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Experimental and Numerical Analysis of the Drag Force on Surfboards with Different Shapes By Experimental and Numerical Analysis of the Drag Force on Surfboards with Different Shapes By Omar Lahlou Bachelor of Science University of California, San Diego A thesis submitted to the College of Engineering and Sciences at Florida Institute of Technology in partial fulfillment of the requirement for the degree of Master of Science in Ocean Engineering Melbourne, Florida May, 2019 We the undersigned committee hereby approve the attached thesis, “Experimental and Numerical Analysis of the Drag Force on Surfboards with Different Shapes” by Omar Lahlou, for partial fulfilment of the degree of Master of Science in Ocean Engineering. ___________________________________ Robert J. Weaver, Ph.D. Associate Professor of Ocean Engineering Major Advisor ___________________________________ Stephen Wood, Ph.D. Associate Professor of Ocean Engineering ___________________________________ Yahya Sharaf Eldeen, Ph.D., P.E. Professor of Mechanical Engineering ___________________________________ Ronnal Reichard, Ph.D. Professor of Ocean Engineering ___________________________________ Richard Aronson, Ph.D. Professor and Head Department of Ocean Engineering and Marine Sciences Abstract Title: Experimental and Numerical Analysis of the Drag Force on Surfboards with Different Shapes. Author: Omar Lahlou Committee chair: Robert J. Weaver The aim of the present study is to define the hydrodynamic resistance of surfboards with different designs. Decreasing the drag acting on a board helps the surfer to attain higher surfing performances. It helps to attain higher speeds and maneuvering for the surfer. In this thesis, theoretical, experimental and computational approaches are used to explain how the drag changes with different surfboard shapes. Four surfboards with varying shapes were towed at different speeds and the resulting force was recorded. Two computational models were developed using Star CCM and Flowsquare to confirm the experimental results. The drag force on the boards increases with increasing area. The increased wetted surface area resulted in increased buoyancy but decreased mobility. The drag force could be broken down into frictional and form drag. Form drag is a function of the shape of the object and decreases with improved surfboard design, like pinned tail instead of squared. Longer surfboards attain planing mode at higher speeds. Displacement mode is dominated by form drag whereas planing mode is dominated by frictional drag. Drag coefficients for the various boards ranged from 0.1 at low speeds to 0.05 at high speeds. Shorter surfboards had higher drag coefficients because of higher shear stress. iii Table of Contents Chapter I: Introduction and Motivations ........................................................................ 1 1.1. Introduction and Hypothesis .................................................................................... 1 1.2. History and context................................................................................................... 2 1.3. Thesis approach ........................................................................................................ 3 Chapter II: Theory ........................................................................................................... 4 2.1. The Drag Force ......................................................................................................... 4 2.1.1. Skin Friction and Form Drag ................................................................................ 5 2.1.2. Drag Coefficient .................................................................................................... 6 2.2. Turbulence modeling ................................................................................................ 7 2.2.1. Reynolds Number .................................................................................................. 8 2.2.2. Froude Number .................................................................................................... 11 2.2.3. Boundary effects .................................................................................................. 11 2.2.4. Boundary Layer Thickness ................................................................................. 12 2.2.5. Shear Stress .......................................................................................................... 13 2.3. Displacement and Planing regimes ....................................................................... 14 2.3.1. Hull Speed ............................................................................................................ 15 Chapter III: Methodology .............................................................................................. 17 3.1. Surfing Data Analysis ............................................................................................ 17 3.1.1. Material and Calibration ..................................................................................... 18 3.2. Experimental Analysis using a Load Cell ............................................................. 20 3.2.1. Material and Calibration ..................................................................................... 22 3.2.2. Bronze Aluminum bar dimensions calculation ................................................. 24 3.2.3. Strain Gauges and Wheatstone bridge ............................................................... 24 3.2.4. Load Cell Design ................................................................................................. 28 3.2.5. Load Cell Calibration .......................................................................................... 31 3.3. Star CCM simulation .............................................................................................. 32 iv 3.4. Flowsquare Simulation ........................................................................................... 33 Chapter IV: Results ........................................................................................................ 34 4.1. Surfing Data Analysis ............................................................................................ 34 4.1.1. Hobo Data ............................................................................................................ 34 4.1.2. Spectral density.................................................................................................... 37 4.1.3. Iribarren Number ................................................................................................. 40 4.1.4. Surfboard Data ..................................................................................................... 42 4.1.5 Data Analysis ........................................................................................................ 45 4.1.5.1. Surf Identification............................................................................................. 45 4.1.5.2. Dimensionless parameters ............................................................................... 46 4.1.5.3. Drag Coefficient ............................................................................................... 47 4.2. Experimental Analysis using a Load Cell ............................................................. 48 4.2.1. Load Cell data ...................................................................................................... 48 4.2.2. Drag Force ........................................................................................................... 49 4.2.3. Drag Coefficient .................................................................................................. 56 4.2.4. Magnetometer data and Pitch Angle .................................................................. 57 4.3. Star CCM+ simulation ........................................................................................... 61 4.3.1. Drag Force ........................................................................................................... 62 4.3.2. Drag Coefficient .................................................................................................. 63 4.4. Flowsquare simulation ........................................................................................... 64 Chapter V: Discussion ................................................................................................... 67 5.1. Theoretical and Surfing data analysis ................................................................... 67 5.2. Experimental Analysis using a Load Cell ............................................................. 68 5.3. Experimental and Computational data comparison .............................................. 77 Chapter VI: Conclusion ................................................................................................. 80 References ...................................................................................................................... 82 Appendices ..................................................................................................................... 89 Appendix A: Theoretical Drag analysis for a flat plate ............................................... 89 v Appendix B: Instrumentation calibration and preparation .......................................... 92 B.1. Hobo Pressure Gauge Instrument preparation and calibration ........................... 92 B.2. Hobo calibration certificate ................................................................................... 93 B.3. Lowell Accelerometers/Magnetometer
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