References of "Dimitriadis, Grigorios"
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See detailUsing Proper Orthogonal Decomposition Methods for Comparing CFD Results to Experimental Measurements
Andrianne, Thomas ULg; Guissart, Amandine ULg; Terrapon, Vincent ULg et al

in Proceedings of the 5th Symposium on Integrating CFD and Experiments in Aerodynamics (Integration 2012) (2012, October 03)

This work presents a method for quantitative comparison of numerical results to experimental measurements. It is based on the concept of Proper Orthogonal Decomposition. This technique is selected in ... [more ▼]

This work presents a method for quantitative comparison of numerical results to experimental measurements. It is based on the concept of Proper Orthogonal Decomposition. This technique is selected in order to compare the unsteady aerodynamic flows around static and oscillating bodies obtained from wind tunnel testing and numerical simulations. Two dimensional Time-resolved Particle Image Velocimetry measurements are carried out on the upper surface a 4:1 rectangular cylinder. Simulations are performed using unsteady Reynolds-Averaged Navier-Stokes and an unsteady Discrete Vortex Method. It is demonstrated that the proposed technique is a good preliminary step for comparing the main characteristics of unsteady aerodynamic data. [less ▲]

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See detailDamping identification of linear dynamic systems using Common-base Proper Orthogonal Decomposition
Andrianne, Thomas ULg; Dimitriadis, Grigorios ULg

in Proceedings of ISMA2012 (2012, September 19)

This paper presents a novel identification technique of the modal damping of linear systems. It is based on the Proper Orthogonal Decomposition (POD) of the free response of the system and extended to the ... [more ▼]

This paper presents a novel identification technique of the modal damping of linear systems. It is based on the Proper Orthogonal Decomposition (POD) of the free response of the system and extended to the Common-base POD (CPOD) approach. Different initial conditions are applied to the system and the corresponding free responses are considered simultaneously. The CPOD technique leads to a unique vector basis which is likely to contain more information about the dynamics of the system than a vector basis obtained by the classic POD technique, based on one set of initial conditions only. The ability of the technique to estimate the modal damping is demonstrated on a simulated mass-spring-damper system and an experimental system. Good agreement is shown between the damping estimates of the CPOD technique and the ones of the LSCF technique. The advantage and limitations of the present technique are discussed. [less ▲]

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See detailNumerical simulations of torsional flutter oscillations of a bluff body: Energy issues, GraSMech poster session
Andrianne, Thomas ULg; Dimitriadis, Grigorios ULg

Poster (2012, May)

The possibility of harvesting energy from aeroelastic phenomena is assessed through numerical simulations. The unsteady aerodynamics around an aeroelastic structure are modeled using the Discrete Vortex ... [more ▼]

The possibility of harvesting energy from aeroelastic phenomena is assessed through numerical simulations. The unsteady aerodynamics around an aeroelastic structure are modeled using the Discrete Vortex Method (DVM). The Torsional Flutter oscillations of a rectangular cylinder are studied in this work. The phenomenon is characterized by Limit Cycle Oscillations (LCOs) around the pitch axis of the rectangle. The complete bifurcation behaviour is investigated numerically and compared to wind tunnel measurements for validation. The energy issues are investigated in terms of the sensibility of the energy output to variations in the structural damping. [less ▲]

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See detailExperiments on a pitch-plunge wing undergoing limit cycle oscillation
Norizham, Abdul Razak ULg; Rothkegel Ide, José Ignacio ULg; Dimitriadis, Grigorios ULg

in Proceedings of the 53rd AIAA/ASME/ASCE/AHS/ASC Structures, Structural Dynamics and Materials Conference (2012, April 25)

The aeroelastic behaviour of a wing oscillating in the heave and pitch degrees of freedom is examined by means of wind tunnel experiment. The phenomena of interest are classical flutter and limit cycle ... [more ▼]

The aeroelastic behaviour of a wing oscillating in the heave and pitch degrees of freedom is examined by means of wind tunnel experiment. The phenomena of interest are classical flutter and limit cycle oscillation. Classical flutter is normally associated with the exponential growth of the response amplitude. Linear flutter theory only predicts the critical flutter speed. Any excitation or disturbance beyond the critical speed is assumed to cause exponential growth in the response amplitude. In contrast, any limited amplitude oscillations occurring post-fultter suggest the existence of nonlinear properties in the system. Such properties can originate from the aerodynamic forces in the form of flow separation and reattachment. On the structural side, damping and stiffness can also contribute nonlinear properties. Furthermore, these nonlinearities can manifest themselves even at pre-flutter conditions, depending on the values of some governing parameter. The focus of the present work is the transformation of classical flutter into stall flutter as the equilibrium angle of attack of heaving and pitching wing is increased. The interaction of stall-related nonlinearity with structural nonlinearities is also of interest. The measured aeroelastic responses are analyzed and the bifurcation behavior of the dynamic system is characterized. Structural responses as well as flow field visualization through Particle Image Velocimetry show the origin of nonlinearity does not solely come from the manifestation of separation and the shedding of vortices, but from the structural nonlinearity which limits the response amplitude. [less ▲]

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See detailA Cross-Validation Approach to Approximate Basis Function Selection of the Stall Flutter Response of a Rectangular Wing in a Wind Tunnel
Kukreja, Sunil L.; Vio, Gareth A.; Andrianne, Thomas ULg et al

in Proceedings of the 53rd AIAA/ASME/ASCE/AHS/ASC Structures, Structural Dynamics and Materials Conference (2012, April 25)

The stall flutter response of a rectangular wing in a low speed wind tunnel is modelled using a nonlinear difference equation description. Static and dynamic tests are used to select a suitable model ... [more ▼]

The stall flutter response of a rectangular wing in a low speed wind tunnel is modelled using a nonlinear difference equation description. Static and dynamic tests are used to select a suitable model structure and basis function. Bifurcation criteria such as the Hopf condition and vibration amplitude variation with airspeed were used to ensure the model was representative of experimentally measured stall flutter phenomena. Dynamic test data were used to estimate model parameters and estimate an approximate basis function. [less ▲]

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See detailInitial Investigation of Chordwise Flexible Flat Aerofoil
Vio, Gareth A.; Worotynska, Jagoda; Berci, Marco et al

in Proceedings of the 53rd AIAA/ASME/ASCE/AHS/ASC Structures, Structural Dynamics and Materials Conference (2012, April 25)

The size of small Unmanned Air Vehicles (UAVs) makes them susceptible to gusts, hence an evaluation of their aeroelastic performance is a necessary requirement. With advances in materials, new aircraft ... [more ▼]

The size of small Unmanned Air Vehicles (UAVs) makes them susceptible to gusts, hence an evaluation of their aeroelastic performance is a necessary requirement. With advances in materials, new aircraft are ever lighter and more flexible, both in the span-wise and chord-wise direction. An experiment is set up to evaluate the aeroelastic performance of a 2D chord-wise aerofoil structure under gust loading, where the wing of a small Unmanned Air Vehicle (UAV) is considered by coupling a Finite Element (FE) structural model with a Doublet-Lattice Method (DLM) aerodynamic model; a new semi-analytical modelling approach is also described. The flexible aerofoil structure is then optimised by means of a genetic algorithm (GA) framework for the minimum weight, subject to aeroelastic constraints of flutter, divergence and structural displacement. [less ▲]

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See detailComputational Considerations for the Prediction of Stall Flutter
Watrin, Damien ULg; Dimitriadis, Grigorios ULg; Perry, Tristan et al

in Proceedings of the 53rd AIAA/ASME/ASCE/AHS/ASC Structures, Structural Dynamics and Materials Conference (2012, April 25)

A solver has been developed within the OpenFoam framework to compute large amplitude motion of two-dimensional rigid configurations. The results obtained with this code were successfully validated on ... [more ▼]

A solver has been developed within the OpenFoam framework to compute large amplitude motion of two-dimensional rigid configurations. The results obtained with this code were successfully validated on rigid airfoils at static and dynamic conditions, as well as correlated with experimental data and numerical solutions from similar unsteady solvers. The results demonstrated that while current computational methods are able to predict the self-sustained oscillations characterizing a pitch-dominated stall flutter, including energy transfer, improvements are needed. The influence of grid, temporal integration, turbulence modeling, and flow equations is examined for the stall flutter starting solution of dynamic stall. [less ▲]

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See detailDiscrete Vortex Simulations of the torsional flutter oscillations of a 4:1 rectangular cylinder
Andrianne, Thomas ULg; Dimitriadis, Grigorios ULg

in Proceedings of the 53rd AIAA/ASME/ASCE/AHS/ASC Structures, Structural Dynamics and Materials Conference (2012, April 24)

This work presents aeroelastic simulations of a 2D bluff-body around its pitching degree of freedom. The numerical tool consists in an aerodynamic solver based on the Discrete Vortex Method (DVM), coupled ... [more ▼]

This work presents aeroelastic simulations of a 2D bluff-body around its pitching degree of freedom. The numerical tool consists in an aerodynamic solver based on the Discrete Vortex Method (DVM), coupled with a linear structural model. The shape of the bluff-body is a sharp edged rectangular cylinder with a side ratio equal to 4. The numerical results are compared to the experimental measurements recently obtained by the authors.8 The validation is carried out in three steps: first the frequency content of the flow-field in the wake of the static body is investigated. Then the simulated unsteady flow-field around the imposed pitching motion of the body is compared to experimental flow visualizations. This comparison is performed using Proper Orthogonal Decomposition (POD). Finally, the simulation of the global aeroelastic behaviour, based on the coupling of the DVM code to the structural model of the pitching degree of freedom is carried out and the results are compared to measured aeroelastic responses.. Very good agreements are found between numerical and experimental results, demonstrating the capabilities of the numerical tool to simulate complex unsteady aerodynamics around an oscillating bluff-body. [less ▲]

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See detailComputational Identification of Non-Linear Damping in an Aeroelastic System
Vio, Gareth A.; Dimitriadis, Grigorios ULg

in Proceedings of the 53rd AIAA/ASME/ASCE/AHS/ASC Structures, Structural Dynamics and Materials Conference (2012, April 23)

An energy-based method is proposed to identify damping parameters from time histories of responses to sets of single-frequency harmonic excitation. The method is intended to be practically applicable to ... [more ▼]

An energy-based method is proposed to identify damping parameters from time histories of responses to sets of single-frequency harmonic excitation. The method is intended to be practically applicable to real structures and is able to identify the value of viscous damping, Coulomb friction and eventually other forms of non-linear damping models in aeroelastic systems. The inputs required are simply the accelerometer signals and the forces applied. It will be shown that if the system is undergoing Limit Cycle Oscillations, no external force is required for the identification process. [less ▲]

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See detailDamping identification of lightly damped linear dynamic systems using Common-base Proper Orthogonal Decomposition
Andrianne, Thomas ULg; Dimitriadis, Grigorios ULg

in Mechanical Systems & Signal Processing (2012), 28

This paper presents a new technique to identify the damping of linear systems. It is developed from the Proper Orthogonal Decomposition (POD) of the free response of the system and extended to the ... [more ▼]

This paper presents a new technique to identify the damping of linear systems. It is developed from the Proper Orthogonal Decomposition (POD) of the free response of the system and extended to the recently proposed Common-base POD (CPOD). The present application of CPOD considers simultaneously several free responses of the system to different initial conditions. The eigen-decomposition of the co-variance matrix leads to a unique vector basis which is likely to contain more information about the dynamics of the system than a vector basis obtained by the classic POD technique. The ability of the technique to estimate the mode shapes and the modal damping is demonstrated on a simulated mass-spring-damper system. Two different distributions of masses are considered in order to confront the CPOD analysis to the intrinsic limitation of POD, i.e. that the mode shapes are identified exactly only if the mass matrix is proportional to the identity matrix. It is shown that the identification of the damping is still possible when the modes are not orthonormal. The robustness of the technique is demonstrated in the presence of noise in the responses of the system and through an experimental application with comparison with other identifications techniques. [less ▲]

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See detailUnsteady Navier-Stokes simulation of low-Reynolds stall flutter
Yabili, Sacha ULg; Smith, Marilyn; Dimitriadis, Grigorios ULg

in Proceedings of the 50th AIAA Aerospace Sciences Meeting (2012, January)

A solver has been developed within the OpenFoam framework to compute large amplitude motion of two-dimensional rigid configurations. The results obtained with this code were successfully validated on ... [more ▼]

A solver has been developed within the OpenFoam framework to compute large amplitude motion of two-dimensional rigid configurations. The results obtained with this code were successfully validated on rigid airfoils at static and dynamic conditions, as well as correlated with experimental data and numerical solutions from similar unsteady solvers. The results demonstrate that current computational methods are, within the constraints imposed by spatial grids, temporal integration and turbulence modeling, capable of capturing the self-sustained oscillations characterizing stall flutter event with reasonable accuracy, including the mechanisms of energy transfer. [less ▲]

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See detailBlowing against the tide
Dimitriadis, Grigorios ULg

Article for general public (2011)

Wind tunnels are experimental installations that create an airflow going past a test object. In a controlled and safe environment, they can recreate real-life situations, like an aircraft flying at 30 ... [more ▼]

Wind tunnels are experimental installations that create an airflow going past a test object. In a controlled and safe environment, they can recreate real-life situations, like an aircraft flying at 30,000ft and 950km/h or a storm blasting a bridge deck. In this way the aerodynamic forces acting on structures can be measured, the flow-field around them can be visualised and fundamental research on aerodynamics can be carried out. Wind tunnels have been invaluable tools to aircraft, automobile and civil engineering designers for over 100 years. However, a significant number of wind tunnels have been decommissioned over the last 20 years, both in Europe and in the United States. Some of these closures have made the news, for instance the decommissioning of the NASA Ames facilities. There are many reasons for this phenomenon and a significant number of possible implications. [less ▲]

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See detailFlapping flight aerodynamics for flying animals
Norizham, Abdul Razak ULg; Dimitriadis, Grigorios ULg

Scientific conference (2011, October 10)

Most research into the aerodynamics of flying animals is based on aircraft aerodynamics. Aircraft have rigid wings, therefore such research is mostly suited to the study of the gliding flight of animals ... [more ▼]

Most research into the aerodynamics of flying animals is based on aircraft aerodynamics. Aircraft have rigid wings, therefore such research is mostly suited to the study of the gliding flight of animals. However, many species spend more time flapping than gliding. Some species don’t glide at all. This seminar presents recent work on flapping flight carried out at the University of Liège. [less ▲]

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See detailAerodynamic modeling of horizontal axis wind turbines
Prasad, Chandra Shekhar ULg; Dimitriadis, Grigorios ULg

(2011, July 14)

The paper presents the development of a vortex lattice aerodynamic modeling routine for SAMCEF for Wind Turbine (S4WT). S4WT is a general-purpose commercial code for wind turbine design and analysis. At ... [more ▼]

The paper presents the development of a vortex lattice aerodynamic modeling routine for SAMCEF for Wind Turbine (S4WT). S4WT is a general-purpose commercial code for wind turbine design and analysis. At present, it uses blade element momentum theory (BEM) for the estimation of the aerodynamics loads on HAWT rotor blades. BEM is a simple aerodynamic modeling approach that is currently used by several software packages for wind turbine design (as well as other rotor-based applications). It is based on the assumption that the flow can be treated as quasi-steady and quasi-2D, so that the steady, 2D aerodynamic loads acting on a strip of a rotor blade are used to estimate the instantaneous unsteady, 3D loads acting on a complete blade. This approach ignores the effect of the unsteady wake of the blades on the aerodynamic loads and simplifies the true 3D load distribution over the blades. A higher fidelity calculation of the time varying aerodynamic forces and moments acting on the blades is the main focus of this work. A good compromise between speed and accuracy to calculate these forces is the 3D unsteady vortex lattice method with a freely deforming wake. The vortex lattice results are compared to the BEM results from S4WT. The ultimate aim is to integrate the vortex lattice calculation as a subroutine in S4WT in order to calculate the unsteady aerodynamic forces on the rotor blades during the design process. This new method in S4WT will provide more representative results to the user, which can be very important for designing a more efficient wind turbine. [less ▲]

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See detailInfluence of a static wing wake on the stall flutter behavior of a flexible wing
Li, Jing; Dimitriadis, Grigorios ULg

in Proceedings of the 13th International Conference on Wind Engineering, ICWE13 (2011, July 13)

The subject of this paper is the experimental study of the aeroelastic behavior of a wing undergoing stall flutter in the vicinity of second, static wing. While stall flutter has been the subject of ... [more ▼]

The subject of this paper is the experimental study of the aeroelastic behavior of a wing undergoing stall flutter in the vicinity of second, static wing. While stall flutter has been the subject of several investigations, such work has almost always concentrated on isolated wings. Stall flutter is a phenomenon that is mostly encountered in rotating blades, such as wind turbine or helicopter blades. In such cases, the phenomenon is influenced by the wake of the preceding blade. This paper presents a series of experiments carried out at the Goldstein Laboratory of the University of Manchester, concerning the phenomenon of stall flutter influenced by the proximity of a static wing. The work is an extension of the single wing stall flutter experiments presented by Dimitriadis and Li (2009). [less ▲]

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See detailExperimental Analysis of the Bifurcation Behaviour of a Bridge Deck Undergoing Across-Wind Galloping
Andrianne, Thomas ULg; Dimitriadis, Grigorios ULg

in De Roeck, G.; Degrande, G.; Lombaert, G. (Eds.) et al Proceedings of the 8th International Conference on Structural Dynamics, EURODYN 2011 (2011, July 05)

The phenomenon of aeroelastic galloping is a very important design consideration for bridges and other slender structures. It has been investigated by a number of researchers but, most frequently, the ... [more ▼]

The phenomenon of aeroelastic galloping is a very important design consideration for bridges and other slender structures. It has been investigated by a number of researchers but, most frequently, the analysis is limited to quasi-steady aerodynamic and linearized aeroelastic considerations. Such treatment has been shown to be effective for simple cross-sectional shapes, such as rectangles. In this work, an aeroelastic model of a realistic bridge deck cross-section is tested in a low speed wind tunnel. Both static and dynamic tests are carried out and the resulting force and vibration measurements are presented. The static force results are used to set up a quasi-steady mathematical model. The dynamic responses are used to draw a complete bifurcation diagram within a chosen airspeed range and to discuss the stability of the system. It is shown that the experimental system undergoes a subcritical Hopf bifurcation, its phase space including both a stable and an unstable limit cycle. As consequence, throughout the chosen airspeed, the system can either remain stable or undergo limit cycle oscillations. The quasi-steady analysis fails completely in capturing this type of behaviour. The predicted galloping onset speed is too conservative and the predicted oscillation amplitudes too high. The reason for this failure is the fact that the quasi-steady mathematical model is incapable of modelling subcritical Hopf bifurcations. [less ▲]

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See detailDouble wake vortex lattice modeling of horizontal axis wind turbines
Prasad, Chandra Shekhar ULg; Dimitriadis, Grigorios ULg

in Proceedings of the 15th International Forum on Aeroelasticity and Structural Dynamics, IFASD 2011 (2011, June 29)

This paper presents the development of a vortex lattice based aerodynamic calculation for wind turbine blades. Emphasis is placed on the modeling of flow separation using the vortex lattice approach. This ... [more ▼]

This paper presents the development of a vortex lattice based aerodynamic calculation for wind turbine blades. Emphasis is placed on the modeling of flow separation using the vortex lattice approach. This modeling is carried out by shedding a second free wake at the leading edge, which travels downstream at the local air velocity. The decision on whether to shed a wake or not is taken by looking at the sectional lift characteristics of the blade’s airfoil. If the local angle of attack exceeds the maximum lift angle, then a leading edge wake element is shed. Results from the method are presented for both attached and separated flow and compared with predictions obtained from the Blade Element Momentum theory. It is shown that the shedding of the leading edge wake can increased significantly the agreement between vortex lattice and Blade Element results in cases where there are significant regions of separated flow. This improvement concerns mostly forces acting normal to the rotor plane; tangential forces depend more strongly on the drag and neither method calculates a full representation of the drag. [less ▲]

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See detailWind tunnel analysis of separated aerodynamics leading to different types of torsional flutter in bluff-bodies.
Andrianne, Thomas ULg; Korbahti, Banu; Dimitriadis, Grigorios ULg

in Proceedings of the 15th International Forum on Aeroelasticity and Structural Dynamics, IFASD 2011 (2011, June 29)

A new experimental apparatus has been developed at the wind tunnel laboratory of University of Liège with the objective to study the torsional flutter instability of a rectangular cylinder. Different ... [more ▼]

A new experimental apparatus has been developed at the wind tunnel laboratory of University of Liège with the objective to study the torsional flutter instability of a rectangular cylinder. Different configurations, based on the position of the pitching axis, the stiffness of the restoring springs and the static equilibrium angle of the model have been tested and analysed. Sub- and supercritical bifurcations behaviour has been identified. The dynamics of the system, as well as the aerodynamic phenomena taking place around the model have been measured. The frequency content of the pitching displacement and the longitudinal component of the velocity in the wake showed the occurrence of two types of aerodynamic instability: large separation from the leading edge and vortex shedding in the Strouhal sense. Furthermore, preliminary Particle Image Velocimetry visualisation confirmed the presence of complex aerodynamic phenomena over the upper surface and in the wake of the model. These measurements allow a better understanding of the sources of aerodynamic excitation leading to torsional flutter. [less ▲]

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See detailInteractive boundary layer calculation of separated flows around 2D airfoils
Rothkegel Ide, José Ignacio ULg; Dimitriadis, Grigorios ULg

in Proceedings of the 15th International Forum on Aeroelasticity and Structural Dynamics, IFASD 2011 (2011, June 27)

Stall flutter is an important undesirable phenomenon affecting lifting surfaces such as helicopter and wind turbine blades, but is particularly difficult to model. Existing methodologies are not ... [more ▼]

Stall flutter is an important undesirable phenomenon affecting lifting surfaces such as helicopter and wind turbine blades, but is particularly difficult to model. Existing methodologies are not universally applicable, and have significant limitations concerning applicability, time of computation and accuracy. This paper presents a method that combines interactive boundary layer with point vortex shedding. It is applied to a NACA0012 airfoil, at different angles of attack in the −40◦ < a < 40◦ range, considering a free flow velocity of 20[m/s]. Results are compared to those obtained at the Goldstein Laboratory of the University of Manchester. [less ▲]

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