Experimental Investigation of Transition over a NACA 0018 Airfoil at a Low Reynolds Number

237 Pages·2011·27.74 MB·English

Experimental Investigation of Transition over a
NACA 0018 Airfoil at a Low Reynolds Number
by
Michael S. H. Boutilier
A thesis
presented to the University of Waterloo
in fulfillment of the
thesis requirement for the degree of
Master of Applied Science
in
Mechanical Engineering
Waterloo, Ontario, Canada, 2011
c Michael S. H. Boutilier 2011
(cid:13)

I hereby declare that I am the sole author of this thesis. This is a true copy of the thesis,
including any required final revisions, as accepted by my examiners.
I understand that my thesis may be made electronically available to the public.
iii

Abstract
Shear layer development over a NACA 0018 airfoil at a chord Reynolds number of
100,000 was investigated experimentally. The effects of experimental setup and analysis
tools on the results were also examined.
Thesensitivityoflinearstabilitypredictionsformeasuredseparatedshearlayervelocity
profiles to both the analysis approach and experimental data scatter was evaluated.
Analysis approaches that are relatively insensitive to experimental data scatter were
identified. Stability predictions were shown to be more sensitive to the analysis approach
than to experimental data scatter, with differences in the predicted maximum disturbance
growth rate and corresponding frequency of approximately 35% between approaches.
A parametric study on the effects of experimental setup on low Reynolds number
airfoil experiments was completed. It was found that measured lift forces and vortex
shedding frequencies were affected by the end plate configuration. It was concluded that
the ratio of end plate spacing to projected model height should be at least seven, consistent
with the guideline for circular cylinders. Measurements before and after test section wall
streamlining revealed errors in lift coefficients due to blockage as high as 9% and errors in
the wake vortex shedding frequency of 3.5%.
Shear layer development over the model was investigated in detail. Flow visualization
images linked an observed asymmetry in wake velocity profiles to pronounced vortex roll-
up below the wake centerline. Linear stability predictions based on the mean hot-wire
profiles were found to agree with measured disturbance growth rates, wave numbers, and
streamwise velocity fluctuation profiles. Embedded surface pressure sensors were shown
to provide reasonable estimates of disturbance growth rate, wave number, and convection
speed for conditions at which a separation bubble formed on the airfoil surface. Convection
speeds of between 30 and 50% of the edge velocity were measured, consistent with phase
speed estimates from linear stability theory.
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Acknowledgements
I would like to thank my supervisor, Professor Serhiy Yarusevych, for providing
guidance and sharing his insight at every stage of this investigation. His experience with
experimental techniques was invaluable in the planning and troubleshooting phases of this
project.
I am grateful to Ryan Gerakopulos for developing the airfoil model used in these
experiments. His expertise in mechanical design and meticulous care in developing the
embedded pressure sensor array has ensured the reliability of measurements presented in
this thesis.
I would also like to thank the other students in my research group, Sina Kheirkhah,
Christopher Morton, and Holly Neatby, who were always willing to discuss ideas and
results with me. Holly’s help with renovations to experimental hardware is particularly
appreciated.
Laboratory staff members Jim Merli, Neil Griffett, and Jason Benninger, provided me
continual support with hardware designs and modifications. I would specifically like to
acknowledge the contributions of Neil, who designed and built the Scanivalve mechanical
multiplexer control circuit, and Jason, who manufactured the end plates for the airfoil
model.
Financial support for this work was provided by the Natural Sciences and Engineering
Research Council of Canada and the Ontario Graduate Scholarship Program.
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Table of Contents
List of Tables xiii
List of Figures xix
Nomenclature xxi
1 Introduction 1
2 Background 5
2.1 Low Reynolds Number Airfoil Operation . . . . . . . . . . . . . . . . . . . 5
2.1.1 Structure of Transitional Separation Bubbles . . . . . . . . . . . . . 9
2.2 Stability Theory . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12
2.2.1 Stability of Separated Shear Layers . . . . . . . . . . . . . . . . . . 14
2.2.2 Approaches to Linear Stability Analysis of Measured Separated
Shear Layer Profiles . . . . . . . . . . . . . . . . . . . . . . . . . . 16
2.3 Influence of Experimental Setup . . . . . . . . . . . . . . . . . . . . . . . . 17
2.3.1 End Plates on Aerodynamic Models . . . . . . . . . . . . . . . . . . 17
2.3.2 Test Section Wall Interference . . . . . . . . . . . . . . . . . . . . . 19
2.3.3 Hot-Wire Measurements in Separation Bubbles . . . . . . . . . . . 21
2.4 Surface Pressure Measurements with Embedded Sensors . . . . . . . . . . 23
3 Experimental Methodology 25
3.1 Experimental Setup . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25
3.1.1 NACA 0018 Airfoil Model . . . . . . . . . . . . . . . . . . . . . . . 27
3.1.2 End Plate Design . . . . . . . . . . . . . . . . . . . . . . . . . . . . 29
3.2 Measurement Techniques . . . . . . . . . . . . . . . . . . . . . . . . . . . . 29
3.2.1 Static Pressure Measurements . . . . . . . . . . . . . . . . . . . . . 30
3.2.2 Velocity Measurements . . . . . . . . . . . . . . . . . . . . . . . . . 30
ix
3.2.3 Microphone Measurements . . . . . . . . . . . . . . . . . . . . . . . 31
3.3 Wall Adaptation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 33
3.4 Flow Visualization . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 35
4 Sensitivity of Linear Stability Analysis of Separated Shear Layers 37
4.1 Numerical Methods . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 38
4.2 Rayleigh Equation Solutions using Experimental Profiles Directly . . . . . 40
4.3 Rayleigh Equation Solutions for Curve Fits to Experimental Profiles . . . . 45
4.4 Solutions to the Orr-Sommerfeld Equation for Experimental Profiles . . . . 53
5 Effect of Experimental Setup on Flow Development 63
5.1 End Plate Configuration . . . . . . . . . . . . . . . . . . . . . . . . . . . . 63
5.2 Blockage . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 73
5.3 Intrusive Techniques . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 82
5.3.1 Influence of the Hot-Wire Probe and Traverse . . . . . . . . . . . . 82
5.3.2 Effect of Opening the Wind Tunnel Door . . . . . . . . . . . . . . . 86
6 Boundary Layer Development 87
6.1 Flow Visualization . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 87
6.2 Boundary Layer Measurements . . . . . . . . . . . . . . . . . . . . . . . . 96
6.3 Instability of the Laminar Separated Shear Layers . . . . . . . . . . . . . . 117
7 Conclusions 133
7.1 Sensitivity of Linear Stability Analysis . . . . . . . . . . . . . . . . . . . . 133
7.2 Effect of Experimental Setup . . . . . . . . . . . . . . . . . . . . . . . . . . 135
7.3 Separated Shear Layer Development . . . . . . . . . . . . . . . . . . . . . . 136
8 Recommendations 139
PERMISSIONS 143
REFERENCES 147
APPENDICES 161
A Experimental Uncertainty 163
A.1 Uncertainty in Experimental Setup . . . . . . . . . . . . . . . . . . . . . . 163
A.2 Hot-Wire Uncertainty . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 165
A.3 Uncertainty in Static Pressure Measurements . . . . . . . . . . . . . . . . . 169
x