Aerodynamic forces and vortex structures of flapping wings in forward flight
- Autores: Alejandro Gonzalo Grande
- Directores de la Tesis: Manuel García Villalba Navaridas (dir. tes.), Óscar Flores Arias (codir. tes.)
- Lectura: En la Universidad Carlos III de Madrid ( España ) en 2018
- Idioma: inglés
- Tribunal Calificador de la Tesis: Alfredo Pinelli (presid.), Stefano Discetti (secret.), Joaquín Ortega Casanova (voc.)
- Programa de doctorado: Programa de Doctorado en Mecánica de Fluidos por la Universidad Carlos III de Madrid; la Universidad de Jaén; la Universidad de Zaragoza; la Universidad Nacional de Educación a Distancia; la Universidad Politécnica de Madrid y la Universidad Rovira i Virgili
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- Tesis en acceso abierto en: e-Archivo
- Resumen
In the last two decades, there has been an increasing interest to develop micro air vehicles (MAVs) capable to fly like insects and small birds. These animals have evolved over centuries achieving outstanding flight abilities. Thus, engineers are trying to mimic their flapping motions to develop devices with high maneuverability. Although the unconventional unsteady aerodynamic mechanisms are well known since the 1980s, their systematic applicability to the practical design of MAVs has been proven difficult. The problem arises due to the vast amount of maneuvers performed by these flying animals, which leads to an incredibly large range of kinematics parameters. This, added to the diversity of geometric parameters (body morphology, wing shape, size and weight,...) of the different birds and insects makes really difficult to develop reliable models for the aerodynamic forces.
In order to contribute to the understanding of the aerodynamics of MAVs, in this thesis we study flapping wings in forward flight by means of direct numerical simulations. More specifically, the question we want to address is how the aerodynamic forces change when the wing kinematics are varied. Thus, we consider the transition from wings rotating with respect to their roots (flapping wing) to wings oscillating vertically (heaving wings). To that end, several direct numerical simulations of the flow (at low Reynolds number, Re = 500) around a pair of wings have been performed, varying the distance between an axis parallel to the flying velocity and the wing root (radius of flapping motion, R). Apart from R, which shifts from flapping to heaving motion, another kinematic parameter has been varied. This parameter is the maximum vertical displacement of the outboard wing tip (h), which has been kept fixed for most of the cases studied. Besides, the importance of the wing geometry has also been considered by studying wings with two different aspect ratios (AR). Note that to keep the problem as simple as possible, the same angular frequency has been imposed in all the motions and no other kinematics and geometric complexities (e.g. pitching motion or wing geometric twist) have been considered.
The database generated has been studied in terms of net aerodynamic forces during the cycle, forces distributions on the wings surface and flow structures around the wings. Among these structures, a particular attention has been paid to the leading edge vortex (LEV), which has been characterized qualitatively and quantitatively. For the latter characterization, a methodology to track the position of the LEV core in time and space has been developed. This methodology has also been used to evaluate several flow quantities along the LEV and their relation with the aerodynamic forces on the wings.
The results show that in the configurations studied, the local aerodynamic forces, forces distributions on the wing surface, and flow structures are mainly associated to the local effective angle of attack (AOA). This parameter is defined as the angle formed by chordline and the relative velocity vector, which is obtained with the flight velocity and the local vertical velocity of a corresponding wing section. Note that AOA increases along the wing span with R for cases with equal h and is maximum close to the outboard wing tip for cases with larger h. Thus, cases with larger effective angle of attack averaged along the wing span produce net forces with larger peaks during the cycle when the AR and the h are equal. This is translated into a larger mean lift and a smaller mean drag generation during the downstroke motion. However, cases with higher AR produced larger mean aerodynamic forces during the downstroke even with somewhat smaller AOA values. The forces have been decomposed in normal and tangential contributions, showing that in all cases the former is responsible for almost the whole lift and thrust generation, while the latter produces the drag force.
The importance of the LEV in the lift generation has been observed through the comparison of the forces distributions on the upper wing surface at the mid-downstroke and the vortical structures. For the flapping cases the LEV structure has a conical shape and its intensity increases from the root to the outboard wing tip, where the forces distributions show larger values. Using the methodology developed to characterize the LEV, it has been shown that the local position of the LEV core depends mainly on AOA along the wing span, except close to the wing tips. In fact, cases with different R and AR, but equal AOA show LEVs with similar positions and therefore comparable local aerodynamic forces. The evolution of the LEV during the downstroke seems also to be linked with the AOA of the wing section studied, presenting more separation from the wing and more chordwise distance respect to the leading edge for larger AOA . However, it has been observed that for cases with equal R and AR, but different values of AOA along the wing span, the chordwise position remains almost equal, both along the wing span and during its time evolution. On the other hand, the local lift force coefficient during the downstroke seems to be associated to the LEV circulation (which in turn depends on AOA ), provided that the LEV is sufficiently close to the wing.
