考虑转捩影响的翼型动态失速数值模拟

Numerical simulation of airfoil dynamic stall incorporating transition modeling

用数值求解雷诺平均NS方程的方法来对考虑转捩影响的中低雷诺数下振荡翼型动态失速进行数值模拟,计算中采用了k-ωSST两方程湍流模式,并加入Chen-Thyson转捩模型来模拟流动中的转捩效应。采用该方法分别对雷诺数Re=1.35×105和Re=7.7×104情况下NACA0012翼型的动态失速进行了数值模拟。计算结果显示:计算出的翼型动态失速气动力系数迟滞曲线与实测结果符合较好;对于Re=7.7×104的工况,实测的升力系数迟滞曲线中出现了高频振荡,计算结果有效地捕捉到了这一流动现象,并通过分析瞬时流线的计算结果揭示出尾缘涡的涡脱落是引起该高频振荡的主要原因。此外,通过数值计算分析了中低雷诺数下减缩频率对升力系数迟滞曲线的影响,从结果中看到,随着减缩频率的增加,翼型的失速攻角值会增加,升力系数峰值会增加;当减缩频率减小时,升力系数曲线中的高频振荡频率会增加。通过进一步计算分析知,升力曲线中高频振荡的产生不仅取决于减缩频率,还取决于流动的雷诺数,只有在中低雷诺数、较小的减缩频率下翼型动态失速的升力迟滞曲线中才有可能出现高频振荡。

Dynamic stall of airfoil in medium and low Reynolds number flow is numerically simulated by solving the Reynolds Averaged Navier-Stokes （RANS） equations. In the numerical simulation, the k-ω SST two-equation turbulence model is chosen to evaluate the effect of turbulence and the Chen-Thyson transition model is used to account for the transition and intermittency phenomena. After applying this numerical scheme to two dynamic stall cases of NACA0012 airfoil of Reynolds number 1.35×10^5 and 7.7×10^4, it can be seen from the results that for both cases the calculated hysteresis of airfoil lift coefficient agrees to the experimental values. Especially, for the case of Re = 7.7×10^4, there exists some high frequency oscillations in the experimental lift hysteresis which can be captured by the numerical method to some extent. And it also can be revealed from calculated instantaneous streamlines that these high frequency oscillations might be due to the physics of trailing edge vortex shedding. Moreover, the influence of reduced frequency on the lift hysteresis is numerically investigated and the following two results can be obtained. First, when the reduced frequency becomes larger, the airfoil dynamical stall angle of attack becomes larger and the peak value of lift coefficient also becomes greater. Second, when the reduced frequency decreases, the frequency of the oscillations in the lift hysteresis becomes larger. So, further numerical consideration is taken on the high frequency oscillations in the lift hysteresis and it is found that these oscillations are determined both by the reduced frequency and the Reynolds number. Only for medium and low Reynolds number flow and low reduced frequency case may these high frequency oscillations turn out.