A numerical study on the aerodynamic forces and the wake stability of flapping flight at low Reynolds number
- Autores: Manuel Moriche Guerrero
- Directores de la Tesis: Óscar Flores Arias (dir. tes.), Manuel García Villalba Navaridas (codir. tes.)
- Lectura: En la Universidad Carlos III de Madrid ( España ) en 2017
- Idioma: español
- Tribunal Calificador de la Tesis: Ramón Fernández Feria (presid.), Francisco Javier Rodríguez Rodríguez (secret.), Jan Wissink (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
The unsteady aerodynamics that govern flapping flight at low Reynolds number are not yet properly understood, so there is no theory available to characterize this flight regime. Many works on the aerodynamic forces of flapping airfoils can be found in the literature, but still our capability to predict these forces is limited. Most of these studies focus on flapping airfoils, assuming infinite aspect ratio wings and Two dimensional (2D) flow. To what extent the 2D assumption is valid is uncertain. Furthermore, the number of studies on the effect that Three dimensional (3D) flow structures originated by flow instabilities instead of finite wings is small.
In this work we present Direct Numerical Simulations of heaving and pitching airfoils at low Reynolds number where the airfoil motion is prescribed by sinusoidal laws. The parameter space of this problem is huge, so only the mean pitch angle and the phase shift between the heaving and pitching motions are modified. We generate a database of 18 cases and analyze the integrated values of thrust and lift of each case. Also, a reference case is selected to perform a detailed analysis of the forces and decompose the total aerodynamic force in contributions from body motion, vorticity within the flow and surface vorticity. This analysis is extended to a subset of cases from the database in order to study the influence of the motion parameters on the aerodynamic forces. After that, we proceed to estimate the aerodynamic forces by existing models from the literature and, based on observations made through this work, we propose a modification of the classical Kutta-Joukowsky theorem. Finally, we compute the total aerodynamic force as the combination of the contribution from body motion and vorticity within the flow, neglecting surface vorticity effects. This proposed model shows remarkable results for the prediction of thrust and good results for the lift.
After analyzing the aerodynamic forces of the 2D cases, we proceed to study the three-dimensionality of the flow of part of the database. First, we present a stability analysis of four of the cases from the database. Each case is studied by Floquet stability analysis. The four cases considered display different wake structures resulting in different mean aerodynamic forces. Two cases produce thrust and lift, one case only thrust (with symmetric heaving and pitching) and the remaining case mainly lift (with the highest mean pitch angle). In addition, the latter case displays a period doubling phenomenon, and it is found to be linearly unstable for long wavelengths, with an instability mode that resembles that of mode A found in the wake of cylinders. Other cases, although being linearly stable, present a convective instability at smaller wavelengths. Finally, the unstable case has been studied with a fully 3D DNS to evaluate the effect of the three-dimensionality on the forces. The resulting flow structure is consistent with the linear stability analysis in the near wake. Further downstream nonlinearities lead to a fully 3D wake. Despite this, the aerodynamic forces on the 3D wing are very similar to those obtained in the 2D simulation.
