# Effect of High-Speed Train-Induced Wind on Trackside UAV Thrust Near Railway Bridge
## 1 Introduction  
Railway bridges need to be constructed to support heavy trains such as passenger and freight trains. Furthermore, they are designed to support railway tracks along which the trains run and wherein steel rails are installed with a constant distance between them. Railway bridges need to secure a high level of safety while controlling the vibration of tracks and roadbeds essential to train operation owing to the introduction of high-speed trains that can travel at 200 km/h or higher and ultra-high-speed trains that can travel at 350 km/h or higher [1]. In this context, prestressed concrete (PSC) box girder bridges are commonly constructed for high-speed trains owing to their excellent safety against dynamic structural behavior [2, 3]. A concrete girder, highly resistant to bending stress, is constructed by placing steel wires in concrete and fixing them after firmly pulling them owing to the compressive stress that acts on the concrete. However, regular crack inspections are necessary as cracking occurs over time in some cases because of the drying shrinkage of concrete [4]. Cracking in bridge decks is inspected through visual inspection. However, crack inspection in the lower parts of bridges and piers is difficult as it requires workers to use vehicle-mounted ladders for visual inspection owing to the difficulty in accessing these parts. Further, this type of inspection requires considerable cost and time, and it depends on the subjective judgment of the inspector [5].  
Recently, studies have been conducted to measure cracks by acquiring images by means of various imaging devices attached to unmanned aerial vehicles (UAV) and by analyzing the acquired images. The interference of environmental factors such as uneven natural illumination, non-uniform shooting distance, and non-flat structural surfaces of bridge piers, and conditions where GPS coverage is unavailable has been identified through several studies [6; 7; 8; 9; 10]. The UAV used for the health monitoring of railway bridges is operated close to railway bridges. Therefore, the interference of train-induced wind should be considered in order to ensure safety. However, the effect of train-induced wind on UAVs near railway bridges has not been studied.  
In this study, the characteristics of train-induced wind in three axial directions along the track, train-induced wind velocity based on the distance from the train surface, and the effect of train-induced wind caused by a high-speed train on the UAV thrust were analyzed. This was achieved by installing 3-axis ultrasonic anemometers and a UAV thrust measurement system on top of a PSC box girder bridge. Two types of high-speed trains with different lengths were used for the experiments. The changes in the train-induced wind velocity and direction caused by the leading power car, passenger cars, and the trailing power car were monitored along the train's travel, width, and height direction. Additionally, the train-induced wind was measured at distances of 0.8, 1.3, 2.3, and 2.8 m from the train's surface to analyze wind velocity based on the distance. The effect of the wind caused by the passing of a high-speed train on the UAV thrust at the trackside was experimentally characterized, and this result can be used for research on the stabilization of the thrust of UAVs used for railway bridge health monitoring.