376 research outputs found

    How wavelength affects the hydrodynamic performance of two accelerating mirror-symmetric slender swimmers

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    Fish schools are capable of simultaneous linear acceleration. To reveal the underlying hydrodynamic mechanism, we numerically investigate how Reynolds number Re=1000−2000 Re = 1000 - 2000 , Strouhal number St=0.2−0.7 St = 0.2 - 0.7 and wavelength λ=0.5−2 \lambda = 0.5 - 2 affect the mean net thrust and net propulsive efficiency of two side-by-side hydrofoils undulating in anti-phase. In total, 550 550 cases are simulated using immersed boundary method. The thrust increases significantly with wavelength and Strouhal number, yet only slightly with the Reynolds number. We apply a symbolic regression algorithm to formulate this relationship. Furthermore, we find that mirror-symmetric schooling can achieve a \textit{net} thrust more than ten times that of a single swimmer, especially at low Reynolds numbers. The highest efficiency is obtained at St=0.5 St = 0.5 and λ=1.2 \lambda = 1.2 , where St St is consistent with that observed in the linear-accelerating natural swimmers, \eg Crevalle jack. Six distinct flow structures are identified. The highest thrust corresponds to an asymmetric flow pattern, whereas the highest efficiency occurs when the flow is symmetric with converging vortex streets.Comment: This paper has been accepted by Physics of Fluids. This is the accepted versio

    Force moment partitioning and scaling analysis of vortices shed by a 2D pitching wing in quiescent fluid

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    We experimentally study the dynamics and strength of vortices shed from a NACA 0012 wing undergoing sinusoidal pitching in quiescent water. We characterize the temporal evolution of the vortex trajectory and circulation over a range of pitching frequencies, amplitudes and pivot locations. By employing a physics-based force and moment partitioning method (FMPM), we estimate the vortex-induced aerodynamic moment from the velocity fields measured using particle image velocimetry. The vortex circulation, formation time and vorticity-induced moment are shown to follow scaling laws based on the feeding shear-layer velocity. The vortex dynamics, together with the spatial distribution of the vorticity-induced moment, provide quantitative explanations for the nonlinear behaviors observed in the fluid damping (Zhu et al., J. Fluid Mech., vol. 923, 2021, R2). The FMPM-estimated moment and damping are shown to match well in trend with direct force measurements, despite a discrepancy in magnitude. Our results demonstrate the powerful capability of the FMPM in dissecting experimental flow field data and providing valuable insights into the underlying flow physics.Comment: 21 pages, 11 figure

    The Effect of Wing Shape and Ground Proximity on Unsteady Fluid Dynamics During the Perching Maneuver.

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    While landing, birds often perform a perching maneuver, which involves pitching their wings upwards while decelerating to a complete stop. By performing this perching maneuver, the birds can continue generating higher lift and drag force while slowing down, resulting in a smooth landing. The present study is motivated by the perching maneuver and aims to investigate two critical aspects of it. First, we want to explore how the proximity of the ground affects the unsteady forces and the flow field during the perching flight; and second, we want to analyze how a wing sweep influences a perching maneuver. To explore the first aspect of this dissertation, we investigated the finite flat plate undergoing a perching maneuver in the ground effect. Our results showed that the instantaneous and time-averaged lift force increased as the plate came close to the ground, while the instantaneous peak drag coefficient stayed relatively constant with changes in the ground height. However, the negative drag force, or the parasitic thrust, at the latter stages of the perching maneuver increased with the increase of the ground proximity. We found that performing rapid pitching at the end phase of the decelerating motion, which is done by introducing the time offset between the decelerating and pitch-up motion, significantly reduced the parasitic thrust even when the perching plate was in close proximity to the ground. Our results revealed that the dipole jet induced by the counter-rotating vortices was lower for the pitching case executed at the latter stage of the decelerating motion, which affected the advection of the shed vortices, acceleration of the fluid between the wing and the ground, and varied the unsteady forces during the perching maneuver. For the highest shape change number considered in this study, at a time offset of 0.5, the wing generated a positive averaged drag force and near zero averaged lift force, which is appropriate to land smoothly on the initial perching location without gaining altitude. The second aspect of this dissertation is motivated by the observation that some birds fold their wings to create a wing sweep during such perching. This study aims to find out whether such a wing sweep helps during a perching maneuver. We use two flat plates: one with a sweep and another without any sweep, and consider a deceleration maneuver where both decelerate to a complete stop from a Reynolds number, Re = 13000. We consider two cases: one, where the wings undergo only heaving, and another, where the wings perform both heaving and pitching. The latter maneuver was designed to mimic perching. By performing experiments and simulations, we compare the temporal evolution of the instantaneous forces and the vortex dynamics of both these plates. We show that during a major part of the deceleration, the instantaneous lift forces are higher in the case of the plate with sweep compared to the plate with no sweep during both kinematics. Our results indicate that the higher lift in the swept plate case was contributed by a stable leading edge vortex (LEV) which remains attached to the plate. This increase in stability was contributed by the spanwise vorticity convection caused by a distinct spanwise flow on the swept plate, as revealed by the numerical simulation. We also show that combined pitching and heaving resulted in higher force peaks, and the forces also decayed faster in this case compared to the heave-only case. Finally, by using an analytical model for unsteady flows, we prove that the higher lift characteristics of the swept plate were entirely due to higher circulatory forces. We also developed an analytical model that accounts for the variation of unsteady forces on a flat plate undergoing a perching maneuver. We model the flat plate using unsteady lifting line theory while the effect of ground height is incorporated using image vortices. We used Wagner\u27s theory and the unsteady Kutta condition to model pitching and gradual deceleration. To include the ground effect, we updated the added mass force by accounting for the increase in flow acceleration between the wing and the ground. The model\u27s accuracy was tested against the experimental results on a finite wing undergoing identical kinematics. Our result demonstrates that the present analytical model captures the unsteady variation of forces during a perching maneuver

    Investigation of the Free-Fall Dynamic Behavior of a Rectangular Wing with Variable Center of Mass Location and Variable Moment of Inertia

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    © 2023 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https://creativecommons.org/licenses/by/4.0/).In recent years, the air-drop launch technology of near-space UAVs has attracted much attention. Between downfall from the carrier and the flight control system’s initiation, the UAV presents free-fall movement. This free-fall process is very important for the control effect of the flight control system and is also crucial for the safety of the UAV and the carrier. Focus is required on two important dynamic parameters of the UAV: the moment of inertia and the center of mass position. In this paper, we used a quasi-steady model proposed by predecessors to address the flat-plate falling problem with modifications to describe the freely falling motion of the wing. Computational fluid dynamics (CFD) were used to simulate the free-fall movement of the wing with various parameters, and the wing release behavior was analyzed to check the quasi-steady model. Research shows that the movement characteristics of the falling wing are mostly reflected in the longitudinal plane, and the developed quasi-steady analytical model can more accurately describe the dynamic behavior of free-fall to some extent. By using CFD methods, we further investigated the aerodynamic performance of the free-fall wing. The results show that the wing mainly presents tumbling and fluttering motion. Changing the moment of inertia around the tumbling axis changes the tumbling frequency and the time point as the wing enters tumbling. In contrast, changing the position of the center of mass significantly changes the form of falling and makes the free-fall motion more complex. Therefore, it is necessary to carefully configure the center of mass in the UAV design process.Peer reviewe

    Wave devouring propulsion: an overview of flapping foil propulsion technology

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    A comprehensive review of flapping foils for Wave Devouring Propulsion (WDP) is presented. The flapping foil can effectively utilize wave energy and generate thrust. The development of WDP is discussed, followed by an introduction to the geometry, modes of motion, and operating principles. These research studies are classified as theoretical, experimental, and numerical and are provided in detail. They demonstrate that marine equipment with a flapping foil system can achieve high energy conversion efficiency and low resistance. Several prototypes of the combination of WDP with human-crewed and uncrewed vessels have been shown, including the latest initial concept models and company products. There is a huge prospect for self-driven, pollution-free propulsion of marine devices, and this paper suggests several future studies

    On the Association of Kinematics, Spanwise Instability and Growth of Secondary Vortex Structures in the Wake of Oscillating Foils

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    Three-dimensional wake of an oscillating foil with combined heaving and pitching motion is numerically evaluated at a range of chord-based Strouhal number (0.32 \le Stc \le 0.56) and phase offset (90 deg \le \phi \le 70 deg) at Re = 8000. The changes in \phi and Stc reflect a unique route of transition in mechanisms that govern the origin of spanwise instabilities and growth of secondary wake structures. At lower Stc, heave dominated kinematics demonstrates a strong secondary leading edge vortex (LEV ) as the source of growing spanwise instability on the primary LEV , followed by an outflux of streamwise vorticity filaments from the secondary LEV . With increasing heave domination, the origin of stronger spanwise instability is governed by a counter-rotating trailing edge vortex (TEV ) and LEV that leads to growth of streamwise secondary structures. A decreasing heave domination ultimately coincides with an absence of strong LEV undulations and secondary structures. The consistent transition routes are represented on a phase-space map, where a progression of spanwise instability and growth of secondary structures becomes evident within regimes of decreased heave domination. The increasing strength of circulation for the primary LEV , with increasing Stc, provides a crucial reasoning for this newly identified progression

    Bioinspired fluid-structure interaction problems: gusts, load mitigation and resonance

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    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

    Review of Vortex Lattice Method for Supersonic Aircraft Design

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    © The Author(s), 2023. Published by Cambridge University Press on behalf of Royal Aeronautical Society. This is an Open Access article, distributed under the terms of the Creative Commons Attribution licence (https://creativecommons.org/licenses/by/4.0/There has been a renewed interest in developing environmentally friendly, economically vi-able, and technologically feasible supersonic transport aircraft and reduced order modeling methods can play an important contribution in accelerating the design process of these future aircraft. This paper reviews the use of the vortex lattice method (VLM) in modeling the gen-eral aerodynamics of subsonic and supersonic aircraft. The historical overview of the vortex lattice method is reviewed which indicates the use of this method for over a century for devel-opment and advancements in the aerodynamic analysis of subsonic and supersonic aircraft. The preference of VLM over other potential flow-solvers is because of its low order highly efficient computational analysis which is quick and efficient. Developments in VLM covering steady, unsteady state, linear and non-linear aerodynamic characteristics for different wing planform for the purpose of several different types of design optimization is reviewed. For over a decade classical vortex lattice method has been used for multi-objective optimization studies for commercial aircraft and unmanned aerial vehicle’s aerodynamic performance op-timization. VLM was one of the major potential flow solvers for studying the aerodynamic and aeroelastic characteristics of many wings and aircraft for NASA’s supersonic transport mission (SST). VLM is a preferred means for solving large numbers of computational design parameters in less time, more efficiently, and cheaper when compared to conventional CFD analysis which lends itself more to detailed study and solving the more challenging configu-ration and aerodynamic features of civil supersonic transport.Peer reviewe

    Self-propelled fish locomotion in an otherwise quiescent fluid

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    Since the deep observations by Leonardo da Vinci, understanding fish locomotion in water has always attracted the attention of scientists in many fields, from fluid mechanics to other disciplines concerning environmental sciences. The complexity of this problem is mainly given by the non-linear interaction between the fish body and the surrounding fluid otherwise at rest, leading to the desired forward locomotion and to the unavoidable angular and lateral recoil reactions, which are essential for a correct evaluation of the swimming performance. Despite many advances have been obtained for the study of fish self-propulsion in recent years, from simple mathematical models up to complex numerical solutions, the main mechanisms underlying fish locomotion are not fully clarified and still require further investigations. In this thesis free swimming conditions is deeply analyzed for both steady swimming and fast maneuvers by a theoretical approach which considers the full body-fluid system to obtain the ex- changed internal forces. The focus is on the added mass and the vortex shedding contributions to the locomotion performance and on the role of recoil motions which, together with the prescribed body deformation, define the free swimming behavior. To this purpose, the impulse formulation allows for an easy isolation of the potential contri- bution, related to the added mass, and of the vortical contribution related to bound and released vorticity and a simple two-dimensional numerical model with concentrated vorticity is adopted for the numerical simulations to generate meaningful results able to clarify these physical phenomena. The aim is a unified procedure for both undulatory and oscillatory swimming to obtain valid an- swers for cruising speed, expended energy and kinematics, hence for the swimming performance in terms of the cost of transport and propulsive efficiency. The same model is also able to give new insights on the impressive performance characterizing fish fast maneuvers. The extreme turning capability and the large acceleration, so essential to fish survival along pray-predator encounters, are studied by highlighting the potential and the vortical impulses and their interplay induced by recoil motions, to show their relevance for the realization of the maneuver

    A novel methodology for the assessment or wave energy opions at early stages

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    276 p.El aumento de la proporción de generación de electricidad a partir de fuentes renovables es clave para garantizar un sistema energético totalmente descarbonizado y luchar contra el cambio climático. La energía undimotriz es un recurso abundante pero, al mismo tiempo, es la menos desarrollada de todas las tecnologías renovables. El marco de evaluación común desarrollado en la tesis se basa en principios sólidos de ingeniería de sistemas y abarca el contexto externo, los requisitos del sistema y los criterios de evaluación. Se puede aplicar a diferentes niveles de madurez tecnológica y capta los aspectos cualitativos relacionados con las expectativas de las partes interesadas. El enfoque novedoso guía las decisiones de diseño a lo largo del proceso de desarrollo para la gestión adecuada del riesgo y la incertidumbre, y facilita la selección y evaluación comparativa de la tecnología undimotriz a diferentes niveles de madurez de manera controlada. Los métodos propuestos en esta investigación brindan información valiosa para enfocar los esfuerzos de innovación en aquellas áreas que tienen la mayor influencia en el desempeño de la tecnología. La incorporación de estrategias de innovación eficaces en el desarrollo de la energía undimotriz ayuda a gestionar la complejidad del sistema y canalizar la innovación hacia mejoras útiles.Tecnali
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