Active Flow Separation Control of a Laminar Airfoil at Low Reynolds Number DISSERTATION Presented in Partial Fulfillment of the Requirements for the Degree Doctor of Philosophy in the Graduate School of The Ohio State University By Nathan Owen Packard, B.S, M.S Graduate Program in Aeronautical and Astronautical Engineering The Ohio State University 2012 Dissertation Committee: Dr. Jeffrey P. Bons, Advisor Dr. Jen-Ping Chen Dr. Mohammad Samimy Dr. Andrea Serrani Copyright by Nathan Owen Packard 2012 Abstract Detailed investigation of the NACA 643-618 is obtained at a Reynolds number of 6.4x104 and angle of attack sweep of -5° < α < 25°. The baseline flow is characterized by four distinct regimes depending on angle of attack, each exhibiting unique flow behavior. Active flow control is exploited from a row of discrete holes located at five percent chord on the upper surface of the airfoil. Steady normal blowing is employed at four representative angles; blowing ratio is optimized by maximizing the lift coefficient with minimal power requirement. The range of effectiveness of pulsed actuation with varying frequency, duty cycle and blowing ratio is explored. Pulsed blowing successfully reduces separation over a wide range of reduced frequency (0.1-1), blowing ratio (0.5–2), and duty cycle (0.6–50%). A phase-locked investigation, by way of particle image velocimetry, at ten degrees angle of attack illuminates physical mechanisms responsible for separation control of pulsed actuation at a low frequency and duty cycle. Temporal resolution of large structure formation and wake shedding is obtained, revealing a key mechanism for separation control. The Kelvin-Helmholtz instability is identified as responsible for the formation of smaller structures in the separation region which produce favorable momentum transfer, i assisting in further thinning the separation region and then fully attaching the boundary layer. 4 Closed-loop separation control of an oscillating NACA 643-618 airfoil at Re = 6.4x10 is investigated in an effort to autonomously minimize control effort while maximizing aerodynamic performance. High response sensing of unsteady flow with on-surface hot- film sensors placed at zero, twenty, and forty percent chord monitors the airfoil performance and determines the necessity of active flow control. Open-loop characterization identified the use of the forty percent sensor as the actuation trigger. Further, the sensor at twenty percent chord is used to distinguish between pre- and post- leading edge stall; this demarcation enables the utilization of optimal blowing parameters for each circumstance. The range of effectiveness of the employed control algorithm is explored, charting the practicality of the closed-loop control algorithm. To further understand the physical mechanisms inherent in the control process, the transients of the aerodynamic response to flow control are investigated. The on-surface hot-film sensor placed at the leading edge is monitored to understand the time delays and response times associated with the initialization of pulsed normal blowing. The effects of angle of attack and pitch rate on these models are investigated. Black-box models are developed to quantify this response. The sensors at twenty and forty percent chord are also monitored for a further understanding of the transient phenomena. ii Dedicated to Telisha. iii Acknowledgments First, and foremost, I acknowledge my beautiful wife, Telisha. I thank her for standing by my side and supporting me. I thank her for joining me on this difficult yet extremely rewarding experience. I also want to thank my children: Sidney, 6, Owen, 4, and Tanner, 3. I hope to instill in you the desire to get all the education you can, and to never stop learning. I need to thank my parents, Paul and Sherrie, for instilling in me the importance of doing well in school. I must acknowledge Dr. Jeffrey Bons. He has been an inspiration and guide, both professionally and personally. I thank him for his tireless efforts, and recognize how inadequate this work would be without his assistance. Lastly, I acknowledge God for his support and direction. To him I owe all that I have and am. iv VITA April 2007……………………………………….B.S. Applied Physics, Minor Mathematics, Brigham Young University August 2009……………………………………..M.S. Mechanical Engineering, Brigham Young University, Thesis Title: Numerical Characterization of the Inlet Flow of Eleven Radial Flow Turbomachines PUBLICATIONS Packard, N. O., Thake, M. P., Bonilla, C. H., Gompertz, K., Bons, J. P., “Active Control of Flow Separation on a Laminar Airfoil,” Under Review, AIAA Journal, Feb. 2012. Packard, N. O. and Bons, J. P., “Pulsed Blowing on a Laminar Airfoil at Low Reynolds Number,” AIAA 2011-3173, 29th AIAA Applied Aerodynamics Conference, Honolulu, HW, 27-30 June, 2011. Packard, N. O. and Bons, J. P., “Closed-Loop Separation Control of Unsteady Flow on an Airfoil at Low Reynolds Number,” AIAA-2012-754, 50th AIAA Aerospace Sciences Meeting, Nashville, TN, 9-12 Jan, 2012. FIELDS OF STUDY Major Field: Aeronautical and Astronautical Engineering Specialization: Experimental Fluid Dynamics v Table of Contents Abstract ........................................................................................................................... i Acknowledgments ......................................................................................................... iv VITA .............................................................................................................................. v Table of Contents .......................................................................................................... vi List of Tables ............................................................................................................... viii List of Figures ............................................................................................................... ix Nomenclature ............................................................................................................ xviii Chapter 1: Introduction .................................................................................................. 1 Chapter 2: Experimental Setup ....................................................................................... 9 2.1 Wind Tunnel ......................................................................................................... 9 2.2 Airfoil ................................................................................................................... 9 2.3 Flow Control ....................................................................................................... 10 2.4 Data Acquisition ................................................................................................. 18 Chapter 3: Results ........................................................................................................ 24 3.1 Baseline Airfoil Characterization ....................................................................... 24 vi 3.2 Open-loop Flow Control ..................................................................................... 30 3.3 Instability Classification ..................................................................................... 55 3.4 Closed-Loop Separation Control ........................................................................ 68 3.5 Transients of the Aerodynamic Response to Flow Control ................................ 86 Chapter 4: Future Considerations ............................................................................... 112 Chapter 5: Conclusions .............................................................................................. 119 References .................................................................................................................. 122 vii List of Tables Table 1 Combinations of frequencies and duty cycles used in search of dynamic motion ............................................................................................................................... 47 Table 2. FOPDT model parameters for 2° ≤ α ≤ 10°, along with a nominal model for 5° ≤ α ≤ 9° ......................................................................................................................... 92 Table 3. Comparison of model parameters for the static airfoil versus a dynamic airfoil at nominal pitch rate and double pitch rate for α = 5°, 7° and 9°. .................................. 111 viii List of Figures Figure 1. The flow structure of a laminar separation bubble. Taken from [39]. ............ 3 Figure 2. Typical surface pressure distributions with and without a separation bubble. Taken from [5]. ................................................................................................................... 4 Figure 3. NACA 643-618 airfoil with pressure tap and flow control locations. ............. 5 Figure 4. Internal structure of the NACA 643-618 airfoil with pressure taps and active flow control. ...................................................................................................................... 10 Figure 5. Streamlines and normalized mean velocity magnitude for BR = 0.5, depicting the domain of slow moving reverse flow formed behind the jet. Taken from [21]. ......... 11 Figure 6. Streamlines and normalized mean velocity magnitude for BR = 2.5, depicting the advection of the jet far from the wall. Taken from [21]. ............................................ 12 Figure 7. Cartoon depicting