The Dimple Effect on Flettner Rotors for the Wind Power Drive System of a Sailing Merchant Ship

Wind-powered ship propulsion is investigated in relation to reducing ship exhaust emissions, such as the application of Flettner rotor systems. How to increase rotor thrust's contribution to ship speed is an intriguing research question. This paper investigates the influence of dimples on the surface of a Flettner rotor on its ability to generate thrust for a ship. The CFD code, which includes the Reynolds average Navier-Stokes equation and the k-ω model for turbulent flow, is utilized to analyse three rotor designs: one with a smooth surface and two with dimple surfaces of varying diameters. Lift and drag are computed by taking into account varying spin ratios and the incoming wind angle. The CFD results are validated by comparing them to experimental data gathered in a wind tunnel and determining that their error is less than 5%. The addition of dimpled surfaces to the rotor increases lift by 69.6% and decreases drag by 14.8% due to the smooth surface. For the case study, the two rotors are mounted on the sailing ship 900 DWT and can generate the most thrust to give the ship a speed of 8.5 knots with a spin ratio of 4 and apparent wind angles of 90^o or 270^o relative to its speed.