7 research outputs found
Influence of the porosity pattern on the aerodynamics of a square plate
The evolution of the normal aerodynamic coefficient of 19 configurations of
square plates with various porosity patterns, ranging from solid plate to
homogeneous porous plate, is experimentally characterized. The variation of the
porosity pattern is obtained by partially covering the holes of a commercial
fly-swatter using adhesive tape. Evolution of the normal aerodynamic
coefficient is assessed from the measurement of the angular position of the
porous plate, placed as a freely rotating pendulum swept by a flow in a wind
tunnel. These angular measurements are also supported by PIV measurements of
the structure of the wake. We show that the porosity pattern determines whether
or not an abrupt stall occurs. In particular, the details of the porosity
pattern on the edges of the plate are decisive for the existence of abrupt
stall
Interactions fluide-structure de systĂšmes pendulaires multistables
Lorsquâun objet est placĂ© dans un Ă©coulement, celui-ci est dĂ©viĂ© et de cette dĂ©viation rĂ©sultent lâapparition de tourbillons et la gĂ©nĂ©ration de forces de rĂ©action du fluide, telles que la traĂźnĂ©e et la portance. Si maintenant, lâobjet en question est libre de se mouvoir, un couplage sâinstalle entre les mouvements de lâobjet et de lâĂ©coulement environnant, du domaine des interactions fluide-structure. Dans cette thĂšse, le couplage entre un systĂšme pendulaire et un Ă©coulement dâair est Ă©tudiĂ© expĂ©rimentalement et thĂ©oriquement. PlacĂ© dans une soufflerie, un pendule circulaire prĂ©sente notamment une bistabilitĂ© sur une gamme de vitesse de vent, tandis quâun rectangle nâen prĂ©sente aucune. En variant le rapport dâaspect du pendule rectangulaire et en visualisant le sillage dâun disque fixe, nous proposons une origine Ă lâexistence ou non de la bistabilitĂ©, en lien avec le phĂ©nomĂšne de dĂ©crochage. Lâinfluence de la turbulence incidente sur ce phĂ©nomĂšne est ensuite approfondie ainsi que le lien entre les fluctuations dâangle du pendule et celles de lâĂ©coulement, amont comme aval. En revenant Ă la bistabilitĂ© elle-mĂȘme, des transitions spontanĂ©es dâun Ă©tat stable Ă lâautre sont observĂ©es, dont le cadre de la transition vers la turbulence permet une modĂ©lisation et suggĂšre certains mĂ©canismes Ă lâĆuvre dans le sillage, notamment des Ă©vĂšnements aĂ©rodynamiques rares. La modification des paramĂštres gĂ©omĂ©triques du pendule permet de varier la plage de vitesses pour laquelle la bistabilitĂ© existe, et ainsi observer des sauts entre les deux positions, Ă mĂȘme vitesse dâĂ©coulement. Enfin, lorsque le pendule est Ă©quilibrĂ©, son mouvement est dictĂ© par lâĂ©coulement uniquement et alors quâun modĂšle quasi-statique est insuffisant Ă la description de la dynamique rĂ©elle du pendule, nous Ă©tablissons deux maniĂšres de prendre en compte la rĂ©troaction dynamique de lâĂ©coulement, lâune empirique et lâautre basĂ©e sur les vibrations induites par vortex.When in a flow, an object deviates it and from this deviation are generated vortices and flow reaction forces, such as drag and lift. If yet the object is free to move, its movement can couple with the surrounding flow, falling into the domain of fluid-structure interactions. In this PhD thesis, the coupling between a pendular system and an air flow is studied both experimentally and theoretically. Placed in a wind tunnel, a disk pendulum presents a bistability for a range of flow velocity, while a rectangular one does not. By varying the aspect ratio of such rectangle and visualizing the wake behind a fixed disk, we propose an origin on whether or not the bistability emerges, linking it to stall phenomenon. The influence of ambient turbulence on this phenomenon is then investigated together with the link between angular fluctuations and flow variations, both upstream and downstream. Going back to the bistability itself, spontaneous transitions between stable states are observed and a model inspired from the transition to turbulence suggest certain mechanisms in the wake triggering such transitions, in particular rare aerodynamical events. Modifying geometrical parameters of the pendulum enables the adjustment of the range of velocity for which the bistability occurs, and with it, we could observe jumps between both transitions at the same flow rate. Finally, when the pendulum is balanced, its movement is only driven by the flow and while a quasi-static modelling is not sufficient to describe the real dynamics of the pendulum, we introduce two ways of accounting for the dynamical retroaction of the flow in the equation of movement, one empirical and the other based on vortex-induced vibration theory
Interactions fluide-structure de systĂšmes pendulaires multistables
When in a flow, an object deviates it and from this deviation are generated vortices and flow reaction forces, such as drag and lift. If yet the object is free to move, its movement can couple with the surrounding flow, falling into the domain of fluid-structure interactions. In this PhD thesis, the coupling between a pendular system and an air flow is studied both experimentally and theoretically. Placed in a wind tunnel, a disk pendulum presents a bistability for a range of flow velocity, while a rectangular one does not. By varying the aspect ratio of such rectangle and visualizing the wake behind a fixed disk, we propose an origin on whether or not the bistability emerges, linking it to stall phenomenon. The influence of ambient turbulence on this phenomenon is then investigated together with the link between angular fluctuations and flow variations, both upstream and downstream. Going back to the bistability itself, spontaneous transitions between stable states are observed and a model inspired from the transition to turbulence suggest certain mechanisms in the wake triggering such transitions, in particular rare aerodynamical events. Modifying geometrical parameters of the pendulum enables the adjustment of the range of velocity for which the bistability occurs, and with it, we could observe jumps between both transitions at the same flow rate. Finally, when the pendulum is balanced, its movement is only driven by the flow and while a quasi-static modelling is not sufficient to describe the real dynamics of the pendulum, we introduce two ways of accounting for the dynamical retroaction of the flow in the equation of movement, one empirical and the other based on vortex-induced vibration theory.Lorsquâun objet est placĂ© dans un Ă©coulement, celui-ci est dĂ©viĂ© et de cette dĂ©viation rĂ©sultent lâapparition de tourbillons et la gĂ©nĂ©ration de forces de rĂ©action du fluide, telles que la traĂźnĂ©e et la portance. Si maintenant, lâobjet en question est libre de se mouvoir, un couplage sâinstalle entre les mouvements de lâobjet et de lâĂ©coulement environnant, du domaine des interactions fluide-structure. Dans cette thĂšse, le couplage entre un systĂšme pendulaire et un Ă©coulement dâair est Ă©tudiĂ© expĂ©rimentalement et thĂ©oriquement. PlacĂ© dans une soufflerie, un pendule circulaire prĂ©sente notamment une bistabilitĂ© sur une gamme de vitesse de vent, tandis quâun rectangle nâen prĂ©sente aucune. En variant le rapport dâaspect du pendule rectangulaire et en visualisant le sillage dâun disque fixe, nous proposons une origine Ă lâexistence ou non de la bistabilitĂ©, en lien avec le phĂ©nomĂšne de dĂ©crochage. Lâinfluence de la turbulence incidente sur ce phĂ©nomĂšne est ensuite approfondie ainsi que le lien entre les fluctuations dâangle du pendule et celles de lâĂ©coulement, amont comme aval. En revenant Ă la bistabilitĂ© elle-mĂȘme, des transitions spontanĂ©es dâun Ă©tat stable Ă lâautre sont observĂ©es, dont le cadre de la transition vers la turbulence permet une modĂ©lisation et suggĂšre certains mĂ©canismes Ă lâĆuvre dans le sillage, notamment des Ă©vĂšnements aĂ©rodynamiques rares. La modification des paramĂštres gĂ©omĂ©triques du pendule permet de varier la plage de vitesses pour laquelle la bistabilitĂ© existe, et ainsi observer des sauts entre les deux positions, Ă mĂȘme vitesse dâĂ©coulement. Enfin, lorsque le pendule est Ă©quilibrĂ©, son mouvement est dictĂ© par lâĂ©coulement uniquement et alors quâun modĂšle quasi-statique est insuffisant Ă la description de la dynamique rĂ©elle du pendule, nous Ă©tablissons deux maniĂšres de prendre en compte la rĂ©troaction dynamique de lâĂ©coulement, lâune empirique et lâautre basĂ©e sur les vibrations induites par vortex
Interactions fluide-structure de systĂšmes pendulaires multistables
When in a flow, an object deviates it and from this deviation are generated vortices and flow reaction forces, such as drag and lift. If yet the object is free to move, its movement can couple with the surrounding flow, falling into the domain of fluid-structure interactions. In this PhD thesis, the coupling between a pendular system and an air flow is studied both experimentally and theoretically. Placed in a wind tunnel, a disk pendulum presents a bistability for a range of flow velocity, while a rectangular one does not. By varying the aspect ratio of such rectangle and visualizing the wake behind a fixed disk, we propose an origin on whether or not the bistability emerges, linking it to stall phenomenon. The influence of ambient turbulence on this phenomenon is then investigated together with the link between angular fluctuations and flow variations, both upstream and downstream. Going back to the bistability itself, spontaneous transitions between stable states are observed and a model inspired from the transition to turbulence suggest certain mechanisms in the wake triggering such transitions, in particular rare aerodynamical events. Modifying geometrical parameters of the pendulum enables the adjustment of the range of velocity for which the bistability occurs, and with it, we could observe jumps between both transitions at the same flow rate. Finally, when the pendulum is balanced, its movement is only driven by the flow and while a quasi-static modelling is not sufficient to describe the real dynamics of the pendulum, we introduce two ways of accounting for the dynamical retroaction of the flow in the equation of movement, one empirical and the other based on vortex-induced vibration theory.Lorsquâun objet est placĂ© dans un Ă©coulement, celui-ci est dĂ©viĂ© et de cette dĂ©viation rĂ©sultent lâapparition de tourbillons et la gĂ©nĂ©ration de forces de rĂ©action du fluide, telles que la traĂźnĂ©e et la portance. Si maintenant, lâobjet en question est libre de se mouvoir, un couplage sâinstalle entre les mouvements de lâobjet et de lâĂ©coulement environnant, du domaine des interactions fluide-structure. Dans cette thĂšse, le couplage entre un systĂšme pendulaire et un Ă©coulement dâair est Ă©tudiĂ© expĂ©rimentalement et thĂ©oriquement. PlacĂ© dans une soufflerie, un pendule circulaire prĂ©sente notamment une bistabilitĂ© sur une gamme de vitesse de vent, tandis quâun rectangle nâen prĂ©sente aucune. En variant le rapport dâaspect du pendule rectangulaire et en visualisant le sillage dâun disque fixe, nous proposons une origine Ă lâexistence ou non de la bistabilitĂ©, en lien avec le phĂ©nomĂšne de dĂ©crochage. Lâinfluence de la turbulence incidente sur ce phĂ©nomĂšne est ensuite approfondie ainsi que le lien entre les fluctuations dâangle du pendule et celles de lâĂ©coulement, amont comme aval. En revenant Ă la bistabilitĂ© elle-mĂȘme, des transitions spontanĂ©es dâun Ă©tat stable Ă lâautre sont observĂ©es, dont le cadre de la transition vers la turbulence permet une modĂ©lisation et suggĂšre certains mĂ©canismes Ă lâĆuvre dans le sillage, notamment des Ă©vĂšnements aĂ©rodynamiques rares. La modification des paramĂštres gĂ©omĂ©triques du pendule permet de varier la plage de vitesses pour laquelle la bistabilitĂ© existe, et ainsi observer des sauts entre les deux positions, Ă mĂȘme vitesse dâĂ©coulement. Enfin, lorsque le pendule est Ă©quilibrĂ©, son mouvement est dictĂ© par lâĂ©coulement uniquement et alors quâun modĂšle quasi-statique est insuffisant Ă la description de la dynamique rĂ©elle du pendule, nous Ă©tablissons deux maniĂšres de prendre en compte la rĂ©troaction dynamique de lâĂ©coulement, lâune empirique et lâautre basĂ©e sur les vibrations induites par vortex
Rare Event-Triggered Transitions in Aerodynamic Bifurcation
International audienc
Influence of the porosity pattern on the aerodynamics of a square-shaped fly-swatter
International audienceThe evolution of the normal aerodynamic coefficient of 19 configurations of square plates with various porosity patterns, ranging from solid plate to homogeneous porous plate, is experimentally characterized. The variation of the porosity pattern is obtained by partially covering the holes of a commercial fly-swatter using adhesive tape. Evolution of the normal aerodynamic coefficient is assessed from the measurement of the angular position of the porous plate, placed as a freely rotating pendulum swept by a flow in a wind tunnel. These angular measurements are also supported by particle image velocimetry (PIV) measurements of the structure of the wake. We show that the porosity pattern determines whether or not an abrupt stall occurs. In particular, the details of the porosity pattern on the edges of the plate are decisive for the existence of abrupt stall
Fluidelastic modeling of a weathercock stabilization in a uniform flow
International audienceThe relaxation dynamics of a weathercock free-to-rotate, in the presence of a uniform flow, as it aligns with the flow direction, is investigated experimentally in a wind-tunnel. The dynamics is observed to conveniently follow a damped harmonic oscillator behavior. At first order, the frequency is set by the aerodynamic coefficients. We show that a quasi static approach fails to precisely describe the relaxation dynamics and that non-stationary corrections are required to model the dynamics. A first strategy is to introduce added mass, added stiffness and added damping to the quasi-static approximation, following what is usually done in the context of vortex-induced vibrations. A second strategy is to introduce empirical corrections, whose scaling is obtained from the analysis of the experimental data. Finally, these two strategies are compared and we discuss the physical interpretations of the non-stationary corrections