Experimental study on the effect of rough surface on aerodynamic characteristics and flow field of low Reynolds number airfoil
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摘要: 为探究粗糙表面对低雷诺数翼型气动特性和流场的影响及作用机理，本文针对SD8020翼型开展了实验研究（Re = 4 × 104），测量了翼型的气动力，并采用荧光油膜、烟线流动可视化和热线技术观测了绕翼流场。研究结果表明：在小迎角（0°～3°）范围内，光滑翼型升力系数增长具有非线性特征，翼面层流分离泡结构的突变是导致低雷诺数下翼型升力系数非线性特征的主要原因；过小的前缘粗糙度（Sa+ = 0.00025）不会对流场产生明显影响，适当大小的前缘粗糙度（Sa+ = 0.0051、0.013）能够延缓边界层分离、加快剪切层再附，缩小甚至消除层流分离泡，显著降低气动阻力并增大升阻比，与光滑翼型相比，最大升阻比分别提升35.7%和41.4%；Sa+ = 0.013的前缘粗糙度能够减弱小迎角范围内升力系数增长的非线性特征，显著提高小迎角下的升力系数（迎角2°时提高约219.5%）；粗糙前缘加快了扰动增长（表现为高频速度脉动、T–S波的增长），将壁面涡量更快地上卷至流场中，更早发展形成涡结构，涡结构可以加强法向对流，提高边界层对逆压梯度的抵抗能力，延缓分离；边界层分离之后，涡结构在分离剪切层转捩过程中起主导作用，加快流动转捩，流动提前再附。Abstract: In order to explore the influence of rough surfaces on aerodynamic characteristics and the flow field of the low Reynolds number airfoil and to deeply understand the action mechanism of rough surfaces, the SD8020 airfoil was used for experimental research (Re = 4 × 104). The aerodynamic force of the airfoil was measured in the experiment, and the flow field around the airfoil was observed in detail by using luminescent oil-film, smoke wire flow visualization and hot wire technology. The results show that the lift coefficient of the smooth airfoil has nonlinear characteristics in the range of α = 0°～3°, and the abrupt change of the laminar flow separation bubble structure is the main reason for the nonlinear characteristics of the lift coefficient of the airfoil at low Reynolds number. Too small leading edge roughness (Sa+ = 0.00025) does not have significant influence on the flow field, but appropriate leading edge roughness (Sa+ = 0.0051, 0.013) can delay the separation of boundary layer, accelerate the reattachment of shear layer, and reduce or even eliminate laminar separation bubbles. So it can significantly reduce aerodynamic drag and increase the lift-drag ratio. The lift-drag ratio of the smooth airfoil is increased by 35.7% and 41.4%, respectively. When Sa + =0.013, the nonlinear characteristics of the lift coefficient growth is weakened at a small attack angle and the lift coefficient can increase significantly (e. g. the lift coefficient increases by 219.5% when α=2°). The roughness of the leading edge accelerates the growth of the disturbance (manifested as the growth of high-frequency velocity pulsation and T–S wave), rolls the vorticity of the wall into the flow field faster, and forms the vortex structure earlier. The vortex structure can strengthen the normal convection, improve the resistance of the boundary layer to the adverse pressure gradient, and delay the separation. After the boundary layer separation, the vortex structure plays a leading role in the transition process of the separated shear layer, speeding up the transition of flow to turbulence and flow reattachment in advance.
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图 13 弦向瞬态烟线流动可视化（α = 2°）
Figure 13. The chordwise transient smoke wire flow visualization (α = 2°)
图 14 展向瞬态烟线流动可视化（α = 2°）
Figure 14. The spanwise transient smoke wire flow visualization (α = 2°)
图 15 弦向瞬态烟线流动可视化（α = 3°）
Figure 15. The chordwise transient smoke wire flow visualization (α = 3°)
图 16 展向瞬态烟线流动可视化（α = 3°）
Figure 16. The spanwise transient smoke wire flow visualization (α = 3°)
图 20 α = 3°时，间隔7张图片选取的展向烟线图
Figure 20. The spanwise smoke wire picture was extracted at the interval of 7 pictures when α = 3°
表 1 ATI天平各分量参数表
Table 1. ATI balance component parameter table
Fx, Fy/N Fz/N Tx, Ty, Tz/（N·m） 量程 20 60 1 分辨率 1/200 1/100 1/8000 
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表 2 翼型表面状态简称对应表
Table 2. Correspondence table for airfoil surface states
状态简称 翼面状态 Sa + Con 0 名义光滑，全翼面粗糙度Sa = 0.05 μm 6.25 × 10−6 Con 1 前缘粗糙度Sa = 2 μm 0.00025 Con 2 前缘粗糙度Sa = 41 μm 0.0051 Con 3 前缘粗糙度Sa = 106 μm 0.013
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