Noise control of serrated trailing edge airfoil under small incidence angle
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摘要: 受猫头鹰寂静飞行能力的启发，锯齿尾缘设计被认为是一种有效的控制湍流边界层−尾缘干涉噪声的方法。本文采用隐式大涡模拟法，详细研究了嵌入式锯齿尾缘对NACA 0012翼型绕流的近场流动和噪声特性的影响，雷诺数为$9.6 \times {10^4}$，远场马赫数为0.1631，攻角为4°，计算采用的非结构化网格具有约7000万的自由度。在实际计算时，为促进流动快速转捩，在直尾缘和锯齿尾缘算例的翼型表面均布置了锯齿形粗糙元转捩带。研究结果表明：相比于0°攻角状态，${4^ \circ }$攻角下的噪声辐射增强，主辐射方向发生偏转，在该方向上锯齿尾缘实现了约2.5 dB的降噪，且在小攻角（4°）下，锯齿也会诱导出有利于降噪的侧边涡对结构。针对壁面压力脉动的分析表明：锯齿主要改变了尾缘附近的时空关联特性，且压力场不能直接由现有针对速度场的Taylor或椭圆近似模型定量描述；此外，锯齿在抑制尾缘噪声的同时，对翼型气动性能造成了一定损失。Abstract: Inspired by the silent flight capability of owls, the serrated trailing edge design is considered as an effective method to reduce the turbulent boundary layer-trailing edge interference noise. In this study, the near-field flow and noise characteristics of a NACA 0012 airfoil with additional serrated trailing edges are investigated in detail using an implicit large eddy simulation approach with Reynolds number Re = $9.6 \times {10^4}$, far-field Mach number Ma = 0.1631, and angle of attack $\alpha = {4^ \circ }$. The simulation adopts unstructured grids with 70 million degrees of freedom. In this particular calculation, a small sawtooth-shaped rough strip is added to the airfoil surface to facilitate the fast transition to turbulence for both straight and serrated trailing edge cases. At an angle of attack of 4°, an increase in noise radiation is observed with respect to that at an angle of attack of 0°, with a deflection of the primary radiation direction and a noise reduction of about 2.5 dB in this direction. The flow analysis shows that the sawtooth induces the regularly distributed vortex pair structures at its sides, which facilitates noise reduction in the far-field. The analysis of the wall pressure fluctuation shows that the sawtooth mainly changes the space-time correlation properties near the trailing edges, and the space-time correlation properties of the pressure cannot be described by the existing velocity-based Taylor or elliptical correlation models. In addition, the sawtooth suppresses the noise radiation while causing some loss to the aerodynamic performance of the airfoil.
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Key words:
- aeroacoustics /
- serrated trailing edges /
- noise control /
- compressible turbulence
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图 1 计算几何构型示意图
Figure 1. Schematic diagram of the configuration for the numerical simulation
图 2 ${ R_{ \mathrm{s}}} = 10l$位置处的噪声强度分布
Figure 2. The noise intensity with ${R_{\mathrm{s}}} = 10l $ from the trailing edge
图 3 $R_{\mathrm{s}} = 10l $的圆周上声功率的指向性分布
Figure 3. The acoustic power directivity on a circle with $R_{\mathrm{s}} = 10l $ from the trailing edge in different ranges
图 4 近场流动结构示意图。基于Q准则的等值面（Q = 1）识别的旋涡结构，以流向速度着色，背景为张量表征的噪声辐射
Figure 4. Schematic diagram of the near-field flow structure. The vortex structures are identified based on the Q criterion （Q = 1） and flooded by the flow velocity, with the background being the noise radiation characterized by the dilatation
图 5 时均旋涡等值面与流线图
Figure 5. Iso-surfaces of time-averaged vortex and illustration of streamlines
图 6 直尾缘和锯齿尾缘算例中雷诺应力分量的空间分布
Figure 6. The spatial distribution of Reynolds stress components for straight and serrated trailing edges
图 7 尾缘附近的对流速度分布示意图
Figure 7. Schematic diagram of convection velocity near the trailing edges
图 8 直尾缘和锯齿尾缘算例上翼面位置时空关联特性的空间分布
Figure 8. The spatial distribution of space-time correlations on the upper airfoil surface for straight and serrated trailing edge cases
图 9 直尾缘和锯齿尾缘算例尾缘附近时空关联特性的空间分布
Figure 9. The spatial distribution of space-time correlations in vicinity of the trailing edge for straight and serrated trailing edge cases
表 1 计算参数设置
Table 1. The settings of computational parameters
算例 控制方式 攻角/$\left( ^\circ \right) $ 网格自由度 $\Delta {y^ + }$ h0 直尾缘 0 74501080 0.87 h6 锯齿尾缘 0 75903900 0.87 h0α4 直尾缘 4 74501080 0.87 h6α4 锯齿尾缘 4 75903900 0.87 
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表 2 气动特性对比
Table 2. Comparison of aerodynamic performance
算例 h0 h6 h0α4 h6α4 浸润面积S变化 — +6% — +6% 升力L变化 — — — +2.94% 阻力D变化 — +6.24% — +6.93% 升力系数${C_L}$变化 — — — −2.89% 阻力系数${C_D}$变化 — +0.23% — +0.87% 摩擦阻力占比 84.07% 85.29% 72.78% 74.40% 压差阻力占比 15.93% 14.71% 27.22% 25.60%
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