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A wind turbine blade equipped with root spoilers is analysed using time domain aeroelastic blade element momentum (BEM) simulations to assess the impact of passive devices on the turbine annual energy production (AEP) and lifetime. Previous 2D computational fluid dynamics (CFD) showed a large unsteadiness in aerodynamic coefficients associated with the spoiler, and such behaviour is captured by the OpenFAST simulations when all degrees of freedom are switched off. Once the turbine is fully flexible, a novel way to account for aerofoil-generated unsteadiness in the fatigue calculation is proposed and detailed. The outcome shows that spoilers, on average, can increase the AEP of the turbine. However, the structural impacts on the turbine can be severe if not accounted for initially in the turbine design.

The complexity of the flow over a wind turbine blade makes its understanding and monitoring a challenging task, especially on operating wind turbines. The innovative electronic TellTale (e-TellTale) sensor is developed for that purpose – detecting the flow separation on wind turbines blades. In this paper, high-Reynolds-number wind tunnel tests have been performed with different configurations of full-scale e-TellTale sensors and wall pressure measurements on a wind turbine blade section. A comparison between the lift curve and the e-TellTale signal was used to evaluate the ability of the sensor to detect flow separation. Results show different interesting properties of the sensor response depending on its size, position along the chord and fitting process that could be used in real applications.

International audience; Aerodynamic loads of wind turbine blades are often predicted by manufacturers using the Blade Element Momentum (BEM) theory, for which many corrections have been proposed in the literature. The physical impacts of such corrections on field measurements have seldom been assessed because of the relative unavailability of dedicated measurements. Based on the unique full scale database of the DANAERO project, available through the IEA (International Energy Agency) Task 29, this work incrementally applies on aerodynamic field measurement, improvements of the BEM theory: atmospheric boundary layer vertical velocity gradient, neighbouring wake, yaw misalignment, wind inflow location, tower shadow effect, cone angles modelling, blade aeroelastic deformation and dynamic wake. This is performed using the iBEM method (inverse Blade Element Momentum), which back-calculates the aerodynamic coefficients (lift - CL and drag - CD) using aerodynamic loads from field tests.

A commercial wind turbine blade equipped with root spoilers is analysed using 2D URANS computational fluid dynamics (CFD) to assess the unsteady impact of passive devices. In this work, we present the 2D CFD unsteady results from a non-rotating single thick section located at the root end of the blade with and without spoiler. Computations were performed at the chord-based Reynolds number Rec=3×106. The analysed spoiler is of commercial size with a height of approximately 33 % of the local chord. Comparing to existing literature, it is at least 1 order of magnitude larger than the size of the well-known Gurney flaps. The analysis is first performed in the steady state at a single angle of attack using global aerodynamic forces, the local pressure distributions, and flow field analysis. Analyses are then continued accounting for the flow unsteadiness. The spoiler induces an important wake behaviour linked to the apparition of global load fluctuations. Using the wall pressure distributions and the associated spatio-temporal organisation of the flow field, those fluctuations are well characterised. Globally, in terms of lift gain, adding a spoiler is found to be detrimental for the negative angles of attack while of high interest for higher angles of attack. Another drawback of the spoiler addition is the unsteady effects. A dominant peak frequency is emerging in the aerodynamic polar coefficients, when adding a spoiler, which corresponds to a vortex shedding organisation.

A wind turbine blade equipped with root spoilers is analysed using 2D Computational Fluid Dynamics (CFD) to assess the unsteady impact of passive devices. Several metrics such as lift and drag coefficients, pressure and instantaneous velocity field around the aerofoil, Power Spectral Density and Strouhal number are used in the 2D unsteady analysis. The spoiler is found to efficiently rearrange the flow, adding lift throughout the positive angles of attack. However, the drawback is a high drag penalty coupled with high unsteadiness of the aerodynamic forces.

The complexity of the flow over a wind turbine blade makes its understanding and monitoring a challenging task, especially on operating wind turbines. The innovative e-Telltale sensor is developed for that purpose : detecting the flow separation on wind turbines blades. In this paper, high Reynolds wind tunnel tests have been performed with different configurations of full scale e-Telltale sensors and wall pressure measurements on a wind turbine blade section. A comparison between the lift curve and the e-Telltale signal was used to evaluate the ability of the sensor to detect flow separation. Results show different interesting properties of the sensor response depending on its size, position along the chord and its fitting process that could be used in real applications.

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The performance of lifting bodies such as hydrofoils is determined by the flow state and particular attention should be paid to flow separation, as this greatly affects the generated lift and drag. Sailors are used to look at telltales (woolies) to trim their sails or steer their yacht. A French company developed an electronic telltale for sails based on a strain gauge activated by a silicon strand, with the appropriate signal processing to deliver the same information as a classical wool-made telltale, basically attached or separated flow. This new sensor proved useful when woolies are not visible or to deliver a signal to feed a control system, such as the autopilot for example. It was also applied to wind turbines to control the blade pitch. Mer Agit\'ee is now developing an equivalent hydrodynamic e-Telltale to be used on hydrofoils and rudders to help trimming and controlling. The present work presents the investigation of a foil section fitted with this new sensor in a water tunnel, combining force and PIV measurements with the sensor signal, on a wide range of angle of attack. Results show that the hydro e-Telltale enables detecting the flow separation and anticipate stall, and possibly allows for detecting the boundary layer transition to turbulence. In many cases of fluid flow over a lifting body, it is interesting to get some real-time feedback from the flow in order to help optimizing performance and controlling the system. The feedback from this new sensor could be used in a closed-loop controlling system, for example feeding a reduced-order model of the flow around the foil.

International audience; The performance of lifting bodies such as hydrofoils is determined by the flow state and particular attention should be paid to flow separation, as this greatly affects the generated lift and drag. Sailors are used to look at telltales (woolies) to trim their sails or steer their yacht. A French company developed an electronic telltale for sails based on a strain gauge activated by a silicon strand, with the appropriate signal processing to deliver the same information as a classical wool-made telltale, basically attached or separated flow. This new sensor proved useful when woolies are not visible or to deliver a signal to feed a control system, such as the autopilot for example. It was also applied to wind turbines to control the blade pitch. Mer Agitée is now developing an equivalent hydrodynamic e-Telltale to be used on hydrofoils and rudders to help trimming and controlling. The present work presents the investigation of a foil section fitted with this new sensor in a water tunnel, combining force and PIV measurements with the sensor signal, on a wide range of angle of attack. Results show that the hydro e-Telltale enables detecting the flow separation and anticipate stall, and possibly allows for detecting the boundary layer transition to turbulence. The feedback from this new sensor could be used for example in a control loop to make an “Anti-Stall System” on a rudder or a foil.

Monitoring the flow features over wind turbine blades is a challenging task that has become more and more crucial. This paper is devoted to demonstrate the ability of the e-TellTale sensor to detect the flow stall/reattachment dynamics over wind turbine blades. This sensor is made of a strip with a strain gauge sensor at its base. The velocity field was acquired using TR-PIV measurements over an oscillating 2D blade section equipped with an e-TellTale sensor. PIV images were post-processed to detect movements of the strip, which was compared to movements of flow. Results show good agreement between the measured velocity field and movements of the strip regarding the stall/reattachment dynamics.; Monitoring the flow features over wind turbine blades is a challenging task that has become more and more crucial. This paper is devoted to demonstrate the ability of the e-TellTale sensor to detect the flow stall/reattachment dynamics over wind turbine blades. This sensor is made of a strip with a strain gauge sensor at its base. The velocity field was acquired using TR-PIV measurements over an oscillating 2D blade section equipped with an e-TellTale sensor. PIV images were postprocessed to detect movements of the strip, which was compared to movements of flow. Results show good agreement between the measured velocity field and movements of the strip regarding the stall/reattachment dynamics.

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Wind turbines operate in the turbulent atmospheric boundary layer and can be additionally exposed to the wakes of upstream wind turbines. Hence the inflow encountered by wind turbine rotor blades is turbulent. One way to gain more knowledge on the impact of the turbulence on the airfoil aerodynamics is by studying a 2d airfoil section of a real wind turbine blade against inflows of various turbulence intensities and Reynolds numbers. Therefore, for the purpose of this paper, experiments were performed and aerodynamic coefficients were calculated for chord based Reynolds numbers between 1.3 × 105 and 3 × 105 and angles of attack between -10° and 24° for inflows of three different averaged turbulence intensities, 0.3%, 3% and 5.5%. Results were compared with the already available result of measurements done at a higher chord based Reynolds number of 4.7×106 for the same airfoil type. The results show that, for a given Reynolds number, an increase in turbulence intensity suppresses the flow separation. It was also found that an increase in the turbulence intensity of the inflow does reduce the dependence of aerodynamic coefficients on the Reynolds number. The Comparison between experimental results for lower chord-based Reynolds numbers and high chord-based Reynolds numbers shows differences which implies that an increase of the turbulence intensity is not enough to bypass the low Reynolds number effect completely.

This work addresses the problem of developing control algorithms for the control of the aerodynamic lift of wind turbine blades using air injection, taking into account disturbances caused by turbulent perturbations. For this, a test bench is used where the lift of a 2D blade section in a wind tunnel can be controlled by a set of micro-jets close to the trailing edge. Through a continuous, local identification of the lift variations a model-free control that does not need any prior knowledge of the system is proposed. It allows the control of the flow of the micro-jets and stabilizes the lift around a tracking reference. The ability of the proposed control algorithm to track the lift reference when subjected to external perturbations, i.e., gusts, is discussed. In particular, this work demonstrates that the lift can be set to particular values using the proposed control strategy, and can be re-stabilized to pre-gust lift conditions. Experimental results illustrate globally the feasibility of such a control.

We present a new system for the generation of rapid, strong flow perturbations in the aerodynamic wind tunnel at École Centrale de Nantes. The system is called the chopper, and it consists of a rotating bar cutting through the inlet of a wind tunnel test section, thus generating an inverse gust that travels downstream. The flow generated by the chopper is investigated with respect to the rotational frequency using an array equipped with hot-wires that is traversed downstream in the flow field. It is found that the gust can be described as a superposition of the mean gust velocity, an underlying gust shape, and additional turbulence. Following this approach, the evolution of the mean gust velocity and turbulence intensity are presented, and the evolution of the underlying inverse gust shape is explained. The turbulence is shown to be characterized by an integral length scale of approximately half the chopper blade width and a turbulence decay according to E(f)∝f-5/3.

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In boundary layer flow around rotating machines, a radial (or cross-flow) velocity exists due to Coriolis and centrifugal forces. This velocity component can be of great importance for laminar-turbulent transition. A series of direct numerical simulations (DNS) are performed to study the boundary layer flow transition on a rotating Horizontal Axis Wind Turbine blade. To quantify the effect of blade rotation, results are compared with that from airfoil DNS, where the section is taken from 3D blades and does not rotate. It is shown that the rotation gives rise to a small radial velocity and slightly modifies the shape of unstable waves. However, the transition location and mechanism of 3D blade boundary layer flow resemble 2D flow for the investigated case.

Wind turbines operate in the naturally turbulent atmospheric boundary layer. Due to strong flow variations, the aerodynamics at the rotor blades are complex. Therefore, to gain a better understanding of the effect of strong velocity and angle fluctuations on the aerodynamic behavior of an airfoil, we present a new system capable of generating rapid, strong gusts in a wind tunnel, the chopper. It consists of a rotating bar cutting through the inlet of the wind tunnel, thus generating turbulent, strong flow perturbations. Using this system and exposing an airfoil to its flow, we investigate the lift variations caused by the simultaneous, rapid velocity and angle variations. The results show that the lift response of the airfoil is directly correlated with the velocity. The lift response to changes of the angle of attack is determined not only by the change of the angle, but also by the rapidity with which it changes.

International audience; An experimental campaign is performed to the study of feedback lift control applied to a wind turbine blade. A 5-digit NACA profile whose trailing edge is rounded for circulation control purposes is used in combination with fluidic actuation. It is first shown in this article how the modified profile performs, in terms of aerodynamic forces, in both natural and manipulated cases. Then, the dynamics of controlled pressure (and thus lift) establishment is identified. A discussion is given on the speed of lift dynamics in comparison with previous studies and on its ability to overcome lift perturbations due to external perturbations. Finally, a feedback lift control experiment is performed showing the feasibility of such control in a wind turbine environment.

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Colloque avec actes et comité de lecture. Internationale.; International audience; Wind turbines are installed in the strongly inhomogeneous and unsteady turbulent atmospheric boundary layer. This induces unsteady mechanical loads at different characteristic time scales from seconds to minutes which limit significantly their life time. The present work, supported by the SMARTEOLE ANR project, focuses on the flow control strategies at the blade scale, to manipulate lift and thus alleviate some of these loads. For this purpose, a NACA654-421 airfoil profile has been modified : the trailing edge has been rounded to take advantage of Coanda effects and the camber has been increased to compensate the loss of lift due to the trailing edge modifications. The lift control is obtained by fluidic injection via 42 1x1 mm micro-jets placed at the trailing edge along the entire span of the wing. An experiment has been conducted to identify both static and dynamic performances of the proposed control mechanism. The experimental campaign consisted in chordwise unsteady pressure measurements as well as aerodymics forces measurements. The preliminary results of the mean quantities indicate that the lift gain obtained is proportional to the fluidic injection, which is of interest when closed-loop control is to be considered. In a second series of measurements, we focus on the step-response of the flow to the actuation. The lift response is shown to behave as a first order dynamics and we show that the response time of lift is of the order of 3 convective time units. This is about three times faster than what is usually observed for boundary layer reattachement process.

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In order to reduce the aerodynamic load fluctuations on wind turbine blades by innovative control solutions, strategies of active circulation control acting at the blade airfoil trailing edge are studied, allowing lift increase and decrease. This study presents a comparison of results obtained by performing surface plasma and continuous fluidic jet actuation on a blade airfoil designed with a rounded trailing edge. In the present study, both actuator types are located at the trailing edge. Plasma actuators act uniformly in the spanwise direction, whereas fluidic jets blow through small squared holes distributed along the span, and therefore, provide a three-dimensional action on the flow. Load and velocity field measurements were performed to assess the effectiveness of both actuators and to highlight the flow mechanisms induced by both actuation methods for lift-up configurations. Results are presented for a chord Reynolds number of 2. 105 and for a lift coefficient increase of 0.06.

International audience; The turbulent atmospheric boundary layer in which wind turbines are implemented is strongly inhomogeneous and unsteady. This induces unsteady mechanical loads at different characteristic time scales from seconds to minutes which limits significantly their life time. Different control strategies have been proposed in the framework of the French ANR SmartEole project to alleviate the impact of these upstream fluctuations at the farm, wind turbine and blade scales (i.e. characteristic time scales from seconds to minutes). The present work, which is part of this ANR project, focus on the flow control strategies at the blade scale, to manipulate lift and thus alleviate fatigue loads. The design of a NACA654-421 airfoil profile has been modified to be able to implement jet control. Slotted jet and discrete jet configurations were implemented numerically and experimentally respectivelly. Results show the ability of both configurations to increase the lift by up to 30% using a significant redistribution of the mean shear. Efficiency seems to be more important using slotted jets, which however needs to be confirmed from 3D simulations.

The mixing layer–wake interaction is studied experimentally in the framework of two flow configurations. For the first one, the initial conditions of the mixing layer are modified by using a thick trailing edge, a wake effect is therefore superimposed to the mixing layer from its beginning (blunt trailing edge). In the second flow configuration, a canonical mixing layer is perturbed in its asymptotic region by the wake of a cylinder arranged perpendicular to the plane of the mixing layer. These interactions are analyzed mainly by using two-point velocity correlations and the proper orthogonal decomposition (POD). These two flow configurations differ by the degree of complexity they involve: the former is mainly 2D while the latter is highly 3D. The blunt trailing edge configuration is analyzed by using rakes of hot wire probes. This flow configuration is found to be considerably different when compared to a conventional mixing layer. It appears in particular that the scale of the large structures depends only on the trailing edge thickness and does not grow in its downstream evolution. A criterion, based on POD, is proposed in order to separate wake–mixing layer dominant areas of the downstream evolution of the flow. The complex 3D dynamical behaviour resulting from the interaction between the canonical plane mixing layer and the wake of a cylinder is investigated using data arising from particle image velocimetry measurements. An analysis of the velocity correlations shows different length scales in the regions dominated by wake like structures and shear layer type structures. In order to characterize the particular organization in the plane of symmetry, a POD-Galerkin projection of the Navier–Stokes equations is performed in this plane. This leads to a low-dimensional dynamical system that allows the analysis of the relationship between the dominant frequencies to be performed. A reconstruction of the dominant periodic motion suspected from previous studies is then retrieved.

International audience; The flow over a ramp model is characterised in detail in the present study. In the selected configuration, a turbulent boundary layer developed on a flat plate is first accelerated in a curved contraction. It is then submitted to a mild adverse pressure gradient on a flat plate followed by a separation above a flap. Inlet boundary conditions and pressure distribution are provided to allow numerical simulations. The flow in the mild adverse pressure gradient region is characterised with hot-wire anemometry. In this region, the boundary layer thickness is of the order of 20 cm, the momentum Reynolds number is about 11,000 and the Clauser pressure gradient parameter β in the stabilised region is about 0.4. Particular emphasis is laid on the separation to provide quantitative information to evaluate turbulence models. This is achieved through a large streamwise two-dimensional two-component particle image velocimetry (2D2C PIV) plane which contains all the separation bubble and part of the flow upstream and downstream of it. The separation border is detected using the backflow coefficient, resulting in a separation length of about 3.49 Hs (with Hs the flap step height) and a maximum height of about 0.17 Hs. The Reynolds stresses and their main production terms are also determined. A region of high turbulence intensity develops above the separation border for all the measured components. The production of dominates the production of turbulent kinetic energy which implies a redistribution from to to explain the increase observed. The production term drives the production of in the first part of the flap which is not the case for zero pressure gradient boundary layers. Finally, a high similarity is observed between and as the production of the latter is dominated by . This flow appears as a challenging test case for Reynolds averaged Navier–Stokes (RANS) and large eddy simulation (LES) validation.

Open and closed-loop flow control experiments were performed on the transient attachment and separation mechanisms of a thick turbulent boundary layer (TBL). Without actuation, the TBL is subjected to an adverse pressure gradient and separates downstream of a sharp variation in the wall geometry. Departing from a given geometry and steady operations of vortex generator actuators, the control objective was to attach the flow in the separated region with a minimum of injected fluid using adaptation of the closed-loop control. The large scale of the facility (i.e., δ = 20 cm upstream of separation) induces large time scales and large Reynolds numbers of the flow to be controlled. It is found to consequently induce large time scales of the separation/attachment mechanisms, making the dynamic closed-loop implementation easier. Open-loop tests were first performed to extract the adequate input/output variables for closed-loop implementations. The chosen input variable was the Duty Cycle, DC, which enables sending of a control action at least 10 times faster than the time scales of the attachment/separation process. The chosen output variable was the voltage signal from a hot-film probe located on the flap which characterizes the degree of separation. In open loop, both the large scale (i.e., large time scales) of the present facility (Carlier and Stanislas in J Fluid Mech 535(36):143–188, 2005) and the well-defined excitation (Braud and Dyment in Phys Fluids 24:047102, 2012) help to extract the different time scales involved and to identify the whole system (actuators, baseline flow and sensor). Three Reynolds numbers based on the momentum thickness of the boundary layer near the actuators and upstream of separation were investigated (Re θ = 7,500, 10,500 and 12,600) through variation of the free-stream velocity (U ∞ = 5, 8, 10 m/s). These three systems were found to behave like first-order linear systems, with coefficients that need to be adapted depending on the Reynolds number. From Re θ = 7,500 to Re θ = 12, 600, the time scale and static gain of the linear system needed to be almost doubled. A simple controller (Proportional-Integral) was implemented in closed-loop configuration, improving the reactivity of the system. Robustness was tested by varying the free-stream velocity. Closed-loop control based on a fixed reference was unsuccessful as it failed to account for the effect of the Reynolds number. This was successfully overcome by tracking a given state of the flow using a simple P controller to adapt the reference according to variations of Re. The P controller, acting on the DC variable, compensates the corresponding variations of VR (ratio between the free-stream and the jet exit velocity).

International audience; In the present contribution a physical model of the flow occurring inside impulsive actuators is developed. An impulsive actuator is characterized by its sudden flow output. The subsonic actuator under consideration is composed of a high pressure air supply and a sonic orifice driven by a valve and followed by a tube. A theoretical analysis is put forward and assessed using experimental data. It is demonstrated that the operation of such actuators essentially depends on a single parameter consisting of the reservoir to ambient pressure ratio and the throat to tube area ratio. The relationship between this parameter and others, such as the length to diameter ratio of the tube are explored. In the unsteady mode the flow inside the tube is thoroughly analyzed with the help of measurements at the exit of the tube. In addition, the time scales involved are accurately extracted, which is a crucial factor in designing closed-loop control approaches. The results obtained therefore provide an important design tool for engineering problems in flow control. Results are also applicable in the more general context of fluidic networks containing a sonic nozzle.

International audience; The flow in a converging-diverging channel at Reτ = 180 is studied by Direct Numerical and Large Eddy Simulations. Continuous and pulsed jets are applied normally to the wall through a spanwise slot to delay the separation. The influence of the position is first analysed and it is shown that the control efficiency is closely linked to the location of the actuation within the channel. A parametric study on both the intensity and pulsating frequency for the "optimal" position shows that the separation can be drastically reduced and even vanishes with a proper choice of parameters. It is additionally reported that pulsed jets yield better results than continuous jets at an equivalent mass-flux. Moreover, low frequencies are shown to be more efficient than high frequencies. Aphysical analysis finally demonstrates that the efficiency of the low frequency pulsed jets is mainly due to the creation of spanwise structures, that does not occur in other cases.

International audience; The objective of the present work is to analyse the behaviour of a turbulent decelerating boundary layer under the effect of both passive and active jets vortex generators (VGs). The stereo PIV database of Godard and Stanislas [1,2] obtained in an adverse pressure gradient boundary layer is used for this study. After presenting the effect on the mean velocity field and the turbulent kinetic energy, the line of analysis is extended with two points spatial correlations and vortex detection in instantaneous velocity fields. It is shown that the actuators concentrate the boundary layer turbulence in the region of upward motion of the flow, and segregate the near-wall streamwise vortices of the boundary layer based on their vorticity sign.

This paper deals with the fluidic actuators in the pulsed mode used in turbulent boundary layer control, in particular as vortex generator (VG) in order to delay separation. Recently the study by Kostas et al (2007) has shown the importance of the transient phase of the VG actuators. In particular, an enhancement of the vortex-generation mechanism has been observed during the transient period, that is responsible for an increase of turbulence stress levels up to 200% relatively to the non actuated case. A large dependency of the exit velocity on the physical characteristic of the feed tube has been detected. This dependency suggests that a precise quantification of the pulsed jet dynamic during the transient period is necessary. In this work the transient behavior of the fluidic actuators used by Kostas et al (2007) is analyzed and experimented. A model is developed to explain the dynamics of the flow inside the actuator. On the whole, experiments show that the role of all physical parameters is consistent with the foreseen properties. The results obtained help to separate the input dynamic of the controller from the controlled flow. Another perspective of this work is to provide a guide for the design of fluidic actuators.Copyright © 2008 by ASME

The perturbations of localised ultra-clean air delivery systems due to continual access of operators to handle products have been investigated. It was exhibited that the keystone of localised air delivery systems was a turbulent mixing layer modeling the interface between the polluted and the clean air. Furthermore the obstacle penetrating into the clean air flow was represented by a generic shape, the circular cylinder. Hence the problem of perturbation was approached by studying the interaction between the wake of a circular cylinder and a turbulent plane mixing layer. From the analysis of experimental results a local injection of contaminant has beeen highlighted. Actions to prevent that pollution were deduced and studied on the Progressive Flux, a localised delivery system developped by the Cemagref. The inherent drawback of uni-directional flow systems was demonstrated and explained., Les conséquences d'une effraction par un bras d'opérateur, au sein d'un dispositif de protection localisée par flux d'air ultra-propre, sont étudiées. Il est montré que la maîtrise de l'interface de mélange entre l'air pollué ambiant et l'air ultra-propre soufflé est la clé de voûte des dispositifs de protection localisée. L'obstacle pénétrant le flux d'air propre est modélisé par un cylindre circulaire. L'analyse du problème d'effraction revient étudier l'interaction entre une couche de mélange plane turbulente et un sillage de cylindre circulaire. Les observations expérimentales ont permis d'identifier un transfert local de polluants liés à une structuration cohérente de l'écoulement en sillage proche. Des actions pour prévenir cette pollution sont déduites et étudiées sur le cas d'un dispositif de protection localisée, le Flux Progressif. Les inconvnients propres aux systèmes de soufflages uni-directionnels sont montrés et expliqués.