翼尖涡多阶段演化过程及其对气动力的影响

针对高雷诺数和中等马赫数下翼尖涡的近场演化问题,以NACA0012机翼为对象,采用大涡模拟方法,研究了三组不同的马赫数(0.3、0.45、0.6)和雷诺数(5×10~5、1×10~6、2×10~6)下翼尖涡中主涡和次级涡的演化特性以及其对机翼气动力的影响。研究发现,根据主涡和次级涡特性可将翼尖涡近场演化过程分为三阶段:在第一阶段中二者独立生长,主涡涡核处涡量先增后减,次级涡涡核处涡量和流向速度显著变化;在第二阶段中次级涡运动至机翼上表面与主涡相互作用融合,二者涡核处涡量变化分别趋于平缓并最终相同;在第三阶段中主涡与次级涡融合后的共转涡和新生成的二次融合涡离开机翼进入尾迹。马赫数影响主涡和次级涡涡核处流向涡量及"扭结"现象,但不影响主涡和次级涡涡核处流向速度和融合位置;雷诺数影响主涡和次级涡涡核处流向涡量、主涡涡核处无量纲流向速度以及"扭结"现象,但不影响次级涡涡核处流向速度和融合位置。在整个翼尖涡近场演化过程中,与第一阶段相比,第二阶段通过显著改变机翼上表面压力分布,诱导出强烈的下洗现象,主导影响着机翼的气动力,此外翼尖涡能抑制翼尖附近上表面流动分离,在一定程度上减轻其对气动力的不利影响。

Near field multi-stage evolution of wingtip vortex and its impacts on aerodynamic forces

In order to gain insight into the near field evolution of wingtip vortex at high Reynolds numbers and moderate Mach numbers, a study of evolving characteristics of primary wingtip vortex and secondary wingtip vortex as well as their impacts on the aerodynamic forces of a NACA0012 wing for three different Reynolds numbers and Mach numbers has been conducted by using large eddy simulations. The result shows that the near field formation and development of wingtip vortex can be divided into three distinct stages. In the first stage, primary and secondary vortex rolls up and develops independently. Vorticity in primary vortex core undergoes an initial increase and an subsequent decrease in streamwise direction. Vorticy and streamwise velocity in secondary vortex core changes significantly. In the second stage, secondary vortex moves to the upper surface of the wing, interacting and merging with primary vortex. Vorticties in both primary and secondary vortex cores become equivalent in a slow and gradual manner. In the third stage, the huge vortex newly-formed by the merging process leaves the wing and enters the wake region. Mach number affects vorticities in in primary and secondary vortex cores as well as the ''kink'' phenomenon, but has no impact on the streamwise velocity as well as the merging location. Reynolds number affects vorticities in primary and secondary vortex cores, streamwise velocity in primary vortex core as well as the ''kink'' phenomenon, but exerts a negligible influence on the streamwise velocity in secondary vortex core as well as the merging location. In the whole near field evolution process, compared with the first stage, the second stage largely alters pressure distributions on the upper surface of the wing and induces intense downwash, playing a dominant part in impacting aerodynamic forces of the wing. Addtionally, wingtip vortex inhibits flow separations on the upper surface, to some extent mitigating the aerodynamic penalty it brings to the wing.