高雷诺数湍流横侧射流的大涡模拟

Large Eddy Simulation of a High Reynolds Number

关键词:  湍流  横侧射流  大涡模拟  反向旋转涡对  马蹄涡

Key words:  Turbulence  Jet in cross-flow  Large eddy simulation  Counter-rotating vortex pair  Horseshoe vortices

摘要:

采用大涡模拟(LES)方法研究横向主流与壁面射流均为高雷诺数的壁面横侧射流(JICF)，采用动态Smagorinsky涡粘模型对亚格子尺度进行封闭。数值模拟结果中，近场和远场的平均速度、均方根值均与实验结果符合很好。瞬时压力等值面在射流迎风侧与射流迹线成正交关系，表明JICF近场迎风涡是由于射流出口上游剪切层Kelvin?Helmholtz不稳定性引起的。基于长度尺度[RD]，Broadwell 和 Breidenthal提出了射流迹线公式，通过拟合数值模拟结果得到公式中常数[A=1.35]，[B=0.3]。采用平均流线、平均场涡量与Q准则分析三维涡旋结构。横向主流遇到射流后发生夹带现象，在射流出口下游受涡旋结构的影响发生卷起；平均涡量等值面表明，反向旋转涡对(CVP)的涡旋方向与射流出口下游马蹄涡两个分支的涡旋方向相反；由于横向主流和射流雷诺数较高，Q准则表征的涡旋结构在射流出口下游一倍射流直径位置开始发生破碎。

Abstract:

Large eddy simulations (LES) are performed to study a turbulent jet issuing perpendicularly into a turbulent crossflow，where both jet and crossflow are at high Reynolds numbers. Dynamic Smagorinsky sub-grid scale eddy viscosity model is used. The numerical results show that the mean velocity and root mean square velocity in the near-field and far-field are in agreement with the experimental results. The iso-surface of instantaneous pressure at the upstream of jet-exit is orthogonal to the jet streamlines of mean velocity，which indicates that the upwind vortices are generated by the Kelvin Helmholtz shear layer instability. The constant number A and B of the jet trajectory in the Empirical formula based on the length scale [RD]given by Broadwell and Breidenthal are 1.35 and 0.3，respectively，by fitting the simulation results. The three-dimensional vortex structures are visualized and investigated by the streamlines of mean velocity，the average vorticity and the iso-surface of Q-criterion，respectively. The entrainment and rolling up of the crossflow fluid caused by the jet flow are observed respectively by the three-dimensional streamlines of mean velocity. Close to the wall，there is a pair of secondary vortices，rotating in the opposite direction of the counter-rotating vortex pair (CVP) characterized by the average vorticity. Due to the high Reynolds numbers of jet and crossflow，the vortex structures visualized by the Q-criterion，start to break down at about one fuel jet diameter downstream of the jet-exit.