Two-element wingsail as wind-assisted ship propulsor

Abstract

Throughout the last century, high energy-density fossil fuels have been the main energy source for merchant shipping. As the world tries to minimize the effects of global warming, shipowners are forced to significantly reduce their $CO_2$ emissions. Because alternative fuels are expensive, the interest in wind energy, which has been the single energy source for centuries, is renewed. There is a desire for passive and inexpensive wind-assisted ship propulsion (WASP) systems. The two-element wingsail is a promising option. This combination of a wing with a slotted flap can generate more than one and a half times more lift compared to a one-element alternative. It is used in the aviation industry as a high-lift device and has been applied on board yachts in the America's Cup and other races. Used as WASP system places new requirements as limited deck space and the ability to tack. This report describes the effects of the geometry on the performance of the wingsail and advises an ideal configuration that maximizes the overall thrust contribution. Both fixed design parameters such as the profiles and chord lengths and adjustable settings as the angle of attack and flap angle are examined. This is achieved by doing systematical geometry variations at a Reynolds number of 1.86 $\cdot 10^6$ using 2D CFD simulations. The design goal was to maximize the lift coefficient. The variations show that the lift coefficient is very sensitive to the flap angle. Increasing this angle increases the lift, but requires a well-positioned slot to ensure attachment of the flow along the suction side of the flap. The influence of the profile thickness is small. However, a minimum flap thickness was required to avoid flow separation at low angles of attack. Stall was always observed around the leading edge of the flap. The maximum lift coefficient of 2.73 at an angle of attack of 10 degrees was found for a configuration with a NACA0024 wing and NACA0012 flap profile, equal chord lengths, and a flap angle of 25 degrees. The corresponding slot had an offset of 0.5 and a side placement of 2.5 percent of the total chord length. Using the same method, the effects of adding a steering flap downstream of the wingsail were investigated. This third flap should keep the wingsail stable at a specific angle of attack. This ability is interesting for WASP purposes because it eliminates the need for fast-responding software-based control systems. The simulations show that the steering flap, which had a fixed chord length of 30 percent of the wingsail, can stabilize the system. Compared to the standard configuration the decrease of the lift was approximately 10 percent. The steering flap operates in the wake of the wingsail, which causes a decrease of drag of approximately 5 percent. This is an interesting feature because it reduces the negative effects of the steering flap. Depending on their positions, the interaction between all three elements can further enhance the performance of the whole system.