Effects of leading edge tubercles on wing sail performance

In this thesis study, effects of leading edge tubercles on 2013 America's Cup boat wing sail are investigated by using commercial Computational Fluid Dynamics (CFD) code Star-CCM+. Adding tubercles on the leading edge of the sail is inspired by humpback whales, which are fast and maneuverable animals along baleen whales thanks to their distinctive flippers. Understanding the basic principles of sailing physics and difference between upwind and downwind sailing plays important role on designing a boat and its sails. Therefore, as a beginning, theoretical background of sailing physics is studied and explained in the chapter 2. Basic principles of lift generation and underlying flow phenomena are also tried to be explained in this chapter. There are numerous incorrect theories about lift generation which generates misconceptions about this nature phenomenon. Most widely accepted correct theory is lift generation by turning fluid which obtained by Kutta and Joukowski. Wing sail design and current state of the art bio-inspired wing design is explained in the chapter 3 of this thesis. It has been seen from the studies of whale flippers and bio-inspired wings that tubercles on the leading edge of the wings delay stall and provide better Lift/Drag ratio in high angle of attacks (AoA) compared to the plain wing with no undulations. Current theories about the physics of this phenomena are also explained in chapter 3. It has been observed from both in literature review and in this study that tubercles play similar role as vortex generators and exchange momentum in boundary layer which attaches flow to airfoil and creates better pressure distribution. Before the investigation of wing sails, validation study is carried out to determine optimum mesh values, numerical and mathematical models. Firstly, grid independence study is carried out and best mesh setting are determined for further analysis's. NACA 0012 airfoil wing performance is compared to experimental data performed in wind tunnel by Tezel et al (2017). Wing is tested at angles ranging from 10 to 32 degrees by using Unsteady Reynolds Averaged Navier Stokes Equation Solver (U-RANS) of Star CCM+. It has been seen that best results are achieved with implicit unsteady method by using Shear Stress Transport k-w (SST k-w) turbulence model. Same lift and drag behavior is observed between CFD and wind tunnel tests. Also NACA 0012 wing with leading edge tubercles analyzed and compared to wind tunnel tests. Both wind tunnel and CFD study show that modified wing is giving better overall performance than base NACA 0012 wing. Modified wing did not stall as base wing at AoA around 24 degrees and continued to provide lift in high angles of attacks. To conclude, similar results are archived between wind tunnel experiment CFD analysis after validation study. 2013 America's Cup boat wingsail geometry is developed according to the race rules to be able to measure effects of tubercles on its performance. Profile selection, element number and dimensions of sail are determined according to both literature review and rule book. After design of sail geometry, test matrix for CFD analysis is generated with totally six different tubercle dimensions. Sinusoidal tubercles are placed on leading edge with different wave lengths and amplitudes varying as a function of chord length. As a beginning, plain wing sail's lift coefficient, drag coefficient and lift to drag ratio is measured in CFD with different angle of attacks ranging from 140 to 320. It has been observed that stall angle is around 260. Therefore, modified wing sails with tubercles are tested in CFD at angle of attacks 230 and 300 to see performance change in pre-stall and post-stall regimes. Best results are achieved with largest wavelength and smallest amplitude tubercles in test matrix. Stall angle remained same with modified wingsail with tubercles while increasing lift/drag ratio after stall angles and having negligible negative effect in pre-stall regime. As a final step, this modified wingsail is compared with plain wingsail in all angles of attack between 140 to 320 with incremental step of 20. Tubercle geometry on the leading edge increased wing sail performance as expected after post-stall regimes by increasing lift/drag ratio compared to plain wingsail. In small angles of attack performance almost remained same between two wingsails. Final results, comments and future study suggestions are discussed in chapter five.