高雷诺数双螺旋涡尾迹演化特性分析

Evolution characteristic analysis of double-helical vortex wake of high Reynolds number flow

螺旋状尾迹涡是直升机悬停旋翼流场的主导特征之一,其时空演化特性对旋翼气动性能具有重要影响.为了揭示悬停状态下旋翼尾迹涡的演化特征,对两桨叶刚性旋翼在高雷诺数悬停状态下的双螺旋状尾迹涡开展数值研究,采用基于流场特征的网格自适应技术,结合低耗散迎风/中心混合格式以及延迟脱体涡模拟方法对Caradonna-Tung旋翼在桨尖马赫数为0.439、桨尖雷诺数为1.92×10~6的悬停流场进行了高分辨率计算.基于欧拉和拉格朗日两种描述方法对计算结果进行了分析,揭示了双螺旋尾涡系统的演化特性:后缘尾涡面在桨尖附近的反向卷起及其与下游桨尖涡的相互作用是影响涡系稳定性以及涡-涡相互作用的重要因素;涡龄小于720°时,在固连于桨叶上的旋转坐标系中观察,涡系具有时空稳定性,涡管中心处轴向涡量随涡龄按照幂函数规律衰减.在固连于漩涡中心的局部极坐标系中,周向速度分布以及涡核半径随涡龄的变化与理论涡模型相符合,环量随涡龄的变化显示了漩涡的生长、平衡及耗散等演化阶段;模态分析结果表明,除点涡模态外,来流与点涡的复合模态在漩涡演化过程中对流动特征的转变有重要影响;涡系轴截面速度场的拉格朗日拟序结构直观地显示了漩涡场的时空演化过程,揭示了漩涡配对和共旋穿越等流动特征,同时也展示了后缘尾涡面卷起现象在漩涡演化过程中的作用.

High Reynolds number helical vortex system possesses a dominant characteristic of helicopter rotor flow field, whose spatiotemporal evolution is one of the most important factors affecting the aerodynamic performance. In such a type of flow field, vortex interaction due to flow unsteadiness and non-linearity possesses the most common characteristic,whose complexity and tightly coupling property make it very hard to understand its physical behaviors. Also, the multi-scale characteristic of the helical vortex evolution poses a severe challenge to the computational fluid dynamics community. In this paper, a hybrid numerical method, blending 5 th order weighted essentially non-oscillatory and6 th order cenitral schemes, implemented in a finite volume overset grid framework based on adaptive mesh refinement technique, are adopted to capture the evolution of vortical structure in a high resolution manner. The highly-resolved flow field of Caradonna-Tung rotor with two blades in hover, with a tip Mach number of 0.439 and a tip Reynolds number of 1.92 x 10~6, is obtained using delayed detached eddy simulation method. The averaged pressure coefficient distributions at 50%R, 68%R, 80%R, and 96%R stations show good agreement with experiment data, and the vortex trajectories during the stable stage, as well as the instantaneous turbulent kinetic energy distribution in the wake region,and also validate the computed result. In order to reveal the underlying physical mechanism of the helical vortex structure evolution, proper orthogonal decomposition analysis and Lagrangian coherent structures are adopted as a post processing procedure, which brings more details about the unsteady vortex system. The evolution characteristics of the vortex system are revealed as follows. 1） Trailing edge vortex sheet rolling-up and interaction with tip vortex strongly affect vortical stability and downstream nonlinear vortex-vortex behaviors. 2） The vortical system exhibits the spatiotemporal stability at an age less than 720°, and the vorticity decays with age and trajectories by power law,the distribution of circumferential velocity and the evolution of vortex core radius agree well with theoretical models.3） Results of proper orthogonal decomposition analysis show that the mode of free stream and point vortex combination plays critical roles in the state transition of flow field. 4） Lagrangian coherent structure further gives the evolution process of helical vortex, and reveals the flow characteristics of vortex pairing and co-rotating, showing the effect of trailing edge vortex roll up phenomenon in the vortical system evolution.