Resumen tesis:In this thesis we have studied the stationary and non-stationary aerodynamic characteristics of wings at low Reynolds numbers commonly achieved in MAVS vehicles. The lift coefficient for a flat plate at low angles of attack is obtained experimentally for various aspect ratios (AR=1,2,4,8) and moderate Reynolds numbers (Re=40x10^3 to Re=200x10^3). The variation of the lift coefficient with the angle of attack in the pre-stall region is consistent with a linear slope approximation. We consider this slope to be a function of both the aspect ratio and Reynolds number. In this research, we provide a correlation that can predict the lift slope value with an average error of less than 5%.
Further extending this idea, we study experimentally the lift coefficient on NACA0012 wing model at different Reynolds numbers from 40x10^3 to 200x10^4. A non-linearity around the zero angle of attack leading to a shift of sign in the lift was observed for a sufficiently large aspect ratio at Re=40×10^3. The existence of the negative lift for wing models with the largest aspect ratio suggests that the three-dimensional effects are negligible. Therefore, two-dimensional simulations were performed to understand the cause of the negative lift. For the cases with the negative lift, the flow displays an interesting feature of pre-alignment with the chord upstream of the airfoil. Furthermore, it was found that the negative lift is directly related to the positive net circulation (anti-clockwise) around the airfoil.
The following chapters of this thesis focus on the non-intrusive measurement of forces that will eventually be applied to a flapping wing. We introduce a new Particle Image Velocimetry (PIV) software coded in Python that is based on the already existing DPIVSoft program which is a double pass with a window deformation algorithm.
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