Numerical investigation of the effect of airfoil thickness on onset of dynamic stall
Numerical investigation of the effect of airfoil thickness on onset of dynamic stall
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DOI：
10.1017/jfm.2019.235
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发表时间：
2017-04
影响因子：
3.7
通讯作者：
Anupam Sharma;M. Visbal
中科院分区：
文献类型：
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作者：
Anupam Sharma;M. Visbal
采用大涡模拟方法研究了雷诺数为200000时翼型厚度对动态失速起始的影响。本文研究了厚弦比分别为9%、12%、15%和18%的四种对称NACA翼型。三维Navier-Stokes方程求解器FDL 3DI与六阶紧致有限差分格式一起用于空间离散，二阶隐式时间积分和鉴别滤波器以去除未解决的波数。研究了俯仰轴位于翼型四分之一弦上的恒速率上仰机动。模拟分两步进行。在第一步中，翼型在规定的迎角（$=4^{\circ }$）下保持静止。在第二步中，使用斜坡函数将俯仰速率从零平滑地增加到选定值，然后保持俯仰速率恒定，直到迎角超过升力-失速点。通过对NACA 0012翼型绕流的实验验证了该方法的有效性.所有翼型几何形状的静态模拟结果也与XFOIL预测进行了比较，总体上一致。FDL 3DI预测了薄翼型（9%和12%）的两级转捩，这在XFOIL结果中没有观察到。动态模拟表明，在所有情况下，动态失速的开始都是以层流分离泡（LSB）的破裂为标志的。然而，对于所测试的最厚翼型，回流区扩展到翼型的大部分，并在LSB爆发和动态失速开始之前立即到达LSB位置，这表明失速可能由分离的湍流边界层触发。结果表明，不同类别的动态失速之间的界限，特别是前缘失速与后缘失速，是模糊的。动态失速起始机制随着某些参数（在这种情况下是翼型厚度）的逐渐变化而逐渐从一种变化到另一种。
Effect of airfoil thickness on onset of dynamic stall is investigated using large eddy simulations at chord-based Reynolds number of 200 000. Four symmetric NACA airfoils of thickness-to-chord ratios of 9 %, 12 %, 15 % and 18 % are studied. The three-dimensional Navier–Stokes solver, FDL3DI is used with a sixth-order compact finite difference scheme for spatial discretization, second-order implicit time integration and discriminating filters to remove unresolved wavenumbers. A constant-rate pitch-up manoeuver is studied with the pitching axis located at the airfoil quarter chord. Simulations are performed in two steps. In the first step, the airfoil is kept static at a prescribed angle of attack ( $=4^{\circ }$ ). In the second step, a ramp function is used to smoothly increase the pitch rate from zero to the selected value and then the pitch rate is held constant until the angle of attack goes past the lift-stall point. The solver is verified against experiments for flow over a static NACA 0012 airfoil. Static simulation results of all airfoil geometries are also compared against XFOIL predictions with a generally favourable agreement. FDL3DI predicts two-stage transition for thin airfoils (9 % and 12 %), which is not observed in the XFOIL results. The dynamic simulations show that the onset of dynamic stall is marked by the bursting of the laminar separation bubble (LSB) in all the cases. However, for the thickest airfoil tested, the reverse flow region spreads over most of the airfoil and reaches the LSB location immediately before the LSB bursts and dynamic stall begins, suggesting that the stall could be triggered by the separated turbulent boundary layer. The results suggest that the boundary between different classifications of dynamic stall, particularly leading edge stall versus trailing edge stall, is blurred. The dynamic-stall onset mechanism changes gradually from one to the other with a gradual change in some parameters, in this case, airfoil thickness.