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Identification of multi-degree of freedom non-linear systems using an extended modal space model ; ; Dimitriadis, Grigorios 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 ▲] Detailed reference viewed: 154 (11 ULg)Drag Minimisation Using Adaptive Aeroelastic Structures ; ; et al in Proceedings of the 48th AIAA/ASME/ASCE/AHS/ASC Structures, Structural Dynamics, and Materials Conference (2007, April) This paper describes the latest developments in a research program investigating the development of “adaptive internal structures” to enable adaptive aeroelastic control of aerospace structures. Through ... [more ▼] This paper describes the latest developments in a research program investigating the development of “adaptive internal structures” to enable adaptive aeroelastic control of aerospace structures. Through controlled changes of the second moment of area, orientation or position of the spars, it is possible to control the bending and torsional stiffness characteristics of aircraft wings or tail surfaces. The aeroelastic behaviour can then be controlled as desired. A number of different adaptive internal structure concepts (rotating, moving and split spars) are compared here using a simple rectangular wing structure in order to determine which are the most effective for achieving minimum drag at different points in a representative flight envelope. A genetic algorithm approach is employed to determine the optimal spar orientation for rotating spars concept. It is shown that it is feasible to adjust the structure and trim characteristics of such wing structures in order to achieve minimum drag at all conditions. [less ▲] Detailed reference viewed: 52 (1 ULg)Non-Linear Identification in Modal Space Using a Genetic Algorithm Approach for Model Selection ; ; et al in International Journal of Applied Mathematics & Mechanics (2007), 3(1), 72-89 The Non-Linear Resonant Decay Method is an approach for the identification of non-linear systems with large numbers of degrees of freedom. The identified non-linear model is expressed in linear modal ... [more ▼] The Non-Linear Resonant Decay Method is an approach for the identification of non-linear systems with large numbers of degrees of freedom. The identified non-linear model is expressed in linear modal space and comprises the modal model of the underlying linear system with additional terms representing the non-linear behaviour. Potentially, a large number of these non-linear terms will exist but not all of them will be significant. The problem of deciding which and how many terms are required for an accurate identification has previously been addressed using the Forward Selection and Backward Elimination techniques. In this paper, a Genetic Algorithm optimisation is proposed as an alternative to those methods. A simulated lumped parameter non-linear dynamic system is used to demonstrate the proposed optimisation. The use of separate data sets for the identification and validation of the modal model is also investigated. It is found that the Genetic Algorithm approach yields significantly better results than the Backward Elimination and Forward Selection algorithms in many cases. [less ▲] Detailed reference viewed: 54 (2 ULg)Aeroelastic system identification using transonic CFD data for a wing/store configuration ; Dimitriadis, Grigorios ; et al in Aerospace Science and Technology (2007), 11(2-3), 146-154 This paper is part of a study investigating the prediction of the aeroelastic behaviour of aircraft subjected to non-linear aerodynamic forces. The main objective of the work is the characterization of ... [more ▼] This paper is part of a study investigating the prediction of the aeroelastic behaviour of aircraft subjected to non-linear aerodynamic forces. The main objective of the work is the characterization of the dynamic response of aeroelastic models resulting from coupled Computational Fluid Dynamic and Finite Element calculations. Of interest here is the identification of the flight condition at which the response bifurcates to limited or divergent amplitude self-sustained oscillations without carrying out a comprehensive set of full, computationally expensive, time-marching calculations. The model treated in this work is a three-dimensional wing in a transonic flowfield. Short datasets of pre-bifurcation behaviour are analysed to determine the system’s stability and degree of non-linearity. It is found that the calculated responses on the run-up to a transonic Limit Cycle Oscillation show little or no evidence of non-linearity. The non-linearity appears abruptly at the bifurcation flight condition. The variation of the local Mach number over the wing’s surface in the steady-state case is used to demonstrate that the non-linearity is due to a shock wave that can move along the surface. At Mach numbers where this is not possible the system behaves in a linear manner and its stability can be analysed using linear methods. [less ▲] Detailed reference viewed: 84 (5 ULg)Bifurcation analysis and limit cycle oscillation amplitude prediction methods applied to the aeroelastic galloping problem ; Dimitriadis, Grigorios ; in Journal of Fluids & Structures (2007), 23(7), A global stability and bifurcation analysis of the transverse galloping of a square section beam in a normal steady flow has been implemented. The model is an ordinary differential equation with ... [more ▼] A global stability and bifurcation analysis of the transverse galloping of a square section beam in a normal steady flow has been implemented. The model is an ordinary differential equation with polynomial damping nonlinearity. Six methods are used to predict bifurcation, the amplitudes and periods of the ensuing Limit Cycle Oscillations: (i) Cell mapping, {ii} Harmonic Balance, (iii) Higher Order Harmonic Balance,(iv) Centre Manifold linearization, (v) Normal Form and (vi) Numerical Continuation. The resulting stability predictions are compared with each other and with results obtained from numerical integration. The advantages and disadvantages of each technique are discussed. It is shown that, despite the simplicity of the system, only two of the methods succeed in predicting its full response spectrum. These are Higher Order Harmonic Balance and Numerical Continuation. [less ▲] Detailed reference viewed: 109 (0 ULg)Identification of Structural Free-play Non-linearities using the Non-Linear Resonant Decay Method ; Dimitriadis, Grigorios ; et al in Proceedings of the 2006 International Conference on Noise and Vibration Engineering (2006, September) Structural non-linearities are becoming of increasing importance in determining the performance of a range of vibrating mechanical structures. As a consequence, the identification of systems with non ... [more ▼] Structural non-linearities are becoming of increasing importance in determining the performance of a range of vibrating mechanical structures. As a consequence, the identification of systems with non-linearities is starting to become a necessary part of vibration testing procedures. NL-RDM (Non-Linear Resonant Decay Method) is an approach for the identification of non-linear multi-degree of freedom systems in modal space on a mode by mode basis, using an appropriated sine excitation to isolate modes or groups of modes. However, the application of NL-RDM to a multi-degree of freedom system with a discontinuous free-play non-linearity has not been attempted yet, except to treat it using high order polynomial terms. The difficulty of using NL-RDM when seeking a discontinuous free-play model lies in the choice of the so-called underlying linear model to be used in uncoupling the linear equations. In this paper, a simulated two degree of freedom lumped parameter system with a free-play non-linearity [less ▲] Detailed reference viewed: 65 (1 ULg)Comment on Flutter Prediction from Flight Flutter Test Data Dimitriadis, Grigorios ; in Journal of Aircraft (2006), 43(3), 862-863 In a previous paper entitled “Flutter Predictions from Flight Flutter Test Data” the authors applied a number of different flutter prediction methods to data from two simulated aeroelastic aircraft models ... [more ▼] In a previous paper entitled “Flutter Predictions from Flight Flutter Test Data” the authors applied a number of different flutter prediction methods to data from two simulated aeroelastic aircraft models and compared the resulting flutter predictions. The two simulated models were a simple three-degree-of-freedom Hancock wing model and the Sim-2 model of a generic four-engined civil transport. One of the methods examined in the paper was the Nissim and Gilyard method (NGM). Because of difficulties encountered with the Sim-2 model, the authors failed to apply the NGM successfully to it, and only results for the Hancock model were presented in the paper. With the aid of Eli Nissim, the authors have now succeeded in applying the method to the Sim-2 model and to obtain quality flutter predictions from it. In this short Comment, the initial problems encountered will be described and then the solutions will be outlined. Finally, flutter predictions for the method will be presented and compared to the flutter predictions obtained from the other methods by Dimitriadis and Cooper. [less ▲] Detailed reference viewed: 145 (10 ULg)Flutter Clearance of a Non-linear aircraft ; ; Dimitriadis, Grigorios et al in Proceedings of the 2005 International Forum on Aeroelasticity and Structural Dynamics (2005, June) Flight flutter testing is always carried out under the assumption that aircraft are linear. Recently, this assumption has started to come under question, especially as far as military aircraft are ... [more ▼] Flight flutter testing is always carried out under the assumption that aircraft are linear. Recently, this assumption has started to come under question, especially as far as military aircraft are concerned. This paper deals with possible methodologies for flight flutter testing of aircraft that are no longer assumed linear. Simulated flight testing is performed for a simple non-linear aeroelastic system with cubic stiffness. The flutter speeds predicted using some of the classical linear flutter prediction methods as well as a non-linear method are compared. It is shown that, for non-linear system undergoing Hopf Bifurcations, classical linear flutter prediction can predict the flutter envelope with reasonable accuracy. However, fully non-linear system identification and stability analysis can not only predict the flutter point but also determine whether it is a linear or non-linear flutter point (i.e. whether divergent or Limit Cycle Oscillations will ensue). Additionally, the non-linear method can predict the amplitudes of LCOs that will occur post-critically. The application of the nonlinear method was successful for noise free data, but the problem of noise corruption still needs further investigation. [less ▲] Detailed reference viewed: 31 (1 ULg)On the use of control surface excitation in flutter testing ; ; et al in Proceedings of the Institution of Mechanical Engineers - Part G - Journal of Aerospace Engineering (2003), 217(6), 317-332 Flutter testing is aimed at demonstrating that the aircraft flight envelope is flutter free. Response measurements from deliberate excitation of the structure are used to identify and track frequency and ... [more ▼] Flutter testing is aimed at demonstrating that the aircraft flight envelope is flutter free. Response measurements from deliberate excitation of the structure are used to identify and track frequency and damping values against velocity. In this paper, the common approach of using a flight control surface to provide the excitation is examined using a mathematical model of a wing and control surface whose rotation is restrained by a simple actuator. In particular, it is shown that it is essential to use the demand signal to the actuator as a reference signal for data processing. Use of the actuator force (or strain) or control angle (or actuator displacement) as a reference signal is bad practice because these signals contain response information. It may also be dangerous in that the onset of flutter may not be seen in the test results. [less ▲] Detailed reference viewed: 39 (3 ULg)A time–frequency technique for the stability analysis of impulse responses from nonlinear aeroelastic systems Dimitriadis, Grigorios ; in Journal of Fluids & Structures (2003), 17(8), 11811201 A time–frequency method is proposed for the analysis of response time histories from nonlinear aeroelastic systems. The approach is based on a time-varying curve-fit of the short time Fourier transform of ... [more ▼] A time–frequency method is proposed for the analysis of response time histories from nonlinear aeroelastic systems. The approach is based on a time-varying curve-fit of the short time Fourier transform of the impulse response. It is shown that the method can be used in order to obtain a clear picture of the sub-critical stability of a number of aeroelastic systems with a variety of structural and aerodynamic nonlinearities. Additionally, frequency and amplitude information can be obtained for both the linear and nonlinear signatures of the response signals in the sub- and postcritical regions. Finally, it is shown that, given certain types of nonlinear functions, sub-critical damping trends can be extrapolated to predict bifurcation airspeeds. [less ▲] Detailed reference viewed: 16 (1 ULg)Identification of the nonlinear function in a vibrating nonlinear beam Dimitriadis, Grigorios ; in Lees, A. W.; Prells, U.; Norton, J. (Eds.) Proceedings of the Third International Conference on Identification in Engineering Systems (2002, April) The Constant Level Identification (CLI) method for nonlinear dynamical systems has been shown to identify accurately a wide variety of systems with discrete degrees of freedom. In this paper, the method ... [more ▼] The Constant Level Identification (CLI) method for nonlinear dynamical systems has been shown to identify accurately a wide variety of systems with discrete degrees of freedom. In this paper, the method is applied to a continuous nonlinear system, namely a simulated model of a vibrating beam attached to a nonlinear spring. A Rayleigh-Ritz series representation of the beam is assumed in order to extract the desired number of modal responses. Then, a combination of modal and physical coordinates are used in the identification process. It is shown that the CLI method can accurately capture the shape of the nonlinearity in the system. [less ▲] Detailed reference viewed: 10 (1 ULg)Blade-Tip Timing Measurement Of Synchronous Vibrations Of Rotating Bladed Dimitriadis, Grigorios ; ; et al in Mechanical Systems & Signal Processing (2002), 16(4), 599-622 Blade-tip timing (BTT) is a promising method for the detection, measurement and analysis of blade vibrations in rotating bladed assemblies. However, the intricacies of the method when applied to real ... [more ▼] Blade-tip timing (BTT) is a promising method for the detection, measurement and analysis of blade vibrations in rotating bladed assemblies. However, the intricacies of the method when applied to real rotating structures undergoing synchronous (Engine Ordered) vibrations are not yet fully understood. In this paper, a mathematical model is developed to simulate data from typical BTT tests of rotating assemblies. The simulator is then used in order to provide a qualitative analysis of several phenomena that can be associated with the synchronous vibrations of rotating assemblies, including mistuning, coupling, excitation at multiple Engine Orders and simultaneous synchronous and asynchronous responses. It is concluded that none of these phenomena on its own will render the identification of the frequency and amplitude of blade vibrations impossible. However, there is no single BTT data analysis method that is able to deal with all of these phenomena. [less ▲] Detailed reference viewed: 140 (0 ULg)Online Flight Flutter Testing Dimitriadis, Grigorios ; in Link, M. (Ed.) Proceedings of the International Conference on Structural System Identification (2001, September) A number of different identification methods are considered as candidate approaches to undertake online flight flutter testing. Rather than flying the test aircraft to a new flight condition and then ... [more ▼] A number of different identification methods are considered as candidate approaches to undertake online flight flutter testing. Rather than flying the test aircraft to a new flight condition and then performing a data analysis at a constant flight condition, it is shown how it is feasible to track changes in frequencies and damping ratios as the flight speed is increased. It is also possible to provide an updated estimate of the predicted flutter speed, giving the flight engineers added confidence in the margin of safety. Both frequency and time domain methods are evaluated on a simulated 3 DOF aeroelastic system. These preliminary results show that such an approach is feasible although further work is required to determine the best testing and analysis approach. [less ▲] Detailed reference viewed: 41 (4 ULg)A comparison of blade tip timing data analysis methods ; ; et al in Proceedings of the Institution of Mechanical Engineers - Part G - Journal of Aerospace Engineering (2001), 215(6), 301-312 The experimental determination of the vibration characteristics of rotating engine blades is very important for fatigue failure considerations. One of the most promising techniques for measuring the ... [more ▼] The experimental determination of the vibration characteristics of rotating engine blades is very important for fatigue failure considerations. One of the most promising techniques for measuring the frequency of blade vibrations is blade tip timing. In this paper, three vibration analysis methods were specifically formulated and applied to the tip timing problem for the first time, using data obtained from a simple mathematical blade tip timing simulation. The results from the methods were compared statistically in order to determine which of the techniques is more suitable. One of the methods, the global autoregressive instrumental variables approach, produced satisfactory results at realistic noise levels. However, all of the techniques produced biased results under certain circumstances. [less ▲] Detailed reference viewed: 134 (2 ULg)Flutter Prediction from Flight Flutter Test Data Dimitriadis, Grigorios ; in Journal of Aircraft (2001), 38(2), 355-367 The most common approach to flight flutter testing is to track estimated modal damping ratios of an aircraft over a number of flight conditions. These damping trends are then extrapolated to predict ... [more ▼] The most common approach to flight flutter testing is to track estimated modal damping ratios of an aircraft over a number of flight conditions. These damping trends are then extrapolated to predict whether it is safe to move to the next test point and also to determine the utter speed. In the quest for more reliable and efficient flight flutter testing procedures, a number of alternative data analysis methods have been proposed. Five of these approaches are compared on two simulated aeroelastic models. The comparison is based on both the accuracy of prediction and the efficiency of each method. It is found that, for simple aeroelastic systems, the Nissim and Gilyard method (Nissim, E., and Gilyard, G. B., “Method for Experimental Determination of Flutter Speed by Parameter Identification,” AIAA Paper 89-1324, 1989) yields the best flutter predictions and is also the least computationally expensive approach.However, for larger systems, simpler approaches such as the damping fit and envelope function methods are found to be most reliable. [less ▲] Detailed reference viewed: 93 (4 ULg)Characterization of the Behaviour of a Simple Aeroservoelastic System with Control Nonlinearities Dimitriadis, Grigorios ; in Journal of Fluids & Structures (2000), 14(8), 1173-1193 The characterization of the behaviour of nonlinear aeroelastic systems has become a very important research topic in the Aerospace Industry. However, most work carried to-date has concentrated upon ... [more ▼] The characterization of the behaviour of nonlinear aeroelastic systems has become a very important research topic in the Aerospace Industry. However, most work carried to-date has concentrated upon systems containing structural or aerodynamic nonlinearities. The purpose of this paper is to study the stability of a simple aeroservoelastic system with nonlinearities in the control system and power control unit. The work considers both structural and control law nonlinearities and assesses the stability of the system response using bifurcation diagrams. It is shown that simple feedback systems designed to increase the stability of the linearized system also stabilize the nonlinear system, although their effects can be less pronounced. Additionally, a nonlinear control law designed to limit the control surface pitch response was found to increase the flutter speed considerably by forcing the system to undergo limit cycle oscillations instead of fluttering. Finally, friction was found to affect the damping of the system but not its stability, as long as the amplitude of the frictional force is low enough not to cause stoppages in the motion. [less ▲] Detailed reference viewed: 20 (1 ULg)Limit Cycle Oscillation Control and Suppression Dimitriadis, Grigorios ; in Aeronautical Journal (1999), 103(1023), 257-263 The prediction and characterization of the Limit Cycle Oscillation (LCO) behaviour of nonlinear aeroelastic systems has become of great interest recently. However, much of this work has concentrated on ... [more ▼] The prediction and characterization of the Limit Cycle Oscillation (LCO) behaviour of nonlinear aeroelastic systems has become of great interest recently. However, much of this work has concentrated on determining the existence of LCOs. This paper concentrates on LCO stability. By considering the energy present in di®erent limit cycles, and also using the Harmonic Balance Method, it is shown how the stability of limit cycles can be determined. The analysis is then extended to show that limit cycles can be controlled, or even suppressed, by the use of suitable excitation signals. A basic control scheme is developed to achieve this, and is demonstrated on a simple simulated nonlinear aeroelastic system. [less ▲] Detailed reference viewed: 160 (6 ULg)A method for identification of non-linear multi-degree-of-freedom systems Dimitriadis, Grigorios ; in Proceedings of the Institution of Mechanical Engineers - Part G - Journal of Aerospace Engineering (1998), 212(4), 287-298 System identification methods for non-linear aeroelastic systems could find uses in many aeroelastic applications such as validating finite element models and tracking the stability of aircraft during ... [more ▼] System identification methods for non-linear aeroelastic systems could find uses in many aeroelastic applications such as validating finite element models and tracking the stability of aircraft during flight flutter testing. The effectiveness of existing non-linear system identification techniques is limited by various factors such as the complexity of the system under investigation and the type of non-linearities present. In this work, a new approach is introduced which can identify multi-degree-of-freedom systems featuring any type of non-linear function, including discontinuous functions. The method is shown to yield accurate identification of three mathematical models of aeroelastic systems containing a wide range of structural non-linearities. [less ▲] Detailed reference viewed: 30 (4 ULg) |
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