Relationship between Rotor Wake Structures and Performance Characteristics over a Range of Low-Reynolds Number Conditions THESIS Presented in Partial Fulfillment of the Requirements for the Degree Master of Science in the Graduate School of The Ohio State University By Mark Louis Sutkowy Jr. Graduate Program in Aeronautical and Astronautical Engineering The Ohio State University 2018 Thesis Committee: Dr. James W. Gregory, Advisor Dr. Jeffrey P. Bons Dr. Matthew H. McCrink Copyrighted by Mark Louis Sutkowy Jr. 2018 Abstract Small-scale rotors exhibit degraded aerodynamic efficiency, which has been linked to non-ideal losses within their wake. Many small unmanned aircraft systems (UAS) are powered by such rotors, and are currently at the forefront of aerospace research for a multitude of innovative applications. As such, a comprehensive understanding of their operational capabilities is critical for implementation in the field. A great deal of attention has been given to characterize the performance of large- scale rotor models. However, similar studies for small-scale, low Reynolds number (Re) applications has received relatively little attention. This work seeks to gather insight into the behavior of the rotor wake structures as a function of Re, relate this to performance capabilities and the corresponding far-field acoustic signature. Two-component particle image velocimetry (PIV), performance, and acoustic measurements were performed using three small-scale, NACA 0012 rotors operated over a range of low-Reynolds number conditions. Rotor geometry and operational speed (Ω) were varied to obtain the desired Re variation. Spanwise PIV has demonstrated an absence of tip vortex formation as the operational thrust coefficient (CT) is increased, suggesting the presence of outboard tip stalling. Phase-locked, chordwise PIV has confirmed this hypothesis, showing the development of flow separation and a highly turbulent downstream wake. Thrust and ii torque measurements show degraded rotor performance at the onset of these conditions, especially at low Re. A vortex identification scheme was used to locate downstream tip vortices and characterize their size, swirl velocity, and aperiodic wandering behavior for different operational conditions. When observed at constant wake age, the wandering motion of the vortices behaved independently of vortex Reynolds number (Rev) scaling. The normalized standard deviation of the tip vortex wander was found to match well with historically observed trends, but a difference in magnitude suggests aperiodicity depends strongly on blade number instead. Chordwise PIV measurements revealed the wake characteristics at moderate collective angles (θ) produce periodic counter-rotating structures which are attributed to laminar boundary layer vortex-shedding. Acoustic measurements at similar operating conditions show significant broad-band, high frequency peaks. The broad-band peaks were found to correlate well with a physical quantification of the shedding phenomenon. iii Dedication FOR MY FAMILY. THEIR SELFLESSNESS MADE HIGHER EDUCATION POSSIBLE. FOR MY FRIENDS, WHO ARE STILL HERE WHILE MY ATTENTION WAS ELSEWHERE. FOR AVA, THE BRIGHTEST LIGHT AT THE END OF THE TUNNEL. MOST OF ALL, FOR MY LORD AND SAVIOR, JESUS CHRIST. HE IS THE ONE WHO CREATED THE QUESTIONS FOR WHICH SCIENTISTS SEEK ANSWERS, AND HE IS THE ONE WHO ENABLES US TO ANSWER THEM. IN HIS WISDOM, I CAME TO SEE THAT HOURS SPENT IN A LAB ARE MINUTE COMPARED TO ETERNITY IN PARADISE. iv Acknowledgments In my experience, I have come to learn that progress in scientific research demands resilience and curiosity. For fueling these qualities, I express gratitude to my adviser, Dr. Gregory, and mentor, Dr. McCrink. Their own accomplishments and passion for aerospace have shown that work can be an enjoyable and gratifying experience. Their guidance has been instrumental in my development as a researcher and engineer. I am grateful to my colleagues for sharing their expertise and unique skill sets which were critical to the completion of this work. I am appreciative of Achal Singhal for his efforts in developing my fluency with experimental techniques. His patience is truly remarkable. Anshuman Pandey and Braxton Harter played critical roles in the acquisition, discussion, and interpretation of the data. Collaboration with Ryan Thorpe and Wenbo Zhu was also valuable during course of my graduate work. Josh Gueth and Ken Fout are appreciated for sharing their machining expertise which enabled the design, fabrication, and assembly of my experiments. This research was partially funded by the Government under Agreement No. W911W6-17-2-0002. The U.S. Government is authorized to reproduce and distribute reprints for Governmnet purposes notwithstanding any copyright notation thereon. The views and conclusions contained in this document are those of the authors and should not v be interpreted as representing the official policies, either expressed or implied, of the Aviation Development Directorate or the U.S. Government. I am also grateful to colleagues with the Georgia Institute of Technology Vertical Lift Research Center of Excellence, including Narayanan Komerath, Ganesh Rajagopalan, Gloria Yamauchi, Oliver Wong, and Tom Thompson for helpful discussions on this work. vi Vita 2013................................................................Co-op Employee, Passport 20 Systems, GE Aviation, Cincinnati, Ohio 2015................................................................Intern, Manufacturing Engineer, Ford Motor Company, Cleveland, Ohio 2015................................................................B.S. Mechanical Engineering, University of Dayton, Dayton, Ohio 2016................................................................Research Intern, Inlets and Nozzles Division, NASA Glenn Research Center, Cleveland, Ohio 2016 to present ...............................................Graduate Research Assistant, Mechanical and Aerospace Engineering, Aerospace Research Center, The Ohio State University, Columbus, Ohio vii Publications Conference Publications 1. Sutkowy, M., Pandey, A., McCrink, M., and Gregory, J., “Rotor Wake Structure Development in Low Reynolds Number Conditions,” AIAA SciTech Forum, Orlando, Florida, January 8, 2018, DOI: 10.2514/6.2018-1830 2. Sutkowy, M., Harter, B., McCrink, M., and Gregory, J., “Impact of Wake Structure Characteristics on Small-Scale Rotor Performance over a Range of Reynolds Numbers,” American Helicopter Society 74th Annual Forum, Phoenix, Arizona, May 14, 2018, SKU #:74-2018-0135 3. Wang, Z., Pandey, A., Sutkowy, M., Harter, B., McCrink, M., Gregory, J., and Zhuang, M., “A Comprehensive Approach to Study Aerodynamics and Aerocoustics around Small Multicopter Unmanned Aerial Systems,” AIAA SciTech Forum, Orlando, Florida, January 8, 2018, DOI: 10.2514/6.2018-0268 4. Pandey, A., Sutkowy, M., McCrink, M., and Gregory, J., “Aerodynamic Characterization of a Quad-Rotor Helicopter,” AIAA SciTech Forum, Orlando, Florida, January 8, 2018, DOI: 10.2514/6.2018-1526 Fields of Study Major Field: Aeronautical and Astronautical Engineering viii Table of Contents Abstract .............................................................................................................................. ii Dedication ......................................................................................................................... iv Acknowledgments.............................................................................................................. v Vita ................................................................................................................................... vii List of Tables ................................................................................................................... xii List of Figures ................................................................................................................. xiii Nomenclature ................................................................................................................... xx Chapter 1. Introduction ...................................................................................................... 1 Chapter 2. Background ..................................................................................................... 5 2.1 Rotor Performance .................................................................................................... 6 2.2 Rotor Wake Features............................................................................................... 11 2.3 Acoustic Signature .................................................................................................. 17 Chapter 3. Description of the Experiment ....................................................................... 18 3.1 Rotor Characteristics ............................................................................................... 18 3.2 Test Stand Design ................................................................................................... 20 3.3 Performance Measurements .................................................................................... 21 ix 3.3.1 Thrust ............................................................................................................... 21 3.3.2 Torque .............................................................................................................. 23 3.4 Flowfield Measurements ........................................................................................