Exploring on Aerodynamic Characteristics of Low Reynolds Number Airfoil in Time-varying Wind Field

时变风场中低雷诺数翼型气动特性研究

¹ School of Aeronautics, Northwestern Polytechnical University, Xi’an 710072, China
² UAV Science and Technology Laboratory, Northewestern Ploytechnical University, Xi’an 710065, China

Received: 2 March 2018

Abstract

A solar-powered unmanned aerial vehicle generally encounters the problems that it has low Reynolds effects and is highly susceptible to gust response. Therefore, the grid velocity method was used to analyze the gust response characteristics of the airfoil FX63-137 under low Reynolds number. First, the reliability of the numerical simulation method at low Reynolds number and grid velocity method were verified with experimental data. Second, the gust response characteristics of FX63-137 airfoil under different Reynolds numbers and different angles of attack were numerically simulated. The results show that the magnitude of incremental lift coefficient in gust response decreases because laminar separation bubbles are complete as the Reynolds number decreases at a small angle of attack. They also show that laminar separation bubbles have an unloaded effect on gust response. At a high angle of attack, as the airfoil enters into stalling stage, the incremental lift coefficient begins to decline before reaching maximum gust disturbance. Because of the stalling of the airfoil, when the gust disappears, the incremental lift coefficient has a negative value. What's more, although the effective angle of attack is equal, the flow structure of the airfoil is somewhat different in upstream and downstream moments. Compared with the downstream moment, the incremental lift coefficient at the upstream moment is generally larger, and the incremental lift coefficient curve of the airfoil forms a non-closed hysteresis loop.

摘要

太阳能无人机普遍具有低雷诺数效应显著，对突风敏感的问题。以此为背景，采用网格速度法，对低雷诺数翼型FX63-137在低雷诺数下的阵风响应特性进行了研究。首先，通过与实验数据和参考文献对比，对低雷诺数下的数值模拟方法以及网格速度法进行了验证。接着对FX63-137翼型在不同雷诺数以及不同迎角下的阵风响应特性进行了数值模拟。研究结果表明：在小迎角情况下，随着雷诺数的减小，翼型表面分离泡变得饱满，翼型在阵风扰动下的升力系数增量减小，层流分离泡对阵风响应幅值具有卸载作用。在大迎角情况下，由于翼型进入失速区，升力系数增量在未达到阵风扰动最大值时就开始下降。并且在阵风扰动消失时，升力系数增量为负值。同时，在有效迎角相同的上行和下行时刻，翼型流场结构存在较大差异，翼型升力系数增量在上行时刻要大于下行时刻，形成一个不封闭的迟滞环。