[论文解读] Numerical Investigation of the Effect of Airfoil Thickness on Onset of Dynamic Stall

Anupam Sharma, Miguel R. Visbal|arXiv (Cornell University)|Apr 29, 2017

Fluid Dynamics and Turbulent Flows被引用 4

一句话总结

本研究利用雷诺数为200,000的壁面解析大涡模拟（LES）研究了机翼厚度对动态失速起始的影响。结果表明，薄机翼的失速起始由层流分离泡（LSB）破裂主导，而厚机翼则表现为湍流尾缘分离与LSB的相互作用，导致传统意义上的前缘失速与后缘失速分类变得模糊。

ABSTRACT

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 3-D 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 maneuver is studied with the pitching axis located at the airfoil quarter chord point. 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. Comparisons against XFOIL for the static simulations show good agreement in predicting the transition location. 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 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 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 are 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.

研究动机与目标

• 研究在中等雷诺数下，机翼厚度对动态失速起始机制的影响。
• 确定动态失速是否主要由层流分离泡（LSB）破裂或与湍流边界层分离的相互作用触发。
• 评估随机翼厚度变化，非定常流事件序列（包括转捩、LSB形成、动态失速涡DSV生成）的变化规律。
• 评估传统动态失速分类（如前缘失速与后缘失速）在不同厚度范围内的适用性。

提出的方法

• 采用FDL3DI求解器进行大涡模拟（LES），空间离散采用六阶紧凑有限差分格式，时间积分采用二阶精度。
• 在雷诺数为200,000的条件下，对四种对称NACA机翼（厚度比分别为9%、12%、15%、18%）进行模拟。
• 模拟了恒定速率的抬头俯仰运动，俯仰轴位于弦长1/4处，采用斜坡函数平滑增加俯仰速率。
• 通过区分高阶低通滤波器进行空间滤波，以隐式方式模拟亚格子尺度应力。
• 在α = 4°时进行静态模拟，并与XFOIL对比验证转捩和压力系数（Cp）预测的准确性。
• 通过展向相干性分析确认，10%弦长的展向长度足以捕捉失速起始，且展向非相干性可忽略。

实验结果

研究问题

• RQ1 机翼厚度如何影响动态失速的起始机制，特别是层流分离泡（LSB）破裂与湍流边界层相互作用的相对作用？
• RQ2 在壁面解析LES中，转捩过程（尤其是两阶段转捩）随机翼厚度如何变化？
• RQ3 动态失速涡（DSV）的形成与传播速度是否依赖于机翼厚度？
• RQ4 失速起始前边界层中逆向流动的范围与机翼厚度有何关联？
• RQ5 传统的动态失速分类（如前缘失速与后缘失速）是否仍然适用，还是随着厚度增加而变得模糊？

主要发现

• 对于NACA-0009和NACA-0012，FDL3DI模拟预测了XFOIL未捕捉到的两阶段转捩，表明高分辨率模拟对薄机翼转捩物理更敏感。
• 在所有机翼中，层流分离泡（LSB）破裂紧随其后即为动态失速起始，证实其作为普遍前兆的作用。
• 对于NACA-0018，湍流边界层分离区域扩展至吸力面约50%的范围，并在失速前抵达LSB位置，表明失速起始由相互作用触发。
• 随着机翼厚度增加，最大吸力系数（Cp）升高，在最厚机翼（NACA-0018）时达到最大值。
• 动态失速涡（DSV）的传播速度在最厚机翼（NACA-0018）时最高，表明其具有更强的非定常涡动力学特征。
• 壁面摩擦系数（Cf）在失速前出现急剧上升，且其与Cp_rms的空间相关性表明，Cp_rms可有效检测转捩与失速起始。