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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 device to measure the aerodynamic loads. The position control setup can pitch the wing for a range of frequencies, amplitude, and offset levels. In this work, a NACA-0018 wing profile was used with an aspect ratio of 1.8. The position control and the pressure measurement setups operate independently of each other, necessitating advanced signal processing techniques to synchronize the pitch angle and the lift force. Furthermore, there is a (not well-documented) issue with the (sampling) clock frequency of the pressure measurement setup, which was resolved using a fully automated spectral analysis technique. The wing was pitched using a simple harmonic sine excitation signal at eight different offset levels (between 6° and 21°) for a fixed amplitude variation (std) of 6°. At each offset level, the wing was pitched at five different frequencies between 0.1 Hz and 2 Hz (that correspond to reduced frequencies k ranging from 0.006 to 0.125). All the experiments were conducted at a fixed chord-based Reynolds number of 2.85 × 105. The choice of operating parameters invokes the linear and nonlinear behavior of the wing. The linear unsteady measurements agreed with the analytical results. The unsteady pressure measurements at higher offset levels revealed the nonlinear aerodynamic phenomenon of dynamic stall. This confirms that a nonlinear and dynamic model is required to capture the salient characteristics of the lift force on a pitching wing.

The present paper investigates the effectiveness of artificial neural networks for the identification of nonlinear state-space models in fluid dynamics. The test case under consideration consists in the modeling of the unsteady lift force of a pitching NACA 0018 airfoil at a Reynolds number Re = 3 x 10^5. Data used for model training and validation are obtained through Computational Fluid Dynamics (CFD) simulations performed in OpenFOAM. The State-Space Neural Network (SS-NN) model is trained using swept sines performed at several angle-of-attack ranges, and it is then validated using different sine sweeps as well as simple harmonic motions. The study demonstrates that the selected SS-NN model represents a powerful tool to accurately predict the unsteady aerodynamic loads of a pitching airfoil, both in pre-stall and post-stall conditions. In particular, the model succeeds in correctly capturing highly nonlinear flow features such as the delay in flow separation, and the formation and shedding of the dynamic stall vortex. The accuracy and evaluation speed of this technique make it particularly valuable for engineering applications involving design optimization and real-time control of systems based on lift. The obtained results represent a stepping-stone towards the integration of flexible, data-driven SS-NN models representing unsteady aerodynamics inside dynamic multi-physics models.

In many engineering applications, aerofoils experience elevated free-stream turbulence levels. The present work experimentally investigates the effect of free-stream turbulence on the aerodynamic characteristics of a sinusoidally pitching NACA 0018 aerofoil at the transitional Reynolds number of 2.8×105. Wind tunnel tests are conducted in quasi-static and dynamic conditions at different turbulence intensities between 0.3% and 8.2%, considering reduced frequencies up to 0.1. The dynamic experiments investigate multiple angle-of-attack ranges in order to quantify the influence of free-stream turbulence in the attached-flow regime, in the stalled regime, and in-and-out of stall. The study demonstrates that high free-stream turbulence drastically changes the flow physics over the aerofoil for both static and dynamic conditions, producing large deviations in the lift and moment coefficients. The quasi-static experiment performed at low free-stream turbulence features a large stall hysteresis linked to the breakdown of the leading-edge laminar separation bubble, whereas static hysteresis is not present at higher turbulence levels. Moreover, dynamic experiments at high angles of attack show a strong dependency on the incoming turbulence intensity, which is found to delay flow separation during the upstroke and enhance reattachment during the downstroke. The work also reveals the intrinsic difficulty of predicting the dynamic stall behaviour under different turbulence conditions, and gives insight into why the existing empirical dynamic stall models are unlikely to succeed in accurately predicting the aerodynamic loads across different onset turbulence intensities.

While modern wind turbines have become by far the largest rotating machines on Earth with further upscaling planned for the future, a renewed interest in small wind turbines is fostering energy transition and smart grid development. Small machines have traditionally not received the same level of aerodynamic refinement of their larger counterparts, resulting in lower efficiency, lower capacity factors, and therefore a higher cost of energy. In an effort to reduce this gap, research programmes are developing worldwide. With this background, the scope of the present study is twofold. In the first part of this paper, an overview of the current status of the technology is presented in terms of technical maturity, diffusion, and cost. The second part of the study proposes five grand challenges that are thought to be key to fostering the development of small wind turbine technology in the near future, i.e.: (1) improve energy conversion of modern SWTs through better design and control, especially in the case of turbulent wind; (2) better predict long-term turbine performance with limited resource measurements and prove reliability; (3) improve the economic viability of small wind energy; (4) facilitate the contribution of SWTs to the energy demand and electrical system integration; (5) foster engagement, social acceptance, and deployment for global distributed wind markets. To tackle these challenges, a series of unknowns and gaps are first identified and discussed. Based on them, improvement areas are suggested within which ten key enabling actions are finally proposed.

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 pitching wings. The forces acting on the pitching wing can be considered a nonlinear dynamic function of the pitching angle and therefore require a nonlinear dynamic model. In this work, a nonlinear data-driven model is developed for a pitching wing. The proposed model structure is a polynomial nonlinear state-space model (PNLSS), which is an extension of the classical linear state-space model with nonlinear functions. The PNLSS model is trained on experimental data of a pitching wing. The experiments are performed using a dedicated wind tunnel setup. The pitch angle is considered as the input to the model, while the lift coefficient is considered as the output. Three models are trained on swept-sine signals at three offset angles with a fixed pitch amplitude and a range of reduced frequencies. The three training datasets are selected to cover the linear and nonlinear operating regimes of the pitching wing. The PNLSS models are validated on single-sine experimental data at the respective pitch offset angles. The PNLSS models are able to capture the nonlinear aerodynamic forces more accurately than a linear and semi-empirical models, especially at higher offset angles.

Understanding unsteady aerodynamic forces on a lifting surface is crucial for many engineering applications. This work demonstrates the use of system identification techniques for the data-driven modelling of the aerodynamic lift force on a pitching wing. This lift force can be considered as a (nonlinear) dynamic function of the angle of attack. The proposed approach consists of multiple steps: 1) excitation by the means of broadband excitation, 2) pre-processing of the measurement, and 3) data-driven modelling techniques. The considered excitation signal is the so-called multisine (also known as periodic pseudo- random noise). However, dynamic stall is known to exhibit important cluster-to-cluster variations and this is no different when utilizing multisines. Therefore, we cluster the data before the actual data-driven Preliminary proceedings modeling and validation. We show that the obtained models can capture the nonlinear aerodynamic forces more accurately than the classical linear and semi-empirical models.

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 blades, as well as the effect on the output power and the blade root bending moment. The tower shadow effect is not only due to the change in wind speed but also to the change in angle of attack. A one-dimensional description of the flow field is therefore bound to be inadequate. The impact of the tower diameter and blade-tower clearance on the rotor power and blade root bending moment can be expressed as a quadratic function of a non-dimensional parameter. Apart from a mild Reynolds-number effect, the results presented are independent of turbine size.

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 vortex shedding are modelled.

In this paper, the performance of propellers is evaluated with 3D RANS and Vortex Theory for a wide range of geometries and advance ratios at high-altitude conditions. For this purpose, the results of a high-altitude propeller optimization are used as a CFD database and discussed briefly. A comparison is made between the predictions of 3D RANS and Vortex Theory with a goal to examine the agreement between the two models. Moreover, a variance-based sensitivity analysis is performed to determine geometrical or operational features of the propeller designs that influence the error between the two models. In the second part, the data from the two physical models are used to train different multi-fidelity surrogate models, using the data produced from 3D RANS, as the high-fidelity dataset and Vortex Theory, as the low-fidelity one. Different performance metrics are used to evaluate the predictive capabilities of the multi-fidelity and single-fidelity surrogate models in new propeller geometries. The predictions ofthese data-driven models are finally compared to the predictions of Vortex Theory.

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 grid is placed upstream of the wind turbines to increase the level of free stream turbulence. The wind tunnel tests confirm the increase in power coefficient of paired VAWTs in turbulent flows compared to smooth flows. Additionally, this study analyses the near-wake downstream of the paired turbines in both smooth and turbulent flows to better understand the flow physics.

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 understanding of the aerodynamic fluctuations, and to study the capability of a lower surface microtab to mitigate such fluctuations. For this, aerodynamic load variations caused by the presence of the tower on the rotor blades were reproduced and studied in the wind tunnel using a setup that mimics the interaction between the tower and the blade. Anemometers and flow visualisation techniques were used to explore the influence of the tower on the surrounding flow field. Real-time measurements show that the alterations in the lift force are mainly caused by the alterations in the flow angle and have proven that a lower surface microtab can partially mitigate aerodynamic load variations in the case of an upwind rotor blade. Experimental results are validated using a tower shadow model that is based on potential flow theory. The tower shadow model shows a reduction in 3p power oscillations by a factor of 2.

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 flow, and one where the facing blades move against the wind. The wind tunnel tests confirm a net increase in the power coefficient of the paired configuration compared with twice the power coefficient of the individual turbine. We found average relative increases in the power coefficients between 13% and 16%, which is consistent with numerical studies available in the literature.

A persisting challenge in nonlinear dynamical modelling is parameter inference from data. Provided that an appropriate model structure was selected, the identification problem is profoundly affected by a choice of initialisation. A particular challenge that may arise is initialisation within a region of the parameter space where the model is not contractive. Exploring such regions is not feasible using the conventional optimisation tools for they require a bounded evaluation of the cost. This work proposes an unconstrained multiple shooting technique, able to mitigate stability issues during the optimisation of nonlinear state-space models. The technique is illustrated on simulation results of a Van der Pol oscillator and benchmark results on a Bouc-Wen hysteretic system.

A surrogate-based, multipoint and multiobjective optimization was performed in this work for a High-Altitude Propeller. For each blade design, the performance was evaluated with 3D RANS at three advance ratios for cruise and one advance ratio for climb, while the ability to take off, was assessed with Vortex Theory. Moreover, a different sampling strategy for the DoE phase was tested as an alternative to the Latin Hypercube Sampling of the whole design space. In this optimization problem, two objective functions were defined, one for the aerodynamic performance and one for the propeller weight, while Kriging was used to find the response surface of each objective. The Expected Improvement Matrix with the Euclidean distance was used as an infill criterion and the Kriging Believer algorithm was adopted in order to parallelize the procedure. The Pareto front formation is discussed as, well as the characteristics of some specific designs. In the end, a variance based sensitivity analysis is performed on the evaluated designs to give insight into the importance of the different design variables.

During the last three decades, rapid growth of wind energy has led to questions regarding the possible impacts of wind farms on local weather and microclimates. Physically, the increased turbulence due to the wind turbine operation affects the mixing processes, may slightly disturb the pressure and temperature distributions downstream of wind farms and may have an impact on natural ecosystems such as the famous mastic tree population located on the island of Chios in the North Aegean Sea. This study explores the wind farms and their wake effects downstream with a particular focus on the effect on the southern part of the island where the mastic trees cultivation is located. The analysis is carried out with the use of the commercial CFD code ANSYS Fluent. Steady state computations of full 3D Navier–Stokes equations, using the k-ε turbulence closure scheme are carried out. The development of the multiple wake effects of the wind farms and their propagation downstream is examined under low and high turbulence intensities. Results clearly indicate that for both test cases there is no impact to the local microclimate and to the mastic Tree population.

Nonlinear state-space modelling is a very powerful black-box modelling approach. However powerful, the resulting models tend to be complex, described by a large number of parameters. In many cases interpretability is preferred over complexity, making too complex models unfit or undesired. In this work, the complexity of such models is reduced by retrieving a more structured, parsimonious model from the data, without exploiting physical knowledge. Essential to the method is a translation of all multivariate nonlinear functions, typically found in nonlinear state-space models, into sets of univariate nonlinear functions. The latter is computed from a tensor decomposition. It is shown that typically an excess of degrees of freedom are used in the description of the nonlinear system whereas reduced representations can be found. The method yields highly structured state-space models where the nonlinearity is contained in as little as a single univariate function, with limited loss of performance. Results are illustrated on simulations and experiments for: the forced Duffing oscillator, the forced Van der Pol oscillator, a Bouc-Wen hysteretic system, and a Li-Ion battery model., submitted to Mechanical Systems and Signal Processing

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 deflected, the direction of rotation significantly influences the wake of paired VAWTs. The wake of counter-rotating VAWTs where the adjacent blades move downwind, exhibits a similar length, width and replenishment as the wake of an isolated VAWT. The wake of counter-rotating VAWTs with adjacent upwind moving blades, however, significantly differs from an isolated VAWT wake. While its wake length is similar to an isolated VAWT wake, its width and replenishment are not. Because of appealing wake characteristics, paired VAWTs exhibit unique advantages for wind farm applications, and especially for offshore floating wind farms.

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 analysis (FAST) is used to distinguish odd from even nonlinear behaviour. The information which is obtained from the nonlinear analysis is explicitly used in constructing a nonlinear model of the polynomial nonlinear state-space (PNLSS) type. The latter results in a reduction of the number of parameters and an increased accuracy compared to the generic modelling approach where typically no such information of the nonlinearity is used. The obtained model is able to accurately simulate time series of the transverse force coefficient over a wide range of the frequency-amplitude plane of imposed cylinder motion., Comment: Accepted for publication in Journal of Fluids and Structures

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 of such environments. Starting from these insights, this study presents a combined numerical and experimental analysis aimed at assessing the physical phenomena that take place during the operation of a Darrieus VAWT in turbulent flows. Wind tunnel experiments provided a quantification of the performance variation of a two-blade VAWT rotor for different levels of turbulence intensity and length scale. Furthermore, detailed experiments on an individual airfoil provided an estimation of the aerodynamics at high turbulence levels and low Reynolds numbers. Computational fluid dynamics (CFD) simulations were used to extend the experimental results and to quantify the variation in the energy content of turbulent wind. Finally, the numerical and experimental inputs were synthetized into an engineering simulation tool, which can nicely predict the performance of a VAWT rotor under turbulent conditions.

The increasing sophistication of wind turbine design and control generates a need for high-quality data. Therefore, the relatively limited set of measured wind data may be extended with computer-generated surrogate data, e.g. to make reliable statistical studies of energy production and mechanical loads. This paper presents a data-driven, statistical model for the generation of realistic surrogate time series that is based on the phase-randomised Fourier transform. The proposed model simulates an ergodic, pseudo-random process that makes use of an iterative rank-reordering procedure to yield synthetic time series that possess the power spectral density of the target data and concurrently converges to the probability distribution of the target data with an arbitrary, user-defined precision. A comparison with two established data-driven modelling techniques for generating surrogate wind speeds is presented. The proposed model is tested under the same input conditions given in the test cases of the selected models, and its performance is investigated in terms of the agreement with the target statistical descriptors. Simulation results show that the proposed model can reproduce with high fidelity the statistical descriptors of the input datasets and is able to capture the nonstationary diurnal and seasonal variations of the wind speed.

Trabajo presentado a Science of Making Torque from Wind 2020, TORQUE 2020 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 lead to a more efficient grid integration of this renewable source. This paper proposes a statistical model for generating synthetic wind speeds consistent with both the probability density function and the spectral density function of a measured wind-speed dataset and that simulates accurately its average diurnal variation. To test the proposed methodology, multiple synthetic time series are generated using three long-term wind-speed time series recorded at a meteorological site in the Netherlands. The accuracy in terms of the statistical descriptors of the generated time series and their average diurnal variation is assessed with respect to the target data. We show that the average diurnal cycles present in all the three measured time series are always reproduced accurately, and that the statistical descriptors of the target dataset are constantly matched with high accuracy. Possible advantages of the present approach in terms of power system modelling are discussed.

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 economic comparison between PV panels and wind turbines has already largely been addressed, in general, the space constraint of a rooftop has never been taken into account. In this work, the authors propose a methodology to compare the energy production and the return on investment both for rooftop-mounted PV panels and wind turbines. The comparison is made for relatively tall buildings (≥60 m) with good wind conditions (≥5 m/s annual mean wind speed). Using a brute-force approach, this study presents the results of the methodology applied to a case study: the Brussels Region. On tall rooftops, considering the space already taken by other installations and assuming an average wind speed of 5 m/s, small building-mounted wind turbines are shown to produce more energy than PV panels. Nevertheless, their return on investment is always lower than that of the PV panels.

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 modelling it inside wind tunnels for prototype testing. Previous experiments showed a considerable boost in VAWT performance when turbulence intensity increases, but the explanation of this increase still has to be determined. This study analyses the effect of turbulent flows on the performance of a NACA0018 airfoil, using a blade model provided with pressure tabs and a traverse system to analyze the wake. The model is subjected to the same angles of attack and Reynolds numbers that would be found in normal VAWT operation, while turbulence intensity and integral length scale are kept at levels similar to those found in urban environments. The effect of turbulence is evident as it considerably delays the stall angle of the blade. Using a Single Streamtube model, the results from this parametric study are compared with the overall turbine ratings, in order to find a way to optimize turbine blades using a simplified set-up.

Multivariate polynomials are omnipresent in black-box modelling. They are praised for their flexibility and ease of manipulation yet typically fall short in terms of insight and interpretability. Hence, often an alternative representation is desired. Translating the coupled polynomials into a decoupled form, containing only univariate polynomials has hence become a popular option. In this letter, two new polynomial decoupling techniques are introduced. The features and performance of both methods are illustrated on a nonlinear state-space model identified from data of the forced Duffing oscillator.

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. However, hybrid PV-wind systems apparently have never been studied for these limited spaces. In this work, we study the technical, economic and environmental performance of hybrid PV/wind systems installed on tall buildings. From the rooftop plan, the building electricity consumption and the solar and wind resources of a roof, we developed an algorithm that generates hybrid retrofitting configurations and calculates their global performance. We then use the algorithm for two high-rise buildings in the Brussels-Capital Region. For these two real case studies, in which all building and energy characteristics are known (i.e. rooftop plan, energy consumption) and where we measured wind and solar resources, we observe that hybrid PV/wind systems show good technical, economic and environmental performance. They also offer a good trade-off between economic and environmental objectives. However, we observe that the shadow impact from the turbines on PV panels is much larger than for ground-based hybrid installations (about 1% to 8% annual loss of irradiance).

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-rotating VAWTs. Because of the unsteady aerodynamics, the wake of an isolated VAWT deflects towards the region behind its upwind moving blade. The direction of rotation thus directly influences the deflection of the wake. For paired configurations, it is possible to use this wake deflection as an advantage. A pair of counter-rotating VAWTs where the upwind moving blades are at the centre of the pair, exhibits a particularly narrow wake. With the benefit of a power improvement and a narrow wake, closely spaced counter-rotating VAWTs exhibit promising characteristics to improve the power density in a wind farm.

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 configurations could benefit from increased power output and reduced wake losses. In this article, we report on wind tunnel experiments to study the mechanical power output of a reference VAWT scale model, tested individually and in a closely-spaced pair of VAWTs. The power output of the individual VAWT configuration is compared with a pair of VAWTs spaced 1.3 diameters apart for two counterrotating directions. A net power increase in the power coefficient for the paired configuration of up to 17.0 % compared with two individual rotors has been observed.

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 effectively used in the design phase or for control applications. Alternative methods use heuristic mathematical models of the fluid forces, but these lack the accuracy (they often assume the system to be linear) or flexibility to be useful over a wide operating range. In this work we show that it is possible to build an accurate, flexible and low-computational-cost mathematical model using nonlinear system identification techniques. This model is data driven: it is trained over a user-defined region of interest using data obtained from experiments or simulations, or both. Here we use a Van der Pol oscillator as well as CFD simulations of an oscillating circular cylinder to generate the training data. Then a discrete-time polynomial nonlinear state-space model is fit to the data. This model relates the oscillation of the cylinder to the force that the fluid exerts on the cylinder. The model is finally validated over a wide range of oscillation frequencies and amplitudes, both inside and outside the so-called lock-in region. We show that forces simulated by the model are in good agreement with the data obtained from CFD.

The development of large-scale wind energy projects has created the demand for increasingly accurate and efficient models that limit a project's uncertainties and risk. Wake effects are of great importance and are relevant for the optimization of wind farms. Despite a growing body of research, there are still many open questions and challenges to overcome. In computational modelling, there are always numerous input parameters such as material properties, geometry, boundary conditions, initial conditions, turbulence modelling etc. whose estimation is difficult and their values are often inaccurate or uncertain. Due to the lack of information of several sources, e.g., uncertainties present in operating conditions as well as in the mathematical modelling, the computational output is also uncertain. It is therefore very important to validate the mathematical models with experiments performed in controlled conditions. In the present paper, the single wake characteristics of a Horizontal-Axis Wind Turbine Rotor (HAWT) and their spatial evolution are investigated with different Computational Fluid Dynamics (CFD) modelling approaches and compared to experimental measurements. The steady state 3-D Reynolds-Averaged Navier Stokes (RANS) equations are solved in the open-source platform OpenFOAM, using different turbulence closure schemes. For the full-rotor CFD simulations, the Multiple Reference Frames (MRF) approach was used to model the rotation of the blades. For the simplified cases, an actuator disk model was used with the experimentally measured thrust (CT) and power (CP) coefficient values. The performance of each modelling approach is compared with experimental wind tunnel wake measurements from the 4th blind test organized by NOWITECH and NORCOWE in 2015. Numerical results are compared with experimental data along three horizontal lines downstream, covering all the wake regions. Wake predictions are shown to be very sensitive to the choice of the RANS turbulence model. For most cases, the ADM under-predicts the velocity deficit, except for the case of RNG k-ε which showed a superb performance in the mid and far wake. The full wind turbine rotor simulations showed good agreement to the experimental data, mainly in the near wake, amplifying the differences between the simplified models.

Polynomial nonlinear state-space (PNLSS) models have proven to be very useful in modelling highly nonlinear systems, encountered over a variety of engineering applications. In this work, we focus on modelling the kinematics of an oscillating circular cylinder, submerged in a low Reynolds number flow. Such a set up is typically used to study vortex shedding phenomena and their related forces. The power of the PNLSS model class comes from its large flexibility in candidate nonlinear basis functions. Flexibility, however, comes at a price. The number of parameters generally grows large, hampering the identification process and leading to a loss of insight in the nonlinear functions. The objective of this work is to investigate how prior knowledge of the nonlinearity can be introduced in the basis functions of these nonlinear models and how this affects the accuracy of the estimated model. In particular, the usage of polynomial functions in terms of states and the input is compared to nonlinear functions in termsof the output variable. An improved model was obtained when a deliberate choice of basis functions was chosen based on prior knowledge of the nonlinearity. In addition, promising results were obtained from using dedicated nonlinear basis functions in terms of the output on a system closely related to the vortex shedding system. © 2017 International Forum on Aeroelasticity and Structural Dynamics (IFASD).

The potential of vertical-axis wind turbines (VAWTs) for specific applications (floating off-shore or urban installations) 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 important is the design of the experimental setup. The objective of the present study is to elaborate on the complete development process of a VAWT wind tunnel test setup, from design to aerodynamic interpretation. The effect of the design choices and structural characteristics of the setup can be traced in the measured torque. The influence of structural dynamics of an experimental setup are proven to influence unsteady measurements significantly through the analysis of the spectral content of the measured torque. An extensive modal analysis of the VAWT rotor allows to link modal vibrations to their contaminating effect on the measured torque. In addition, the dynamics of the sensors and setup are found to be important contributions to the contamination of dynamic measurements. To interpret unsteady torque measurements, the complete experimental setup needs to be dynamically characterised.

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 of turbulence intensity, integral length scales and Reynolds number. Passive grids are used to increase the wind tunnel free stream turbulence up to Iu = 15%, with integral length scales of Lux = 0.18 m and Reynolds numbers of ReD ≈ 300,000. The measurements are repeated in two wind tunnels of different size, which strengthens the results and helps quantify the effect of blockage on the turbine ratings. The results show power coefficient increases up to 20% from smooth (Iu = 0.5%) to turbulent (Iu = 15%) flows, an effect that is enhanced at low λ but that fades as ReD > 400,000. The study of the crossed influence of ReD, Iu and Lux offers valuable data in the process of optimizing the operation of small VAWTs inside urban environments.

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 oscillation frequencies and amplitudes to construct a transfer function that relates the displacement of the cylinder and the resulting flow field. This transfer function can then be inverted to ‘predict’ the displacement of the cylinder given the flow field (as determined by simulations or measurements). We investigate this technique in the so-called lock-in region, where the vortex shedding frequency is synchronised with the oscillation frequency of the cylinder.

HD 167971 is a triple system consisting of a 3.3-day eclipsing binary (O5-8 V + O5-8 V) and an O8 supergiant. It is also a well known non-thermal radio emitter. We observed the radio emission of HD 167971 with the Very Large Array (VLA) and the Australia Telescope Compact Array (ATCA). By combining these data with VLA archive observations we constructed a radio lightcurve covering a 20-yr time-range. We searched for, but failed to find, the 3.3-day spectroscopic period of the binary in the radio data. This could be due to the absence of intrinsic synchrotron radiation at the colliding-wind region between the two components of the eclipsing binary, or due to the large amount of free-free absorption that blocks the synchrotron radiation. We are able to explain many of the observed characteristics of the radio data if the non-thermal emission is produced in a colliding-wind region between the supergiant and the combined winds of the binary. Furthermore, if the system is gravitationally bound, the orbital motion occurs over a period of ~ 20 years, or longer, as suggested by the long-term variability in the radio data. We argue that the variability is due to the free-free absorption that changes with orbital phase or may also in part be due to changes in separation, should the orbit be eccentric., Comment: 10 pages; 3 figures; accepted by A&A

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 flutter or the vortex-induced vibration (VIV) of bluff bodies. We study a two-dimensional circular cylinder, restricted to move only in the transverse direction to the incoming flow. To describe the coupling that exists between the oscillation of the structure and the wake, we propose a model structure composed of three transfer functions. One transfer function captures all the aerodynamic forces that act on the structure during oscillation. A second transfer function translates these forces into an oscillatory motion, taking into account the structural parameters of the set up (mass, stiffness, damping). The third transfer function relates measurements from flow variables in the wake (velocity, pressure, vorticity) to the oscillation of the cylinder. We use imposed-oscillation CFD simulations to generate time series to construct the aero- dynamic transfer function. The imposed oscillation (a sinusoid, or a specific combination of sinusoids called a multisine) is used as the input signal, while the measured forces on the cylinder are the output signal.

We present radio and submillimetre observations of the O4I(n)f star ζ Pup, and discuss structure in the outer region of its wind (~ 10-100 R*). The properties of bremsstrahlung, the dominant emission process at these wavelengths, make it sensitive to structure and allow us to study how the amount of structure changes in the wind by comparing the fluxes at different wavelengths. To look for variability, we acquired 3.6 and 6 cm observations with the Australia Telescope Compact Array (ATCA). We supplemented these with archive observations from the NRAO Very Large Array (VLA). We did not find variability at more than the ± 20 % level. The long integration time does allow an accurate determination of the fluxes at 3.6 and 6 cm. Converting these fluxes into a mass loss rate, we find M = 3.5 x 10-6 M⊙/yr. This value confirms the significant discrepancy with the mass loss rate derived from the Hα profile, making ζ Pup an exception to the usually good agreement between Hα and radio mass loss rates. We also observed ζ Pup at 850 μm with the James Clerk Maxwell Telescope (JCMT) and at 20 cm with the VLA. A smooth wind model shows that the millimetre fluxes are too high compared to the radio fluxes. While recombination of helium in the outer wind cannot be discounted as an explanation, the wealth of evidence for structure strongly suggests this as the explanation for the discrepancy. Model calculations show that the structure needs to be present in the inner ~ 70 R* of the wind, but that it decays significantly, or maybe even disappears, beyond that radius.

We report the results of an XMM-Newton observation of the Lagoon Nebula (M 8). Our EPIC images of this region reveal a cluster of point sources, most of which have optical counterparts inside the very young open cluster NGC 6530. The bulk of these X-ray sources are probably associated with low and intermediate mass pre-main sequence stars. One of the sources experienced a flare-like increase of its X-ray flux making it the second brightest source in M 8 after the O4 star 9 Sgr. The X-ray spectra of most of the brightest sources can be fitted with thermal plasma models with temperatures of kT af ew keV. Only a few of the X-ray selected PMS candidates are known to display H emission and were previously classified as classical T Tauri stars. This suggests that most of the X-ray emitting PMS stars in NGC 6530 are weak-line T Tauri stars. In addition to 9 Sgr, our EPIC field of view contains also a few early-type stars. The X-ray emission from HD 164816 is found to be typical for an O9.5 III-IV star. At least one of the known Herbig Be stars in NGC 6530 (LkH 115) exhibits a relatively strong X-ray emission, while most of the main sequence stars of spectral type B1 and later are not detected. We also detect (probably) diuse X-ray emission from the Hourglass Region that might reveal a hot bubble blown by the stellar wind of Herschel 36, the ionizing star of the Hourglass Region.

We report the results of a multi-wavelength investigation of the O4 V star 9 Sgr (= HD 164794). Our data include observations in the X-ray domain with XMM-Newton, in the radio domain with the VLA as well as optical spectroscopy. 9 Sgr is one of a few presumably single OB stars that display non-thermal radio emission. This phenomenon is attributed to synchrotron emission by relativistic electrons accelerated in strong hydrodynamic shocks in the stellar wind. Given the enormous supply of photospheric UV photons in the wind of 9 Sgr, inverse Compton scattering by these relativistic electrons is a priori expected to generate a non-thermal power law tail in the X-ray spectrum. Our EPIC and RGS spectra of 9 Sgr reveal a more complex situation than expected from this simple theoretical picture. While the bulk of the thermal X-ray emission from 9 Sgr arises most probably in a plasma at temperature ~ 3 x 106 K distributed throughout the wind, the nature of the hard emission in the X-ray spectrum is less clear. Assuming a non-thermal origin, our best fitting model yields a photon index of >=2.9 for the power law component which would imply a low compression ratio of =2 x 107 K. Our VLA data indicate that the radio emission of 9 Sgr was clearly non-thermal at the time of the XMM-Newton observation. Again, we derive a low compression ratio (1.7) for the shocks that accelerate the electrons responsible for the synchrotron radio emission. Finally, our optical spectra reveal long-term radial velocity variations suggesting that 9 Sgr could be a long-period spectroscopic binary. Based on observations with XMM-Newton, an ESA Science Mission with instruments and contributions directly funded by ESA Member states and the USA (NASA). Also based on observations collected at the European Southern Observatory (La Silla, Chile) and with the Very Large Array. The VLA is a facility of the National Radio Astronomy Observatory which is operated by the Associated Universities Inc. under cooperative agreement with the National Science Foundation. ispartof: Astronomy & Astrophysics vol:394 issue:3 pages:993-1008 status: published

In common with other early-type stars, epsilon Ori (B0 Ia) shows evidence for structure in its stellar wind. Variations in optical and ultraviolet line profiles reveal the presence of large-scale structure in the inner wind. The detection of X-rays and the existence of black troughs in saturated ultraviolet lines are indicative of small-scale structure. The geometric extent of both types of structure is poorly known. In principle, large-scale structure can be detected directly from very high spatial resolution observations that resolve the stellar wind. A simpler technique is to look for the presence of additional flux compared to that expected from a smooth wind. The run of this excess flux as a function of wavelength indicates how fast structure decays in the wind. If there is variability in the excess flux, it shows us that the structure must be large-scale. Such variability is suggested by two previous 6 cm radio observations of epsilon Ori: Abbott et al. (1980) found 1.6 +/- 0.5 mJy, while Scuderi et al. (1998) measured only 0.60 +/- 0.06 mJy. This could indicate that the large-scale structure persists beyond similar to50 R-*. To further investigate this variability, we used the Very Large Array (VLA) to monitor epsilon Ori over a 5-day period in February 1999. We supplemented our data with observations from the VLA archive. In an attempt to resolve the stellar wind, we also obtained a series of high spatial resolution observations with the Multi-Element Radio Linked Interferometer Network (MERLIN) during January-March 1999. From this combined material we find no evidence for variability and we conclude that the Abbott et al. (1980) flux determination is in error. The data do show substantial excess flux at millimetre wavelengths, compared to a smooth wind. This excess is confirmed by a submillimetre observation which we obtained with the James Clerk Maxwell Telescope (JCMT). The behaviour of epsilon Ori is therefore similar to what had been found previously for alpha Cam, delta Ori A, kappa Ori and zeta Pup. While the present data do not allow very strong constraints, they show that considerable structure must persist up to at least similar to10 R-* in the wind of epsilon Ori. The combined radio fluxes are used to derive a mass-loss rate of log (M) over dot (M./yr) = 5.73 +/- 0.04. This value is in good agreement with the Halpha mass-loss rate. The good agreement between Halpha and radio mass-loss rates for hot stars in general remains puzzling, as it implies that the same amount of structure is present in very different formation regions.

Compared to horizontal-axis wind turbines (HAWTs), vertical-axis wind turbines (VAWTs) exhibit a different behaviour when built in tight arrays. In this research we build a model based on a free vortex model and empirical wake models, which are the Jensen wake model and multiple wakes combination models. The Jensen wake model and multiple wakes combination models are used to calculate the power production of wind farms, where the power output of single turbine is calculated using the free vortex model at a given wind speed and wind direction. The simulation results are compared with the field test results from a literature. The goal is to investigate the main factors impacting the power density of wind farms of VAWT pairs. The results indicate that the power density of wind farms using VAWTs could be much higher than existing wind farms, mainly because of enhanced power performance of closely-spaced counter-rotating VAWT pairs and because of large height-diameter-ratio of VAWTs. However, wind direction has a crucial influence on the power production of wind farms using turbine pairs.

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 in the estimation (so-called polyreference estimation) has the advantage that it allows to separate closely spaced modes. The main drawback of the LSCF estimator is that it is theoretically inconsistent, i.e. the estimates do not converge to the true values when the number of measurements increases to infinity. However, the noise information available from most measurements can be used to construct a maximum likelihood-like weighting for the LSCF estimators, giving consistent estimates. The results are fast, polyreference, and consistent weighted generalised total-least-squares (WGTLS) estimators. The iterative quadratic maximum likelihood (IQML) estimator is practically consistent for high signal-to-noise ratios, with the additional advantage that it yields clear stabilisation charts. The performance of the presented WGTLS and IQML estimators is evaluated by means of ground vibration test data and demonstrated on flight flutter test data.

Ambient noise is generally seen as an unwanted excitation that disturbs the estimation of vibration parameters. Av- eraging 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 (broadband) noise excites vibration modes that are not well excited by the artificially applied forces. Those modes are missed by clas- sical estimation methods. Recently, classical EMA and OMA were combined into the so-called OMAX framework. In this framework both the artificial force and the ambient excitation are considered useful in determining the modal parameters. In this paper it is shown that the classical frequency-domain modal parameter estimators (rational fraction polynomial based and state space based) can be used without changing them, if the correct non-parametric preprocessing is applied to calculate the frequency response function (FRF) and the power spectrum (PSD). Special attention is paid to the case of structure-exciter interaction, where a direct OMAX approach would result in erroneous results. Also the importance of scaling the FRF and PSD is discussed. The approach is demonstrated on a typical OMAX case: flight flutter test of an airplane wing.

During the recent years several new tools have been introduced in the field of Operational Modal Analysis (OMA) such as the transmissibility-based approach and the OMAX concept. Most OMA estimators assume that the ambient forces are random noise sources with a flat amplitude spectrum. The transmissibility-based approach does not require this assumption: it works for almost any kind of ambient forces. The main advantage of the OMAX concept is the fact that it combines the advantages of Operational and Experimental Modal Analysis: ambient (unknown) forces as well as artificial (known) forces are processed simultaneously resulting in improved modal parameters. The main drawback of the existing OMAX estimators is related to the assumption that the ambient forces are white noise. In this paper, the transmissibility-based output-only approach will be combined with the input/output OMAX concept. This should result in a new methodology in the field of operational modal analysis allowing the estimation of (scaled) modal parameters in presence of arbitrary ambient (unknown) forces and artificial (known) forces. One important application is aeroelastic flutter. Flight flutter tests are necessary to assure that newly developed aircraft are free from flutter throughout the entire flight envelope. The airplane is excited by externally applied forces, but also due to turbulence. This turbulence excitation is not white at all, and can vary a lot between two flight phases. The transmissibility OMA approach benefits from changing operational loading conditions, and this enhances the accuracy of the estimated modal parameters. Our approach is tested on simulations and real-life measurements.

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Many stars continuously eject material and thus surround themselves with a stellar wind. We can observe this wind indirectly through its characteristic effects on the stellar spectrum. In particular, this mass flow influences the line profiles. These so called P-Cygni profiles can be observed in the visual part of the spectrum of early-type stars and Wolf-Rayet stars, as well as - and even more clearly - in the ultraviolet part. In order to study the stellar wind structure, a computer programme was written which calculates theoretical P-Cygni profiles using the Comoving Frame Method. As input, this code can use the results obtained from other programmes, such as a non-LTE code. Through parameter adjustments a fit to an observed profile can be made.

We present one-dimensional, time-dependent models of the clumps generated by the linedeshadowing instability. In order to follow the clumps out to distances of more than 1000 stellar radii, we use an efficient moving-box technique. We show that, within the approximations, the wind can remain clumped well into the formation region of the radio continuum.