Abstract This paper simulates and analyzes a stationary cylindrical rigid wind sail, termed JetSailTM (JetSail), which employs CoFlow Jet (CFJ) active flow control (AFC) with rotating tip cap and a fence at near tip span to enhance aerodynamic lift and energy efficiency. Unlike a Flettner rotor that relies on rotating the cylinder to generate lift, a JetSail uses a stationary cylinder with an internal fan system that withdraws a small amount of air mass flow on the leeward side, pressurizes it, and ejects it back to the main flow tangentially upstream at the suction peak location to energize the boundary layer and attach the flow around the cylinder. Three-dimensional CFD simulations were performed using a high-fidelity unsteady Improved Delayed Detached Eddy Simulation (IDDES) solver based on the Spalart–Allmaras one-equation turbulence model. For a JetSail with an aspect ratio of 8, cap tipspeed ratio CTSR of 2.55, and a fence positioned at 85% the CFD simulation shows that the JetSail can reach an extradentary lift coefficient CL of 16.8. Furthermore, under such a high lift coefficient, it achieves excellent aerodynamic efficiency CL/CD of 3.19 and lift-power efficiency CL/PC of 8.129, where PC is the coefficient of the power required for CFJ AFC. However, the optimal aerodynamic efficiency and lift-power efficiency are obtained at a lower CL level of 8.1 to 10.6 with CL/CD of 5.4 and CL/PC of 25.3. These characteristics provide an optimal control law to operate JetSail at high lift high power mode (e.g. CL = 16) for side apparent wind and low lift high power mode (e.g. CL = 10) for headwind. In addition to the advantages of CFJ AFC, the high performance is benefited from the rotating cap at an RPM of 162, which increases the CL by 20%, CL/CD by 7%, and most importantly augments CL/PC by 68%. The disk rotation power is not counted and is estimated to be no greater than 5% of the CFJ power. More detailed study on the rotating power will be performed in the next step. The fence brings a smaller benefit with a 5% increase of CL and a slight increase of CL/CD. For a Rotterdam–New Jersey route-level analysis for a 3 m × 24 m JetSail using ERA5 wind matrix subject to the maximum power extraction of 200 kW from the ship engines, a JetSail produces about 149 kW net propulsive power at a ship speed of 12 kn, varying from 126 kW at 10 kn to 189 kW at 16 kn. This study demonstrates that the JetSail using stationary cylinders with adaptive CFJ flow control and tip treatment provides a novel high potential rigid wind sail technology that achieves unprecedently high thrust, high propulsive power, and high energy efficiency. Keywords CoFlow Jet (CFJ); JetSail; active flow control; wind-assisted propulsion; available effective power; tip-vortex mitigation
Ren et al. (Tue,) studied this question.