Tim De Troyer

• Doctor of Philosophy
• Professor (Assistant) at Vrije Universiteit Brussel

Vertical‐axis wind turbines (VAWTs) operate in the atmospheric boundary layer and are frequently subjected to turbulent flow. The objective of this study is to investigate the effect of free‐stream turbulence on the power performance and wake characteristics of a pair of counter‐rotating VAWTs. The research was conducted using wind tunnel experimen...

This paper presents a structural optimization of a 3D-printed thermoplastic-core propeller blade for high-altitude unmanned aerial vehicles using a genetic algorithm. It aims to minimize the weight of the blade by creating longitudinal holes and spars and to minimize the blade tip deflection during operation at an altitude of 16 km. The objective i...

This research explores cycle-to-cycle variability in dynamic stall through wind tunnel experiments conducted on a pitching NACA 0018 aerofoil at a Reynolds number of 2.8×10 ⁵ . Multiple cycles are considered, and different clusters are identified based on inspection of the lift time series. Experiments reveal that the blind application of a convent...

Recent studies have shown that the placement of paired vertical-axis wind turbines (VAWTs) in close proximity is a promising approach to design efficient VAWT farm layouts, mainly due to the increased power output. This study aims to experimentally investigate the impact of the rotational direction of paired counter-rotating VAWTs on their wake cha...

The aerodynamic interaction of vertical-axis wind turbines (VAWTs) in paired configurations can be advantageous in wind farms. However, these turbines operate in the atmospheric boundary layer which is characterised by high turbulence and may also face high-turbulence levels associated with downstream wakes. Therefore, the main goal of this study i...

The development of simple, low-order and accurate unsteady aerodynamic models represents a crucial challenge for the design optimisation and control of fluid dynamical systems. In this work, wind tunnel experiments of a pitching NACA 0018 aerofoil conducted at a Reynolds number $Re = 2.8 \times 10^5$ and at different free-stream turbulence intensit...

A known challenge when building nonlinear models from data is to limit the size of the model in terms of the number of parameters. Especially for complex nonlinear systems, which require a substantial number of state variables, the classical formulation of the nonlinear part (e.g. through a basis expansion) tends to lead to a rapid increase in the...

In this work, an advanced 2D nonparametric correlogram method is presented to cope with output-only measurements of linear (slow) time-varying systems. The proposed method is a novel generalization of the kernel function-based regularization techniques that have been developed for estimating linear time-invariant impulse response functions. In the...

Modal analysis has developed into a major technology for the study of structural dynamics in the past several decades. Through it, complex structural dynamics phenomena can be represented in terms of structural invariants, i.e., the modal parameters: natural frequencies, damping ratios and mode shapes. Operational Modal Analysis (OMA) deals with th...

In recent years, the developments in transportation and industrialization have constituted air pollution as one of the main widespread environmental and health issues. Air quality forecasting has become an important aspect of smart city planning and management [1]. Digital twin technologies, such as sensor networks coupled with machine learning alg...

This paper presents an original approach to the structural design and analysis of a 3D-printed thermoplastic-core propeller blade for high-altitude UAVs. A macroscale linear isotropic numerical model for the behavior of 3D-printed parts (in Tough PLA, as well as ABS) is fed with values from tensile and bending testing on standard specimens (ISO 527...

This work discusses the experimental challenges and processing of unsteady experiments for a pitching wing in the low-speed wind tunnel of the Vrije Universiteit Brussel. The setup used for unsteady experiments consisted of two independent devices: (a) a position control device to steer the pitch angle of the wing, and (b) a pressure measurement de...

The aim of this paper is to introduce an identification method for industrial vibro-acoustic systems with multiple inputs using (very) short measurement. The classical time-consuming phase resonance (or normal modes) testing procedures are nowadays almost fully substituted by frequency response functions (FRFs) methods which are used to obtain para...

Accurate unsteady aerodynamic models are essential to estimate the forces on rapidly pitching wings and to develop model-based controllers. As system identification is arguably the most successful framework for model predictive control in general, in this paper we investigate whether system identification can be used to build data-driven models of...

This paper aims to improve the understanding of the wind turbine tower shadow effect by developing a model that combines potential flow and blade-element momentum theory. The change in the flow field due to the presence of the tower is discussed in detail, together with the resulting variations in the aerodynamic forces acting on the rotating blade...

Engineers and scientists want mathematical models of the observed system for understanding, design, and control. Many mechanical and civil structures are nonlinear. This paper illustrates a combined nonparametric and parametric system identification framework for modelling a nonlinear vibrating structure. First step of the process is the analysis:...

The focus of this study is to investigate under high turbulence intensity the performance and wake characteristics of a pair of closely spaced counter-rotating vertical axis wind turbines (VAWTs) in wind tunnel-controlled conditions. To achieve this, wind tunnel experiments for an isolated VAWT and dual VAWTs are conducted. A squared-mesh wooden gr...

This paper presents unsteady aeroelastic simulation of a 3D-Printed propeller blade designed for High-Altitude Pseudo-Satellites by using Fluid-Structure Interaction FSI. Aerodynamic simulations are carried out in ANSYS Fluent considering an altitude of 16 km, and the spring-based smoothing method is used for updating the mesh during the coupling....

Engineers and scientists want mathematical models of the observed system for understanding, design and control. Many mechanical and civil structures are nonlinear. This paper illustrates a combined nonparametric and parametric system identification framework for modeling a nonlinear vibrating structure. First step of the process is the analysis: me...

The increasing sophistication of wind turbine design and control generates a need for high‐quality wind data. The relatively limited set of available measured wind data may be extended with computer generated data, for example, to make reliable statistical studies of energy production and mechanical loads. Here, a data‐driven model for the generati...

PID controllers are the most frequently used controllers. Theoretical understanding of the system to be controlled and the working principle of PID controllers are crucial for closed-loop control. This fundamental knowledge is - in principle - learned by all engineering students at the undergraduate level. However, understanding the practical aspec...

The integration of renewable and residual energy sources requires a detailed level of knowledge of district heating components such as thermal pipes, pipes, pumps, and heat exchanger of a substation. The thermal energy stored in these components can be utilized to match the heat demand and heat generation in time for next generation district heatin...

This paper describes the first results of the modelling of a plate heat exchanger which is installed in the VUB district heating network. A properly controlled energy exchange through the heat exchanger improves the economy of the entire system, minimizes the pollutant emissions and fossil fuel consumption, in accordance with the main goals of the...

Quasi-static and dynamic stall control experiments, using dielectric barrier discharge plasma actuators, were performed on a square-tipped aspect-ratio-one wing, with a NACA 0015 profile, under harmonic pitching, at a Reynolds number of 0.3M. Relatively low O(1) and relatively high O(10) pulse-modulated reduced excitation frequencies were introduce...

View Video Presentation: https://doi.org/10.2514/6.2021-2490.vid This paper presents a steady aeroelastic simulation of a 3D Printed propeller blade designed for a High Altitude Platform Station (HAPS) by using the Fluid-Structure Interaction (FSI) method. On the fluid side, steady RANS simulations are carried out in ANSYS Fluent at an altitude of...

This paper illustrates a combined nonparametric and parametric system identification framework for modeling nonlinear vibrating structures. First step is the analysis: multiple-input multiple-output measurements are (semi-automatically) preprocessed, and a nonparametric Best Linear Approximation (BLA) method is performed. The outcome of the BLA ana...

In this work recurrent neural networks of the LSTM type are used to describe unsteady fluid loading. Indirect load measurements are especially useful for structural health monitoring applications. Relying on discrete spatiotemporal measurements of the velocity field, both the forces and the corresponding displacement of a cylinder subjected to vort...

Dynamic fluid loading is of major concern to the off shore industry. Impacted structures may exhibit accelerated fatigue and even failure. Load monitoring has therefore become an integral part of structural health monitoring. Obtaining direct load measurements can be cumbersome, e.g. obtaining strain gauge or pressure taps measurements. In this wor...

Many engineering applications involve lifting surfaces (wings) that oscillate about in pitch, heave, and/or stream-wise (referred to as surge). For small pitch amplitudes, under the stall angle of attack, the lift force will respond linearly to such oscillations. However, at sufficiently large amplitudes, unsteady flow separation and dynamic stall...

Two vertical-axis wind turbines (VAWTs) benefit from a power increase when placed side by side in close proximity. To study the potential of paired VAWTs for integration in wind farms, wind tunnel wake measurements of lift-driven VAWTs are compared for isolated and three counter-rotating configurations. Because the wake of an isolated VAWT is defle...

Closely spaced counter-rotating vertical-axis wind turbines (VAWTs) exhibit a considerable power improvement compared to the same turbines in isolation. It thus makes sense to study their potential for wind farm production optimisation. With this objective in mind, the wake of an isolated VAWT is compared experimentally to the wake of counter-rotat...

In aerodynamics, as in many engineering applications, a parametrised mathematical model is used for design and control. Often, such models are directly estimated from experimental data. However, in some cases, it is better to first identify a so-called nonparametric model, before moving to a parametric model. Especially when nonlinear effects are p...

Synthetic wind-speed generators can provide a detailed characterisation of the wind variability at different time scales. A keen interest in the availability of synthetic wind speeds has recently risen in wind power modelling applications. In particular, a proper simulation of the diurnal and annual variability of the wind speed is sought that can...

As the structural limits for the conventional horizontal‐axis wind turbine are being reached, innovative concepts are needed to ensure increased reliability and reduced costs. One option is limiting the fatigue to which a rotor is subjected. This study, therefore, intends to experimentally investigate the tower shadow effect for a deeper understand...

A nonlinear model relating the imposed motion of a circular cylinder, submerged in a fluid flow, to the transverse force coefficient is presented. The nonlinear fluid system, featuring vortex shedding patterns, limit cycle oscillations and synchronisation, is studied both for swept sine and multisine excitation. A nonparametric nonlinear distortion...

A nonlinear model relating the imposed motion of a circular cylinder, submerged in a fluid flow, to the transverse force coefficient is presented. The nonlinear fluid system, featuring vortex shedding patterns, limit cycle oscillations and synchronisation, is studied both for swept sine and multisine excitation. A nonparametric nonlinear distortion...

Small Darrieus vertical-axis wind turbines (VAWTs) have recently been proposed as a possible solution for adoption in the built environment as their performance degrades less in complex and highly-turbulent flows. Some recent analyses have even shown an increase of the power coefficient for the large turbulence intensities and length scales typical...

In cities, the tallest rooftops have the best potential for electricity production. They often combine good wind and solar conditions with available space. Rooftop-mounted photovoltaic (PV) panels have already proven their efficiency and profitability. For tall rooftops with good wind conditions, also small wind turbines can be economically viable....

Dynamic stall is exploited to produce positive torque on a large-chord/radius-ratio (c∕R) vertical-axis wind turbine. An elementary kinematic analysis shows that the blades experience large variations in angle of attack that produce deep dynamic stall, combined with large relative dynamic pressure variations, that are phase shifted by approximately...

This paper introduces a nonparametric, nonlinear system identification toolbox called SAMI (simplified analysis for multiple input systems) developed for industrial measurements of vibro-acoustic systems with multiple inputs. It addresses the questions related to the user-friendly (semi-)automatic processing of multiple-input, multiple-output measu...

This work proposes a deterministic methodology to generate synthetic wind-speed time series consistent with both a probability density function and a spectral density function. The method relies on the random-phase multisine signal to generate an initial sequence conforming to the target spectral density. A following iterative rank-reordering proce...

An elementary kinematic analysis of large chord-radius ratio (c/R) vertical axis wind turbines was performed and a small two-bladed model turbine (0.4c/R0.6) was built and tested. The analysis shows that these machines operate at low blade-speed/wind-speed ratios and hence the blades experience large changes in angle of attack, that produce dynam...

As rooftops in cities are mostly underused, they have a large potential for decentralised electricity production. In that context, photovoltaic (PV) panels have proven to be an effective solution. Meanwhile, the market of small wind turbines is increasing, and some building owners have already installed one or more units on their roof. While the ec...

The potential of vertical-axis wind turbines (VAWTs) for specific applications (floating off-shore or urban in- stallations) has been reconsidered in recent years. This renewed interest has led to an increasing number of experimental studies on VAWTs. A challenge of experimental work is correctly interpreting experimental results but just as import...

Turbulence is one of the main characteristics of urban flows, but its effect on the performance of small VAWTs (often used in urban wind installations) has not been thoroughly researched. This experimental study focuses in testing a H-Darrieus VAWT prototype in different turbulent conditions inside the wind tunnel, in order to study the influence o...

Wind tunnel tests have been performed of individual and paired H-type Darrieus vertical-axis wind turbines. The turbines in the paired configuration are closely spaced, at 1.2 and 1.3 rotor diameters shaft to shaft, and are counter-rotating. Two directions of rotation were studied, one where the facing (inner) blades move along with the incoming fl...

In this paper, a time-domain equivalent source method (ESM) for acoustic scattering prediction is suggested, based on acoustic velocity formulations. Incident and scattered acoustic predictions are realized by acoustic velocity formulation V1A of Ghorbaniasl. Formulation V1A is based on the FW-H equation and allows direct evaluation of the boundary...

Vertical-Axis Wind Turbines are receiving the attention of the wind energy community for urban wind harvesting. However, their practical application is still far from maturity, due to the lack of understanding of urban flows. High turbulence is one of the main characteristics of wind in complex environments, so special attention has been paid to mo...

In this paper, high-speed impulsive noise radiated from helicopter rotors in hover and forward flight is computed by using a convected FW-H equation in the frequency domain with a rotating permeable data surface. Compared to the time-domain formulations, the frequency-domain formula avoids retarded time computation and flow data interpolation with...

In this paper, a convected frequency-domain approach for acoustic scattering prediction is suggested, for sources and scattering surfaces in a uniform constant flow. Frequency-domain moving-medium formulations are used for prediction of the incident acoustic pressure and acoustic pressure gradient. The latter is required for evaluation of the hardw...

In this work we study the effect of using different types of excitation signals as training data when constructing black box nonlinear models. We focus in particular on the class of nonlinear systems which exhibit autonomous oscillations. Three type of excitation signals are considered: random-phase multisines, filtered white noise and swept sines....

Vertical-axis wind turbines (VAWTs) have received a renewed interest in the wind energy research community, mainly for off-shore applications. One advantage is that installing a pair of counterrotating VAWTs on the same floating platform would result in thrust reduction and potential cancellation of the mooring yaw moment. In addition, such configu...

Small Darrieus Vertical-Axis Wind Turbines (VAWTs) are presently seen as a relevant research topic for the wind energy community, since they are thought to perform better than horizontal-axis rotors in the complex and highly-turbulent flow typical of the urban environment. Indeed, a preliminary wind tunnel test campaign on a H-Darrieus VAWT showed...

The flow-induced vibration of bluff bodies is an important problem of many marine, civil, or mechanical engineers. In the design phase of such structures, it is vital to obtain good predictions of the fluid forces acting on the structure. Current methods rely on computational fluid dynamic simulations (CFD), with a too high computational cost to be...

The flow-induced vibration of bluff bodies is an important problem of many marine, civil, or mechanical engineers. In the design phase of such structures, it is vital to obtain good predictions of the fluid forces acting on the structure. Current methods rely on computational fluid dynamic simulations (CFD), with a too high computational cost to be...

This study presents an innovative wind tunnel approach to evaluate the efficiency of Vertical-Axis Wind Turbines (VAWT) in turbulent flows, to study their integration in urban environments.The first part of the research is devoted to obtaining highly-turbulent wind profiles in the wind tunnel, with the use of different configurations of square grid...

This paper presents a time-domain method for noise prediction of supersonic rotating sources in a moving medium. The proposed approach can be interpreted as an extensive time-domain solution for the convected permeable Ffowcs Williams and Hawkings equation, which is capable of avoiding the Doppler singularity. The solution requires special treatmen...

This study presents an innovative wind tunnel approach to evaluate the efficiency of Vertical-Axis Wind Turbines (VAWT) in turbulent flows, to study their integration in urban environments. The first part of the research is devoted to obtaining highly-turbulent wind profiles in the wind tunnel, with the use of different configurations of square gri...

Aeroacoustic scattering prediction generally relies on boundary integral methods which require evaluation of the impermeability condition on the scattering surface. The boundary condition implies zero normal velocity relative to the scattering surface. This condition has been expressed by relating the acoustic velocity to the acoustic pressure grad...

In this work a black-box identification approach is used to identify a nonlinear model for nonlinear systems exhibiting autonomous oscillations. We focus on the proces of vortex-induced vibrations where fluid-structure interactions cause vibrations. A polynomial nonlinear state space model structure is proposed and fitted to the data acquired from...

Recently, a new maximum likelihood modal model-based (ML-MM) modal parameter estimator has been proposed [1,2]. One major drawback of this estimator is the modeling error, which can be caused by the effects of out-of-band modes (i.e. lower and upper residual effects). The ML-MM estimator uses the modal model as a parameterization form. This modal m...

A new modal parameter estimation method to directly establish modal models of structural dynamic systems satisfying two physically motivated constraints will be presented. The constraints imposed in the identified modal model are the reciprocity of the frequency response functions (FRFs) and the estimation of normal (real) modes. The motivation beh...

Vertical-axis wind turbines (VAWTs) have experienced a renewed impulse during the last few years with important research efforts focused on them. This work explores whether the global profitability of VAWTs can be improved through improved manufacturing techniques. We studied how large-series production techniques from the sheet-metal industry can...

In this work we explain the basic ideas of a novel modelling methodology for unsteady aerodynamics based on system identification techniques. We aim to construct transfer functions (as they are often used in noise and vibration engineering) to model periodic unsteadiness. This is of direct use for the study of limit-cycle oscillations (LCO) in flut...

In this paper, the ML-MM estimator, a multivariable frequency-domain maximum likelihood estimator based on a modal model formulation, will be represented and improved in terms of the computational speed and the memory requirements. Basically, the design requirements to be met in the ML-MM estimator were to have accurate estimate for both of the mod...

The key challenge behind the Polymax Plus modal parameter estimator is to keep the property of clear stabilization diagram constructed by the well-known Polymax estimator (Polymax user friendliness) and at the same time to have improved, statistically optimal, modal parameter estimates (Maximum likelihood (ML) accuracy). The Polymax Plus estimator...

In this paper we address the use of system identification techniques as a means for constructing a model relating fluid variables (pressure, velocity, vorticity) measured in the wake to the oscillatory behaviour of a cylinder in a flow. We describe how the model can be decomposed in three separate transfer functions, linked in a loop configuration....

Recently, a new maximum likelihood modal model-based (ML-MM) modal parameter estimator has been proposed [1, 2]. One major drawback of this estimator is the modelling error, which can be caused by the effects of out-of-band modes (i.e. lower and upper residual effects). The ML-MM estimator uses the modal model as a parameterization form. This modal...

Vertical axis wind turbines have experienced a renewed impulse during the last few years, with important research efforts focused on them. This work is focused in the development of new blade designs capable of notably reduce their manufacturing cost and, consequently, improve global profitability. Thanks to the aerodynamic and structural analyses...

A classical problem in vortex-induced vibration is to know the flow field past an oscillating cylinder. In this paper we use system theory to identify the oscillatory behaviour of a circular cylinder from flow variables in the wake. We use numerical simulations (CFD) of the flow past a cylinder oscillating in the cross-flow direction at different o...

In this paper we present the results of a feasibility study about small wind turbines in Flanders. We performed wind speed measurements at various sites and conducted an exhaustive review of small wind turbines to compare the different characteristics. Combining these two aspects, we predicted the annual energy production and compared the payback t...

In building a reliable aeroelastic model of a newly developed aircraft, one key factor is the coherence between ground vibration and flight flutter tests. This complex relation entails the complementarity of the ground and flight tests, and the integration of the numerical model with experimental (ground and flight) data. Recent research projects h...

In this paper, consistent multiple-input frequency-domain estimators will be presented based on a right matrix-fraction description of the frequency response function. This matrix-fraction description leads to a fast algorithm in the same way as the least-squares complex frequency-domain estimator (LSCF). The use of multiple inputs simultaneously i...

In this paper, a multivariable frequency-domain maximum likelihood estimator based on a modal model formulation is proposed. The proposed approach is mainly introduced to improve the accuracy of the modal parameters estimated by the poly-reference least squares complex frequency-domain (i.e. pLSCF) estimator and to have their confidence intervals a...

During the recent years several new tools have been introduced by the Vrije Universiteit Brussel in the field of Operational Modal Analysis (OMA) such as the transmissibility based approach and the the frequency-domain OMAX concept. One advantage of the transmissibility based approach is that the ambient forces may be coloured (non-white), if they...

A numerical study of the combined implementation of terrain roughness and velocity shear of the atmospheric boundary layer is presented in this paper. The consistency of these boundary conditions is tested by simulating the flow over a flat terrain. The results are described in the first part of this paper. Once these conditions are tuned, they are...

Ambient noise is generally seen as an unwanted excitation that disturbs the estimation of vibration parameters. Averaging techniques are then used to decrease as much as possible the influence of the noise. However, this noise also excites the mechanical structure and thus increases the vibration response level. Moreover, it is possible that (broad...

This article uses frequency domain transmissibility functions for detecting and locating damage in operational conditions. In recent articles numerical and experimental examples were presented and the possibility to use the transmissibility concept for damage detection seemed quite promising. In the work discussed so far, it was assumed that the op...

One major drawback of the PolyMAX estimator, a polyreference least-squares frequency-domain estimator, is the lack of confidence bounds on the estimated modal parameters. This paper proposes a fast two-step approach to determine the variances on the estimated polynomial parameters and next on the resonance frequencies and damping ratios. The approa...

The least-squares complex frequency-domain (LSCF) estimator—commercially known as the PolyMAX estimator—nowadays is used intensively in various modal analysis applications. The main advantages are the very clear stabilization diagrams and even more important, the speed. In this paper it is shown that confidence intervals of the modal parameter esti...

The PolyMAX estimator is used intensively in modal analysis applications nowadays. The main advantages are its speed and the very clear stabilization diagrams it yields. Recently, an algorithm allowing a fast calculation of confidence intervals has been derived for frequency-domain least-squares estimators based on a common-denominator model. In th...

To estimate multi-reference frequency response matrices with the classical spectral techniques (H 1, H 2, H v, ⋯) the number of measurements (averages) should be at least equal to the number of inputs (or references). Moreover, the input (or reference) signals should be sufficiently uncorrelated to obtain a reliable estimate of the frequency respon...

Flutter monitoring is stated as a statistical hypothesis testing problem. This results in an on-line flutter onset detection algorithm that combines a subspace-based residual with the CUSUM test statistics to monitor a flutter instability indicator (damping, flutter margin...). In this paper, this flutter monitoring approach is adjusted to get rid...

This paper discusses a new flutter speed prediction method. Flutter is a dynamic aeroelastic instability of surfaces exposed to wind, e.g. aircraft wings. Current flight flutter tests trace damping ratios evolution with aircraft speed and then extrapolate these linearly. The critical flutter speed is found when one of the damping ratios becomes zer...

In experimental modal analysis (EMA), all input and output signals are measurable. In operational modal analysis only the output signal are measurable; the operational forces are unknown and usually assumed to be white noise sources. The OMAX framework is a unifying approach combining experimental and operational modal analysis. OMAX stands for Ope...

In this paper, improved poly-reference frequency-domain estimators will be presented for application in the field of flutter analysis. Flight flutter measurements are heavily contaminated by noise. To adequately handle such data, a stochastic approach, which takes the noise information into account, is required. Starting from the deterministic Poly...

At the end of the development cycle, a new aircraft is certified by means of in-flight flutter tests. These tests consist of flying the aircraft at different airspeeds and measuring the accelerations at a limited number of locations on the aircraft structure. The scope is to open the flight domain by verifying that the aircraft does not suffer from...

A basic assumption when using modal analysis parameter identification procedures is that the structure under test is time invariant. In many practical structural dynamics applications, however, these assumptions are violated (because either the boundary conditions or the structural parameters change in time). In this paper we will introduce a new a...

One of the most demanding applications of modal analysis is flight flutter testing. The classical approach is to expand the flight envelope of an airplane by performing a vibration test at constant flight conditions, curve-fit the data to estimate the resonance frequencies and damping ratios, and then to plot these estimates against flight speed. T...

In experimental modal analysis (EMA), all input and output signals are measurable. In operational modal analysis only the output signal are measurable; the operational forces are unknown and usually assumed to be white noise sources. The OMAX framework is a unifying approach combining experimental and operational modal analysis. OMAX stands for Ope...