PREDICTION OF AERODYNAMIC NOISE GENERATED BY WIND TURBINE BLADES By Rohith Giridhar Submitted to the graduate degree program in Aerospace Engineering and the Graduate Faculty of the University of Kansas School of Engineering in partial fulfillment of the requirements for the degree of Masters of Science. Committee: ________________________ Dr. Saeed Farokhi, Chairperson ________________________ Dr. Ray Taghavi, Committee Member ________________________ Dr. Shawn Keshmiri, Committee Member. Date Defended: 05/09/2016 The Thesis Committee for Rohith Giridhar certifies that this is the approved version of the following thesis: PREDICTION OF AERODYNAMIC NOISE GENERATED BY WIND TURBINE BLADES Committee: ________________________ Dr. Saeed Farokhi, Chairperson ________________________ Dr. Ray Taghavi, Committee Member ________________________ Dr. Shawn Keshmiri, Committee Member. Date Defended: 05/09/2016 i Abstract The preliminary step in the computational study of mitigating the aerodynamic noise generated by wind turbine blades involves accurate prediction of aerodynamic noise generated by a wind turbine rotor which can be used a basis for comparison. The NREL Phase VI HAWT rotor has been chosen to perform this study. This is achieved by first predicting the three dimensional flow field around the rotor through CFD analysis using SST k-ω turbulence model for wind speeds of 7m/s, 10m/s, 13m/s and 15m/s. CFD analysis has been performed using the rotating reference frame method at steady state conditions which resulted in predicting the flow field accurately with less computational time. The rotational periodic boundary condition with 1800 symmetry has been used with which one blade has been simulated instead of two. This reduced the mesh size and thus computational costs to perform the CFD analysis. To validate the prediction of flow field obtained through CFD analysis, performance characteristics and aerodynamic characteristics such as torque generated and trends of pressure coefficients at different span locations are validated against the time averaged experimental results and other results pertaining to the same published in previous computational study. The results obtained through CFD analysis show good agreement with both experimental results and previous computational results. Based on the trends of pressure coefficients predicted for different wind speeds we see that it is most accurate at a wind speed of 7m/s and this accuracy gradually decreases with increase in wind speed. Once the flow field was accurately predicted, this was used to predict both the location and magnitude of aerodynamic noise generated by the blade using the Curle broadband noise source model. Aeroacoustic analysis indicates that major noise sources are located near the tip of the blade and it gradually decreases as we move towards its root. This trend is observed at all four wind speed conditions. It is also observed that with increase in wind speeds, there is increase in the intensity of noise generated by the blades and thus increase in Sound Power Level across the blade. ii Acknowledgements Firstly, I would like to thank my advisor, Dr. Saeed Farokhi for giving me the opportunity to work on this research project on applied aerodynamics under his guidance. I thank him for both his valuable advice and his constant support and encouragement that he gave me throughout this project. I would also like to thank him for all his invaluable guidance that I received throughout my Master’s program. I would like to thank the other member of my committee Dr. Ray Taghavi and Dr. Shawn Keshmiri for taking their time to review this thesis and for providing comments and suggestions. I would like to thank Dr. Scott Schreck from the National Renewable Energy Laboratory for providing me with the experimental data necessary to validate this computational study. I would like to thank Mrs. Kristin Renells, Mr. Robert Curry and Dr. Jennifer Delgado for giving me the opportunity to work for the Physics and Astronomy Department as a Graduate Teaching Assistant all these semesters which gave me a great experience and has also provided me with financial support during my Master’s program. Finally I would like to thank Mrs. Radha Giridhar, Mr. S. Giridhar, Sri. R. Seshadri and Ms. Amirtha Giridhar, my family. It was through their constant support, love and encouragement that I have been able to complete this Master’s program. iii Table of Contents Abstract ..................................................................................................................................... ii Acknowledgements .................................................................................................................. iii List of Figures ......................................................................................................................... vii List of Tables ........................................................................................................................... ix Nomenclature .............................................................................................................................x 1. Introduction ............................................................................................................................1 1.1 Classification of Wind turbines ..........................................................................................3 1.2 Motivation and Objectives ..................................................................................................6 1.3 Overview of the thesis.........................................................................................................7 2. Literature Review ...................................................................................................................8 2.1. Literature review pertaining to CFD simulation of NREL Phase VI rotor ........................8 2.1.1. Ece Sagol, Marcelo Reggio and Adrian Ilinca ...............................................................8 2.1.2. Jang Oh MO, Young-Ho Lee .........................................................................................9 2.1.3. N.N Sorensen, J.A Michelsen, S. Schreck .....................................................................9 2.1.4. Y. Song and J.B Perot ..................................................................................................10 2.1.5. R P J O M vaj Rooij and E.A Arens ............................................................................11 2.2. Literature review pertaining to wind turbine noise ..........................................................11 2.2.1. Masoud Ghasemian and Amir Nejat ............................................................................12 2.2.2. A. Tadamasa and M. Zangeneh ...................................................................................12 2.2.3. Seunghoon Lee and Soogab Lee ..................................................................................13 2.2.4. R.C Ramachandran, G. Raman and R.P Dougherty ....................................................13 2.3. Literature review pertaining to experimental work on NREL Phase VI rotor .................14 3. Wind Turbine Aerodynamics ...............................................................................................19 3.1. Aerodynamic Models .......................................................................................................22 3.2. Basic Definitions ..............................................................................................................24 3.3. CFD Techniques ..............................................................................................................26 3.3.1. SST k-ω turbulence model ...........................................................................................28 iv 4. Wind Turbine noise ..............................................................................................................30 4.1. Mechanical noise .............................................................................................................30 4.2. Aerodynamic noise ..........................................................................................................32 4.2.1. Low frequency noise ....................................................................................................33 4.2.2. In flow turbulence noise ...............................................................................................35 4.2.3. Airfoil self noise ..........................................................................................................37 4.3. Basic Definitions ..............................................................................................................40 4.4. Linear wave equation .......................................................................................................43 4.5. Aeroacoustics ...................................................................................................................45 4.5.1. Curle Broadband noise source model ..........................................................................48 5. Methodology ........................................................................................................................50 5.1. STAR CCM+ ...................................................................................................................50 5.2 3- D CFD analysis of NREL Phase VI blade ....................................................................51 5.2.1. Polyhedral mesh ...........................................................................................................53 5.2.2. Mesh set up ..................................................................................................................54 5.2.3. Modifying the blade trailing edge ................................................................................56 5.2.4. Initial and boundary conditions....................................................................................58 5.2.5. Aeroacoustic simulation set up ....................................................................................60 6. Results and Discussions .......................................................................................................61 6.1. Trends of pressure coefficients at different span locations of the blade ..........................61 6.1.1. At wind speed of 7m/s .................................................................................................62 6.1.2. At wind speed of 10m/s ...............................................................................................65 6.1.3. At wind speed of 13m/s ...............................................................................................68 6.1.4. At wind speed of 15m/s ...............................................................................................71 6.2. Prediction of torque generated by NREL Phase VI blade ...............................................74 6.3. Prediction of noise sources at different wind speeds .......................................................76 6.3.1. At wind speed of 7m/s .................................................................................................76 6.3.2. At wind speed of 10m/s ...............................................................................................77 6.3.3. At wind speed of 13m/s ...............................................................................................79 v 6.3.4 At wind speed of 15m/s ..................................................................................................80 7 Conclusions ...........................................................................................................................82 8 Recommendations for Future Work ......................................................................................83 9 References .............................................................................................................................85 vi List of Figures Figure 1: Global Annual Installed Wind capacity for 2000-2015, Global Wind Energy Council ......... 2 Figure 2: Schematic representation of a Horizontal Axis Wind Turbine............................................... 3 Figure 3: Schematic representations of Vertical Axis Wind Turbines .................................................. 5 Figure 4: Wind Turbine tower present in the 24.4*36.6m NASA Ames wind tunnel .................. 14 Figure 5: NREL Phase VI rotor ..................................................................................................... 15 Figure 6: Linear chord length variation along the blade span ........................................................ 16 Figure 7: Nonlinear twist angle variation along the blade span ..................................................... 16 Figure 8: S809 Airfoil coordinates ................................................................................................ 17 Figure 9: Power curve for Pitch regulated and Stall regulated Wind Turbines ................................... 22 Figure 10: Growth in size of wind turbine over the years ................................................................... 23 Figure 11: Various noise sources from a wind turbine ........................................................................ 31 Figure 12: Flow around the rotor blade ............................................................................................... 33 Figure 13: Flow around a cylindrical wind turbine tower for upwind configuration .......................... 34 Figure 14: Flow around a cylindrical wind turbine tower for downwind configuration ..................... 34 Figure 15: Low frequency in flow turbulence noise ............................................................................ 36 Figure 16: High frequency inflow turbulence noise ............................................................................ 36 Figure 17: Trailing edge noise ............................................................................................................. 38 Figure 18: Laminar boundary layer vortex shedding noise ................................................................. 38 Figure 19: Tip noise ............................................................................................................................. 39 Figure 20: Stalled flow noise ............................................................................................................... 40 Figure 21: Blunt trailing edge noise ..................................................................................................... 40 Figure 22: CAD model of blade geometry ........................................................................................... 52 Figure 23: Semi-Cylindrical domain containing NREL Phase VI Blade ............................................ 53 Figure 24: Mesh generated on blade surface ....................................................................................... 54 Figure 25: Blade refinement region ..................................................................................................... 55 Figure 26: Density region around the blade surface ............................................................................ 56 Figure 27: Blunt trailing edge created along blade span ...................................................................... 57 Figure 28: Plot of Reynolds number based on chord versus wind speeds at three span locations ...... 57 vii Figure 29: Pressure coefficient comparison at 30% blade span for 7m/s ............................................ 62 Figure 30: Pressure coefficient comparison at 46.6% blade span for 7m/s ......................................... 62 Figure 31: Pressure coefficient comparison at 63.3% blade span for 7m/s ......................................... 63 Figure 32: Pressure coefficient comparison at 80% blade span for 7m/s ............................................ 63 Figure 33: Pressure coefficient comparison at 95% blade span for 7m/s ............................................ 64 Figure 34: Pressure coefficient comparison at 30% blade span for 10m/s .......................................... 65 Figure 35: Pressure coefficient comparison at 46.6% blade span for 10m/s ....................................... 65 Figure 36: Pressure coefficient comparison at 63.3% blade span for 10m/s ....................................... 66 Figure 37: Pressure coefficient comparison at 80% blade span for 10m/s .......................................... 66 Figure 38: Pressure coefficient comparison at 95% blade span for 10m/s .......................................... 67 Figure 39: Pressure coefficient comparison at 30% blade span for 13m/s .......................................... 68 Figure 40: Pressure coefficient comparison at 46.6% blade span for 13m/s ....................................... 68 Figure 41: Pressure coefficient comparison at 63.3% blade span for 13m/s ....................................... 69 Figure 42: Pressure coefficient comparison at 80% blade span for 13m/s .......................................... 69 Figure 43: Pressure coefficient comparison at 95% blade span for 13m/s .......................................... 70 Figure 44: Pressure coefficient comparison at 30% blade span for 15m/s .......................................... 71 Figure 45: Pressure coefficient comparison at 46.6% blade span for 15m/s ....................................... 71 Figure 46: Pressure coefficient comparison at 63.3% blade span for 15m/s ....................................... 72 Figure 47: Pressure coefficient comparison at 80% blade span for 15m/s .......................................... 72 Figure 48: Pressure coefficient comparison at 95% blade span for 15m/s .......................................... 73 Figure 49: Comparison of torque prediction ........................................................................................ 75 Figure 50: Sound Power Level distribution of noise sources on pressure surface of blade for 7m/s .. 76 Figure 51: Sound Power Level distribution of noise sources on suction surface of blade for 7m/s .... 77 Figure 52: Sound Power Level distribution of noise sources on pressure surface of blade for 10m/s 77 Figure 53: Sound Power Level distribution of noise sources on suction surface of blade for 10m/s .. 78 Figure 54: Sound Power Level distribution of noise sources on pressure surface of blade for 13m/s 79 Figure 55: Sound Power Level distribution of noise sources on suction surface of blade for 13m/s .. 79 Figure 56: Sound Power Level distribution of noise sources on pressure surface of blade for 15m/s 80 Figure 57: Sound Power Level distribution of noise sources on suction surface of blade for 15m/s .. 80 viii List of Tables Table 1: Specifications of the blade required to create the CAD model .......................................17 Table 2: Operating conditions used at different wind speeds ........................................................18 Table 3: Mesh and CFD settings ....................................................................................................59 Table 4: Aeroacoustic settings .......................................................................................................60 ix