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http://hdl.handle.net/10985/11396
Sharp Transition in the Lift Force of a Fluid Flowing Past Nonsymmetrical Obstacles: Evidence for a Lift Crisis in the Drag Crisis Regime
BOT, Patrick; RABAUD, Marc; THOMAS, Goulven; LOMBARDI, Alessandro; LEBRET, Charles
Bluff bodies moving in a fluid experience a drag force which usually increases with velocity. However in a particular velocity range a drag crisis is observed, i.e., a sharp and strong decrease of the drag force. This counterintuitive result is well characterized for a sphere or a cylinder. Here we show that, for an object breaking the up-down symmetry, a lift crisis is observed simultaneously to the drag crisis. The term lift crisis refers to the fact that at constant incidence the time-averaged transverse force, which remains small or even negative at low velocity, transitions abruptly to large positive values above a critical flow velocity. This transition is characterized from direct force measurements as well as from change in the velocity field around the obstacle.
Fri, 01 Jan 2016 00:00:00 GMThttp://hdl.handle.net/10985/113962016-01-01T00:00:00ZBOT, PatrickRABAUD, MarcTHOMAS, GoulvenLOMBARDI, AlessandroLEBRET, CharlesBluff bodies moving in a fluid experience a drag force which usually increases with velocity. However in a particular velocity range a drag crisis is observed, i.e., a sharp and strong decrease of the drag force. This counterintuitive result is well characterized for a sphere or a cylinder. Here we show that, for an object breaking the up-down symmetry, a lift crisis is observed simultaneously to the drag crisis. The term lift crisis refers to the fact that at constant incidence the time-averaged transverse force, which remains small or even negative at low velocity, transitions abruptly to large positive values above a critical flow velocity. This transition is characterized from direct force measurements as well as from change in the velocity field around the obstacle.Fluid Structure Interaction of Yacht Sails in the Unsteady Regime
http://hdl.handle.net/10985/12555
Fluid Structure Interaction of Yacht Sails in the Unsteady Regime
AUGIER, Benoit; BOT, Patrick; HAUVILLE, Frédéric; DURAND, Mathieu
The dynamic Fluid Structure Interaction (FSI) of yacht sails submitted to a harmonic pitching motion is numerically investigated to address both issues of aerodynamic unsteadiness and structural deformation. The model consists in an implicit dynamic coupling algorithm between a Vortex Lattice Method model for the aerodynamics and a Finite Element Method model for the structure dynamics. It is shown that the dynamic behaviour of a sail plan subject to yacht motion clearly deviates from the quasi-steady theory. The aerodynamic forces oscillate with phase shifts with respect to the motion. This results in hysteresis phenomena, which show aerodynamic equivalent damping and stiffening effects of the unsteady behaviour. The area of the hysteresis loop corresponds to the amount of energy exchanged by the system and increases with the motion reduced frequency and amplitude. In the case of a rigid structure, the aerodynamic forces oscillations and the exchanged energy are lower than for a flexible structure.
Tue, 01 Jan 2013 00:00:00 GMThttp://hdl.handle.net/10985/125552013-01-01T00:00:00ZAUGIER, BenoitBOT, PatrickHAUVILLE, FrédéricDURAND, MathieuThe dynamic Fluid Structure Interaction (FSI) of yacht sails submitted to a harmonic pitching motion is numerically investigated to address both issues of aerodynamic unsteadiness and structural deformation. The model consists in an implicit dynamic coupling algorithm between a Vortex Lattice Method model for the aerodynamics and a Finite Element Method model for the structure dynamics. It is shown that the dynamic behaviour of a sail plan subject to yacht motion clearly deviates from the quasi-steady theory. The aerodynamic forces oscillate with phase shifts with respect to the motion. This results in hysteresis phenomena, which show aerodynamic equivalent damping and stiffening effects of the unsteady behaviour. The area of the hysteresis loop corresponds to the amount of energy exchanged by the system and increases with the motion reduced frequency and amplitude. In the case of a rigid structure, the aerodynamic forces oscillations and the exchanged energy are lower than for a flexible structure.Transducteur adapté à la génération de forces en fonction de la vitesse d'écoulement d'un fluide
http://hdl.handle.net/10985/12674
Transducteur adapté à la génération de forces en fonction de la vitesse d'écoulement d'un fluide
BOT, Patrick
L’invention concerne un dispositif détecteur d’une vitesse seuil de déplacement d’un fluide, le dispositif détecteur comprenant un transducteur et configuré pour exercer une première force non nulle dans une première direction lorsque la vitesse du fluide est inférieure à la vitesse seuil et pour exercer une seconde force non nulle dans une seconde direction lorsque la vitesse du fluide est supérieure à la vitesse seuil , la première et la seconde direction étant identiques ou sensiblement identiques et la première et la deuxième force étant dirigées dans des sens opposés.
Sun, 01 Jan 2017 00:00:00 GMThttp://hdl.handle.net/10985/126742017-01-01T00:00:00ZBOT, PatrickL’invention concerne un dispositif détecteur d’une vitesse seuil de déplacement d’un fluide, le dispositif détecteur comprenant un transducteur et configuré pour exercer une première force non nulle dans une première direction lorsque la vitesse du fluide est inférieure à la vitesse seuil et pour exercer une seconde force non nulle dans une seconde direction lorsque la vitesse du fluide est supérieure à la vitesse seuil , la première et la seconde direction étant identiques ou sensiblement identiques et la première et la deuxième force étant dirigées dans des sens opposés.Development of a 6-DOF Dynamic Velocity Prediction Program for offshore racing yachts
http://hdl.handle.net/10985/20167
Development of a 6-DOF Dynamic Velocity Prediction Program for offshore racing yachts
KERDRAON, Paul; HOREL, Boris; BOT, Patrick; LETOURNEUR, Adrien; LE TOUZÉ, David
Thanks to high lift-to-drag ratios, hydrofoils are of great interest for high-speed vessels. Modern sailing yachts fitted with foils have thus reached impressively high speeds on the water. But this hydrodynamic efficiency is achieved at the expense of stability. Accurate tradeoffs are therefore needed to ensure both performance and safety. While usual Velocity Prediction Programs (VPPs) are inadequate to assess dynamic stability, the varying nature of the offshore racing environment further complicates the task. Dynamic simulation in the time-domain is thus necessary to help architects assess their designs. This paper presents a system-based numerical tool which aims at predicting the dynamic behavior of offshore sailing yachts. A 6 degrees of freedom (DOF) algorithm is used, calculating loads as a superposition of several components (hull, appendage, sails). Part of them are computed at runtime while the others use pre-computed dataset, allowing a good compromise between efficiency and flexibility. Three 6DOF simulations of an existing offshore trimaran (a maneuver, unsteady wind conditions and quartering seas) are presented. They underline the interest of dynamic studies, demonstrating how important the yacht state history is to the understanding of her instantaneous behavior and showing that dynamic simulations open a different field of optimization than VPPs.
Wed, 01 Jan 2020 00:00:00 GMThttp://hdl.handle.net/10985/201672020-01-01T00:00:00ZKERDRAON, PaulHOREL, BorisBOT, PatrickLETOURNEUR, AdrienLE TOUZÉ, DavidThanks to high lift-to-drag ratios, hydrofoils are of great interest for high-speed vessels. Modern sailing yachts fitted with foils have thus reached impressively high speeds on the water. But this hydrodynamic efficiency is achieved at the expense of stability. Accurate tradeoffs are therefore needed to ensure both performance and safety. While usual Velocity Prediction Programs (VPPs) are inadequate to assess dynamic stability, the varying nature of the offshore racing environment further complicates the task. Dynamic simulation in the time-domain is thus necessary to help architects assess their designs. This paper presents a system-based numerical tool which aims at predicting the dynamic behavior of offshore sailing yachts. A 6 degrees of freedom (DOF) algorithm is used, calculating loads as a superposition of several components (hull, appendage, sails). Part of them are computed at runtime while the others use pre-computed dataset, allowing a good compromise between efficiency and flexibility. Three 6DOF simulations of an existing offshore trimaran (a maneuver, unsteady wind conditions and quartering seas) are presented. They underline the interest of dynamic studies, demonstrating how important the yacht state history is to the understanding of her instantaneous behavior and showing that dynamic simulations open a different field of optimization than VPPs.Experimental Study of the Flow in a Compact Heat Exchanger Channel with Embossed-type Vortex Generators
http://hdl.handle.net/10985/9495
Experimental Study of the Flow in a Compact Heat Exchanger Channel with Embossed-type Vortex Generators
DUPONT, Frédéric; GABILLET, Céline; BOT, Patrick
The isothermal flow in a model channel of plate-fin heat exchanger with periodically arranged embossed-like vortex generators is investigated. Velocity measurements are performed by LDA in the transitional regime (Reynolds number from 1000 up to 5000). Strong longitudinal vortices are observed downstream of each vortex generator. The vortex roll-up process is highlighted by the evolution of the velocity vector field in the cross section of the flow. The modifications of the vortex characteristics after successively encountered generators are investigated. This work shows most of the flow features which are known to produce heat transfer enhancement, and shows that these smooth shaped vortex generators are very promising for enhanced heat exchangers.
Wed, 01 Jan 2003 00:00:00 GMThttp://hdl.handle.net/10985/94952003-01-01T00:00:00ZDUPONT, FrédéricGABILLET, CélineBOT, PatrickThe isothermal flow in a model channel of plate-fin heat exchanger with periodically arranged embossed-like vortex generators is investigated. Velocity measurements are performed by LDA in the transitional regime (Reynolds number from 1000 up to 5000). Strong longitudinal vortices are observed downstream of each vortex generator. The vortex roll-up process is highlighted by the evolution of the velocity vector field in the cross section of the flow. The modifications of the vortex characteristics after successively encountered generators are investigated. This work shows most of the flow features which are known to produce heat transfer enhancement, and shows that these smooth shaped vortex generators are very promising for enhanced heat exchangers.Performance enhancement of downwind sails due to leading edge flapping: A wind tunnel investigation
http://hdl.handle.net/10985/13736
Performance enhancement of downwind sails due to leading edge flapping: A wind tunnel investigation
AUBIN, Nicolas; AUGIER, Benoit; DEPARDAY, Julien; SACHER, Matthieu; BOT, Patrick
This work presents a wind tunnel experimental study on the effect of the leading edge flapping on the aerodynamic performance of a spinnaker. Four J80-class spinnaker models, combining two different assembling structures (panel layout) and two different sail materials are tested at various wind speeds and wind angles in a wind tunnel. Results show that, for the wind angle range the spinnaker is designed for, the sustained periodic flapping of the sail leading edge has a significant benefit on performance, with 10% increase in drive force. In these model-scale tests, the sail structural properties did not show significant differences in performance, but affect the point where flapping sets in: a model with a stiffer material and a cross-cut panel layout starts flapping for a longer sheet length, compared to a lighter cloth and a tri-radial layout. Finally, it is shown that the nondimensional flapping frequency is rather constant 0.4 in the design range of wind angle, but it varies with the wind speed and sail structural properties on a smaller wind angle where the spinnaker is more stretched.
Mon, 01 Jan 2018 00:00:00 GMThttp://hdl.handle.net/10985/137362018-01-01T00:00:00ZAUBIN, NicolasAUGIER, BenoitDEPARDAY, JulienSACHER, MatthieuBOT, PatrickThis work presents a wind tunnel experimental study on the effect of the leading edge flapping on the aerodynamic performance of a spinnaker. Four J80-class spinnaker models, combining two different assembling structures (panel layout) and two different sail materials are tested at various wind speeds and wind angles in a wind tunnel. Results show that, for the wind angle range the spinnaker is designed for, the sustained periodic flapping of the sail leading edge has a significant benefit on performance, with 10% increase in drive force. In these model-scale tests, the sail structural properties did not show significant differences in performance, but affect the point where flapping sets in: a model with a stiffer material and a cross-cut panel layout starts flapping for a longer sheet length, compared to a lighter cloth and a tri-radial layout. Finally, it is shown that the nondimensional flapping frequency is rather constant 0.4 in the design range of wind angle, but it varies with the wind speed and sail structural properties on a smaller wind angle where the spinnaker is more stretched.Wind-tunnel pressure measurements on model-scale rigid downwind sails
http://hdl.handle.net/10985/14915
Wind-tunnel pressure measurements on model-scale rigid downwind sails
BOT, Patrick; MARIA VIOLA, Ignazio; FLAY, Richard G.J.; BRETT, Jean-Sébastien
This paper describes an experiment that was carried out in the Twisted Flow Wind Tunnel at The University of Auckland to measure a detailed set of pressure distributions on a rigid 1/15th scale model of a modern asymmetric spinnaker. It was observed that the pressures varied considerably up the height of the spinnaker. The fine resolution of pressure taps allowed the extent of leading edge separation bubbles, pressure recovery region, and effect of sail curvature to be observed quite clearly. It was found that the shape of the pressure distributions could be understood in terms of conventional aerodynamic theory. The sail performed best at an apparent wind angle of about 55°, which is its design angle, and the effect of heel was more pronounced near the head than the foot.
Tue, 01 Jan 2013 00:00:00 GMThttp://hdl.handle.net/10985/149152013-01-01T00:00:00ZBOT, PatrickMARIA VIOLA, IgnazioFLAY, Richard G.J.BRETT, Jean-SébastienThis paper describes an experiment that was carried out in the Twisted Flow Wind Tunnel at The University of Auckland to measure a detailed set of pressure distributions on a rigid 1/15th scale model of a modern asymmetric spinnaker. It was observed that the pressures varied considerably up the height of the spinnaker. The fine resolution of pressure taps allowed the extent of leading edge separation bubbles, pressure recovery region, and effect of sail curvature to be observed quite clearly. It was found that the shape of the pressure distributions could be understood in terms of conventional aerodynamic theory. The sail performed best at an apparent wind angle of about 55°, which is its design angle, and the effect of heel was more pronounced near the head than the foot.Full-scale flying shape measurement of offwind yacht sails with photogrammetry
http://hdl.handle.net/10985/11250
Full-scale flying shape measurement of offwind yacht sails with photogrammetry
DEPARDAY, Julien; BOT, Patrick; HAUVILLE, Frédéric; AUGIER, Benoit; RABAUD, Marc
Yacht downwind sails are complex to study due to their non-developable shape with high camber and massively detached flow around thin and flexible membranes. Numerical simulations can now simulate this strong fluid-structure interaction, but need experimental validation. It remains complex to measure spinnaker flying shapes partly because of their inherent instability, like luff flapping. This work presents full-scale experimental investigation of spinnaker shapes with simultaneous measurement of aerodynamic loads on the three sail corners, with navigation and wind data. The experimental set-up and photogrammetric method are presented. Results are analysed in the whole range of apparent wind angle for this sail. The spinnaker shape shows dramatic variations and high discrepancies with the design shape. The photogrammetric measurement produces the full 3D flying shape with a satisfactory accuracy. Even if only steady state results are given here, this new system enables time-resolved measurement of flying shapes and thus flapping of spinnakers to be investigated, which is valuable for yacht performance optimisation. On top of sailing yacht applications, the method is useful in any application where a non-developable 3D shape is to be determined, and particularly when it results from the Fluid Structure Interaction of a flexible structure with a complex flow.
Fri, 01 Jan 2016 00:00:00 GMThttp://hdl.handle.net/10985/112502016-01-01T00:00:00ZDEPARDAY, JulienBOT, PatrickHAUVILLE, FrédéricAUGIER, BenoitRABAUD, MarcYacht downwind sails are complex to study due to their non-developable shape with high camber and massively detached flow around thin and flexible membranes. Numerical simulations can now simulate this strong fluid-structure interaction, but need experimental validation. It remains complex to measure spinnaker flying shapes partly because of their inherent instability, like luff flapping. This work presents full-scale experimental investigation of spinnaker shapes with simultaneous measurement of aerodynamic loads on the three sail corners, with navigation and wind data. The experimental set-up and photogrammetric method are presented. Results are analysed in the whole range of apparent wind angle for this sail. The spinnaker shape shows dramatic variations and high discrepancies with the design shape. The photogrammetric measurement produces the full 3D flying shape with a satisfactory accuracy. Even if only steady state results are given here, this new system enables time-resolved measurement of flying shapes and thus flapping of spinnakers to be investigated, which is valuable for yacht performance optimisation. On top of sailing yacht applications, the method is useful in any application where a non-developable 3D shape is to be determined, and particularly when it results from the Fluid Structure Interaction of a flexible structure with a complex flow.On the Uncertainty of CFD in Sail Aerodynamics
http://hdl.handle.net/10985/8697
On the Uncertainty of CFD in Sail Aerodynamics
VIOLA, Ignazio Maria; BOT, Patrick; RIOTTE, Matthieu
A verification and validation procedure for yacht sail aerodynamics is presented. Guidelines and an example of application are provided. The grid uncertainty for the aerodynamic lift, drag and pressure distributions for the sails is computed. The pressures are validated against experimental measurements, showing that the validation procedure may allow the identification of modelling errors. Lift, drag and L2 norm of the pressures were computed with uncertainties of the order of 1%. Convergence uncertainty and round-off uncertainty are several orders of magnitude smaller than the grid uncertainty. The uncertainty due to the dimension of the computational domain is computed for a flat plate at incidence and is found to be significant compared with the other uncertainties. Finally, it is shown how the probability that the ranking between different geometries is correct can be estimated knowing the uncertainty in the computation of the value used to rank.
Tue, 01 Jan 2013 00:00:00 GMThttp://hdl.handle.net/10985/86972013-01-01T00:00:00ZVIOLA, Ignazio MariaBOT, PatrickRIOTTE, MatthieuA verification and validation procedure for yacht sail aerodynamics is presented. Guidelines and an example of application are provided. The grid uncertainty for the aerodynamic lift, drag and pressure distributions for the sails is computed. The pressures are validated against experimental measurements, showing that the validation procedure may allow the identification of modelling errors. Lift, drag and L2 norm of the pressures were computed with uncertainties of the order of 1%. Convergence uncertainty and round-off uncertainty are several orders of magnitude smaller than the grid uncertainty. The uncertainty due to the dimension of the computational domain is computed for a flat plate at incidence and is found to be significant compared with the other uncertainties. Finally, it is shown how the probability that the ranking between different geometries is correct can be estimated knowing the uncertainty in the computation of the value used to rank.Upwind sail aerodynamics : A RANS numerical investigation validated with wind tunnel pressure measurements
http://hdl.handle.net/10985/8685
Upwind sail aerodynamics : A RANS numerical investigation validated with wind tunnel pressure measurements
BOT, Patrick
The aerodynamics of a sailing yacht with different sail trims are presented, derived from simulations performed using Computational Fluid Dynamics. A Reynolds-averaged Navier-Stokes approach was used to model sixteen sail trims first tested in a wind tunnel, where thepressure distributions on the sails were measured. An original approach was employed byusing two successive simulations: the first one on a large domain to model the blockage due to the wind tunnel walls and the sails model, and a second one on a smaller domain to model the flow around the sails model. A verification and validation of the computed aerodynamic forces and pressure distributions was performed. The computed pressure distribution is shown to agree well with the measured pressures. The sail surface pressure was correlated with the increase of turbulent viscosity in the laminar separation bubble, the flow reattachment and the trailing edge separation. The drive force distribution on both sails showed that the fore part of the genoa (fore sail) provides the majority of the drive force and that the effect of the aft sail is mostly to produce an upwash effect on the genoa. An aerodynamic model based on potential flow theory and a viscous correction is proposed. This model, with one free parameter to be determined, is shown to fit the results better than the usual form drag and induced drag only, even if no friction drag is explicitly considered.
A novel method similar to marching technique was used to model wind tunnel tests. ► Sail trim criteria based on their interactive effect are identified. ► Areas of separated flow were characterised. ► Local flow field was correlated with sail surface pressures. ► An aerodynamic model based on potential flow with viscous correction is proposed.
Sun, 01 Jan 2012 00:00:00 GMThttp://hdl.handle.net/10985/86852012-01-01T00:00:00ZBOT, PatrickThe aerodynamics of a sailing yacht with different sail trims are presented, derived from simulations performed using Computational Fluid Dynamics. A Reynolds-averaged Navier-Stokes approach was used to model sixteen sail trims first tested in a wind tunnel, where thepressure distributions on the sails were measured. An original approach was employed byusing two successive simulations: the first one on a large domain to model the blockage due to the wind tunnel walls and the sails model, and a second one on a smaller domain to model the flow around the sails model. A verification and validation of the computed aerodynamic forces and pressure distributions was performed. The computed pressure distribution is shown to agree well with the measured pressures. The sail surface pressure was correlated with the increase of turbulent viscosity in the laminar separation bubble, the flow reattachment and the trailing edge separation. The drive force distribution on both sails showed that the fore part of the genoa (fore sail) provides the majority of the drive force and that the effect of the aft sail is mostly to produce an upwash effect on the genoa. An aerodynamic model based on potential flow theory and a viscous correction is proposed. This model, with one free parameter to be determined, is shown to fit the results better than the usual form drag and induced drag only, even if no friction drag is explicitly considered.