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Experimental investigation of vortex shedding of an airfoil at post-stall incidences
Proceedings of the Institution of Mechanical Engineers, Part G: Journal of Aerospace Engineering ( IF 1.1 ) Pub Date : 2022-07-11 , DOI: 10.1177/09544100221112718
Niosha Fallahpour, Mahmoud Mani, Mohadese Lorestani

The harmonic vortex shedding from airfoil happens in various incidences. It has noticeable effects on the structure design and aerodynamic performance. In this paper, wind tunnel tests were conducted on a stationary NACA4412 airfoil at angles of attack ranging from 0° to 90° and Re number between Re=9.8×104 and Re=1.5×105 to examine the vortex shedding frequency and Strouhal number. The wake dynamics at post-stall incidences were investigated by surface pressure, wake flow velocity measurement, and smoke flow visualization. Three phases of the wake dynamics were observed with increasing the incidences beyond the stall: (i) the tiny vortices are shed from the airfoil’s suction side with scattered frequencies, (ii) the shear layer is separated from the LE, rolls up over the airfoil’s suction side and forms the harmonic vortex street, and (iii) the separation point moves from the airfoil’s suction side to the pressure side and leads to the vortex shedding like the bluff bodies. Frequency analysis of aerodynamic loads shows that the flow field’s low-frequency feature has a substantial effect normal to the surface while the vortex street unsteadiness impacts both perpendicular and parallel to the surface. The base pressure coefficient increases suddenly by the vortex street onset in the region (ii) and reaches about 0.4. The universal Strouhal number of 0.18 that is independent of Reynolds number was captured for the angles of attack well beyond the stall. Flow visualization shows that the vortex street establishes longitudinally closer to the airfoil at lower freestream velocity compared to the upper one.



中文翻译:

失速后翼型涡流脱落的实验研究

翼型的谐波涡旋脱落发生在不同的发生率中。对结构设计和气动性能有显着影响。在本文中,风洞测试是在一个固定的 NACA4412 机翼上进行的,攻角范围为0 °90 °Re 之间的数Re = 9.8 × 10 4R e = 1.5 × 10 5 _检查涡旋脱落频率和斯特劳哈尔数。通过表面压力、尾流流速测量和烟流可视化研究了失速后的尾流动力学。随着失速后发生率的增加,观察到尾流动力学的三个阶段:(i)微小的涡流以分散的频率从翼型的吸力侧脱落,(ii)剪切层与 LE 分离,在翼型的上方卷起吸力侧形成谐波涡街,(iii)分离点从翼型的吸力侧移动到压力侧,并像钝体一样导致涡流脱落。气动载荷的频率分析表明,流场的低频特征对垂直于表面有很大的影响,而涡街的不稳定性影响垂直和平行于表面。区域(ii)的涡街开始时,基础压力系数突然增加并达到约0.4。与雷诺数无关的通用 Strouhal 数 0.18 被捕获用于远离失速的攻角。流动可视化显示,与上层相比,涡街在较低的自由流速度下纵向建立更靠近翼型。与雷诺数无关的 18 被捕获的攻角远远超出失速。流动可视化显示,与上层相比,涡街在较低的自由流速度下纵向建立更靠近翼型。与雷诺数无关的 18 被捕获的攻角远远超出失速。流动可视化显示,与上层相比,涡街在较低的自由流速度下纵向建立更靠近翼型。

更新日期:2022-07-13
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