低雷诺数下翼型不同分离流态的大涡模拟

Large-eddy simulation of low Reynolds number airfoil with different separating flow regime

采用高精度大涡模拟方法,对5°来流迎角、马赫数0.4、三个不同雷诺数(55 000、100 000和150 000)的NACA 0025翼型进行仿真,研究低雷诺数条件下翼型的气动特性。通过对比分析3种工况的计算结果,发现翼型绕流存在两种不同的分离流态:Re=55 000和100 000时,翼型上表面出现大尺度的开式分离,形成宽的尾迹区;Re=150 000时,上表面边界层分离后再附到翼型表面,形成时均化的闭式分离泡,尾迹宽度明显减小。无论哪种流态,Kelvin-Helmholtz(K-H)不稳定性均对层流分离诱导转捩过程起重要作用,雷诺数的增加导致转捩过程加速,时均化的分离区也从开式变为闭式。

Large-eddy simulation （LES） technique was employed to simulate flows around an isolated NACA 0025 airfoil at three different Reynolds numbers 55000, 100000 and 150000. The Mach number and angle of attack are 0.4 and 5 degree, respectively. Numerical results in three working conditions were compared with each other and analyzed to investigate the aerodynamic characteristics of the separated flows. Two different flow regimes exist around airfoil. A large open separated bubble appears on the upper surface of the airfoil when the Reynolds number is 55000 or 100 000. As the Reynolds number increases to 150 000, the time-averaged open separated region becomes a smaller closed separation bubble, and the wake width significantly decreased. For two flow patterns, the Kelvin-Helmholtz （K-H） instability plays an important role in the process of laminar separation induced transition. The increment of Reynolds number leads to accelerated transition and a closed time-mean separation zone altered from an open one.