The complex aerodynamic interactions between the rotor and the duct has to be accounted for the design of ducted wind turbines (DWTs). A numerical study to investigate the characteristics of flow around the DWT using a simplified duct–actuator disc (AD) model is carried out. Inviscid and viscous flow calculations are performed to understand the effects of the duct shape and variable AD loadings on the aerodynamic performance coefficients. The analysis shows that the overall aerodynamic performance of the DWT can be increased by increasing the duct cross-sectional camber. Finally, flow fields using viscous calculations are examined to interpret the effects of inner duct wall flow separation on the overall DWT performance.

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We present a multidisciplinary design optimization method for the profile and structural reinforcement layout of a ram‐air kite rib. The aim is to minimize the structural elastic energy and to maximize the traction power of a ram‐air kite used for airborne wind energy generation. Because of the large deformations occurring during flight, a fluid‐structure interaction (FSI) routine is included in the optimization, which determines the actual deformed rib geometry and its corresponding aerodynamic characteristics. A qualitative comparison between FSI inclusion and exclusion in the optimization is given. Discrepancies in airfoil profile and structural layout are observed.@en

This thesis investigates the efficiency of wind energy conversion from three aerodynamic perspectives. The first, purely theoretical, assumes steady inviscid flow conditions to identify multi-rotor setups that can extract more power per unit area than isolated rotors. The second, more practical perspective, assesses the extent to which site-tailored airfoils could reduce the cost of wind energy. Finally, the third perspective considers data-driven opportunities to better predict airfoil and vortex-generator flows relevant to wind turbine load calculations.@en

This paper aims to study the aerodynamic performance of ducted wind turbines (DWT) using inviscid and viscous flow calculations by accounting for the mutual interaction between the duct and the rotor. Two generalized duct cross section geometries are considered while the rotor is modelled as an actuator disc with constant thrust coefficient. The analysis shows the opportunity to significantly increase the overall aerodynamic performance of the DWT by a correct choice of the optimal rotor loading for a given duct geometry. Present results clearly indicate that the increased duct cross section camber leads to an improved performance for a DWT. Finally, some insights on the changes occurring to the performance coefficients are obtained through a detailed flow analysis.

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The accuracy of airfoil polar predictions is limited by the usage of imperfect turbulence models. Can machine-learning improve this situation? Will airfoil polars teach the effect of turbulence on skin-friction? We try to answer these questions by refining turbulence treatment in the Rfoil code: boundary layer closure relations are learned from airfoil polar data. Two turbulent closure relations, for skin friction and energy shape factor, are parametrized with a class-shape transformation. An experimental database is then used to define code inaccuracy measures that are minimized with an interior point gradient algorithm. Results show that airfoil polars contain exploitable information about turbulent phenomena. Inferred closures agree with direct numerical simulation results of skin friction and the new code predicts drag more accurately. Maximum lift remains under-predicted but Rfoil maintains its robustness and suitability for optimization of wind energy airfoils.

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We describe a probabilistic approach to design airfoils for wind energy applications. An analytical expression is derived for the probability of perturbations to the operational blade-section angle of attack. It includes the combined influence of wind shear, yaw-misalignment, and turbulence intensity. The theoretical fluctuations in angle of attack are validated against an aero-structural simulation of a 10 MW horizontal axis wind turbine, operating under different inflow conditions. Finally we incorporate the probabilistic approach into a multi-objective airfoil optimization problem, which is solved with a genetic algorithm. The results illustrate the compromise between airfoil performance for a specific angle of attack and robustness of airfoil performance over a large range of angle of attack fluctuations@en

The practice of ducting wind turbines has shown a beneficial effect on the overall performance, when compared to an open turbine of the same rotor diameter1. However, an optimization study specifically for ducted wind turbines (DWT’s) is missing or incomplete. This work focuses on a numerical optimization of the duct orientation and the ideal loading coefficient for the rotor. A 2D planar geometry was employed to model the DWT and the rotor is modelled as an uniformly loaded actuator disc (AD). The flow-field around the DWT is obtained through numerical solutions of Reynolds-averaged-Navier-Stokes (RANS) equations2 and a steady state Lagrangian approach based on vortex ring method3 . The study determines the optimal angle of attack for the duct corresponding to the AD loading, in order to achieve the optimal performance for a given DWT configuration. @en

Flow augmentation consists in modifying mass flow across the actuation plane of a rotor to enhance energy extraction or propulsive efficiency. The talk sketches the distinction between passive and active rotor augmentation strategies. Power coefficient trends are compared analytically while numerical results illustrate differences in flow topology. Rotors are stylized as actuator disks that exert homogeneous normal forces on the steady flow of inviscid fluids to highlight the distinctive features of each augmentation principle. Passive augmentation principles have been well documented because they guide the design of ducted, shrouded and diffuser-augmented wind turbines1-6. These axisymmetric bodies decrease average static pressures on the rotor plane to increase mass flux and power coefficient. Rotor-body interactions are dominated by conservative forces5,7: the bodies don’t exchange energy with the fluid but act as augmenting devices and affect global energy balance by changing rotor state. Virtual work arguments show that bodies exert streamwise forces4,6 that can be related with the power coefficient through the law of de Vries1,6. Active flow augmentation is a rather recent theoretical concept8. Its simplest energy extraction embodiment consists of an upstream actuator that accelerates flow onto a downstream actuator. This augmentation strategy is coined as active because the upstream actuator injects (spends) energy into the flow for the downstream actuator to extract (produce) energy from a greater mass flux than if it were alone. The interaction mechanism depends on the action of non-conservative forces and actuators interact exclusively through changes in total flow enthalpy when they are sufficiently far apart. No pressure interactions occur in this asymptotic case and a closed solution exists together with an analytical power coefficient law. Parallels can be drawn with wake ingestion propeller setups9 but no practical energy extraction realizations have been attempted yet. Passive and active flow augmentation concepts are different but we hope that parallels between them shed further light on the physics of energy extraction from ideal fluid flows. The communication concludes with a few reflections meant to trigger an open discussion about the implications and applicability of the discussed theories. @en

The framework of self-similar laminar boundary layer flow solutions is extended to include the effect of actuation with body force fields resembling those generated by DBD plasma actuators. The deduction line is similar to previous work investigating the effect of porous wall suction on laminar boundary layers. The starting point of the analysis is a generalised form of the Boundary Layer Partial Differential Equations (BL-PDEs) that includes volume force terms. Actuation force distributions are defined such that the volume force term of the BL-PDE equations conforms to the requirements of similarity. New similarity parameters for the plasma strength and thickness are identified. The procedure yields a general similarity equation which includes the effect of pressure gradients, wall transpiration and DBD plasma actuation. Select numerical solutions of the new similarity equation are presented to develop instinctive understanding and prompt a discussion on the construction of new closure relations for integral boundary layer models.

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In 2016 Uwe Fechner presented the paper "Downscaling of Airborne Wind Energy Systems" [1] at the Torque conference in Munich. Here, we report on our efforts in the development of small-scale Airborne Wind Energy (AWE) systems with vertical launch and landing. We are trying to accelerate development of small-scale Airborne Wind Energy systemsby providing the key components that are always needed but not yet available off-the-shelf. We aim at reducing development costs for any AWE startup, but also to provide components and systems for educational purposes. The first part our presentation we explain how cooperation with other startups is shaping the structure of our R&D programs. We have been supplying control systems and simulation models to the Dutch startup ekite and the Swiss startup SkyPull. A case study discusses how component-suppliers help AWE system integrators achieve leaner development cycles. The role of scale economies, specialization and institutional arrangements is assessed in terms of their impact on product development cycle and cost. The second part of the presentation discusses technical challenges of hardware and software component development for the AWE industry. We plan to offer four products in 2017: a fast and reliable wireless link (Ariadne), a flight control computer tailored for the needs of the airborne wind energy industry (Athena), a ground control computer and a small scale ground-station (1.4 kW continuous electrical power, total mass below 30 kg). Distributed control challenges will be reviewed together with opportunities for recycling know-how from the conventional drone industry. We are integrating Pixhawk (hardware) and PX4 (software) stacks within a Linux based framework that was developed from scratch to handle the specific needs of airborne wind energy. All systems are resilient and share the transversal concern of enabling safe launch and landing procedures in both regular, strong and turbulent wind conditions. Wind tunnel validation is planned for the end of the year and involves both internal aerodynamic know-how and cooperation with leading European wing designers. @en

In 2016 Uwe Fechner presented the paper "Downscaling of Airborne Wind Energy Systems" [1] at the Torque conference in Munich. Here, we report on our efforts in the development of small-scale Airborne Wind Energy (AWE) systems with vertical launch and landing. We are trying to accelerate development of small-scale Airborne Wind Energy systemsby providing the key components that are always needed but not yet available off-the-shelf. We aim at reducing development costs for any AWE startup, but also to provide components and systems for educational purposes. The first part our presentation we explain how cooperation with other startups is shaping the structure of our R&D programs. We have been supplying control systems and simulation models to the Dutch startup ekite and the Swiss startup SkyPull. A case study discusses how component-suppliers help AWE system integrators achieve leaner development cycles. The role of scale economies, specialization and institutional arrangements is assessed in terms of their impact on product development cycle and cost. The second part of the presentation discusses technical challenges of hardware and software component development for the AWE industry. We plan to offer four products in 2017: a fast and reliable wireless link (Ariadne), a flight control computer tailored for the needs of the airborne wind energy industry (Athena), a ground control computer and a small scale ground-station (1.4 kW continuous electrical power, total mass below 30 kg). Distributed control challenges will be reviewed together with opportunities for recycling know-how from the conventional drone industry. We are integrating Pixhawk (hardware) and PX4 (software) stacks within a Linux based framework that was developed from scratch to handle the specific needs of airborne wind energy. All systems are resilient and share the transversal concern of enabling safe launch and landing procedures in both regular, strong and turbulent wind conditions. Wind tunnel validation is planned for the end of the year and involves both internal aerodynamic know-how and cooperation with leading European wing designers. @en

This study describes a methodology for designing airfoils suitable to employ actuation in a wind energy environment. The novel airfoil sections are baptized wind energy actuated profiles (WAP). A genetic algorithm-based multi-objective airfoil optimizer is formulated by setting two cost functions: one cost function for wind energy performance and the other representing actuation suitability. The wind energy cost function compares the candidate airfoils' performance with 'reference' wind energy airfoils, considering a probabilistic approach to include the effects of turbulence and wind shear. The actuation suitability cost function is developed considering horizontal axis wind turbines active stall control, including two different control strategies designated by 'enhanced' and 'decreased' performance. Two different actuation types are considered, namely, boundary layer transpiration and dielectric barrier discharge plasma. Results show that using WAP airfoils provides much higher control efficiency than adding actuation on reference wind energy airfoils, without detrimental effects in non-actuated operation. The WAP sections yield an actuator employment efficiency that is two to four times larger than those obtained with reference wind energy airfoils, at equivalent wind energy performance. Regarding geometry, and compared with typical wind energy airfoils, WAP sections for decreased performance display an upper surface concave aft region, while for increased performance, a convex upper surface aft region is obtained. The present study emphasizes that there is much to gain in designing airfoils from the beginning to include actuation effects, especially compared with employing actuation on already existing airfoils. The results demonstrate the potential of including actuation effects in the airfoil design process, thus enabling novel horizontal axis wind turbines control strategies.

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This study describes a methodology for designing airfoils suitable to employ actuation in a wind energy environment. The novel airfoil sections are baptized wind energy actuated profiles (WAP). A genetic algorithm-based multi-objective airfoil optimizer is formulated by setting two cost functions: one cost function for wind energy performance and the other representing actuation suitability. The wind energy cost function compares the candidate airfoils' performance with 'reference' wind energy airfoils, considering a probabilistic approach to include the effects of turbulence and wind shear. The actuation suitability cost function is developed considering horizontal axis wind turbines active stall control, including two different control strategies designated by 'enhanced' and 'decreased' performance. Two different actuation types are considered, namely, boundary layer transpiration and dielectric barrier discharge plasma. Results show that using WAP airfoils provides much higher control efficiency than adding actuation on reference wind energy airfoils, without detrimental effects in non-actuated operation. The WAP sections yield an actuator employment efficiency that is two to four times larger than those obtained with reference wind energy airfoils, at equivalent wind energy performance. Regarding geometry, and compared with typical wind energy airfoils, WAP sections for decreased performance display an upper surface concave aft region, while for increased performance, a convex upper surface aft region is obtained. The present study emphasizes that there is much to gain in designing airfoils from the beginning to include actuation effects, especially compared with employing actuation on already existing airfoils. The results demonstrate the potential of including actuation effects in the airfoil design process, thus enabling novel horizontal axis wind turbines control strategies.

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We explore integral boundary layer approximations for shear layer flows with vortex generators. The flow field is decomposed to highlight two phenomena: shear over the wall and vortex-driven mixing of the shear layer. The Navier-Stokes Equations are normalized to identify a new adimensional parameter: the vortex strength number (Vg). Usual boundary layer scales are valid when the vortex strength number (Vg) is of order one or smaller. New Boundary Layer Equations comprising the effect of streamwise vortex filaments are obtained and integrated accross a periodic vortex cell. The new integral equations share their structure with the original Von Karmann Integral Equations but use different variables. The deduction concludes with an approximate interaction equation for the construction of generalized closures from the classic set of Swafford turbulent closure relations. The new formulation is solved numerically and it is compatible with future integration in the Xfoil or Rfoil viscous-inviscid airfoil analysis codes.@en

A fast, linear scaling vortex method is presented to study inviscid incompressible flow problems involving one or more actuator disks. Building upon previous efforts that were limited to axi-symmetric flow cases, the proposed methodology is able to handle arbitrary configurations with no symmetry constraints. Applications include the conceptual study of wake interaction mechanisms in wind farms, and the correction of wind tunnel blockage effects in test sections of arbitrary shape. Actuator disks represent wind turbines through the shedding of a deformable vortex wake, discretized with a plaid of triangular distributed dipole singularities. An iterative method is adopted to align the wake with the local flow field, which is reconstructed from the vorticity field with a Green function approach. Interactions are computed with a Fast Multipole Method (FMM), effectively overcoming the quadratic scaling of computational time associated with traditional panel methods. When compared to direct computation, the use of an FMM algorithm reduced solution time by a factor 30 when studying the wake of a single actuator disk with 60000 panels. In the same case, the mass flux of the actuator streamtube was conserved to 0:002%. Finally, the presence of round and square impermeable walls around the actuator is considered to demonstrate the code applicability to wind tunnel wall interference correction problems. @en

Pumping-mode airborne wind energy systems (PM-AWE) consist of an airborne drone (UAV) that flies tethered to a ground station. The tethered UAV is expected to generate high lifts when reeling the tether out, descend with low force and damp gusts during take-off or landing events. These missions take place in unpredictable environmental conditions. Winds can be more or less turbulent, ground proximity prompts leading edge soiling and aerostructural effects induce shape deformations. Conventional aviation airfoils were not designed for these conditions but wing designers miss specialized alternatives. The need for specialized airborne wind energy airfoils was first identified by Venturato [1]. He redesigned the Clark Y airfoil with a genetic optimization approach based on a simple performance goal and a RANS CFD solver. This type of CFD model cannot capture important physical phenomena like laminar bubbles or turbulent transition but Venturato’s work had the groundbreaking merrit of raising awareness about specialized airfoils. Here we present a collection of airfoils designed for the needs of the airborne wind energy industry. The design exercise builds upon an established airfoil optimization framework with a proven track record in the conventional wind turbine industry [2,3,4,5]. The method is known to provide a broad coverage of the design space thanks to the use of a CST parametrization, a tuned version of the RFOIL viscous-inviscid solver and multi-objective genetic optimization algorithms. Candidate airfoils are assessed in terms of conflicting structural and aerodynamic goals. Pareto fronts quantify compromises between glide ratio, maximum lift, building height, stall harshness and resilience to leading edge soiling. Finally, desirable pitching moment characteristics are framed within the broader question of planform design, a question on which we hope to engage the audience in a lively discussion. @en

There are many ways to learn from data. Our first experiment consisted in reproducing the way aerodynamicists work [2] with a genetic optimizer. The data pool was too narrow and asymptotic tendencies were unreliable. Our 2nd Experiment, a simple version of [4], had a virtually unlimited data pool and used neural networks. Results were better, but computationally expensive. Data assimilation approaches used in EO [ 7] could yield better results..@en

The article seeks to unify the treatment of conservative force interactions between axi-symmetric bodies and actuators in inviscid ow. Applications include the study of hub interference, di_user augmented wind turbines and boundary layer ingestion propeller con_gurations. The conservation equations are integrated over in_nitesimal streamtubes to obtain an exact momentum model contemplating the interaction between an actuator and a nearby body. No assumptions on the shape or topology of the body are made besides (axi)symmetry. Laws are derived for the thrust coe_cient, power coe_cient and propulsive e_ciency. The proposed methodology is articulated with previous e_orts and validated against the numerical predictions of a planar vorticity equation solver. Very good agreement is obtained between the analytical and numerical methods@en