the first rotor blade row of an axial

flow compressor.

Designed to make the blade row vibrate at the desired

frequency and in the desired mode shape.

Injects air into compressor flow path thereby causing

velocity perturbations.

• Stellenbosch University responsible for demonstrating

capabilities of vibration excitation system.

• Vibration excitation system was therefore fitted to the

Rofanco compressor test bench.

4/19

Background (continued)

Vibration Excitation System

Background

Goal of Project

FSI Modelling

Approaches

Experimental

Work

Numerical

Modelling

Results

Conclusions

• Vibration excitation system fitted to Rofanco test bench

(images from Van der Spuy et al (2012)):

• Rofanco compressor (manufactured by Royston Fan Co. Ltd.):

Three identical stages (43 rotor blades, 41 stator blades).

36 inlet guide vanes (removed for excitation system).

• Each exciter consists of a DC servo motor fitted with a special

rotor disk.

• Two types of rotor disk: 32 hole rotor disk, 16 hole rotor disk. 5/19

Background (continued)

Blade Row Vibration Modes

Background

Goal of Project

FSI Modelling

Approaches

Experimental

Work

Numerical

Modelling

Results

Conclusions

• Phase difference = 360

x (no. of NDs)/(no. of blades)

• Vibration excitation system designed to excite 0 ND, +1 ND,

+2 ND, +3 ND, -1 ND, -2 ND and -3 ND modes.

• Nodal diameter (ND) modes:

+ −

Rotation

Wave propagation

ND

ND

Rotation

2 ND mode 0 ND mode

6/19

Goal of Project

Background

Goal of Project

FSI Modelling

Approaches

Experimental

Work

Numerical

Modelling

Results

Conclusions

• Goal of thesis project:

Construct a FSI model of the vibration excitation system.

• Purpose of FSI model was two-fold:

Numerical tool for carrying out experiments digitally.

Will complement the existing experimental data.

• Restrictions placed on FSI model due to time constraints:

Single setting simulated: excitation frequency of 660 Hz

and a supply pressure of 2.5 bar.

Needs to only be able to simulate the 0 ND mode and

the +2 ND mode of the system.

Must be able to accurately recreate component of blade

motion occuring at excitation frequency (660 Hz).

7/19

FSI Modelling Approaches

Background

Goal of Project

FSI Modelling

Approaches

Experimental

Work

Numerical

Modelling

Results

Conclusions

• Monolithic approach:

• Staggered approach:

Structural equations

+

Fluid equations

Structural equations

Fluid equations

Single dedicated solver

CFD solver

FE solver

Data transfer

• Staggered approach preserves software modularity.

• Ansys CFX and Ansys Mechanical available at start of project.

• Staggered approach already demonstrated for turbomachinery

by Im and Zha (2012), Gnesin et al (2000). 8/19

Experimental Work

Background

Goal of Project

FSI Modelling

Approaches

Experimental

Work

Numerical

Modelling

Results

Conclusions

• Measurements of velocity perturbations required for

boundary conditions of FSI model.

• Velocity profile measured for a frequency of 650 Hz and

supply pressure of 2.5 bar.

• Velocity profiles measured for 0 ND mode of the system:

32 hole rotors 16 hole rotors

9/19

Numerical Modelling

FE Model of First Rotor Blade Row

Background

Goal of Project

FSI Modelling

Approaches

Experimental

Work

Numerical

Modelling

Results

Conclusions

• FE model created of a single blade and verified.

• Multiple copies of single blade FE model then combined:

• Single blade FE model created using SOLSH 190 elements.

• Each blade constrained in cantilever fashion at root.

• Material properties were those of aluminium.

3 copies (0 ND FE model) 21 copies (+2 ND FE model)

10/19

Numerical Modelling (continued)

CFD Model of Vibration Excitation System

Background

Goal of Project

FSI Modelling

Approaches

Experimental

Work

Numerical

Modelling

Results

Conclusions

• To save computation time, number of cells kept to a minimum.

Set up for model of 14 exciters and 42 rotor blades.

Periodic boundaries used to reduce model to three rotor

blades and a single exciter (0 ND CFD model).

• Each exciter nozzle jet modelled by applying a sinusoidal

velocity to patch boundary

• Sinusoidal velocity

selected so that velocity

profile at interface

matched experimental

profile.

0 ND CFD model

• Approach already

demonstrated by

Raubenheimer (2011).

11/19

Numerical Modelling (continued)

CFD Model of Vibration Excitation System

Background

Goal of Project

FSI Modelling

Approaches

Experimental

Work

Numerical

Modelling

Results

Conclusions

• When vibrating in the

+2 ND mode, period of

travelling wave is half of

rotor blade row.

• Model must therefore

contain half of rotor.

• Seven copies of 0 ND

CFD model used to make

+2 ND CFD model.

• Results in model of

7 exciters, 21 rotor blades

• Nozzle jets set to fire out

of phase.

+2 ND CFD model

12/19

Results

FFTs of blade deformation

Background

Goal of Project

FSI Modelling

Approaches

Experimental

Work

Numerical

Modelling

Results

Conclusions

• Two modes simulated:

Frequency of 650 Hz, Pressure of 2.5 bar, 0 ND mode.

Frequency of 650 Hz, Pressure of 2.5 bar, +2 ND mode.

• Run for 4500 time steps at a time step size of 5.4112 x 10-5 s.

+2 ND FSI model 0 ND FSI model

13/19

Results (continued)

FFTs of blade deformation

Background

Goal of Project

FSI Modelling

Approaches

Experimental

Work

Numerical

Modelling

Results

Conclusions

• Data of Van der Spuy et al (2012) shows amplitude of 660 Hz

component of tip displacement perpendicular to root should

be:

0.089 mm for 0 ND mode for the 32 hole rotors.

0.105 mm for +2 ND mode for the 32 hole rotors.

• In both cases, amplitudes predicted by FSI models all within

6% of experimental data.

• Data of Van der Spuy et al (2012) shows amplitude of 660 Hz

component of bending strain, 6.1 mm from root, should be:

0.093 mm/m for 0 ND mode for the 32 hole rotors.

0.109 mm/m for +2 ND mode for the 32 hole rotors.

• As with tip displacement, amplitudes predicted by FSI

models all within between 10% and 20 % of experimental

data for both cases.

14/19

Results (continued)

Blade formation for 0 ND mode

Background

Goal of Project

FSI Modelling

Approaches

Experimental

Work

Numerical

Modelling

Results

Conclusions

• Phase angles of 660 Hz components for the 0 ND mode:

Blade 2 Blade 3 Blade 8 Blade 14

Ideal 17.1

34.3

120

223

FSI model 18.8

31.3

118

222

32 hole rotors 16 hole rotors

Blade 2 Blade 3 Blade 2 Blade 3

Ideal 0

0

0

0

FSI model -2

4

-3

3

• Phase angles from 0 ND FSI model all within 5° of ideal

values. Blades deemed to be vibrating in 0 ND mode.

• Phase angles of 660 Hz components for the +2 ND mode:

• Phase angles from +2 ND FSI model all within 3° of ideal

values. Blades deemed to be vibrating in +2 ND mode. 15/19

Results (continued)

Visualisation of Blade Deformation

Background

Goal of Project

FSI Modelling

Approaches

Experimental

Work

Numerical

Modelling

Results

Conclusions

• Simulation of scenario where vibration excitation system is

set to 660 Hz, 2.5 bar and the 0 ND mode:

16/19

• Phase angles showed that the 660 Hz components of

motions of the blades are all in phase.

• However, visualisation shows that overall motions of the

blades are not in phase.

Conclusions

Background

Goal of Project

FSI Modelling

Approaches

Experimental

Work

Numerical

Modelling

Results

Conclusions

• Correlation between results of FSI models and experimental

data was satisfactory:

660 Hz components of tip displacement perpendicular to

root all within 6% of experimental data.

660 Hz components of bending strain all within between

10% and 20% of experimental data.

• Both 0 ND FSI model and +2 ND FSI model therefore an

acceptable recreation of vibration excitation system.

• Phase angles of 660 Hz components of blade motions show:

Vibration excitation system should be able to excite the

0 ND mode and the +2 ND mode.

Provided excitation frequency is close to 660 Hz.

17/19

Acknowledgements

• The financial assistance of the National Research Foundation (NRF)

towards this research is hereby acknowledged. Opinions expressed

and conclusions arrived at, are those of the author and are not

necessarily to be attributed to the NRF.

• Thank you to Project BALLAST for the financial assistance provided

for this thesis project.

18/19

Thank you

19/19