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Papers by Ramiro Godoy-Diana

Bulletin of the American Physical …, 2006

We study experimentally the vortex streets produced by a high-aspect-ratio pitching foil placed i... more We study experimentally the vortex streets produced by a high-aspect-ratio pitching foil placed in a hydrodynamic tunnel. Particle image velocimetry (PIV) measurements give access to the spatio-temporal characteristics of the vorticity field in the wake and allow for a calculation ...

18ème Congrès Français de …, 2007

18 ?me Congr?s Fran?ais de M?canique Grenoble, 27-31 ao?t 2007 Transition tra?n?e?propulsion et b... more 18 ?me Congr?s Fran?ais de M?canique Grenoble, 27-31 ao?t 2007 Transition tra?n?e?propulsion et brisures de sym?trie dans le sillage d?un aileron oscillantRamiro Godoy-Diana, Jean-Luc Aider & Jos? Eduardo Wesfreid ...

Journal of Fluid Mechanics, 2009

We study experimentally the vortex streets produced by a high-aspect-ratio pitching foil placed i... more We study experimentally the vortex streets produced by a high-aspect-ratio pitching foil placed in a hydrodynamic tunnel. Particle image velocimetry (PIV) measurements give access to the spatio-temporal characteristics of the vorticity field in the wake and allow for a calculation of the spatial distribution of velocity fluctuations as well as an indirect estimate of the forces on the foil. A

Reflets de la physique, 2011

Refl ets de la Physique n° 24 4Avancées de la recherche Brève La plupart des insectes, des oiseau... more Refl ets de la Physique n° 24 4Avancées de la recherche Brève La plupart des insectes, des oiseaux et des poissons utilisent des ailes ou des nageoires battantes afi n de se déplacer dans le fl uide environnant. Le nombre de Strouhal permet d'évaluer leur performance propulsive : il est défi ni par le rapport entre la vitesse de battement de l'aile ou de la nageoire et la vitesse d'avancée de l'animal. Ce nombre varie peu dans la nature, et correspond à un rendement énergétique maximal [1,2]. Certains mécanismes par lesquels ces animaux sont capables de générer une force de propulsion peuvent être spectaculairement reproduits avec une expérience modèle simple [3].

Physics of Fluids, 2011

We study experimentally the impulse response of a cylinder wake below the critical Reynolds numbe... more We study experimentally the impulse response of a cylinder wake below the critical Reynolds number of the Bénard-von Kármán instability. In this subcritical regime, a localized inhomogeneous region of convective instability exists which causes initial perturbations to be transiently amplified. The aim of this work is to quantify the evolution resulting from this convective instability using twodimensional particle image velocimetry in a hydrodynamic tunnel experiment. The velocity fields allow us to describe the evolution of wave packets in terms of two control parameters: the Reynolds number and the magnitude of the imposed perturbation. The temporal evolution of energy exhibits a transient algebraic growth at short times followed by an exponential decay.

Journal of Fluid Mechanics, 2012

The wake of a flexible foil undergoing pitching oscillations in a low-speed hydrodynamic tunnel i... more The wake of a flexible foil undergoing pitching oscillations in a low-speed hydrodynamic tunnel is used to examine the effect of chord-wise foil flexibility in the dynamical features of flapping-based propulsion. We compare the regime transitions in the wake with respect to the case of a rigid foil and show that foil flexibility inhibits the symmetry breaking of the reverse Bénard-von Kármán wake reported in the literature. A momentum balance calculation shows the average thrust to be up to three times greater for the flexible foil than for the rigid foil. We explain both of these observations by analyzing the vortex dynamics in the very near wake. arXiv:1207.6949v1 [physics.flu-dyn] 30 Jul 2012

The vortex structures in the near wake of a apping foil are studied using 2D Particle Image Veloc... more The vortex structures in the near wake of a apping foil are studied using 2D Particle Image Velocimetry (PIV) from a hydrodynamic tunnel experiment. We study in particular the mechanisms that determine the symmetry breaking of the reverse Bénard-von Kármán vortex street and propose a semi-empirical model.

Aps Division of Fluid Dynamics Meeting Abstracts, Nov 1, 2008

We study experimentally the impulse response of a cylinder wake below the critical Reynolds numbe... more We study experimentally the impulse response of a cylinder wake below the critical Reynolds number of the Bénard-von Kárman instability. In this subcritical regime, a localized region of convective instability exists which determines an initial perturbation to be transiently amplified. Previous experimental works [Le Gal and Croquette, Phys. Rev. E 62, 4424 (2000)] have used the spatiotemporal evolution of streaklines from dye visualizations to analyze the evolution of a wave packet, but this has not permitted to give a correct picture of the transient energy growth. The aim of this work is to quantify the evolution of this convective instability using 2D particle image velocimetry in a hydrodynamic tunnel experiment. The velocity fields allow us to describe the evolution of the wave packet in terms of two control parameters: the Reynolds number and the strength of the imposed perturbation. The energy exhibits a transient algebraic growth followed by an exponential decay. A scaling law with respect to the Reynolds number was evidenced for the later decay, but not for the initial growth, which is consistent with the picture of transient growth in inhomogeneous media governed by the interaction of non-normal modes.

Nousétudions expérimentalement la réponse impulsionnelle du sillage d'un cylindre en régime sous-... more Nousétudions expérimentalement la réponse impulsionnelle du sillage d'un cylindre en régime sous-critique, dans un tunnel hydrodynamique. L'évolution du paquet d'ondes généré, en fonction du nombre de Reynolds et de la force de perturbation, estétudiée quantitativement avec la méthode PIVà deux dimensions et permet de rendre compte de la croissance transitoire de l'énergie, dueà l'inhomogénéité du milieu.

Physics of Fluids, 2011

We study experimentally the impulse response of a cylinder wake below the critical Reynolds numbe... more We study experimentally the impulse response of a cylinder wake below the critical Reynolds number of the Bénard-von Kármán instability. In this subcritical regime, a localized inhomogeneous region of convective instability exists which causes initial perturbations to be transiently amplified. The aim of this work is to quantify the evolution resulting from this convective instability using twodimensional particle image velocimetry in a hydrodynamic tunnel experiment. The velocity fields allow us to describe the evolution of wave packets in terms of two control parameters: the Reynolds number and the magnitude of the imposed perturbation. The temporal evolution of energy exhibits a transient algebraic growth at short times followed by an exponential decay.

Bulletin of the American Physical …, 2010

Hydrodynamic tunnel experiments with flexible flapping foils of 4:1 span-to-chord aspect ratio ar... more Hydrodynamic tunnel experiments with flexible flapping foils of 4:1 span-to-chord aspect ratio are used in the present work to study the effect of foil compliance in the dynamical features of a propulsive wake. The average thrust force produced by the foil is estimated from 2D PIV ...

Nousétudions expérimentalement la réponse impulsionnelle du sillage d'un cylindre en régime sous-... more Nousétudions expérimentalement la réponse impulsionnelle du sillage d'un cylindre en régime sous-critique, dans un tunnel hydrodynamique. L'évolution du paquet d'ondes généré, en fonction du nombre de Reynolds et de la force de perturbation, estétudiée quantitativement avec la méthode PIVà deux dimensions et permet de rendre compte de la croissance transitoire de l'énergie, dueà l'inhomogénéité du milieu.

Bulletin of the American Physical Society, 2009

We examine the propulsive performance of a flapping-wing device turning on a ``merry-go-round &#x... more We examine the propulsive performance of a flapping-wing device turning on a ``merry-go-round '' type base. The two-wing flapper is attached to a mast that is ball-bearing mounted to a central shaft in such a way that the thrust force produced by the wings makes the flapper turn ...

Bulletin of the American Physical …, 2010

Wing flexibility governs the flying performance of flapping wing flyers. Here we use the self-pro... more Wing flexibility governs the flying performance of flapping wing flyers. Here we use the self-propelled flapping-wing model mounted on a ``merry-go-round'' described by Thiria and Godoy-Diana (Phys. Rev. E 82, 015303, 2010) to investigate the effect of chord-wise wing compliance ...

EPL (Europhysics Letters), 2014

ABSTRACT Confined geometries usually involve reflected waves interacting together to form a spati... more ABSTRACT Confined geometries usually involve reflected waves interacting together to form a spatially stationary pattern. A recent study on bio-locomotion, however, has reported that propagating wave kinematics can naturally emerge in a forced elastic rod, even with boundary conditions involving significant reflections. It has been shown that this particular behavior is observed only in the presence of strong damping. Based on those observations, this study aims at giving a quantitative description of the mechanism involved to prevent the build-up of standing waves and generate traveling solutions. The question is discussed here in the framework of handmade artificial swimmers as an example of practical application but we believe that its potential is beyond this scope.

Journal of Fluid Mechanics, 2012

The wake of a flexible foil undergoing pitching oscillations in a low-speed hydrodynamic tunnel i... more The wake of a flexible foil undergoing pitching oscillations in a low-speed hydrodynamic tunnel is used to examine the effect of chord-wise foil flexibility in the dynamical features of flapping-based propulsion. We compare the regime transitions in the wake with respect to the case of a rigid foil and show that foil flexibility inhibits the symmetry breaking of the reverse Bénard-von Kármán wake reported in the literature. A momentum balance calculation shows the average thrust to be up to three times greater for the flexible foil than for the rigid foil. We explain both of these observations by analyzing the vortex dynamics in the very near wake. arXiv:1207.6949v1 [physics.flu-dyn] 30 Jul 2012

Bulletin of the American Physical …, 2006

We study experimentally the vortex streets produced by a high-aspect-ratio pitching foil placed i... more We study experimentally the vortex streets produced by a high-aspect-ratio pitching foil placed in a hydrodynamic tunnel. Particle image velocimetry (PIV) measurements give access to the spatio-temporal characteristics of the vorticity field in the wake and allow for a calculation ...

18ème Congrès Français de …, 2007

18 ?me Congr?s Fran?ais de M?canique Grenoble, 27-31 ao?t 2007 Transition tra?n?e?propulsion et b... more 18 ?me Congr?s Fran?ais de M?canique Grenoble, 27-31 ao?t 2007 Transition tra?n?e?propulsion et brisures de sym?trie dans le sillage d?un aileron oscillantRamiro Godoy-Diana, Jean-Luc Aider & Jos? Eduardo Wesfreid ...

Journal of Fluid Mechanics, 2009

We study experimentally the vortex streets produced by a high-aspect-ratio pitching foil placed i... more We study experimentally the vortex streets produced by a high-aspect-ratio pitching foil placed in a hydrodynamic tunnel. Particle image velocimetry (PIV) measurements give access to the spatio-temporal characteristics of the vorticity field in the wake and allow for a calculation of the spatial distribution of velocity fluctuations as well as an indirect estimate of the forces on the foil. A

Reflets de la physique, 2011

Refl ets de la Physique n° 24 4Avancées de la recherche Brève La plupart des insectes, des oiseau... more Refl ets de la Physique n° 24 4Avancées de la recherche Brève La plupart des insectes, des oiseaux et des poissons utilisent des ailes ou des nageoires battantes afi n de se déplacer dans le fl uide environnant. Le nombre de Strouhal permet d'évaluer leur performance propulsive : il est défi ni par le rapport entre la vitesse de battement de l'aile ou de la nageoire et la vitesse d'avancée de l'animal. Ce nombre varie peu dans la nature, et correspond à un rendement énergétique maximal [1,2]. Certains mécanismes par lesquels ces animaux sont capables de générer une force de propulsion peuvent être spectaculairement reproduits avec une expérience modèle simple [3].

Physics of Fluids, 2011

We study experimentally the impulse response of a cylinder wake below the critical Reynolds numbe... more We study experimentally the impulse response of a cylinder wake below the critical Reynolds number of the Bénard-von Kármán instability. In this subcritical regime, a localized inhomogeneous region of convective instability exists which causes initial perturbations to be transiently amplified. The aim of this work is to quantify the evolution resulting from this convective instability using twodimensional particle image velocimetry in a hydrodynamic tunnel experiment. The velocity fields allow us to describe the evolution of wave packets in terms of two control parameters: the Reynolds number and the magnitude of the imposed perturbation. The temporal evolution of energy exhibits a transient algebraic growth at short times followed by an exponential decay.

Journal of Fluid Mechanics, 2012

The wake of a flexible foil undergoing pitching oscillations in a low-speed hydrodynamic tunnel i... more The wake of a flexible foil undergoing pitching oscillations in a low-speed hydrodynamic tunnel is used to examine the effect of chord-wise foil flexibility in the dynamical features of flapping-based propulsion. We compare the regime transitions in the wake with respect to the case of a rigid foil and show that foil flexibility inhibits the symmetry breaking of the reverse Bénard-von Kármán wake reported in the literature. A momentum balance calculation shows the average thrust to be up to three times greater for the flexible foil than for the rigid foil. We explain both of these observations by analyzing the vortex dynamics in the very near wake. arXiv:1207.6949v1 [physics.flu-dyn] 30 Jul 2012

The vortex structures in the near wake of a apping foil are studied using 2D Particle Image Veloc... more The vortex structures in the near wake of a apping foil are studied using 2D Particle Image Velocimetry (PIV) from a hydrodynamic tunnel experiment. We study in particular the mechanisms that determine the symmetry breaking of the reverse Bénard-von Kármán vortex street and propose a semi-empirical model.

Aps Division of Fluid Dynamics Meeting Abstracts, Nov 1, 2008

We study experimentally the impulse response of a cylinder wake below the critical Reynolds numbe... more We study experimentally the impulse response of a cylinder wake below the critical Reynolds number of the Bénard-von Kárman instability. In this subcritical regime, a localized region of convective instability exists which determines an initial perturbation to be transiently amplified. Previous experimental works [Le Gal and Croquette, Phys. Rev. E 62, 4424 (2000)] have used the spatiotemporal evolution of streaklines from dye visualizations to analyze the evolution of a wave packet, but this has not permitted to give a correct picture of the transient energy growth. The aim of this work is to quantify the evolution of this convective instability using 2D particle image velocimetry in a hydrodynamic tunnel experiment. The velocity fields allow us to describe the evolution of the wave packet in terms of two control parameters: the Reynolds number and the strength of the imposed perturbation. The energy exhibits a transient algebraic growth followed by an exponential decay. A scaling law with respect to the Reynolds number was evidenced for the later decay, but not for the initial growth, which is consistent with the picture of transient growth in inhomogeneous media governed by the interaction of non-normal modes.

Nousétudions expérimentalement la réponse impulsionnelle du sillage d'un cylindre en régime sous-... more Nousétudions expérimentalement la réponse impulsionnelle du sillage d'un cylindre en régime sous-critique, dans un tunnel hydrodynamique. L'évolution du paquet d'ondes généré, en fonction du nombre de Reynolds et de la force de perturbation, estétudiée quantitativement avec la méthode PIVà deux dimensions et permet de rendre compte de la croissance transitoire de l'énergie, dueà l'inhomogénéité du milieu.

Physics of Fluids, 2011

We study experimentally the impulse response of a cylinder wake below the critical Reynolds numbe... more We study experimentally the impulse response of a cylinder wake below the critical Reynolds number of the Bénard-von Kármán instability. In this subcritical regime, a localized inhomogeneous region of convective instability exists which causes initial perturbations to be transiently amplified. The aim of this work is to quantify the evolution resulting from this convective instability using twodimensional particle image velocimetry in a hydrodynamic tunnel experiment. The velocity fields allow us to describe the evolution of wave packets in terms of two control parameters: the Reynolds number and the magnitude of the imposed perturbation. The temporal evolution of energy exhibits a transient algebraic growth at short times followed by an exponential decay.

Bulletin of the American Physical …, 2010

Hydrodynamic tunnel experiments with flexible flapping foils of 4:1 span-to-chord aspect ratio ar... more Hydrodynamic tunnel experiments with flexible flapping foils of 4:1 span-to-chord aspect ratio are used in the present work to study the effect of foil compliance in the dynamical features of a propulsive wake. The average thrust force produced by the foil is estimated from 2D PIV ...

Nousétudions expérimentalement la réponse impulsionnelle du sillage d'un cylindre en régime sous-... more Nousétudions expérimentalement la réponse impulsionnelle du sillage d'un cylindre en régime sous-critique, dans un tunnel hydrodynamique. L'évolution du paquet d'ondes généré, en fonction du nombre de Reynolds et de la force de perturbation, estétudiée quantitativement avec la méthode PIVà deux dimensions et permet de rendre compte de la croissance transitoire de l'énergie, dueà l'inhomogénéité du milieu.

Bulletin of the American Physical Society, 2009

We examine the propulsive performance of a flapping-wing device turning on a ``merry-go-round &#x... more We examine the propulsive performance of a flapping-wing device turning on a ``merry-go-round '' type base. The two-wing flapper is attached to a mast that is ball-bearing mounted to a central shaft in such a way that the thrust force produced by the wings makes the flapper turn ...

Bulletin of the American Physical …, 2010

Wing flexibility governs the flying performance of flapping wing flyers. Here we use the self-pro... more Wing flexibility governs the flying performance of flapping wing flyers. Here we use the self-propelled flapping-wing model mounted on a ``merry-go-round'' described by Thiria and Godoy-Diana (Phys. Rev. E 82, 015303, 2010) to investigate the effect of chord-wise wing compliance ...

EPL (Europhysics Letters), 2014

ABSTRACT Confined geometries usually involve reflected waves interacting together to form a spati... more ABSTRACT Confined geometries usually involve reflected waves interacting together to form a spatially stationary pattern. A recent study on bio-locomotion, however, has reported that propagating wave kinematics can naturally emerge in a forced elastic rod, even with boundary conditions involving significant reflections. It has been shown that this particular behavior is observed only in the presence of strong damping. Based on those observations, this study aims at giving a quantitative description of the mechanism involved to prevent the build-up of standing waves and generate traveling solutions. The question is discussed here in the framework of handmade artificial swimmers as an example of practical application but we believe that its potential is beyond this scope.

Journal of Fluid Mechanics, 2012

The wake of a flexible foil undergoing pitching oscillations in a low-speed hydrodynamic tunnel i... more The wake of a flexible foil undergoing pitching oscillations in a low-speed hydrodynamic tunnel is used to examine the effect of chord-wise foil flexibility in the dynamical features of flapping-based propulsion. We compare the regime transitions in the wake with respect to the case of a rigid foil and show that foil flexibility inhibits the symmetry breaking of the reverse Bénard-von Kármán wake reported in the literature. A momentum balance calculation shows the average thrust to be up to three times greater for the flexible foil than for the rigid foil. We explain both of these observations by analyzing the vortex dynamics in the very near wake. arXiv:1207.6949v1 [physics.flu-dyn] 30 Jul 2012