Unsteady RANS and IDDES study on a telescopic crescent-shaped windsail

Paper in proceeding, 2022

Throughout the years, a large number of research projects have been carried out to develop and evaluate different concepts for wind-assisted propulsion, which indicated optimistic fuel-saving rates. The propulsive performance of a single rigid windsail in the view of time-averaged properties has been analyzed in previous studies. However, the unsteady characteristics of the external loads are believed to induce vibration of the structure, thus, also need to be resolved. Meanwhile, to predict the multi-sail interaction, the wake flow is also important. Therefore, it is necessary to improve the CFD model. In this study, full-scale simulations with both unsteady RANS and IDDES turbulence models are performed and compared to analyze the flow field, especially the unsteady characteristics and the wake flow. The flow separation and the vortex shedding at the suction side, development and dissipation of the vortices in the wake regions, as well as the reduction of the lift due to tip vortices are discussed. Meanwhile, the telescopic function of the windsail is also studied by analyzing sails with several different spanwise lengths. It is concluded that unsteady RANS and IDDES simulations provide similar predictions in the view of time-averaged loads, but the unsteady characteristics of the flow field are significantly different. Besides, from IDDES simulations, a much more complex vortex structure in the wake region is predicted, leading to high-frequency oscillations of the external loads, which probably put forward higher requirements on the structure.