Flapping wing aerodynamics in micro-aerial vehicles There is an ever-increasing interest in the field of Micro- Aerial Vehicles (MAV’s) all around the world with recent developments in micro- and nano-technologies. MAV’s are defined as small Unmanned Aerial Vehicles (UAV’s) with length scales similar to birds and insects. In addition to comparable physical dimensions, MAV’s also have similar flight mechanisms with those in nature, such as flapping wing propulsion. A local example of a flapping wing MAV is the ‘The DelFly’, which was born in 2005 as a student project at TU Delft. The design was inspired by nature, particularly the ‘dragonfly’, and it was really promising. After two years of development, it evolved into a lighter and smaller model, the ‘DelFly II’. It has a wing-span of 28cm and it weighs only seventeen grams with an onboard camera that is employed for observation and vision-based control purposes. However, the ultimate goal of the project is to build a DelFly of even smaller proportions, with a wing- span of five cm. To reach this goal, the first step was taken in the building of the ‘DelFly Micro’. It is still the smallest member of the DelFly family and has a wing-span of ten cm and weighs three grams, still being equipped with an onboard camera. The goal of the project is to understand the underlying aerodynamic mechanisms of flapping wing mechanisms, particularly ‘clap-and-peel’ motion of the DelFly wings, and use this information for futher development. Special focus is given on the effect of wing flexibility which is shown to be a crucial parameter in flapping wing aerodynamics. Fundamental Understanding of Flapping Wing Aerodynamics In addition to the experiments performed on the DelFly, simplified experiments were performed in the context of the project. In accordance with the activities of NATO AVT- 202 panel, Tomographic Particle Image Velocimetry (Tomo- PIV) measurements were carried out in a water during the accelarating phase of rotating flat plate. The aim of this project is to explore the evolution and stability of the Leading Edge Vortex (LEV), which is regarded as the major element in the generation of forces. Publications - M. Percin, Y. Hu, B.W. van Oudheusden, B. Remes, and F. Scarano, “Wing Flexibility Effects in Clap-and-Fling”, International Journal of Mico Air Vehicles, 3, 4, J.H.S. de Baar, M. Percin, R.P. Dwight, B.W. van Oudheusden, and H. Bijl, “Kriging regression of PIV data using a local error estimate,” submitted to Experiments in Fluids, DelFly II in flight Wake Flow of the DelFly II Understanding wake flow in the flapping wing systems can reveal variety of characteristics related to the force generation mechanisms. Especially, it is crucial particularly for the DelFly, which has two wing pairs and a tail that is presumably interacting with the wake structures of the wings. In order to gain better understanding of this phenomenon, a detailed experimental study was performed in a wind tunnel. Time-resolved velocity measurements were performed in the wake of the flapping wings of the DelFly II in forward flight configuration, using the Stereoscopic Particle Image Velocimetry (Stereo-PIV) technique. Images were recorded at several spanwise oriented planes in the wake of the flapping wings with three high speed cameras and a high speed laser. Then, three-dimensional reconstruction of the wake was perfomed by use of Kriging regression technique with local error esimates [2]. A simple error model that is derived from the local PIV correlation map was used to estimate local error for each velocity vector. Reconstructed 3D wing geometry As a result of these experiments, the general wake topology is described in conjunction with the behavior of distinctive flow structure, in particular, the occurrence and development of tip vortex, trailing edge vortex, and root vortex. Different vortex mechanisms are observed for different flapping parameters. PhD Candidate: Mustafa Percin Department: AWEP Section: Aerodynamics Supervisor: B. W. van Oudheusden Promoter: F. Scarano Start date: Funding: STW Cooperations: NATO STO Aerospace Engineering Sketch of the top view of the experimental setup Iso-surfaces of vorticity magnitude z vorticity (w z ) at the end of upstroke (k=0.5 (left) and k=1 (right)) How much do the DelFly Wings Deform? It is known that flexibility plays an important role in the generation of forces in flapping wing mechanisms. Previous studies on the DelFly wings also showed that force generation and force production strongly depends on the wing thickness, stiffener position, and orientation. Therefore, assesing deformation of the wings during the flapping motion is crucial and also valuable as an input for the numerical simulations. Two high speed cameras oriented at an angle with respect to each other recorded the DelFly II wings flapping at high recording rates. This time, the DelFly II wings had a structured grid of 89 small markers on each of them. A white background was generated by use of a strong light source and a diffusive surface underneath the DelFly wings. Images of markers were captured at high recording rates. The processing of the images involves image pre- processing, detection of lines (leading edge and stiffeners) and points (grid points), and triangulation of detected points to find their coordinates in three-dimensional space. As a final step, the complete wing geometry is reconstructed by use of Kriging regression technique with local triangulation uncertainities. During the regression, a special attention is given to the conservation of wing surface area between the markers. Images taken by two high speed cameras at an instant of the flapping motion Experimental setup for Nato AVT-202 Basic Test Case Measurements The same setup was also used to investigate the effect of wing flexibility on the vortical structures generated during the clap-and-peel type flapping motion. It was shown that the flexibility affects the magnitude and the behaviour of the LEV. Iso-surfaces of vorticity magnitude at f=0.75 Hz Future Works Although the flow in the wake of the DelFly II wings are visualized for a wide set of parameters, force measurements are still missing. A new balance system has been realized recently which will be used for force and torque measurements simultaneously with the deformation measurements. A water submergible force sensor will also be integrated to the water tank system for a better interpretation of the visualization data.