References of "Dimitriadis, Grigorios"
     in
Bookmark and Share    
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 ULiege; Bernal, Luis; Dimitriadis, Grigorios ULiege 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: 65 (36 ULiège)
Full Text
Peer Reviewed
See detailAeroservoelastic Simulations for Horizontal Axis Wind Turbines
Prasad, Chandra Shekhar ULiege; Chen, Qiong Zhong; Bruls, Olivier ULiege 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: 75 (25 ULiège)
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 ULiege; 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: 33 (7 ULiège)
Full Text
See detailDynamic interactions of a supercritical aerofoil in the presence of transonic shock buffet
Giannelis, N. F.; Vio, G. A.; Dimitriadis, Grigorios ULiege

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: 83 (6 ULiège)
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 ULiege et al

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

Detailed reference viewed: 49 (8 ULiège)
Full Text
See detailPassive flutter suppression using a nonlinear tuned vibration absorber
Verstraelen, Edouard ULiege; Gourc, Etienne ULiege; Kerschen, Gaëtan ULiege 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: 102 (16 ULiège)
Full Text
See detailInfluence of propeller configuration on propulsion system efficiency of multi-rotor Unmanned Aerial Vehicles
Theys, Bart; Dimitriadis, Grigorios ULiege; 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: 39 (5 ULiège)
Full Text
See detailExperimental Passive Flutter Mitigation Using a Linear Tuned Vibrations Absorber
Verstraelen, Edouard ULiege; Habib, Giuseppe ULiege; Kerschen, Gaëtan ULiege 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: 60 (11 ULiège)
Full Text
See detailPIV-based estimation of unsteady loads on a flat plate at high angle of attack using momentum equation approaches
Guissart, Amandine ULiege; Bernal, Luis; Dimitriadis, Grigorios ULiege 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: 80 (17 ULiège)
Full Text
See detailSuppression of simulated self-excited oscillation using smart materials on flexible wing structure
Abdul Razak, N.; Nasip, N. S.; Hawari, A. F. et al

in Proceedings of the International Forum on Aeroelasticity and Structural Dynamics, IFASD 2015 (2015, July 01)

Suppression of simulated self-excited oscillation due to aeroelastic effects using piezoelectric patches is reported. The focus of the present work is suppressing simulated flutter oscillation using ... [more ▼]

Suppression of simulated self-excited oscillation due to aeroelastic effects using piezoelectric patches is reported. The focus of the present work is suppressing simulated flutter oscillation using piezoelectric patches bonded to the wing structure. First, a clean wing is exposed to airflow in the wind tunnel where it experienced limited amplitude oscillation. The responses were recorded. This is followed by bonding piezoelectric patches to the identical wing that was tested in the wind tunnel. Two of the patches were used as actuators to simulate self-excited oscillation in a control manner. The selected mode for excitation is 1st bending mode. The other two patches were used as suppressor for active control using negative velocity feedback Single-Input, Single-Output approach. The single input signal for negative feedback is also sensed using piezoelectric patches. The controller manages to suppress the simulated flutter response to a lower oscillation amplitude values. This works demonstrated the used of piezoelectric material as actuator to reproduce the oscillation amplitude during self-excited oscillation and suppress the oscillation at the same time. [less ▲]

Detailed reference viewed: 79 (5 ULiège)
Full Text
Peer Reviewed
See detailExperimental and numerical study of the flight of geese
Dimitriadis, Grigorios ULiege; Gardiner, James; Tickle, Peter et al

in Aeronautical Journal (2015), 119(1217), 1-30

The flight of barnacle geese at airspeeds representing high-speed migrating flight is investigated using experiments and simulations. The experimental part of the work involved the filming of three ... [more ▼]

The flight of barnacle geese at airspeeds representing high-speed migrating flight is investigated using experiments and simulations. The experimental part of the work involved the filming of three barnacle geese (Branta Leucopsis) flying at different airspeeds in a wind tunnel. The video footage was analysed in order to extract the wing kinematics. Additional information, such as wing geometry and camber was obtained from a 3D scan of a dried wing. An unsteady vortex lattice method was used to simulate the aerodynamics of the measured flapping motion. The simulations were used in order to successfully reproduce the measured body motion and thus obtain estimates of the aerodynamic forces acting on the wings. It was found that the mean of the wing pitch angle variation with time has the most significant effect on lift while the difference in the durations of the upstroke and downstroke has the major effect on thrust. The power consumed by the aerodynamic forces was also estimated; it was found that increases in aerodynamic power correspond very closely to climbing motion and vice versa. Root-mean-square values of the power range from 100 W to 240 W. Finally, it was observed that tandem flying can be very expensive for the trailing bird. [less ▲]

Detailed reference viewed: 122 (24 ULiège)
Full Text
See detailAsymmetric limit cycle oscillations in systems with symmetric freeplay
Dimitriadis, Grigorios ULiege

in Proceedings of the International Forum on Aeroelasticity and Structural Dynamics, IFASD 2015 (2015, June 30)

In this paper, a simple 2D aeroelastic system with degrees of freedom in pitch, plunge and control surface deflection is investigated with freeplay in the pitch degree of freedom. It is shown that this ... [more ▼]

In this paper, a simple 2D aeroelastic system with degrees of freedom in pitch, plunge and control surface deflection is investigated with freeplay in the pitch degree of freedom. It is shown that this system features a fixed point at the origin as well as two anti-symmetric fixed points. Asymmetric limit cycles that span only two of the three piecewise linear subdomains of the phase plane can orbit these anti-symmetric fixed points. A single three-domain symmetric cycle and two two-domain cycles appear as a result of a grazing bifurcation occurring at the flutter speed of the underlying linear system. The two-domain cycles can undergo further fold, period doubling and torus bifurcations. They can cause both periodic and aperiodic oscillations, including highly chaotic responses at parameter values where they interact strongly. [less ▲]

Detailed reference viewed: 87 (9 ULiège)
Full Text
See detailUsing 2D-PIV measurements to compute unsteady aerodynamic loads on a flat plate at high angle of attack
Guissart, Amandine ULiege; Bernal, Luis; Dimitriadis, Grigorios ULiege et al

in Proceedings of the 16th International Forum on Aeroelasticity and Structural Dynamics, IFASD 2015 (2015, June 30)

This work exposes and discusses results obtained for aerodynamic forces using an indirect calculation based on Particle Image Velocimetry (PIV) measurements. The methodology used is based on the integral ... [more ▼]

This work exposes and discusses results obtained for aerodynamic forces using an indirect calculation based on Particle Image Velocimetry (PIV) measurements. The methodology used is based on the integral formulation of the Navier-Stokes equations and is applied to spatio-temporal data for different flows around a plate with a 16:1 chord-to-thickness ratio at high angle of attack. Experimental data are obtained in a water channel for both a static and a pitching plate. In addition to PIV data, direct measurements of aerodynamic loads are carried out to assess the quality of the indirect calculation. It is demonstrated that this indirect method is able to compute the mean and the temporal evolution of the lift and drag coefficients with a reasonable accuracy. It is also shown that the noise sensitivity of the method can be partly alleviated through the use of Dynamic Mode Decomposition (DMD) as a pre-processing step to smooth the spatio-temporal data. [less ▲]

Detailed reference viewed: 82 (23 ULiège)
Full Text
See detailTheoretical and experimental investigation of a structurally and aerodynamically nonlinear pitch and flap wing
Verstraelen, Edouard ULiege; Boutet, Johan ULiege; Grappasonni, Chiara ULiege et al

in Proceedings of the International Forum on Aeroelasticity and Structural Dynamics, IFASD 2015 (2015, June 29)

This paper presents and experimental and theoretical investigation of a novel nonlinear aeroelastic system. It consists of a wing with pitch and flap degrees of freedom, suspended from a leaf spring ... [more ▼]

This paper presents and experimental and theoretical investigation of a novel nonlinear aeroelastic system. It consists of a wing with pitch and flap degrees of freedom, suspended from a leaf spring secured in a nonlinear clamp. Both the structural and the aerodynamic forces acting on the wing can be nonlinear, depending on the amplitude of oscillations. Wind tunnel experiments show that the system undergoes a supercritical Hopf bifurcation that leads to small amplitude limit cycle oscillations. At a particular airspeed, the pitch amplitude jumps to a much higher value and dynamic stall starts to occur. Three mathematical models of the system are formulated, one based on linear aerodynamics and two based on the Leishman-Beddoes dynamic stall model. The objective of the modelling is to determine whether the jump in pitch oscillation amplitude is due to dynamic stall. The predictions for amplitude, frequency and mean angle of the limit cycle oscillations are compared to the experimental observations. All three models predict the small amplitude oscillations with satisfactory accuracy. The complete Leishman-Beddoes model predicts the occurrence of a jump in pitch amplitude but the magnitude of this jump is signi cantly overestimated. The other two models completely fail to model the jump. The failure of the Leishman-Beddoes model to predict the correct post-jump oscillation amplitude may be due to the values selected for the model parameters. [less ▲]

Detailed reference viewed: 130 (36 ULiège)
Full Text
See detailUnsteady Viscous-Inviscid Coupling Simulations of Separated Laminar Flows Around 2D Airfoils
Rothkegel Ide, José Ignacio ULiege; Dimitriadis, Grigorios ULiege

in Proceedings of the International Forum on Aeroelasticity and Structural Dynamics, IFASD 2015 (2015, June 29)

An interactive boundary layer model has been developed in 2D in order to solve the unsteady flow around an airfoil. The inviscid problem is solved using a panel method, by the discretization of the ... [more ▼]

An interactive boundary layer model has been developed in 2D in order to solve the unsteady flow around an airfoil. The inviscid problem is solved using a panel method, by the discretization of the airfoil into linear-varying vortex panels. The solution of the boundary layer equations is carried out using a finite volume scheme. Viscous-inviscid coupling is preformed by imposing a permeation velocity on the skin panels of the airfoil and the addition of a free wake at each separation point. [less ▲]

Detailed reference viewed: 92 (15 ULiège)
Full Text
See detailFlapping around in a wind tunnel
Dimitriadis, Grigorios ULiege

Conference (2015, March 17)

Aerodynamic investigations of animal flight have usually concentrated on static conditions. These are good enough to analyse gliding flight but not other types of flight, such as cruise (migrations), take ... [more ▼]

Aerodynamic investigations of animal flight have usually concentrated on static conditions. These are good enough to analyse gliding flight but not other types of flight, such as cruise (migrations), take-off and landing or manoeuvres. In this work we present wind tunnel experiments on geese flying in a wind tunnel and unsteady aerodynamic simulations of these flights, based on the Vortex Lattice Method. [less ▲]

Detailed reference viewed: 68 (3 ULiège)
Full Text
See detailInternal Resonance and Stall-Flutter in a Pitch-Flap Wing in the Wind-Tunnel
Verstraelen, Edouard ULiege; Kerschen, Gaëtan ULiege; Dimitriadis, Grigorios ULiege

in Proceedings of the SEM IMAC XXXII (2015, February)

Nonlinear aeroelastic phenomena such as store-induced LCOs, transonic buzz and stall flutter are the burden or modern aircraft: they reduce the performance and can even limit the flight envelope in both ... [more ▼]

Nonlinear aeroelastic phenomena such as store-induced LCOs, transonic buzz and stall flutter are the burden or modern aircraft: they reduce the performance and can even limit the flight envelope in both civil and military cases. Several nonlinear setups were studied experimentally in the last decades by the scientific community but most of them have pitch and plunge degrees of freedom and feature a rigid wing. In this paper, we study a new nonlinear aeroelastic apparatus that features nonlinear pitch and flap degrees of freedom, coupled with a flexible wing. The model is tested experimentally in the wind tunnel to determine its dynamic behaviour. Preliminary observations demonstrate that the system undergoes a supercritical Hopf bifurcation due to the hardening nonlinearity followed by an amplitude jump that is the consequence of either dynamic stall (i.e. stall flutter) or internal resonance (i.e. interaction between the hardening nonlinearity and higher modes). [less ▲]

Detailed reference viewed: 86 (33 ULiège)
Full Text
Peer Reviewed
See detailImpact of roughness and circularity-defect on bridge cables stability
Benidir, Adel; Flamand, Olivier; Gaillet, Laurent et al

in Journal of Wind Engineering & Industrial Aerodynamics (2015), 137

The stay cables on cable-stayed bridges have been the subject of considerable research in order to understand the origins of the dry galloping phenomenon. Surface irregularity is one of the last ... [more ▼]

The stay cables on cable-stayed bridges have been the subject of considerable research in order to understand the origins of the dry galloping phenomenon. Surface irregularity is one of the last parameters that have not been thoroughly examined. This paper focuses on static wind tunnel tests on original High Density Polyethylene cable covers in a range of Reynolds numbers from the sub-critical regime to the critical, corresponding to values ranging from Re = 9.6x104 to Re = 3.3x105. The experiment consists in testing cable covers of various diameters in order to investigate the effect of surface irregularity (roughness and circularity defect) on the mechanism of dry galloping excitation. Previous studies reported that dry galloping is caused by the appearance of a negative pressure bubble on one side of the circular cylinder at the critical Reynolds number range, leading to a rapid drop in the drag coefficient and the appearance of a non-negligible lift force. The results of the present investigation demonstrate that there is a clear correlation between the single bubble pressure pattern and the circularity defect along the tube. They further show that surface roughness has little effect on the location of the bubble. The paper also treats the spatial and temporal correlation of the instantaneous pressure pattern along the tube with respect to the circularity defect. [less ▲]

Detailed reference viewed: 83 (2 ULiège)