Investigation of Orographic Updraft Characteristics and Horizontal Distribution Model

Unmanned aerial vehicles (UAVs) can soar like birds by harvesting natural wind energy, significantly extending flight endurance. Thermal updrafts and orographic updrafts are the two primary wind fields exploited by birds during migration and foraging, with the latter being more prevalent in mountainous terrain. Due to computational constraints of current onboard avionics, a simple yet accurate model that adequately characterizes the horizontal distribution of orographic updrafts has not yet been established, limiting existing autonomous energy-harvesting flight techniques to thermal updraft environments. This paper investigates the height-dependent horizontal distribution of vertical winds over isolated and continuous range using RANS numerical simulations. The results show that the horizontal structure of updrafts over an isolated hill closely resembles that of thermal updrafts, allowing direct adoption of existing thermal updraft models. A new horizontal distribution model for orographic updrafts is proposed, which demonstrates high precision with a maximum RMSE of only 0.088 m/s at various altitudes when compared to numerical simulation results. Compared with existing models for continuous ridges, this model significantly improves the consistency in describing the horizontal distribution patterns of orographic updrafts and provides a more reasonable characterization of the updraft distribution in regions above the ridgeline; consequently, it is well-suited for the real-time and efficient prediction of terrain-induced updrafts for small UAVs.