Experimental and Numerical Assessment of the Inflow Conditions for a Wind Propulsion Unit Onboard an Ocean-Going Vessel

Abstract Wind-assisted propulsion using retrofitted wind propulsion units (WPUs) is gaining renewed attention as a means to reduce fuel consumption and emissions in commercial shipping. The effectiveness of a WPU depends critically on the characteristics of the ambient wind field, which are shaped by both large-scale atmospheric dynamics and local interactions with vessel-induced flow. In this study, we investigate the inflow conditions for a WPU on an ocean-going vessel using both onboard measurements and computational fluid dynamics (CFD) simulations. A measurement campaign provides an extensive dataset for validation. It includes a light detection and ranging (LiDAR) system capturing vertical profiles of wind speed and direction up to 230 m above sea level, and synchronised vessel state data. The LiDAR observations are used to assess the true wind shear and veer in the atmospheric surface layer, confirming that the most commonly modelled neutral stratification is the least frequently encountered offshore. Two independent CFD approaches are employed to simulate the vessel across different apparent wind conditions, following guidelines and standards for wind propulsion. Both frameworks include apparent wind, enabling the twist induced by vessel motion to be modelled. When twist is included, the numerical results show good agreement with the measured WPU inflow conditions. The simulations also highlight the significant impact of strong flow deflections and speed increase induced by the hull on the inflow experienced by a WPU. Overall, the results demonstrate that modelling the vessel under realistic apparent wind conditions is essential for capturing the onboard flow conditions correctly. This has direct implications for predicting aerodynamic loads, estimating performance, and developing trimming strategies for WPUs. Keywords wind propulsion; atmospheric boundary layer; hull interactions; wingsail