Unsteady Aerodynamics of Wings at Low Reynolds Numbers

该研究项目旨在全面分析低雷诺数（即低于100 000）下小展弦比的纵倾和俯仰机翼周围的非定常流动发展。所获得的结果可用于开发新的设计方法的扑翼飞行机构的有效产生的升力和信任在低雷诺数制度。此外，该项目还涉及小型飞行器的机动性和阵风敏感性。随着雷诺数的减小，机翼的空气动力学越来越取决于大的涡流结构。因此，该项目着重于三维非定常流场的描述，为此，采用并进一步发展了实验和数值模拟方法。在该项目的第一个资助期内，研究了具有椭圆和矩形平面形状、展弦比为1和2的平板机翼，同时进行了谐倾伏和俯仰运动。为了从内力天平的数据计算气动载荷，研究了确定惯性力的不同方法，包括新的光学测量方法。在几个平面上用PIV测量的流场提供了三维流动发展的洞察力，并用于与伴随的流动模拟结果进行比较。还计划进行层析PIV测量，以测量翼尖的三维流场，显示分离区前缘涡流与翼尖涡流的相互作用。在即将到来的资助期内，初步计划使用第一个资助期获得的数据对谐波情况进行定量分析。发展的涡识别方法允许跟踪以及计算单个涡的环流强度，从而可以定量分析涡的增长和涡的相互作用。然而，这一资助期的主要部分集中在非谐机翼运动的研究上。首先，瞬态情况下的脉冲暴跌运动被认为是允许的垂直阵风的影响分析的机翼载荷和流动拓扑结构，构成了一个主要问题的小型飞行器。第二，周期性运动被认为是通过应用长阶段的恒定的攻角和暴跌速度与短的转向阶段，在上，下死点的运动。在这种情况下，与第一个资助期的正弦情况相比，更高的推力产生效率以及在转弯阶段强制产生涡流都是令人感兴趣的。运动参数的仔细调整可以导致性能的改善，因此，底层的流动物理也将被研究。

The research project aims to analyse comprehensively the unsteady flow development around plunging and pitching wings of low aspect ratio at low Reynolds numbers, i.e. below 100 000. The obtained results can be used for the development of new design methods of flapping flight mechanisms for an efficient generation of lift and trust in the low Reynolds number regime. Additionally, the project addresses also aspects of manoeuvrability and gust sensitivity of small air vehicles. As the Reynolds number reduces the wing aerodynamics are increasingly determined by large vortical flow structures. Therefore, the project places emphasis on a description of the three-dimensional unsteady flow fields, for which experimental as well as numerical simulation methods are adapted and further developed.In the first funding period of this project, flat plate wings of elliptical and rectangular planforms and aspect ratios of one and two are investigated while undergoing harmonic plunge and pitch motions. For the calculation of the aerodynamic loads from data of an internal force balance, different methods are investigated to determine inertial forces including a new optical measurement approach. Flow fields measured with PIV in several planes give insight in the three-dimensional flow development and are used for comparisons with results of accompanying flow simulations. Tomographic PIV measurements are planned as well to measure the three-dimensional flow fields at the wing tips showing the interaction of leading edges vortices of separation regions with the wing tip vortices.In the forthcoming funding period it is initially planned to analyse the harmonic cases quantitatively using the data obtained in the first funding period. Developed vortex identification methods allow a tracking as well as a calculation of the circulation strengths of individual vortices, with which the growing of vortices and vortex interactions can be quantitatively analysed . The major part of this funding period, however, concentrates on the investigation of non-harmonic wing motions. First, transient cases of impulsive plunge motions are considered that allow for an analysis of the influence of vertical gusts on the wing loads and flow topology which constitute a major issue for small air vehicles. Second, periodic motions are considered by means of applying long phases of constant angle of attack and plunge velocity with short turning phases in the upper and lower dead centre of the motion. For this case the higher efficiencies of thrust generation in comparison to the sinusoidal cases of the first funding period are of interest as well as the forced generation of vortices during the turning phases. A careful adjustment of kinematic parameters can lead to improved performances and, therefore, the underlying flow physics will be investigated as well.