Mención Internacional en el título de doctorNature often serves as a reference for the design and development of sustainable solutions in numerous
different fields. The recent development of small-scale robotic vehicles, asMicro-Air Vehicles
(MAVs), is not an exception, and has had an increasingly important impact on society, proposing new
alternatives in areas as surveillance or planetary exploration. Trying to mimic the flight of insects
and small birds, these devices try to offer more efficient designs and with higher manoeuvrability
abilities than the already existing designs. It happens similar with robotic swimmers, with many
different existing prototypes. Indeed, it is even possible to find designs of bioinspired small-scale
wind turbines based on auto-rotating seeds looking for a more efficient energy harvesting. Besides,
in order to develop sustainable designs, increasing their lifetime and reducing the maintenance costs
are crucial factors. Depending on the device to design, different methodologies may be followed in
order to achieve these two goals while meeting the design requirements. One clear example can be
found in the development of wind turbines. Their blades must be designed to withstand not only
maximum loads and stresses but also the fatigue caused by the fluctuations around the load required
to operate correctly. Reducing fatigue issues by limiting the amplitude of those fluctuations using
passive or active control is a viable option to improve their lifetime.
The aimof this dissertation is to contribute to the understanding of the underlying physics in
biolocomotion. To this end, direct numerical simulations of different examples and problems at low
Reynolds number, Re, have been performed using an existing fluid-structure interaction (FSI) solver.
This FSI solver relies on the coupling of an incompressible-flow solver with robotic algorithms for the
computation of the dynamics of a system of connected rigid bodies. The particularities of this solver
are detailed in the thesis.
The second part of the thesis includes the analysis of these examples and problems mentioned
above.More in detail, the aerodynamic and aeroelastic behaviour of airfoils and wings at Re Æ 1000
in various conditions and environments has been analysed.
Natural flyers and swimmers are immersed in turbulent and gusty environments which affect
their aerodynamic behaviour. The first problem that has been studied is that of the unsteady response
of airfoils impacted by vortical gusts. This first example focuses on how the impact of viscous vortices
of different size and intensity on two-dimensional airfoils modify their response. Although in a
simplified framework, this analysis allows to gather relevant information about the aerodynamic
performance of the airfoils. This aerodynamic response is seen to be self-similar, and the work
proposes a semi-empirical model to determine the temporal evolution of the lifting forces based on an integral definition of the vertical velocity induced by the gust, which can be known a priori.
The target of the second problem is to analyse the load that can be mitigated in airfoils undergoing
oscillations in the angle of attack using passive-pitching trailing edge flaps. This corresponds, for
example, to a simplification of the problem of load mitigation in small-scale wind turbines. The
use of passive-pitching trailing edge flaps is a strategy that has recently been recently proposed for
large-scale wind turbines. Here, we investigate the validity of this strategy on a completely different
scenario. Contrary to what happens in experiments at higher Reynolds numbers, whose results
match the predictions of a quasi-steady linear model when the kinematics are within the range of
applicability of this model, the load mitigation obtained in this work differs from the values of this
theory. The load mitigated is larger or smaller than the predicted values depending on the amplitude
of the oscillations in the angle of attack. However, the results of this work show that an increase in
the length of the flap while the chord of the airfoil is kept constant leads to an equal change in the
reduction of load, in line with the predictions of the quasi-steady model. The development of vortical
structures is clearly affected by the flap when it is sufficiently large, which also involves changes in the
dynamics of the flap and the forces seen by the airfoil. The repercussion that several of the variables
defining the parametric space have on the aerodynamic behaviour of the foil and the dynamics of
the flap are analysed. This allows to gather more information for an appropriate selection of those
variables.
Finally, the third and fourth problems involve the study of the effects of spanwise flexibility on
both isolated wings and pairs of wings arranged in horizontal tandem undergoing flapping motions.
The wings are considered to be rectangular flat plates, and the spanwise flexibility is modelled
discretizing these flat plates in a finite number of rigid sub-bodies that are connected using torsional
springs. The wings are considered to be rigid in the chordwise direction. Isolated spanwise-flexible
wings find an optimal propulsive performance when a fluid-structural resonance occurs. At this
flexibility, the time-averaged thrust is maximum and twice the value yielded by the rigid case, and
the increment in efficiency is around a 15%. Flexibility and the generation of forces are coupled, such
that the structural response modifies the development of the vortical structures generated by the
motion of the wing, and vice versa. The optimal performance comes from a combination of larger
effective angles of attack, properly timed with the pitching motion such that the projection of the
forces is maximum, with a delayed development of the vortical structures. Besides, while aspect
ratio effects are important for rigid wings, this effect becomes small when compared to flexibility
effects when the wings become flexible enough. In fact, while the increase in thrust coefficient for
rigid wings with aspect ratio 4 is 1.2 times larger than that provided by rigid wings with aspect ratio
equal to 2, the value of this coefficient for resonant wings is twice the value yielded by rigid wings
of aspect ratio 4. While forewings of the tandem systems are found to behave similarly to isolated
wings, the aeroelastic response of the hindwings is substantially affected by the interaction with the
vortices developed and shed by the forewings. This wake capture effect modifies the flexibility at
which an optimal propulsive behaviour is obtained. This wake capture effect is analysed through an estimation of the effective angle of attack seen by both forewings and hindwings, linking the
optimal behaviour with the maximisation of the effective angle of attack at the right instants. Based
on the obtained results, a proof-of-concept study has been carried out analysing the aerodynamic
performance of tandem systems made of wings with different flexibility, which suggests that the
latter could outperformsystems of equally flexible wings.This thesis has been carried out in the Aerospace Engineering Department at Universidad Carlos III
de Madrid. The financial support has been provided by the Universidad Carlos III de Madrid through
a PIPF scholarship awarded on a competitive basis, and by the Spanish Ministry of Economy and
Competitiveness through grant DPI2016-76151-C2-2-R (AEI/FEDER, UE).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 VirgiliPresidente: José Ignacio Jiménez González.- Secretaria: Andrea Ianiro.- Vocal: Manuel Moriche Guerrer
