The hydraulic transient and thermodynamic characteristics of water flows impacting an air pocket at the vertical end of an elbow pipe are investigated. The CLSVOF (coupled level-set and volume of fluid) and URANS (unsteady Reynolds-averaged Navier–Stokes) equations with energy equation are implemented, where the RNG (Re-Normalization Group) k–ε turbulence model is adopted and the pressure–density equation is introduced to allow for the water-phase compressibility. All numerical predictions are consistent with the experimental in the literature. The evolution characteristics and mechanism of the water–air interface are analyzed based on the Froude number and dimensionless water–air mixing degree. For air-pocket Type I and Type II with the water–air mixing degree threshold of 10%, based on the first law of engineering thermodynamics and related process laws and basic theory of statistics, the applicability of the ideal gas model in the thermodynamic process of air pocket, the polytropic index regulation in the evolution process and its association with relevant typical thermodynamic processes are systematically analyzed. The polytropic index of air-pocket Type I fluctuates a little with the averaged median of 1.35 in different transient periods under different initial pressure conditions, while it fluctuates largely with the averaged median of 1.26 and 1.21 under the low and the high initial pressure conditions, respectively.

研究了水流冲击弯管垂直端气穴的水力瞬态和热力学特性。CLSVOF（耦合水平集和流体体积）和 URANS（非定常雷诺平均 Navier-Stokes）方程与能量方程被实现，其中 RNG（重新归一化组）k – ε采用湍流模型并引入压力-密度方程以考虑水相可压缩性。所有数值预测都与文献中的实验一致。基于弗劳德数和无量纲水气混合度，分析了水气界面的演化特征和机理。对于水气混合度阈值为10%的气袋Ⅰ型和Ⅱ型，基于工程热力学第一定律及相关过程定律和统计学基础理论，理想气体模型在热力学过程中的适用性系统分析了气穴演化过程中的多变指数规律及其与相关典型热力学过程的关联。