Unsteady Aerodynamics of Wings at Low Reynolds Numbers

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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.