Simulation and Characterization of Low-Frequency Flow Oscillations Over a Symmetrical Sail at Large Attack Angles for an Unmanned Sailboat

Abstract

Abstract Unmanned sailboats harvest wind energy as their main thrust power. This ability makes them outstanding for continuous marine hydrometeorological research on open seas. Different from airplanes, where small attack angles are generally used to avoid a stall, sailboats typically sail with large attack angles (more than 20 degrees) in downwind conditions. Although stall happens and the lift drops dramatically, the drag now acts as the main contributor to thrust. However, at low Reynolds numbers (around 10^5) and large attack angles, low-frequency flow oscillations would be easily triggered by vortex shedding. Such periodic or quasi-periodic aerodynamic forces would cause an undesirable change at the moment, which would result in large force fluctuations and random behaviors of the sail. In this study, LES simulations are carried out at Reynolds number 3.6E5, with the attack angle varying from 5 to 90 degrees for a 3D NACA0012 sail. Validation is performed at the same Reynolds number using experimental data. The influence of turbulence intensity is also explored. The results show that the wiggles of both lift and drag coefficients increase with the attack angle. The fully separated flow will form a recirculating flow region on the low-pressure surface of the sail, which triggers vortices at different scales. The flow lacks enough energy to keep a fixed shape, and the vortex shedding causes variations in the pressure leading to oscillations in the lift and drag. The dominant frequency locates in the range of 0.5–10Hz, which straightforwardly indicates a low-frequency oscillation. It also shows that a change in attack angle will alter the frequency of the low-frequency oscillations. Designers of the unmanned sailboat should keep this information in mind when determining the attack angle in a specific condition.