低雷诺数下机翼气动特性研究及控制

高空长航时无人飞行器(HALE UAV)由于飞行环境空气稀薄、雷诺数低导致其气动性能恶化,如何通过流动控制改善机翼低雷诺数气动性能受到越来越多的关注。在低速风洞中通过测力、测压和边界层测试等试验技术开展了NACA 633-421直机翼模型气动特性试验和流动控制研究。天平测力结果表明:随雷诺数降低(Re<1.4×10⁵)机翼气动特性迅速恶化;最大升力系数损失严重,失速迎角急剧降低;分析翼面压力分布结果显示,机翼表面产生层流分离泡(LSB),其长度变化、位置前移和最终发生破裂的发展过程是导致机翼低雷诺数气动性能恶化的主要原因。采用合成微射流(Micro-SJ)对翼面层流分离泡进行流动控制,失速迎角推迟了11°,机翼最大升力系数由0.59提升至1.10,最大升阻比增加了13.6%。合成微射流控制具有选频特性,驱动频率f=200~400 Hz的合成微射流控制效果最佳,更易促进分离剪切层提前转捩,形成湍流再附,使得层流分离泡长度缩短。

Aerodynamic characteristics and flow control on a rectangular wing at low Reynolds number

It is found that the flight environment in thin atmosphere and at low Reynolds number leads to the drop of high-altitude long-endurance unmanned aerial vehicles (HALE UAV) aerodynamic performance, and as a result, how to improve it with flow control has drawn greater attention of researchers. An experimental investigation on the aerodynamic characteristics and flow control of a NACA 633-421 rectangular wing at low Reynolds number is conducted in a low-speed wind tunnel with force measurement,pressure measurement and boundary layer test. It is shown from force measurement result that a sharp deterioration of aerodynamic characteristics of NACA 633-421 wing occurs with the Reynolds number decreasing(Re<1.4×10⁵), which includes severe losses of the maximum lift coefficient with respect to higher Reynolds numbers and the sharp decrease of stall angle. Pressure distribution on upper surface reveals that the emergence and development process of a laminar separation bubble (LSB), which moves forward to the wing leading edge, becomes shorter in length with increasing angles of attack and bursts at the stall angle, finally cause the sharp deterioration of aerodynamic characteristics. The micro synthetic jet (Micro-SJ) is used to control the flow structure of laminar separation bubbles, as a result of which the maximum lift coefficient increases from 0.59 to 1.10, the stall is delayed by 11° and the maximum lift-drag ratio is improved by 13.6% meanwhile. The active flow control of Micro-SJ possesses frequency selectivity, in which case the Micro-SJ at the frequencies between 200 Hz to 400 Hz have the best flow control effects to promote the transition of the separated shear layer and to shorten the length of laminar separation bubbles.