大负荷低压涡轮叶型分离转捩流动的大涡模拟

Large eddy simulation of separated flow transition on a highly-loaded low pressure turbine profile

关键词:  航空发动机  涡轮  叶型  分离流  转捩流  大涡模拟

Key words:  Aircraft engine  Turbine  Blad airfoil  Separated flow  Transition flow  Large eddy simulation+

摘要:

应用动力模式大涡模拟数值方法,对来流无扰动、不可压、雷诺数为5×104(基于进口速度和轴向弦长),定常来流条件下大负荷低压涡轮叶型(Pak B)叶型吸力面非定常分离转捩流动进行了三维数值模拟。在与相关实验数据的对比基础之上,对非定常流动物理信息进行了详细的分析讨论,揭示了计算来流状态下的Pak B叶型吸力面非定常分离转捩流动机理。结果表明,由无粘Kelvin-Helmholtz机制产生的、空间线性增长的初始二维不稳定性在分离剪切内诱导展向旋涡形成并脱落,脱落过程中的展向涡在非线性增长的三维不稳定性作用下发生变形并最终破碎成湍流。计算得到的Kelvin-Helmholtz不稳定性特征频率处于相关实验测量范围内。

Abstract:

Three-dimensional large eddy simulation(LES) of separated flow transition on a highly-loaded low-pressure turbine profile under steady inflow conditions was performed using the dynamic subgrid-scale model.The Reynolds number based on inlet velocity and axial chord length is 5×104,with a free turbulence inflow,and the flow is incompressible.The comparisons of simulated mean quantities with the available experimental data show that the mean bubble length and thickness are a little under-predicted.Because of numerical errors,the freestream turbulence intensity near the profile leading edge is found to be about 0.22%,which triggers the inviscid Kelvin-Helmholtz instability in the free shear layer after separation.The initial two-dimensional instability Kelvin-Helmholtz mechanism grows downstream slowly and linearly,and it induces the formation and shedding of two-dimensional spanwise vortices at some streamwise location where the instability becomes three dimensional and begins to grow quickly and nonlinearly.In the process of vortex shedding,the spanwise vortices become stronger,being more and more severely distorted under the nonlinearly-growing three-dimensional motions,ultimately leading to the flapping free shear layer rolling up in their strongest state,forming the large-scale vortices which eventually break to full turbulence around the mean reattachment location under significant growth of three-dimensional motions.The structures of streamwise vortices are obviously detected at the late stage of transition.