References of "Dimitriadis, Grigorios"
     in
Bookmark and Share    
Full Text
Peer Reviewed
See detailExperimental and Numerical Study of Mini-UAV Propeller Performance in Oblique Flow
Theys, Bart; Dimitriadis, Grigorios ULg; Hendrick, Patrick et al

in Journal of Aircraft (in press)

This paper presents the modelling of the performance of small propellers used for Vertical Take Off and Landing Micro Aerial Vehicles (VTOL MAVs) operating at low Reynolds numbers and in oblique flow ... [more ▼]

This paper presents the modelling of the performance of small propellers used for Vertical Take Off and Landing Micro Aerial Vehicles (VTOL MAVs) operating at low Reynolds numbers and in oblique flow. Blade Element Momentum Theory (BEMT), Vortex Lattice Method (VLM) and momentum theory for oblique flow are used to predict propeller performance. For validation, the predictions for a commonly used propeller for VTOL MAVs are compared to a set of wind tunnel experiments. Both BEMT and VLM succeed in predicting correct trends of the forces and moments acting upon the propeller shaft, although accuracy decreases significantly in oblique flow. For the dataset analysed here, combining the available data of the propeller in purely axial flow with momentum theory for oblique flow and applying a correction factor for the wake skew angle results in more accurate performance estimates at all elevation angles. [less ▲]

Detailed reference viewed: 67 (9 ULg)
Full Text
Peer Reviewed
See detailExperimental passive flutter suppression using a linear tuned vibration absorber
Verstraelen, Edouard ULg; Habib, Giuseppe ULg; Kerschen, Gaëtan ULg et al

in AIAA Journal (in press)

The current drive for increased efficiency in aeronautic structures such as aircraft, wind turbine blades and helicopter blades often leads to weight reduction. A con- sequence of this tendency can be ... [more ▼]

The current drive for increased efficiency in aeronautic structures such as aircraft, wind turbine blades and helicopter blades often leads to weight reduction. A con- sequence of this tendency can be increased flexibility, which in turn can lead to un- favourable aeroelastic phenomena involving large amplitude oscillations and non- linear effects such as geometric hardening and stall flutter. Vibration mitigation is one of the approaches currently under study for avoiding these phenomena. In the present work, passive vibration mitigation is applied to a nonlinear experimental aeroelastic system by means of a linear tuned vibration absorber. The aeroelastic apparatus is a pitch and flap wing that features a continuously hardening restoring torque in pitch and a linear restoring torque in flap. Extensive analysis of the sys- tem with and without absorber at pre-critical and post-critical airspeeds showed an improvement in flutter speed of around 36%, a suppression of a jump due to stall flutter, and a reduction in LCO amplitude. Mathematical modelling of the exper- imental system is used to demonstrate that optimal flutter delay is achieved when two of the system modes flutter at the same flight condition. Nevertheless, even this optimal absorber quickly loses effectiveness as it is detuned. The wind tunnel mea- surements showed that the tested absorbers were much slower to lose effectiveness than those of the mathematical predictions. [less ▲]

Detailed reference viewed: 98 (27 ULg)
Full Text
Peer Reviewed
See detailVortex Lattice simulations of attached and separated flows around flapping wings
Lambert, Thomas ULg; Abdul Razak, Norizham; Dimitriadis, Grigorios ULg

in Aerospace (2017), 4(2), 22

Flapping flight is an increasingly popular area of research, with applications to micro-unmanned air vehicles and animal flight biomechanics. Fast but accurate methods for predicting the aerodynamic loads ... [more ▼]

Flapping flight is an increasingly popular area of research, with applications to micro-unmanned air vehicles and animal flight biomechanics. Fast but accurate methods for predicting the aerodynamic loads acting on flapping wings are of interest for designing such aircraft and optimising thrust production. In this work, the unsteady Vortex Lattice method is used in conjunction with three load estimation techniques in order to predict the aerodynamic lift and drag time histories produced by flapping rectangular wings. The load estimation approaches are the Katz, Joukowski and simplified Leishman-Beddoes techniques. The simulations' predictions are compared to experimental measurements from a flapping and pitching wing presented by Razak and Dimitriadis [1]. Three types of kinematics are investigated, pitch-leading, pure flapping and pitch lagging. It is found that pitch-leading tests can be simulated quite accurately using either the Katz or Joukowski approaches as no measurable flow separation occurs. For the pure flapping tests, the Katz and Joukowski techniques are accurate as long as the static pitch angle is greater than zero. For zero or negative static pitch angles these methods underestimate the amplitude of the drag. The Leishman-Beddoes approach yields better drag amplitudes but can introduce a constant negative drag offset. Finally, for the pitch-lagging tests the Leishman-Beddoes technique is again more representative of the experimental results, as long as flow separation is not too extensive. Considering the complexity of the phenomena involved, in the vast majority of cases the lift time history is predicted with reasonable accuracy. The drag (or thrust) time history is more challenging. [less ▲]

Detailed reference viewed: 41 (12 ULg)
Full Text
See detailIntroduction to Nonlinear Aeroelasticity
Dimitriadis, Grigorios ULg

Book published by Wiley (2017)

Nonlinear aeroelasticity has become an increasingly popular research area over the last 20-30 years. There have been many driving forces behind this development, including faster computers, increasingly ... [more ▼]

Nonlinear aeroelasticity has become an increasingly popular research area over the last 20-30 years. There have been many driving forces behind this development, including faster computers, increasingly flexible structures, automatic control systems for aircraft and other engineering products, new materials with nonlinear characteristics etc. Introduction to Nonlinear Aeroelasticity covers the theoretical basics in nonlinear aeroelasticity and applies the theory to practical problems. As nonlinear aeroelasticity is a combined topic, necessitating expertise from different areas, this book also covers areas such as nonlinear dynamics, bifurcation analysis, numerical methods and others. The emphasis throughout is on the practical application of the theories and methods, so as to enable the reader to apply their newly acquired knowledge [less ▲]

Detailed reference viewed: 50 (10 ULg)
Full Text
See detailFlutter and limit cycle oscillation suppression using linear and nonlinear tuned vibration absorbers
Verstraelen, Edouard ULg; Kerschen, Gaëtan ULg; Dimitriadis, Grigorios ULg

in Proceedings of the SEM IMAC XXXV (2017, February)

Aircraft are more than ever pushed to their limits for performance reasons. Consequently, they become increasingly nonlinear and they are more prone to undergo aeroelastic limit cycle oscillations ... [more ▼]

Aircraft are more than ever pushed to their limits for performance reasons. Consequently, they become increasingly nonlinear and they are more prone to undergo aeroelastic limit cycle oscillations. Structural nonlinearities affect aircraft such as the F-16, which can undergo store-induced limit cycle oscillations (LCOs). Furthermore, transonic buzz can lead to LCOs because of moving shock waves in transonic flight conditions on many aircraft. This study presents a numerical investigation of passive LCO suppression on a typical aeroelastic system with pitch and plunge degrees of freedom and a hardening stiffness nonlinearity. The absorber used is made of a piezoelectric patch glued to the plunge springs and connected to a resistor and an inductance forming a RLC circuit. A mechanical tuned mass damper absorber of similar configuration is also considered. The piezoelectric absorber features significant advantages in terms of size, weight and tuning convenience. The results show that both types of absorber increase the linear flutter speed of the system in a similar fashion but, when optimal, they lead to a sub-critical bifurcation while a super-critical bifurcation was observed without absorber. Finally, it is shown that the addition of a properly tuned nonlinear spring (mechanical absorber) or capacitor (piezo- electric absorber) can restore the super-criticality of the bifurcation. The tuning of the nonlinearity is carried out using numerical continuation. [less ▲]

Detailed reference viewed: 52 (8 ULg)
Full Text
Peer Reviewed
See detailInduced Drag Calculations with the Unsteady Vortex Lattice Method for Cambered Wings
Lambert, Thomas ULg; Dimitriadis, Grigorios ULg

in AIAA Journal (2017), 55(2), 668-672

The Unsteady Vortex Lattice Method (UVLM) is an approach widely used to estimate the aerodynamic loads in unsteady subsonic flows. It is based on modeling the camber surface of a lifting body by means of ... [more ▼]

The Unsteady Vortex Lattice Method (UVLM) is an approach widely used to estimate the aerodynamic loads in unsteady subsonic flows. It is based on modeling the camber surface of a lifting body by means of bound vortex rings. Even though this method has been known and used for several decades, there is little discussion of the modeling of the leading-edge suction in the literature. To address this concern, Simpson et al. [1] presented a comparison of two different ways to model this effect for the case of uncambered airfoils and wings in harmonic pitch or plunge motions. They concluded that the Joukowski method converges significantly faster than the Katz technique as the number of chorwise panels is increased. The present paper is an extension of the study by Simpson et al. to cambered lifting surfaces. It shows that the presence of camber can change radically the convergence performance of the two methods. For cambered wings, the Katz approach converges significantly faster than the Joukowski technique. [less ▲]

Detailed reference viewed: 63 (30 ULg)
Full Text
Peer Reviewed
See detailTwo-domain and three-domain limit cycles in a typical aeroelastic system with freeplay in pitch
Verstraelen, Edouard ULg; Dimitriadis, Grigorios ULg; Dal Ben Rossetto, Gustavo et al

in Journal of Fluids & Structures (2017), 69

Freeplay is a significant source of nonlinearity in aeroelastic systems and is strictly regulated by airworthiness authorities. It splits the phase plane of such systems into three piecewise linear ... [more ▼]

Freeplay is a significant source of nonlinearity in aeroelastic systems and is strictly regulated by airworthiness authorities. It splits the phase plane of such systems into three piecewise linear subdomains. Depending on the location of the freeplay, limit cycle oscillations can result that span either two or three of these subdomains. The purpose of this work is to demonstrate the existence of two-domain cycles both theoretically and experimentally. A simple aeroelastic system with pitch, plunge and control deflection degrees of freedom is investigated in the presence of freeplay in pitch. It is shown that two-domain and three-domain cycles can result from a grazing bifurcation and propagate in the decreasing airspeed direction. Close to the bifurcation, the two limit cycle branches interact with each other and aperiodic oscillations ensue. Equivalent linearization is used to derive the conditions of existence of each type of limit cycle and to predict their amplitudes and frequencies. Comparisons with measurements from wind tunnel experiments demonstrate that the theory describes these phenomena with accuracy. [less ▲]

Detailed reference viewed: 52 (17 ULg)
Full Text
See detailUnsteady lifting line theory using the Wagner function
Boutet, Johan ULg; Dimitriadis, Grigorios ULg

in Proceedings of the 55th AIAA Aerospace Sciences Meeting (2017, January)

A method is presented to model the incompressible, attached, unsteady lift and moment acting on a thin three-dimensional wing in the time domain. The model is based on the combination of Wagner theory and ... [more ▼]

A method is presented to model the incompressible, attached, unsteady lift and moment acting on a thin three-dimensional wing in the time domain. The model is based on the combination of Wagner theory and lifting line theory trough the unsteady Kutta-Joukowsky theorem. The result is a set of closed form linear ordinary di erential equations that can be solved analytically or using a Runge-Kutta-Fehlberg algorithm. The method is validated against numerical predictions from an unsteady Vortex Lattice method for rectangular and tapered wings undergoing step or oscillatory changes in plunge or pitch. As the aerodynamic loads are written in state space form in the proposed method, they can be easily included in aeroelastic and flight dynamic calculations. [less ▲]

Detailed reference viewed: 32 (2 ULg)
Full Text
Peer Reviewed
See detailApplication of a 3D unsteady surface panel method with flow separation model to horizontal axis wind turbines
Prasad, Chandra Shekhar; Dimitriadis, Grigorios ULg

in Journal of Wind Engineering & Industrial Aerodynamics (2017), 166

This work describes the development and application of a 3D unsteady surface panel method with a separation model to the problem of simulating the flow around the blades of a horizontal axis wind turbine ... [more ▼]

This work describes the development and application of a 3D unsteady surface panel method with a separation model to the problem of simulating the flow around the blades of a horizontal axis wind turbine. The present method is intended as a design tool to capture the 3D time-dependent characteristics of both attached and separated flow conditions and is an extension of previous 2D approaches. Flow separation is modelled using a loose coupling procedure between the inviscid panel method and a quasi-3D viscous boundary layer solution. A separated wake is shed at the predicted separation points and propagated at the local flow velocity, just like the trailing edge wake. The methodology is demonstrated on the NREL phase-VI wind turbine test case and the model predictions are compared to experimental measurements. [less ▲]

Detailed reference viewed: 35 (6 ULg)
Full Text
Peer Reviewed
See detailPIV-based estimation of unsteady loads on a flat plate at high angle of attack using momentum equation approaches
Guissart, Amandine ULg; Bernal, Luis; Dimitriadis, Grigorios ULg et al

in Experiments in Fluids (2017), 58(5), 53

This work presents, compares and discusses results obtained with two indirect methods for the cal culation of aerodynamic forces and pitching moment from 2D Particle Image Velocimetry (PIV) measurements ... [more ▼]

This work presents, compares and discusses results obtained with two indirect methods for the cal culation of aerodynamic forces and pitching moment from 2D Particle Image Velocimetry (PIV) measurements. Both methodologies are based on formulations of the momentum balance: the integral Navier-Stokes equations and the “flux equation” proposed by Noca et al. (1999), which has been extended to the computation of moments. The indirect methods are applied to spatio-temporal data for different separated flows around a plate with a 16:1 chord-to-thickness ratio. Experimental data are obtained in a water channel for both a plate undergoing a large amplitude imposed pitching motion and a static plate at high angle of attack. In addition to PIV data, direct measurements of aerodynamic loads are carried out to assess the quality of the indirect calculations. It is found that indirect methods are able to compute the mean and the temporal evolution of the loads for two-dimensional flows with a reasonable accuracy. Nonetheless, both methodologies are noise sensitive and, the parameters impacting the computation should thus be chosen carefully. It is also shown that results can be improved through the use of Dynamic Mode Decomposition (DMD) as a pre-processing step. [less ▲]

Detailed reference viewed: 31 (15 ULg)
Full Text
Peer Reviewed
See detailAeroservoelastic Simulations for Horizontal Axis Wind Turbines
Prasad, Chandra Shekhar ULg; Chen, Qiong Zhong; Bruls, Olivier ULg et al

in Proceedings of the Institution of Mechanical Engineers, Part A: Journal of Power and Energy (2017), 231(2), 103-117

This paper describes the development of a complete methodology for the aeroservoelastic modeling of horizontal axis wind turbines at the conceptual design stage. The methodology is based on the ... [more ▼]

This paper describes the development of a complete methodology for the aeroservoelastic modeling of horizontal axis wind turbines at the conceptual design stage. The methodology is based on the implementation of unsteady aerodynamic modeling, advanced description of the control system and nonlinear finite element calculations in the SWT wind turbine design package. The aerodynamic modeling is carried out by means of fast techniques, such as the Blade Element Method and the unsteady Vortex Lattice Method, including a free wake model. The complete model also includes a description of a doubly fed induction generator and its control system for variable speed operation. The SWT software features a non-linear finite element solver with multi-body dynamics capability. The full methodology is used to perform complete aeroservoelastic simulations of a realistic 2MW wind turbine model. The interaction between the three components of the approach is carefully analyzed and presented here. [less ▲]

Detailed reference viewed: 59 (18 ULg)
Full Text
Peer Reviewed
See detailThe influence of flight style on the aerodynamic properties of avian wings as fixed lifting surfaces
Lees, John J.; Dimitriadis, Grigorios ULg; Nudds, Robert L.

in PeerJ (2016), 4:e2495

The diversity of wing morphologies in birds reflects their variety of flight styles and the associated aerodynamic and inertial requirements. Although the aerodynamics underlying wing morphology can be ... [more ▼]

The diversity of wing morphologies in birds reflects their variety of flight styles and the associated aerodynamic and inertial requirements. Although the aerodynamics underlying wing morphology can be informed by aeronautical research, important differences exist between planes and birds. In particular, birds operate at lower, transitional Reynolds numbers than do most aircraft. To date, few quantitative studies have investigated the aerodynamic performance of avian wings as fixed lifting surfaces and none have focused upon the differences between wings from different flight style groups. Dried wings from 10 bird species representing 3 distinct flight style groups were mounted on a force/torque sensor within a wind tunnel in order to test the hypothesis that wing morphologies associated with different flight styles exhibit different aerodynamic properties. Morphological differences manifested primarily as differences in drag rather than lift. Maximum lift coefficients did not differ between groups, whereas minimum drag coefficients were lowest in undulating flyers (Corvids). The lift to drag ratios were lower than in conventional aerofoils and data from free-flying soaring species; particularly in high frequency, flapping flyers (Anseriformes), which do not rely heavily on glide performance. The results illustrate important aerodynamic differences between the wings of different flight style groups that cannot be explained solely by simple wing-shape measures. Taken at face value, the results also suggest that wing-shape is linked principally to changes in aerodynamic drag, but, of course, it is aerodynamics during flapping and not gliding that is likely to be the primary driver. [less ▲]

Detailed reference viewed: 27 (7 ULg)
Full Text
See detailDynamic interactions of a supercritical aerofoil in the presence of transonic shock buffet
Giannelis, N. F.; Vio, G. A.; Dimitriadis, Grigorios ULg

in Proceedings of the International Conference on Noise and Vibration Engineering, ISMA 2016 (2016, September 19)

Within a narrow transonic flight region, shock-wave/boundary-layer interactions yield large amplitude, self sustained shock oscillations that are detrimental to both platform handling quality and ... [more ▼]

Within a narrow transonic flight region, shock-wave/boundary-layer interactions yield large amplitude, self sustained shock oscillations that are detrimental to both platform handling quality and structural integrity. In this study, the aeroelastic interactions between this transonic buffet instability and a spring-suspended supercritical aerofoil are investigated by means of Reynolds-Averaged Navier-Stokes simulations. Single degree-of-freedom pitching simulations are performed for a range of structural to aerodynamic frequency ratios, sectional mass ratios and levels of structural damping. The results show that for a range of pitch eigenfrequencies above the fundamental buffet frequency, sychronisation of the aerodynamic and structural modes occurs. This so called lock-in phenomenon acts as a mechanism for large amplitude Limit Cycle Oscillation in aircraft structures within the transonic flow regime. The sectional mass and the addition of structural damping are both found to have a pronounced effect on the nature of the limit cycles [less ▲]

Detailed reference viewed: 74 (5 ULg)
Full Text
Peer Reviewed
See detailAssessment of the fluid-structure interaction capabilities for aeronautical applications of the open-source solver SU2.
Sanchez, Ruben; Kline, Heather; Thomas, David ULg et al

in Proceedings of the VII European Congress on Computational Methods in Applied Science and Engineering (2016, September)

Detailed reference viewed: 30 (6 ULg)
Full Text
See detailPassive flutter suppression using a nonlinear tuned vibration absorber
Verstraelen, Edouard ULg; Gourc, Etienne ULg; Kerschen, Gaëtan ULg et al

Poster (2016, July)

A recent study showed that the addition of a linear tuned vibration absorber could increase the flutter speed of a rigid wing with pitch and flap degrees of freedom by about 35%. However, the absorber was ... [more ▼]

A recent study showed that the addition of a linear tuned vibration absorber could increase the flutter speed of a rigid wing with pitch and flap degrees of freedom by about 35%. However, the absorber was turning the initial super-critical bifurcation into a sub-critical one. This work shows numerically that adding a nonlinear restoring force to the absorber can restore the su- percritical behaviour of the bifurcation and further reduce the post-instability limit cycle amplitude. [less ▲]

Detailed reference viewed: 85 (15 ULg)
Full Text
See detailInfluence of propeller configuration on propulsion system efficiency of multi-rotor Unmanned Aerial Vehicles
Theys, Bart; Dimitriadis, Grigorios ULg; Hendrick, Patrick et al

in Proceedings of the International Conference on Unmanned Aircraft Systems, ICUAS 2016 (2016, June)

Multi-rotor Unmanned Aerial Vehicles make use of multiple propellers, mounted on arms, to produce the required lift. This article investigates the influence on propulsion system efficiency in hover due to ... [more ▼]

Multi-rotor Unmanned Aerial Vehicles make use of multiple propellers, mounted on arms, to produce the required lift. This article investigates the influence on propulsion system efficiency in hover due to the configuration of these propellers. Influence of pusher or puller configuration of the propeller, number of blades, shape and dimensions of the arm, coaxial and overlapping propellers, is presented. A dedicated test bench that allows testing of various experimental setups is designed and built in order to realistically represent multi-rotor arms. Test results show that a two-bladed pusher configuration is most efficient and slenderness of the arm has more influence on efficiency than shape. A coaxial propulsion system approaches the efficiency of a single-prop system at high disk loadings. Finally, interference effects due to overlapping propellers are discussed. [less ▲]

Detailed reference viewed: 37 (4 ULg)
Full Text
See detailExperimental Passive Flutter Mitigation Using a Linear Tuned Vibrations Absorber
Verstraelen, Edouard ULg; Habib, Giuseppe ULg; Kerschen, Gaëtan ULg et al

in Proceedings of the IMAC XXXIV Conference (2016)

The current drive for increased efficiency in aeronautic structures such as aircraft, wind turbine blades and helicopter blades often leads to weight reduction. A consequence of this tendency can be ... [more ▼]

The current drive for increased efficiency in aeronautic structures such as aircraft, wind turbine blades and helicopter blades often leads to weight reduction. A consequence of this tendency can be increased flexibility, which in turn can lead to unfavourable aeroelastic phenomena involving large amplitude oscillations and nonlinear effects such as geometric hardening and stall flutter. Vibration mitigation is one of the approaches currently under study for avoiding these phenomena. In the present work, passive vibration mitigation is applied to an experimental aeroelastic system by means of a linear tuned vibration absorber. The aeroelastic apparatus is a pitch and flap wing that features a continuously hardening restoring torque in pitch and a linear one in flap. Extensive analysis of the system with and without absorber at subcritical and supercritical airspeeds showed an improvement in flutter speed around 34%, a suppression of a jump due to stall flutter, and a reduction in LCO amplitude. Mathematical modelling of the experimental system showed that optimal flutter delay can be obtained when two of the system modes flutter simultaneously. However, the absorber quickly loses effectiveness as it is detuned. The wind tunnel measurements showed that the tested absorbers were much slower to lose effectiveness than those of the mathematical predictions. [less ▲]

Detailed reference viewed: 45 (8 ULg)
Full Text
See detailPIV-based estimation of unsteady loads on a flat plate at high angle of attack using momentum equation approaches
Guissart, Amandine ULg; Bernal, Luis; Dimitriadis, Grigorios ULg et al

Conference (2015, November 23)

The direct measurement of loads with force balance can become challenging when the forces are small or when the body is moving. An alternative is the use of Particle Image Velocimetry (PIV) velocity ... [more ▼]

The direct measurement of loads with force balance can become challenging when the forces are small or when the body is moving. An alternative is the use of Particle Image Velocimetry (PIV) velocity fields to indirectly obtain the aerodynamic coefficients. This can be done by the use of control volume approaches which lead to the integration of velocities, and other fields deriving from them, on a contour surrounding the studied body and its supporting surface. This work exposes and discusses results obtained with two different methods: the direct use of the integral formulation of the Navier-Stokes equations and the so-called Noca's method. The latter is a reformulation of the integral Navier-Stokes equations in order to get rid of the pressure. Results obtained using the two methods are compared and the influence of different parameters is discussed. The methods are applied to PIV data obtained from water channel testing for the flow around a 16:1 plate. Two cases are considered: a static plate at high angle of attack and a large amplitude imposed pitching motion. Two-dimensional PIV velocity fields are used to compute the aerodynamic forces. Direct measurements of dynamic loads are also carried out in order to assess the quality of the indirectly calculated coefficients. [less ▲]

Detailed reference viewed: 75 (15 ULg)