Characteristics of air flow driven by the water free surface in a free-flow spillway tunnel

Air is transported downstream by the water flow in a free-flow spillway tunnel, resulting high negative pressure, which causes gate vibration and cavitation. In this research work, physical model of tunnel was used to investigate effects surface velocity area residual cross sections on air movements. The results demonstrate that distribution forms an S-shaped curve. As increases or decreases, increases, relative action height drag force as well. divided into two exponential distributions at position where drops 0.2 times velocity. Two formulas for calculating corresponding are developed. Subsequently, algorithm demand presented, error compared reference data less than ±20%. These provide ventilation shaft design.