References of "Dimitriadis, Grigorios"
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See detailDamping identification in a non-linear aeroelastic structure
Vio, Gareth A.; Prandina, Marco; Dimitriadis, Grigorios ULg

in Proceedings of ISMA2010 (2010, September 20)

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 detailDevelopment and Evaluation of a VTOL Observation Platform
Buysschaert, F.; Yacoubi, M.; Alami, A. et al

in Proceedings of the 27th International Congress of the Aerospace Sciences (2010, September)

An aerodynamic study of the ULB-developed ducted rotor MAV using the results of full-scale wind tunnel tests allowed the determination of the platform’s positive speed envelope, power requirements and ... [more ▼]

An aerodynamic study of the ULB-developed ducted rotor MAV using the results of full-scale wind tunnel tests allowed the determination of the platform’s positive speed envelope, power requirements and endurance characteristics for ISA sea level conditions. In this study, the power consumption appears to be majorly depending on the rotor rotational speed, while an increase in horizontal speed results in a duct operating more as a circular wing, also unveiling a power bucket as is the case with conventional helicopters. A positive influence of the ground proximity on the total thrust has been monitored. [less ▲]

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See detailEtude du comportement aéroélastique des structures soumises à des écoulements décrochés
Andrianne, Thomas ULg; Dimitriadis, Grigorios ULg

Conference (2010, March 02)

L'objectif du travail de recherche est le développement d'un modèle numérique simulant les phénomènes aéroélastiques affectant les corps non-profilés, caractérisés par des effets de décrochage/raccrochage ... [more ▼]

L'objectif du travail de recherche est le développement d'un modèle numérique simulant les phénomènes aéroélastiques affectant les corps non-profilés, caractérisés par des effets de décrochage/raccrochage du fluide.L'application de ce modèle aux tabliers de point est prévue. [less ▲]

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See detailProper Orthogonal Decomposition of Unsteady Aerodynamic Flows
Andrianne, Thomas ULg; Ligot, Jerome; Dimitriadis, Grigorios ULg

Report (2009)

This work constitutes the final report for a project funded by the Fonds de la Recherche Scientifique. The objective of the project was to acquire and install Par- tical Image Velocimetry (PIV) hardware ... [more ▼]

This work constitutes the final report for a project funded by the Fonds de la Recherche Scientifique. The objective of the project was to acquire and install Par- tical Image Velocimetry (PIV) hardware in the University of Liege Wind Tunnel facility in order to carry out the Proper Orthogonal Decomposition (POD) of un- steady aerodynamic flows. The characteristics and installation of the PIV system are described in the present report. Furthermore, the application of POD analysis to three different visualized flow cases is detailed. The flow cases were: flow be- hind a circular cylinder and separated flow over a rectangular wing. In all cases, emphasis was given to the combination of unsteadiness caused by both boundary layer separation and movement of the wind tunnel model. It is shown that the de- composition of flowfields that combine these two sources of unsteadiness is possible and informative. [less ▲]

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See detailA Novel Concept for Helicopter Rotor Drives
Antoine, Hubert; Dimitriadis, Grigorios ULg; Hendrick, Patrick et al

in Proceedings of the 3rd European Conference for Aero-Space Sciences (2009, July)

The REDT (Rotor à Entraînement Direct par Turbine – Direct Turbine Driven Rotor) is a new concept for helicopter rotor drives developed by Sagita in Belgium. It works without any mechanical link between ... [more ▼]

The REDT (Rotor à Entraînement Direct par Turbine – Direct Turbine Driven Rotor) is a new concept for helicopter rotor drives developed by Sagita in Belgium. It works without any mechanical link between the engine on one side and the rotor drive on the other side. It uses a fuselage-mounted compressor that powers two contra-rotating rotor-mounted free turbines. These free turbines drive a pair of contrarotating rotors that are fitted with rigid hingeless main rotor blades. This novel rotor drive eliminates the need for either mechanical transmission or a tail rotor. The aim of the REDT concept is to lower the maintenance costs and the accident rate, as well as to extend the flight envelope towards much higher airspeeds compared to classical helicopters. [less ▲]

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See detailAnalysis of dynamic response of a very flexible Delta wing model in a wind tunnel
Barbason, Mathieu ULg; Andrianne, Thomas ULg; Hickey, Daryl ULg et al

in Proceedings of the 2009 International Forum on Aeroelasticity and Structural Dynamics (2009, July)

Limit cycle oscillations involving Delta wings are an important area of research in modern aeroelasticity. Such phenomena can be the result of geometric nonlin- earity, aerodynamic nonlinearity or under ... [more ▼]

Limit cycle oscillations involving Delta wings are an important area of research in modern aeroelasticity. Such phenomena can be the result of geometric nonlin- earity, aerodynamic nonlinearity or under-wing store nonlinearity. In this paper, a flexible half-Delta wing without stores is tested in a low speed wind tunnel in order to investigate its dynamic response. It is found that, at several combinations of airspeed and angle of attack, the wing undergoes high amplitude limit cycle oscillations. Three types of such oscillations are observed. Type 1 oscillations occur only at low angles of attack and are the result of a Hopf-type bifurcation. Type 2 limit cycle oscillations occur at intermedi- ate angles of attack and are the result of an atypical bifurcation. In other words, these oscillations appear as the airspeed is increased but disappear at even higher airspeeds. Type 3 oscillations occur at even higher angles of attack. A bispectrum analysis shows that type 3 limit cycle oscillations feature quadratic phase coupling. No such coupling was measured for type 2 oscillations, leading to the conclusion that the nonlinearity must be of higher order. [less ▲]

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See detailExperiments on a 3-D Flapping and Pitching Mechanical Model
Norizham, Abdul Razak ULg; Rothkegel Ide, José Ignacio ULg; Dimitriadis, Grigorios ULg

in Proceedings of the 2009 International Forum on Aeroelasticity and Structural Dynamics (2009, June)

The motivation for this paper is to focus on the better understanding of flapping wing flight, including flapping flight involving high pitching amplitudes. The investigation will be primarily ... [more ▼]

The motivation for this paper is to focus on the better understanding of flapping wing flight, including flapping flight involving high pitching amplitudes. The investigation will be primarily experimental. The experiment will utilize a mechanical flapping and pitching wind tunnel model, which is modeled on large migrating birds. The 3-D rectangular wings are forced to flap and pitch sinusoidally in a low speed wind tunnel. The unsteady aerodynamic forces generated are measured using a three-component force balance. The main objective of this work is to investigate the effect that varying the oscillation parameters has on the aerodynamic forces acting on the 3-D wing undergoing flapping and pitching. The parameters in question are reduced frequency, flapping ampli- tude and pitching amplitude. Comparisons are made with aerodynamic force predictions obtained from unsteady vortex lattice calculations. [less ▲]

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See detailUsing the Discrete Vortex Method to Simulate the Stall Flutter Phenomenon
Andrianne, Thomas ULg; Dimitriadis, Grigorios ULg

in Proceedings of the 2009 International Forum on Aeroelasticity and Structural Dynamics (2009, June)

The Discrete Vortex Method (DVM) has been recently used by several researchers in order to simulate the dynamic stall phenomenon on 2D airfoils oscillating at high angles of attack or undergoing stall ... [more ▼]

The Discrete Vortex Method (DVM) has been recently used by several researchers in order to simulate the dynamic stall phenomenon on 2D airfoils oscillating at high angles of attack or undergoing stall flutter oscillations. The aim of the present work is to investigate whether such simulations are representative of the true physics governing such oscillation. A DVM method is described and implemented. A basic validation of the method is performed on the case of an impulsively started static airfoil at a low angle of attack. It is shown that the steady state lift value obtained from the DVM method agrees with experimental measurements. Subsequently, a DVM simulation is performed for the case of a NACA 0012 airfoil undergoing symmetric stall flutter oscillations. The simulation results are compared to experimental results. It is shown that, while the general shape of the simulated lift variation with time agrees with the experiment, there are significant mean value and phase differences between experiment and simulation. Several justifications are suggested and improvements to the DVM simulation proposed. [less ▲]

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See detailWind tunnel investigation of the stall flutter oscillations of a bridge deck
Andrianne, Thomas ULg; Dimitriadis, Grigorios ULg

in Proceedings of the 8th National Congress on Theoretical and Applied Mechanics (2009, May)

Torsional Stall Flutter (TSF) is a one degree of freedom, self-excited phenomenon induced by the periodic separation/re-attachment of the flow around a body. The present paper proposes an experimental ... [more ▼]

Torsional Stall Flutter (TSF) is a one degree of freedom, self-excited phenomenon induced by the periodic separation/re-attachment of the flow around a body. The present paper proposes an experimental investigation of TSF on a bridge deck. A section model free to oscillate in torsion is tested in a low-speed wind tunnel. Static and dynamic tests are performed to enable a complete interpretation of the phenomena involved. For low airspeeds, the Karman vortex shedding excites the structure, resulting in small oscillations at the vortex shedding frequency. For high airspeeds, the deck undergoes Limit Cycle Oscillations (LCO) due to the TSF phenomenon. Nevertheless the Karman vortex shedding excitation is still present and appears in the measured forces signals. Peaks and troughs in the lift coefficient are detected for each pitching period. They are interpreted as the separation/re-attachment of the flow around the deck. The large amplitudes motion of the deck undergoing TSF reduces the amplitude of the aerodynamic lift coefficient. [less ▲]

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See detailIdentification of a Nonlinear Wing Structure Using an Extended Modal Model
Platten, M. F.; Wright, J. R.; Cooper, J. E. et al

in Journal of Aircraft (2009), 46(5), 1614-1626

The nonlinear resonant decay method identifies a nonlinear dynamic system using a model based in linear modal space comprising the underlying linear system and a small number of additional terms that ... [more ▼]

The nonlinear resonant decay method identifies a nonlinear dynamic system using a model based in linear modal space comprising the underlying linear system and a small number of additional terms that represent the nonlinear behavior. In this work, the method is applied to an aircraftlike wing/store/pylon experimental structure that consists of a rectangular wing with two stores suspended beneath it by means of nonlinear pylons with a nominally hardening characteristic in the store rotation degree of freedom. The nonlinear resonant decay method is applied to the system using multishaker excitation. The resulting identified mathematical model features five modes, two of which are strongly nonlinear, one is mildly nonlinear, and two are completely linear. The restoring force surfaces obtained from the mathematical model are in close agreement with those measured from the system. This experimental application of the nonlinear resonant decay method indicates that the method could be suitable for the identification of nonlinear models of aircraft in ground vibration testing. [less ▲]

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See detailIdentification of multi-degree of freedom non-linear systems using an extended modal space model
Platten, Michael F; Wright, Jan Robert; Dimitriadis, Grigorios ULg et al

in Mechanical Systems & Signal Processing (2009), 23(1), 8-29

The identification of non-linear dynamic systems is an increasingly important area of research, with potential application in many industries. Current non-linear identification methodologies are, in ... [more ▼]

The identification of non-linear dynamic systems is an increasingly important area of research, with potential application in many industries. Current non-linear identification methodologies are, in general, mostly suited to small systems with few degrees of freedom and few non-linearities. In order to develop a practical identification approach for real engineering structures, the capability of such methods must be significantly extended. In this paper, it is shown that such an extension can be achieved using multi-exciter techniques in order to excited specific modes or degrees of freedom of the system under investigation. A novel identification method for large non-linear systems is presented, based on the use of a multi-exciter arrangement using appropriated excitation applied in bursts. This proposed Non-linear Resonant Decay Method is applied to a simulated system with 5 degrees of freedom and an experimental clamped panel structure. The technique is essentially a derivative of the Restoring Force Surface method and involves a non-linear curve fit performed in modal space. The effectiveness of the resulting reduced order model in representing the non-linear characteristics of the system is demonstrated. The potential of the approach for the identification of large continuous non-linear systems is also discussed. [less ▲]

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See detailBifurcation Behavior of Airfoil Undergoing Stall Flutter Oscillations in Low-Speed Wind Tunnel
Dimitriadis, Grigorios ULg; Li, Jing

in AIAA Journal (2009), 47(11), 2577-2596

Stall flutter is a nonlinear aeroelastic phenomenon that can affect several types of aeroelastic systems such as helicopter rotor blades, wind turbine blades, and highly flexible wings. Although the ... [more ▼]

Stall flutter is a nonlinear aeroelastic phenomenon that can affect several types of aeroelastic systems such as helicopter rotor blades, wind turbine blades, and highly flexible wings. Although the related aerodynamic phenomenon of dynamic stall has been the subject of many experimental studies, stall flutter itself has rarely been investigated. This paper presents a set of experiments conducted on a NACA0012 airfoil undergoing stall flutter oscillations in a low-speed wind tunnel. The aeroelastic responses are analyzed with the objective of characterizing the local bifurcation behavior of the system. It is shown that symmetric stall flutter oscillations are encountered as a result of a subcritical Hopf bifurcation, followed by a fold bifurcation. The cause of these bifurcations is the occurrence of dynamic stall, which allows the transfer of energy from the freestream to the wing. A second bifurcation occurs at the system’s static divergence airspeed. As a consequence, the wing starts to undergo asymmetric stall flutter bifurcations at only positive (or only negative) pitch angles. The dynamic stall mechanism itself does not change but the flow only separates on one side of the wing. [less ▲]

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See detailData Clustering for the Identification of the Bifurcation Behaviour in Non-Linear Aeroelastic Systems using a Coupled Harmonic Balance/Genetic Algorithm Approach
Vio, Gareth Arthur; Dimitriadis, Grigorios ULg; Cooper, Jonathan Edward

in Proceedings of the 2008 International Conference on Noise and Vibration Engineering (2008, September)

This paper describes an efficient method for calculating the bifurcation behaviour of an aeroelastic system using a Harmonic Balance expansion coupled with a Genetic Algorithm, combined with a clustering ... [more ▼]

This paper describes an efficient method for calculating the bifurcation behaviour of an aeroelastic system using a Harmonic Balance expansion coupled with a Genetic Algorithm, combined with a clustering algorithm in order to determine all the solutions at every single flight condition. It will be shown how it is possible to obtain all the bifurcation branches in one step. Two clustering algorithms, K-Means and PAM, together with a number of cluster index techniques, such as Davies-Boulding, Calinski-Harabasz are investigated. The method is applied to an aeroelastic galloping problem as this phenomenon presents a number of co-existing limit cycles at a range of airspeeds. [less ▲]

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See detailTransonic Limit Cycle Oscillation Prediction From Simulated In-Flight Data
Norizham, Abdul Razak ULg; Dimitriadis, Grigorios ULg

in Proceedings of the 2008 International Conference on Noise and Vibration Engineering (2008, September)

This paper investigates the transonic Limit Cycle Oscillation (LCO) phenomenon from a computational/simulation point of view. A CFD/FE coupled solution is applied to a 3D model of a rectangular wing with ... [more ▼]

This paper investigates the transonic Limit Cycle Oscillation (LCO) phenomenon from a computational/simulation point of view. A CFD/FE coupled solution is applied to a 3D model of a rectangular wing with tip store (referred to as the Goland wing) in transonic flow, in order to study the resulting LCO motions. The aim of the study is to develop a LCO prediction strategy from in-flight data; at this stage of the work the data will be obtained from simulations. The effect of varying the thickness of the Goland wing on the LCO onset flight condition is explored. The work then addresses the presence or absence of nonlinearity prior to the onset of LCOs. This is crucial as the presence of any nonlinearity for this type of wing can act as a telltale sign of impending LCOs. The clearance between the nonlinearity onset flight condition and the LCO onset flight condition is quantified for various values of the wing design parameters. It is shown that this clearance can be used in order to develop a useful ‘oncoming LCO’ warning criterion. [less ▲]

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See detailNumerical Continuation of Aeroelastic Systems: Shooting vs Finite Difference Approach
Dimitriadis, Grigorios ULg

in Limit Cycle Oscillation and Other Amplitude-Limited Self Excited Vibrations (2008, May)

Nonlinearity in aeroelastic systems has become an important area of research. It is widely recognised that nonlinearities due to the structure, control system or aerodynamic forces can affect aeroelastic ... [more ▼]

Nonlinearity in aeroelastic systems has become an important area of research. It is widely recognised that nonlinearities due to the structure, control system or aerodynamic forces can affect aeroelastic systems, such as aircraft, and can cause oscillatory phenomena that cannot be predicted using a purely linear analysis. An important class of methods for predicting such phenomena from the equations of motion of a nonlinear aeroelastic system is numerical continuation. There are several techniques for the application of numerical continuation, each with its advantages and disadvantages. Here, the finite differences and shooting approaches are implemented and compared. Both approaches are applied to a simple pitchplunge nonlinear aeroelastic mathematical system. [less ▲]

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See detailThe aerodynamics of big ears in the brown long-eared bat Plecotus auritus
Gardiner, James; Dimitriadis, Grigorios ULg; Sellers, William et al

in Acta Chiropterologica (2008), 10(2), 313-321

Wings are the most obvious adaptation bats have for powered flight and differences in wing morphology are known to correlate with flight behaviour. However, the function(s) of ancillary structures such as ... [more ▼]

Wings are the most obvious adaptation bats have for powered flight and differences in wing morphology are known to correlate with flight behaviour. However, the function(s) of ancillary structures such as the ears and tail, which may also play an important role during flight, are less well understood. Here we constructed a simplified model of a bat body with ears based upon morphological measurements of a brown long-eared bat (Plecotus auritus) to examine the aerodynamic implications of flying with large ears. The forces and moments produced by the model were measured using a sensitive 6-component force and torque balance during wind tunnel testing. The large ears of the model bat produced positive lift as well as positive drag of the same order of magnitude. At small ears angles (0° to 10°), increasing the angle of the ears resulted in an increase of the lift-to-drag ratio. At higher ear angles (> 10°) separation of the flow occurred which caused a large decrease in the lift-to-drag ratio produced. To maximise the benefit from the ears (i.e., lift-to-drag ratio) our model predicts that a horizontal free flying P. auritus should hold its ears at an approximate angle of 10°. The results of the pitching moment coefficient are inconclusive in determining if the large ears are important as flight control structures. The additional drag produced by the ears has consequences for the foraging behaviour of P. auritus with reductions in its flight speed and foraging range. [less ▲]

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See detailContinuation of Higher Order Harmonic Balance Solutions for Nonlinear Aeroelastic Systems
Dimitriadis, Grigorios ULg

in Journal of Aircraft (2008), 45(2), 523-537

The Harmonic Balance method is a very useful tool for characterizing and predicting the response of nonlinear dynamic systems undergoing periodic oscillations, either self-excited or due to harmonic ... [more ▼]

The Harmonic Balance method is a very useful tool for characterizing and predicting the response of nonlinear dynamic systems undergoing periodic oscillations, either self-excited or due to harmonic excitation. The method and several of its variants have been applied to nonlinear aeroelastic systems over the last two decades. This paper presents a detailed description of several Harmonic Balance methods and a continuation framework allowing the methods to follow the response of dynamic systems from the bifurcation point to any desired parameter value, while successfully negotiating further fold bifurcations. The continuation framework is described for systems undergoing sub-critical and super-critical Hopf bifurcations as well as a particular type of explosive bifurcation. The methods investigated in this work are applied to a nonlinear aeroelastic model of a Generic Transport Aircraft featuring polynomial or freeplay stiffness nonlinearity in the control surface. It is shown that high order Harmonic Balance solutions will capture accurately the complete bifurcation behavior of this system for both types of nonlinearity. Low order solutions can become inaccurate in the presence of numerous folds in the Limit Cycle Oscillation branch but can still yield practical engineering information at a fraction of the cost of higher order solutions. Time domain Harmonic Balance schemes are shown to be more computationally expensive than the standard Harmonic Balance approach. [less ▲]

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See detailBifurcation analysis of aircraft with structural nonlinearity and freeplay using numerical continuation
Dimitriadis, Grigorios ULg

in Journal of Aircraft (2008), 45(3), 893-905

In recent years the aeroelastic research community has carried out substantial work on the characterization and prediction of nonlinear aeroelastic phenomena. Of particular interest is the calculation of ... [more ▼]

In recent years the aeroelastic research community has carried out substantial work on the characterization and prediction of nonlinear aeroelastic phenomena. Of particular interest is the calculation of Limit Cycle Oscillations (LCO), which cannot be accomplished using traditional linear methods. In this paper, the prediction of the bifurcation and post-bifurcation behavior of nonlinear subsonic aircraft is carried out using Numerical Continuation. The analysis does not make use of continuation packages such as AUTO or MatCont. Two different continuation techniques are detailed, specifically adapted for realistic aeroelastic models. The approaches are demonstrated on model of a simple pitch plunge airfoil with cubic stiffness and an aeroelastic model of a transport aircraft with two different types of nonlinearity in the control surface. It is shown that one of the techniques yields highly accurate predictions for LCO amplitudes and periods while the second method trades off some accuracy for computational efficiency. [less ▲]

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See detailDiagnosis of Process Faults in Chemical Systems Using a Local Partial Least Squares Approach
Kruger, Uwe; Dimitriadis, Grigorios ULg

in AIChE Journal (2008), 54(10), 2581-2596

This article discusses the application of partial least squares (PLS) for monitoring complex chemical systems. In relation to existing work, this article proposes the integration of the statistical local ... [more ▼]

This article discusses the application of partial least squares (PLS) for monitoring complex chemical systems. In relation to existing work, this article proposes the integration of the statistical local approach into the PLS framework to monitor changes in the underlying model rather than analyzing the recorded input/output data directly. As discussed in the literature, monitoring changes in model parameters addresses the problems of nonstationary behavior and presents an analogy to model-based approaches. The benefits of the proposed technique are that (i) a detailed mechanistic plant model is not required, (ii) nonstationary process behavior does not produce false alarms, (iii) parameter changes can be non-Gaussian, (iv) Gaussian monitoring statistics can be established to simplify the monitoring task, and (v) fault magnitude and signatures can be estimated. This is demonstrated by a simulation example and the analysis of recorded data from two chemical processes. [less ▲]

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See detailNovel Instrumentation for the Measurement of the Unsteady Pressure Distribution around a Wing Undergoing Stall Flutter Oscillations
Li, Jing; Dimitriadis, Grigorios ULg; Gu, Fengshu et al

in Autumn Conference of the Institute of Acoustics 2007: Advances in Noise and Vibration Engineering (2007, October)

In this work, a wind tunnel model of a wing undergoing stall flutter oscillations is studied. It is a rectangular wing with pitch and plunge degrees of freedom and low pitch stiffness. The objective of ... [more ▼]

In this work, a wind tunnel model of a wing undergoing stall flutter oscillations is studied. It is a rectangular wing with pitch and plunge degrees of freedom and low pitch stiffness. The objective of this study is to promote the understanding of the stall flutter phenomenon by measuring the unsteady pressure distribution around the wing as well as the wing displacement, during unforced motion in two degrees of freedom. Both steady and unsteady pressures must be measured with sufficient accuracy during two types of tests. In the static tests the wing is to be clamped in position and not allowed to move and the steady pressures are to be measured around the centre-span section at different angles of attack. Thus the stall angle of attack can be identified and the stall mechanism characterized. In the dynamic tests the wing will be allowed to move and the unsteady pressures will be measured and recorded during a number of cycles of the oscillation, at a number of free stream airspeeds. [less ▲]

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