relationship between the free shear layer, the wingtip vortex and aerodynamic efficiency

158 Pages·2016·13.13 MB·English

by  Sidaard Gunasekaran

RELATIONSHIP BETWEEN THE FREE SHEAR LAYER, THE WINGTIP
VORTEX AND AERODYNAMIC EFFICIENCY

Dissertation
Submitted to
The School of Engineering of the
UNIVERSITY OF DAYTON

In Partial Fulfillment of the Requirements for
The Degree of
Doctor of Philosophy in Engineering

By
Sidaard Gunasekaran
Dayton, Ohio
August 2016
RELATIONSHIP BETWEEN THE FREE SHEAR LAYER, THE WINGTIP VORTEX
AND AERODYNAMIC EFFICIENCY
Name: Gunasekaran, Sidaard

APPROVED BY:

Dr. Aaron Altman Dr. Markus Peer Rumpfkeil
Advisory Committee Chairman Assistant Professor
Professor Department of Mechanical and
Department of Mechanical and Aerospace Engineering
Aerospace Engineering

Dr. Jose Camberos Dr. Ryan Schmitt
C ommittee Member Research Scientist
Adjunct Professor AFRL
D epartment of Mechanical and Wright Patterson Air Force Base
Aerospace Engineering

Robert J. Wilkens, PhD., P.E. Eddy M. Rojas, PhD., M.A., P.E

Associate Dean for Research and Innovation Dean
Professor School of Engineering
School of Engineering

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© Copyright by
Sidaard Gunasekaran
All rights reserved
2016

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ABSTRACT
RELATIONSHIP BETWEEN THE FREE SHEAR LAYER, THE WINGTIP
VORTEX AND AERODYNAMIC EFFICIENCY
Name: Gunasekaran, Sidaard
University of Dayton
Advisor: Dr. Aaron Altman
The overarching objective of this experimental investigation is to explore the relationship between
the aerodynamic efficiency of the wing and its turbulent wake (both the free shear layer and the
wingtip vortex). Recent evidence of unique turbulent signatures in the free shear layer of a turbulent
generator provided the motivation behind this research. The balance of induced drag and the
parasite drag was hypothesized to be mirrored in the properties of the wingtip vortex and the free
shear layer respectively expanding from classical theoretical descriptions. Experimental
investigations were focused on the wake of wings to understand this balance in the parasite and the
induced drag and to explore the use of the properties in the turbulent wake to increase the
aerodynamic efficiency of the wing. Because of the highly complex nature of the wake, the research
is broken down into several individual sub-studies which explore a) the relationship between the
aerodynamic efficiency and the free shear layer, b) the relationship between the aerodynamic
efficiency and the wingtip vortex, and c) the relationship between the free shear layer and the
wingtip vortex and their correlation to the aerodynamic efficiency. Particle Image Velocimetry
(PIV) was used to measure the velocity in the wake of an SD 7003 wall-to-wall model and an AR
4 flat plate with and without a spanwise boundary layer trip in the Horizontal Free Surface Water
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Tunnel (HFWT) at the Air Force Research Labs (AFRL) and in the Low Speed Wind Tunnel at the
University of Dayton (UD-LSWT).
The results from experimental investigations were Reynolds decomposed to study the mean
and fluctuating quantities in the wake of the wing. The initial prediction of these quantities in the
wake of SD 7003 wall-to-wall model and AR 4 flat plate were made using the existing momentum
deficit and Reynolds stress models (which are derived from simplified Navier-Stokes equations).
Even though the momentum deficit model yielded a good match with the experimental data, the
Reynolds stress model was not able to predict the experimental data because of the asymmetry in
the distribution. The eddy viscosity parameter in the algebraic models was then identified and
incorporated in the algebraic models. The variation in the surrogate eddy viscosity parameter when
compared to the experimental data showed direct correlation with the variation in the aerodynamic
efficiency of the wing. In order to fortify the relationship between the turbulent properties in the
free shear layer and the aerodynamic efficiency, the energy loss in the wake of the SD7003 wall-
to-wall model was quantified by determining the viscous dissipation (Exergy) as a function of
initial conditions upstream. The changes in Exergy mirrored the aerodynamic efficiency of the SD
7003 wall-to-wall model. But in the AR 4 wing wake, there existed a net spanwise momentum due
to the formation of the wingtip vortices. Using orthogonal PIV planes of interrogation in the wingtip
vortex station across several distances downstream, the evolution of the wingtip vortex and its
relationship with aerodynamic efficiency of the wing were investigated. The wake-like to jet-like
transition in the core of the wingtip vortex was not observed at the angle of attack of maximum
aerodynamic efficiency. However the maximum viscous dissipation and the Reynolds stress in the
wingtip vortex shows changes in the slope at the maximum (L/D) location. In the presence of a
spanwise boundary layer trip, the location of the change of slope in the viscous dissipation and
Reynolds stress was changed indicating a direct correlation to the properties in the wingtip vortex
and the aerodynamic efficiency of the wing.
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Significant changes in the boundary layer of the flat plate with the boundary layer trip were
observed at lower angles of attack. The resulting changes in the turbulence character of the wingtip
vortex and the free shear layer were investigated for evidence of an interaction between the free
shear layer and the evolution of the wingtip vortex. The streamwise, cross-stream and spanwise
oriented PIV of the wingtip vortex shows definitive evidence of the free shear layer interaction with
the wingtip vortex at angles of attack lower than maximum (L/D). This interaction was reflected in
the normalized azimuthal velocity profile of the wingtip vortex as well. The composite of the
velocity profiles from the multiple different planes showed a transfer of momentum from the free
shear layer to the wingtip vortex in the vicinity of maximum (L/D) angle of attack. This suggests
that by manipulating the cross-stream flow in the wake of the wing from the wing root to the
wingtip, the balance of induced drag and parasite drag can be altered given initial conditions and
the aerodynamic efficiency can be improved in off-design conditions.
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Dedicated to my parents, my advisor and to all the extraordinary professors at UD

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ACKNOWLEDGEMENTS
I would like to thank my parents for their continued support, patience and unconditional
love they have bestowed me over the years. I consider myself extremely fortunate to be a son of
such great people. I would like to thank my relatives for always encouraging me to pursue my
dreams. Growing up, we look up to our family for motivation, kindness, support and love and they
were a constant source of motivation. This success and any other of mine, belongs to them.
I would like to thank my friends in India and also here in the USA for their constant
expectations of what I would become. Especially I would to thank everyone who was a part of the
research group over the years, Zach Lego, Matt Geyman, Kevin Wabick, Omar Memon, Saad
Qureshi and Garrett Gleason. They gave me a drive to keep pursuing what I love. I would also like
to thank the love of my life Dhuree Seth for her enthusiasm, love and support she gave me while I
was pursuing my PhD. She constantly inspires me to be a better person in every aspect of my life.
I would like to thank all the faculty members in the Mechanical and Aerospace Engineering
Department at the University of Dayton for their support in my masters and in my PhD. I like to
thank Dr. Jose Camberos for being a role model when I first came to UD. I would especially like
to thank Dr. Margaret Pinnell for her unmitigated support, care and knowledge she has given me
throughout my time at the University of Dayton. She is a constant source of inspiration and she
taught me to be a better teacher, a better student and showed me how caring for students go a long
way in changing their lives and most importantly how to remain humble no matter where you are
in life.
Finally, I would like to thank my advisor, my mentor and my guru Dr. Aaron Altman for
teaching me valuable lessons inside and outside the classroom. His life and the way he lives it has
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always been an inspiration to me every single day. Words do not do justice for how much he taught
me over the years. He is the one person whom I always look up to in every aspect of my life. Not
just because he is my advisor but because he is a perfect example of how selfless a person can be.
Whenever I was interested in a research project, he always motivated me to pursue it. He has an
unique way of looking at a problem and analyzing results which amazes me every time. I have
always adored is patience over the years and work ethic and in every way he set the bar so high for
me to shoot for. Dr. Altman is also one of the kindest person I have ever met. He opened his house
and his family to me and he was always there as a shining beacon of light.
I like to quote a true story of the conversation between the Apollo 15 astronaut Dave Scott
and his professor Dr. Lee Silver. Dave Scott was a reluctant student of Professor Silver who
managed to adapt his teaching style to catch Scott’s interest. On his way to the moon on Apollo 15,
Professor Silver came to the mission control to talk to Dave Scott. Professor Silver said “Hey there
Dave. You have done a lovely job. You just don’t know how much we are jumping up and down
down here”. Scott smiled and replied “That’s because I happened to have a very good professor”.
Professor Silver realizes it is not just him and says “A whole bunch of them Dave”. Scott said “We
appreciate everything you did getting us ready for this thing. There is an awful lot to be seen and
done up here”. Professor Silver says “I’ll bet… We think you will find the first site to be revisited
on the moon”. Filled with gratitude Scott says, “Professor.. I hope someday we could get you up
here too”. Professor Silver contemplates the magnitude of it and says “That would be an amazing
adventure”. He paused for a moment and said “But I feel as if I have already been there, thanks to
you”. Scott looked up to the moon out of the window of the Apollo 15 command module and said
“Oh you are with us professor… every step of the way…”.
I am not sure if I’ll go to the moon but everywhere I go on this God given earth, Dr. Altman
will be there, every step of the way
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TABLE OF CONTENTS

ABSTRACT ……………………………………………………………………………….….. iv
DEDICATION ……………………………………………………………………………..… vii
ACKNOWLEDGEMENTS ………………………………………………….…………....… viii
LIST OF TABLES …………………………………………………………..….…………..... xii
LIST OF FIGURES …………….……………………………………………..….……….…. xiii
NOMENCLATURE ……………………..…………………………………………………… xvi

1. INTRODUCTION...................................................................................................................... 1
1. Motivation ................................................................................................................................ 1
2. Research Areas ......................................................................................................................... 3
3. Background Research ............................................................................................................... 7
2. TURBULENCE AND THEORETICAL MODELS ............................................................. 16
1. Derivation of the Governing Equations for Turbulent Flow .................................................. 16
2. Predicting Performance Information from the Free Shear Layer ........................................... 20
3. Exergy ..................................................................................................................................... 28
4. Wingtip Vortex Algebraic Models ......................................................................................... 30
3. EXPERIMENTAL SETUP ..................................................................................................... 32
1. Particle Image Velocimetry (PIV) .......................................................................................... 32
2. Wind/Water Tunnel Specifications ......................................................................................... 38
3. Experimental Setup ................................................................................................................. 40
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