References of "Dimitriadis, Grigorios"
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See detailUnsteady pressure distributions on a 4:1 rectangular cylinder: comparison of numerical and experimental results using decomposition methods.
Guissart, Amandine ULg; Andrianne, Thomas ULg; Dimitriadis, Grigorios ULg et al

Conference (2017, July 04)

Detached flows around bluff bodies are ubiquitous in civil engineering applications. In this work, the flow around a static 4:1 rectangular cylinder at moderate Reynolds number and at different angles of ... [more ▼]

Detached flows around bluff bodies are ubiquitous in civil engineering applications. In this work, the flow around a static 4:1 rectangular cylinder at moderate Reynolds number and at different angles of incidence is studied using both Experimental Fluid Dynamics (EFD) and Computational Fluid Dynamics (CFD). Typically, the integration of EFD and CFD allows a better understanding of the flow of interest by leveraging the complementary of their respective outputs. However, the comparison of computational and experimental results is an important but difficult step of this integration, particularly in the case of local quantities related to unsteady flows. In this work, decomposition methods are used to compare unsteady loads and pressure distributions coming from EFD and CFD. In particular, Proper Orthogonal Decomposition (POD) and Dynamic Mode Decomposition (DMD) are used to extract the dominant structures of the aerodynamic coefficients. The experimental data are obtained from dynamic pressure measurements in wind tunnel while numerical data come from two-dimensional unsteady Reynolds-Averaged Navier-Stokes (uRANS) simulations and tri-dimensional Delayed-Detached Eddy Simulations (DDES). This work shows that the decomposition methods represent a powerful tool enabling the analysis and the quantitative comparison of the main spatial and temporal characteristics of unsteady flows. Moreover, the accuracy of uRANS and DDES results is analyzed in light of the capacity of both CFD techniques to capture the reattachment occurring on the upper part of the rectangular cylinder. [less ▲]

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See detailA Discussion on the Advancement of Blade Tip Timing Data Processing
Kharyton, Vsevolod; Dimitriadis, Grigorios ULg; Defise, Colin

in Proceedings of the Turbomachinery Technical Conference & Exposition, TURBO EXPO 2017 (2017, June 29)

The Blade Tip Timing method (BTT) is a well-known approach permitting individual blade vibration behavior characterization. The technique is becoming increasingly popular among turbomachinery vibration ... [more ▼]

The Blade Tip Timing method (BTT) is a well-known approach permitting individual blade vibration behavior characterization. The technique is becoming increasingly popular among turbomachinery vibration specialists. Its advantages include its non-intrusive nature and its capability of being used for long-term monitoring, both in on-line and off-line analysis. However, the main drawback of BTT is frequency aliasing. Frequency aliasing effects in tip timing can be reduced by means of the application of different methods from digital signal analysis that can exploit the non-uniform nature of the data sampled by BTT. This non-uniformity is due to the fact that an optimization of the circumferential distribution of BTT probes is usually required in order to improve the data quality for targeted modes of blade vibration and/or orders of excitation. The BTT data analysis methods considered in this study are the non-uniform Fourier transform, the minimum variance spectrum estimator approach, a multi-channel technique using in-between samples interpolation, the Lombe-Scargle periodogram and an iterative variable threshold procedure. These methods will be applied to measured data representing quite a large scope of events occurring during gas-turbine compressor operation, e.g. synchronous engine order resonance crossing, rotating stall, suspected limit-cycle oscillations. Finally, the frequency estimates obtained from all these methods will be summarized. [less ▲]

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See detailFreeplay-induced limit cycle oscillation mitigation using linear and nonlinear tuned vibration absorbers
Verstraelen, Edouard ULg; Kerschen, Gaëtan ULg; Dimitriadis, Grigorios ULg

in Proceeding of the IFASD 2017 Conference (2017, June 27)

Structural nonlinearities such as freeplay in control surface bearings and actuators or in connections between wings and external payloads sometimes lead to aeroelastic limit cycle oscillations at ... [more ▼]

Structural nonlinearities such as freeplay in control surface bearings and actuators or in connections between wings and external payloads sometimes lead to aeroelastic limit cycle oscillations at airspeeds lower than the linear flutter speed of the aircraft. In parallel, numerous studies demonstrated the potential of linear and nonlinear tuned vibration absorbers to increase the flutter speed of linear and continuously hardening aeroelastic systems such as two-degree- of-freedom wings or long span bridges. In this work, the effect of linear and nonlinear tuned vibration absorbers is studied on a wing with pitch plunge and control surface deflection degrees of freedom and with freeplay in pitch. Depending on the tuning of the linear absorber, the linear flutter speed of the system can be increased by 10% or the onset of limit cycle oscillations due to the freeplay can be delayed by 7.7% and their amplitude can be significantly decreased. The addition of cubic hardening forces on the absorber can further decrease the limit cycle amplitude in a limited airspeed range at the cost of an increase in limit cycle amplitude in another airspeed range. Conversely, the addition of a freeplay hardening force on the absorber can decrease the limit cycle amplitude without any detrimental effect. [less ▲]

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See detailLimit cycle oscillations of cantilever rectangular flat plates in a wind tunnel
Giannelis, Nicholas; Vio, Gareth A.; Dimitriadis, Grigorios ULg

in Proceedings of the International Forum on Aeroelasticity and Structural Dynamics, IFASD 2017 (2017, June 27)

A closed form state-space model of the nonlinear aeroelastic response of thin cantilevered flat plates is derived using a combination of Von Karman thin plate theory and a linearized continuous time ... [more ▼]

A closed form state-space model of the nonlinear aeroelastic response of thin cantilevered flat plates is derived using a combination of Von Karman thin plate theory and a linearized continuous time vortex lattice aerodynamic model. The modal-based model is solved for the amplitude and period of the limit cycles of the flat plates using numerical continuation. The resulting predictions are compared to experimental data obtained from identical flat plates in the wind tunnel. It is shown that the aeroelastic model predicts the linear flutter conditions and nonlinear response of the plates with reasonable accuracy, although the predicted limit cycle amplitude variation with airspeed is different to the one measured experimentally due to unmodelled physics. [less ▲]

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See detailResearch on Fast Aeroelastic Modeling Methods for the Transonic Regime
Güner, Hüseyin ULg; Dimitriadis, Grigorios ULg; Terrapon, Vincent ULg

in Proceedings of the International Forum on Aeroelasticity and Structural Dynamics, IFASD 2017 (2017, June 27)

Two methods for modeling unsteady transonic flows at low computational cost are presented as a first step towards a fast and accurate aeroelastic calculation methodology for the preliminary design stage ... [more ▼]

Two methods for modeling unsteady transonic flows at low computational cost are presented as a first step towards a fast and accurate aeroelastic calculation methodology for the preliminary design stage in the transonic flow regime. The first approach corresponds to a quasi-steady approximation based on few steady simulations that is improved through the use of an unsteady filter. The second approach is based on the interpolation of dynamic modes between solutions at different frequencies that are obtained either from Dynamic Mode Decomposition (DMD) of unsteady simulations or directly from Harmonic Balance (HB) simulations. The two methods are illustrated in the case of a pitching airfoil in the transonic regime. Results show that the first method is fast and provides a first approximation of the unsteady dynamics. The computational cost of the second approach is higher, but the method provides better results in predicting aerodynamic forces and shock motion for a large range of reduced frequencies. [less ▲]

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See detailDynamic stall onset variation with reduced frequency for three stall mechanisms
Boutet, Johan ULg; Dimitriadis, Grigorios ULg; Amandolese, Xavier

in Proceedings of the International Forum on Aeroelasticity and Structural Dynamics, IFASD 2017 (2017, June 27)

A set of unsteady aerodynamic load measurement is performed on three oscillating airfoils with distinct stall mechanisms: a flat plate, a NACA0012, and a NACA0018. The airfoils are forced to oscillate in ... [more ▼]

A set of unsteady aerodynamic load measurement is performed on three oscillating airfoils with distinct stall mechanisms: a flat plate, a NACA0012, and a NACA0018. The airfoils are forced to oscillate in pitch around the stall angle of attack with prescribed frequency and amplitude. A criterion proposed by Sheng et al. is used to locate the onset of the flow separation process associated with dynamic stall, and quantify its variation with an equivalent reduced pitch rate. The validity of this criterion is tested for the three airfoils at low Reynolds number, Re = 2 × 10^4. Results are compared with the experimental data obtained by Sheng et al. at higher Reynolds number of Re = 1.5 × 10^6. [less ▲]

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See detailSystem Eigenvalue Identification Of Mistuned Bladed Disks Using Least-Squares Complex Frequency-Domain Method
Huang, Yuan; Kielb, Robert E.; Li, Jing et al

in Proceeding of the Turbomachinery Technical Conference & Exposition TURBO EXPO 2017 (2017, June 26)

This paper presents the results from a research effort on eigenvalue identification of mistuned bladed rotor systems using the Least-Squares Complex Frequency-Domain (LSCF) modal parameter estimator. The ... [more ▼]

This paper presents the results from a research effort on eigenvalue identification of mistuned bladed rotor systems using the Least-Squares Complex Frequency-Domain (LSCF) modal parameter estimator. The LSCF models the frequency response function (FRF) obtained from a vibration test using a matrix-fraction description and obtains the coefficients of the common denominator polynomial by minimizing the least squares error of the fit between the FRF and the model. System frequency and damping information is obtained from the roots of the denominator; a stabilization diagram is used to separate physical from mathematical poles. The LSCF estimator is known for its good performance when separating closely spaced modes, but few quantitative analyses have focused on the sensitivity of the identification with respect to mode concentration. In this study, the LSCF estimator is applied on both computational and experimental forced responses of an embedded compressor rotor in a three-stage axial research compressor. the LSCF estimator is first applied to computational FRF data obtained from a mistuned first-torsion (1T) forced response prediction using FMM (Fundamental Mistuning Model) and is shown to be able to identify the eigenvalues with high accuracy. Then the first chordwise bending (1CWB) computational FRF data is considered with varied mode concentration by varying the mistuning standard deviation. These cases are analyzed using LSCF and a sensitivity algorithm is developed to evaluate the influence of the mode spacing on eigenvalue identification. Finally, the experimental FRF data from this rotor blisk is analyzed using the LSCF estimator. For the dominant modes, the identified frequency and damping values compare well with the computational values. [less ▲]

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See detailStaggered strong coupling between existing fluid and solid solvers through a Python interface for fluid-structure interaction problems
Thomas, David ULg; Variyar, Anil; Boman, Romain ULg et al

in Proceedings of the VII International Conference on Coupled Problems in Science and Engineering (2017, June)

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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 (2017), 55(5), 1707-1722

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 ▲]

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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 (2017), 54(3), 1076-1084

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 ▲]

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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 ▲]

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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 ▲]

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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 ▲]

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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 ▲]

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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 ▲]

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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 ▲]

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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 ▲]

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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 ▲]

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